Boosting Photocatalytic Activity Using Carbon Nitride Based 2D/2D van der Waals Heterojunctions
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The document reviews advancements in using carbon nitride-based 2D/2D van der Waals heterojunctions for enhancing photocatalytic activity amid rising energy demands and environmental pollution. It discusses the potential of 2D semiconductor materials, particularly graphitic carbon nitride, in improving charge separation and photon absorption to effectively utilize solar energy for photoredox reactions. The authors conclude by highlighting existing challenges and future research opportunities in the field of photocatalysis.
![of P−C and P−N bonds, resulting in record hydrogen
evolution performance than previously reported for carbon
nitride−phosphorus based systems (2565 μmol g−1
h−1
).210
In
another report, [001]-oriented Hittorf’s phosphorus (HP)
nanorods were fabricated on g-C3N4 using vapor deposition
approach reaching a H2 evolution rate (HER) of 33.2 μmol h−1
from pure water.211
The 2D/2D vdW heterojunction of RP and g-C3N4 is
particularly important as it can overcome the shortcomings of
other dimensionalities due to intimate electronic contact
between two semiconductor surfaces. Wang and coauthors
synthesized 2D/2D RP/CN vdW catalysts via low temperature
(300 °C) in situ phosphorizations on CN sheets (prepared via
hydrothermal exfoliation of bulk CN in NH4OH) (Figure
7a).212
The thermal decomposition of NaH2PO2 followed by
doping and deposition leads to deposition of a uniformly thick
RP layer (∼4.6 nm) on CN (Figure 7b,c). The HR-TEM and
corresponding EDX elemental mapping display the presence of
RP nanosheet fragments on the surface of CN. The XRD peak
of CN was significantly suppressed and slightly shifted after the
formation of the heterostructure, suggesting dense surface
coverage via RP. The RP/CN vdW showed a broad visible
absorption extended up to NIR while XPS peaks in the C 1s
and N 1s region were shifted toward a positive value,
suggesting better intimate contact and charge transfer (Figure
7d). The conduction and the valence band positions of CN
and RP determined using the Mott−Schottky plot were −0.96
and 1.83 vs RHE and −0.22 and 1.87 V, respectively. In
comparison to constituting elements and physical mixture of
RP and CN, the RP/CN vdW heterostructure displayed an
exception with the enhancement of H2 evolution, reaching a
value of 367.0 μmol g−1
h−1
(Figure 7e). Further, the RP/CN
displayed activity in all visible light ranges up to 620 nm with
long hour stability (Figure 7f). However, the stability of the
catalyst was reduced after a long run at high temperatures due
to the formation of H2O2 instead of O2, oxidizing the RP
surface. To validate this hypothesis, when MnO2 was
introduced into the reaction system, the O2 evolution rate
was increased due to the decomposition of H2O2 in the
presence of MnO2 (Figure 7g). A significant PL quenching and
decreased charge transfer resistance suggest a better charge
separation in the RP/CN vdW heterostructure. Due to more
negative CB and VB of CN, a type II (staggered)
heterojunction was formed where the electron flows from
CB of CN to RP, and the holes move in the opposite direction,
leading to better carriers separation (Figure 7h).
2.3. Carbon Nitride−Antimonene. Antimonene (Sb) is a
relatively newly discovered 2D elemental semiconductor that
gained significant attention due to its remarkable electronic
and optical properties.213,214
The 2D structure of antimonene
was first predicted theoretically in 2015 by Zhang et al. along
with arsenene (As).215
However, the experimental synthesis of
Sb was only realized in 2016 by Gibaja et al. by liquid-phase
exfoliation of Sb crystal in a water/ethanol mixture.216
After
that, several procedures to exfoliate Sb have been developed,
including mechanochemical and sonochemical methods.217,218
Additionally, high-quality Sb up to a single atom thickness can
be synthesized using epitaxial growth, which includes van der
Waals epitaxy and molecular beam epitaxy (MBE).219
Employing harsh sonication conditions, using NMP, polyols
such as PEG, and surfactants, well stabilized antimonene
quantum dots can be isolated.220−222
Van der Waals epitaxial
Figure 7. (a) Schematic diagram for fabricating the 2D/2D RP/CN heterostructure. (b) AFM image of RP/CN. (c) Corresponding height profiles
along the denoted lines. (d) UV−vis spectra for CN nanosheets, RP/CN, and commercial RP (inset, the digital photographs). (e) H2 evolution
rates of as-prepared samples under full arc irradiation. (f) H2 evolution rates of RP/CN under the incident light with varied wavelength ranges. (g)
H2 and O2 production over time after the light was turned off (MnO2 added into the reaction suspension after the first 2 h). (h) Schematic diagram
of a photocatalytic process for RP/CN. Reprinted with permission from ref 212. Copyright 2020 American Chemical Society.
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![Table
3.
continued
photocatalyst
synthesis
application
light
source
AQY/STH
remarks
ref
Metal
Oxides
ML
g-C
3
N
4
3200
μmol
g
−1
h
−1
Fe
2
O
3
/g-C
3
N
4
direct
Z-
scheme
Electrostatic
self-assembly
of
g-C
3
N
4
nanosheet
and
Fe
2
O
3
.
Photocatalytic
H
2
evolu-
tion
350
W
Xe
lamp
(λ
≥
420
nm)
-
H
2
398.0
μmol
g
−1
h
−1
(Fe
2
O
3
/g-C
3
N
4
)
331
∼13-fold
that
of
pure
g-
C
3
N
4
(30.1
μmol
g
−1
h
−1
)
2D
α-Fe
2
O
3
@g-C
3
N
4
Ultrasonic
assisted
self-assembly
method:
2D
g-C
3
N
4
and
2D
α-Fe
2
O
3
nanosheets
and
Nafion
solution
as
stabilizing
agents
were
mixed
ultrasonically.
Photocatalytic
degrada-
tion
RhB
500
W
halo-
gen
lamp
(λ
≥
420
nm)
-
α-Fe
2
O
3
@g-C
3
N
4
90%
RhB
degradation
after
120
min
332
g-C
3
N
4
26%
RhB
deg-
radation
after
120
min
g-C
3
N
4
/{010}
facets
BiVO
4
BiVO
4
and
g-C
3
N
4
were
stirred
together.
Photocatalytic
degrada-
tion
of
RhB
500
W
Xe
lamp
-
g-C
3
N
4
/{010}
BiVO
4
88.3%
RhB
degradation
in
30
min
340
BiVO
4
22.66%
RhB
degradation
in
30
min
g-C
3
N
4
/BiVO
4
Z-
scheme
BiCl
3
and
CTAB
solution
in
EG
were
mixed
with
Na
3
VO
4
·12H
2
O
followed
by
the
addition
of
g-C
3
N
4
ultrathin
nanosheets,
and
finally
the
mixture
was
hydrothermally
treated
at
160
°C
for
3
h.
Photocatalytic
CO
2
re-
duction
300
W
Xe
lamp
(λ
≥
420
nm)
-
CH
4
27.43
μmol
g
−1
(g-
C
3
N
4
/BiVO
4
)
341
∼4.8
times
of
g-C
3
N
4
(5.76
μmol
g
−1
)
(CH
4
)
CO31.15
μmol
g
−1
(g-
C
3
N
4
/BiVO
4
)
∼4.4
times
of
g-
C
3
N
4
(7.14
μmol
g
−1
)
(CO)
Porous
g-C
3
N
4
/
Ag
3
VO
4
(Pg-C
3
N
4
/
Ag
3
VO
4
)
Pg-C
3
N
4
and
AgNO
3
were
stirred
for
30
min
followed
by
the
addition
of
Na
3
VO
4
and
stirring
for
6
h;
the
mixture
was
freeze-dried
after
aging
for
4.5
h.
Photocatalytic
degrada-
tion
of
MB
50
W
410
nm
LED
-
40%
Pg-C
3
N
4
/
Ag
3
VO
4
99.3%
MB
degradation
in
8
min
342
Pg-C
3
N
4
5%
MB
degrada-
tion
in
8
min
ZnV
2
O
6
/g-C
3
N
4
[NH
4
VO
3
],
[Zn(O
2
CCH
3
)
2
],
and
DMF
were
mixed,
followed
by
the
addition
of
[H
2
C
2
O
4
·2H
2
O]
in
a
ratio
of
oxalic
acid
to
NH
4
VO
3
of
1:3.
Later,
protonated
g-C
3
N
4
(pCN)
was
added
and
hydrothermally
treated
at
200
°C
for
24
h.
Photocatalytic
CO
2
re-
duction
35
WHID
Xe
lamp
CH
3
OH0.0021;
CO0.028;
H
2
0.0029
(450
nm)
3742.19
μmol
gcat
−1
(ZnV
2
O
6
/100%
pCN)
353
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![yield of 0.2830, which was 3.5 times higher than that of the
binary ZnV2O6/pCN system (0.081).
5. CARBON NITRIDE−LAYERED DOUBLE
HYDROXIDES (LDHS) 2D/2D vdW STRUCTURES
Layered double hydroxides (LDHs), also called hydrotalcite-
like materials (structural similarity with [Mg6Al2(OH)16]CO3·
4H2O), or anionic clays is a class of layered materials with a
chemical formula [M1−x
2+
Mx
3+
(OH)2]2x+
(An−
)x/n·yH2O, where
M2+
is divalent metal cations (Mg2+
, Zn2+
, Co2+
, Mn2+
, Ni2+
, or
Ca2+
) and M3+
is trivalent metal cations (Fe3+
, Cr3+
, and Al3+
,
etc.) occupying octahedral positions within the hydroxide
layers, An−
is the nonframework exchangeable interlayer n-
valent anions with highly chemical reactivity (Cl−
, ClO4
−
,
NO3
−
, CO3
2−
, SO4
2−
, etc.), and x is the molar ratio of M2+
/
(M2+
+ M3+
).355−358
Due to its layered structure, several guest
molecules have been intercalated in between galleries of LDHs
for various applications. The presence of a basic site on LDHs
makes them suitable catalysts to promote base-catalyzed
reactions without using harsh alkaline conditions.359,360
For
example, La-doped Ca−Mg−Al layered double hydroxide (La-
CaMgAl-LDH) can catalyze base-free aerobic oxidation of
HMF to FDCA in water.361
In another example, cobalt
phthalocyanine-assisted oxidation of thiols to disulfide usually
takes place under alkaline conditions. However, when a
magnetically recyclable MgAl-LDHs tethered phthalocyanine
was used, oxidation can occur without using an alkali.362
Interestingly, in the recent years, several visible light active
LDHs have been synthesized by changing the metal
combination (Zn/Cr, Mg/Cr, Zn/Fe, Mg/Fe, Cu/Cr, Co/
Cr), doping (Cu, Ni, Zn), and intercalated ions in between
brucite layers. The simplicity of LDH synthesis by
coprecipitation provides an opportunity to play with
compositions, and many fractional composition LDHs have
been reported for the photocatalytic applications, especially
CO2 reduction due to the weak acidic nature of CO2.363,364
Izumi and co-workers have reported several novel LDHs such
as [Zn3Ga(OH)8]+
2[Cu(OH)4]2−
·mH2O, [Zn1.5Cu1.5Ga-
(OH)8]2
+
[Cu(OH)4]2−
·mH2O, Zn−Al LDH, etc. for photo-
conversion of CO2 to value-added chemicals.365−368
In LDHs, brucite layers remain together due to attraction
between the positively charged brucite layers and the
negatively charged interlayers of H-bonded metal hydroxide
and the oxygen atoms of the intercalated anions. Through
strong force between the LDH layer, methods such as
intercalation with small molecules (formamide, glycine)
followed by sonication, anion exchange, plasma-induced
exfoliation, etc. have been developed to convert the LDHs
into 2D sheets through delamination.369−372
These 2D sheets,
due to high surface area, rich active sites, and excellent visible
absorption, can induce various photochemical reactions. The
face-to-face interaction of LDHs with carbon nitride to make
the vdW heterojunction has been found to boost photo-
catalytic performance.373−375
The potential of such a
combination has already been traversed by Song et al.129
Within a short span, a few more reports have emerged on the
2D/2D heterojunction/close contact of carbon nitrides and
LDHs.376−387
Among various LDHs, bimetallic NiAl-LDH has been
proven as the most efficient catalyst due to its excellent visible
absorption profile originating from ligand-to-metal charge
transfer (LMCT)(O → Ni2+
).388,389
However, d−d transitions
of the Ni2+
interelectronic excitation pathway hinder the
photocatalytic performance due to reduced carrier effi-
ciency.390,391
The charge carrier separation can be improved
by making heterojunctions, i.e., the coupling of n-type NiAl-
LDHs with p-type CuFe2O4 can achieve an excellent charge
separation and produce H2 (345.76 μmol h−1
) almost 7 times
the values of pristine NiAl-LDHs.392
Similarly, the coupling of
NiAl-LDHs with β-In2S3 leads to an increment of CO2
reduction performance. Interestingly, g-C3N4 can make a
2D/2D interface with sharp edges even without converting
LDHs into single-layered sheets.379,393−395
To crop the tunability of carbon nitride to make a
heterojunction, Tonda et al. synthesized a g-C3N4/NiAl-
LDH nanocomposite via in situ hydrothermal depositions of
Figure 17. (a) Schematic illustration of the synthesis process of g-C3N4/NiAl-LDH hybrid heterojunctions. TEM images of (b) NiAl-LDH, (c, d)
CNLDH-10, and (e) HRTEM image of the CNLDH-10 heterojunction. (f) UV−vis DRS of g-C3N4, NiAl-LDH, and g-C3N4/NiAl-LDH
heterojunction samples. (g) Schematic illustration of the proposed mechanism for CO2 photoreduction in the g-C3N4/NiAl-LDH heterojunctions.
Reprinted with permission from ref 396. Copyright 2018 American Chemical Society.
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![NiAl-LDHs on g-C3N4 sheets (Figure 17a, Table 4). Without
using any g-C3N4, the pristine NiAl-LDHs displayed flower-
like morphology with sharp edges (Figure 17b).396
However,
this morphology was disappeared when g-C3N4 was used
during synthesis and 2D sheets of LDHs were grown on the
surface of g-C3N4, which suggests a strong interaction between
g-C3N4 and NiAl-LDHs (Figure 17c−e). The UV−vis bands of
LDHs were slightly blue-shifted in the 2D/2D g-C3N4/NiAl-
LDHs, while the materials still have a strong visible absorption
profile up to 800 nm (Figure 17f). The photocatalytic
application in CO2 reduction using the CNLDH-10 sample
with 10% LDH was found to be optimum and afforded 8.2
μmol g−1
h−1
of CO, which was more than 5 times compared
to g-C3N4 (1.56 μmol g−1
h−1
) (Figure 17g).
In the overall water splitting reaction, the kinetics of water
oxidation remains a rate-determining step due to the
requirement of multiple electron transfer steps followed by
O−H bond cleavage and O−O bond formation. Since carbon
nitride has a poor oxidizing valence band, the amalgamation of
CN with water oxidation catalysts is an appropriate approach
to enhance reaction kinetics.397
Cobalt-based catalysts have
demonstrated promising oxygen evolution performance, which
makes them an ideal candidate for the formation of 2D/2D
heterojunctions with carbon nitride.398−401
Interestingly, the
growth of cobalt hydroxide (Co(OH)2) in the presence of
carbon nitride sheets can lead to the formation of a layered
structure. For example, Zhang et al. have fabricated a
Co(OH)2/g-C3N4 heterojunction by using a cobalt nitrate
precursor in the presence of NH3, which demonstrated
enhanced visible absorption.402
The close contact of Co(OH)2
and g-C3N4 in 2D/2D fashion led to better charge migration
that was evident from the reduced semicircle diameter in the
EIS Nyquist plot and PL quenching. Under optimized
conditions of 3 wt % Co(OH)2 loading and AgNO3 as an
electron acceptor, the oxygen evolution rate was 27.4 μmol h−1
using 300 W solar simulated light. The cobalt-based LDHs
with Ni counterparts were also found to be good water
oxidation catalysts (WOCs).403,404
Zhang et al. synthesized a
2D/2D heterostructure of Ni−Co LDHs (NixCo3−x LDHs;
Ni2+
/Co2+
= 0, 1, 1.5, 2) and carbon nitride nanosheets
(CNU) by a simple ultrasonication approach while keeping the
ratio of Ni−Co LDHs in the range of 1−5 wt %.405
The
diminished PL intensity and enhanced photocurrent density in
PEC measurement suggest better charge separation in the Ni−
Co LDHs/CNU heterojunction. When used as a water
oxidation photocatalyst, the NiCo2 LDHS/CN catalysts with
1:2 Ni/Co stoichiometric composition and 3 wt %
concentration demonstrated the highest O2 evolution rate
(26.7 μmol h−1
), which was 6.5 times higher than pure carbon
nitride sheets.
6. CARBON NITRIDE−PEROVSKITE OXIDE 2D/2D
vdW STRUCTURES
Perovskite materials with a general formula ABX3 or A2BX4 are
constituted of A and B cations coordinated to an X anion. In
this structure, A and B cations have 6- and 12-fold
coordination, surrounded by an octahedron of the X anions
(usually oxygen).406,407
Due to the stable structure of the
perovskite lattice (except halide perovskites), more than 90%
of metal elements have been successfully introduced into the
perovskite lattice.408
When X is a halogen (F, Cl, Br, or I) with
a monovalent (Cs+
, CH3NH3
+
, formamidinium(HC(NH2)2
+
))
and a divalent cation (Pb2+
, Ge2+
, Sn2+
) present in the A and B
sites, it is referred to as a halide perovskite.409
Lead halide-
based perovskites have been widely explored for photovoltaics
reaching an ∼25.2% efficiency in a monolithic cell nearing the
Shockley-Queisser limit of 31.4% due to their excellent visible
absorption, long carrier migration length, easy processability,
etc.410
Halide perovskites have also been used as the
photocatalysts for the various photocatalytic applications
such as CO2 reduction, organic synthesis, photoelectrochem-
ical synthesis, etc.411−413
Unfortunately, halide perovskites
suffer from a trap assisted recombination and structural
stability issue under the ambient conditions such as air and
moisture which further deepen becaise of the toxicity of
lead.414,415
Some lead-free perovskites such as Cs2AgBiBr6 have
been recently reported with enhanced stability and photo-
catalytic performance, but still, the stability is not satisfactory
under operating reaction conditions.412
Several attempts to
stabilize halide perovskites such as surface passivation,
elemental doping, and alloying have been employed; however,
the problem of stability persists. Recently, perovskite nano-
crystal encasing inside of the inorganic shell such as silica,
metal oxides, or wrapping with various types of 2D material
such as graphene, g-C3N4, has been identified to prevent
degradation.416−418
However, such approaches inevitably
reduce visible absorption and enhance surface recombination.
On the other hand, perovskite oxides with a general formula
ABO3, due to their extreme stability, excellent visible
absorption (narrow band gap), and the possibility of
multitudinous elemental combinations are gaining popularity
in electrocatalysis, catalysis, solid oxides fuel cells (SOFCs),
and photocatalysis.34,419−423
Several bi/trimetallic perovskite
materials possessing BO6 type octahedron, such as SrTiO3,
BaTi4O9, CaxTiyO3, CoTiO3, LaMnO3, LaCoO3, K3Ta3Si2O13,
LaNiO3, SrVO3, PbZrO3, Na2Ti6O13, SrNbO3, K2Ti4O9,
K4Nb6O17, and K2La2Ti3O10, have been explored in recent
years for photocatalysis.424−432
Double perovskites
(A2B′B″O6) are another subclass of perovskites that have
shown great promise for photocatalytic applications, especially
fuel cells and water splitting.433−435
Several double perovskite
oxides with stoichiometric and nonstoichiometric composi-
t i o n s s u c h a s S r 2 N i W O 6 ,
Sr2Sc0.125Ti0.875O4Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF), etc. have
been successfully employed in such applications.436,437
Though
an excellent visible absorber, the main challenge associated
with perovskite oxides is their low charge migration distance
which makes bulk recombination very prominent. The use of
reduction and oxidation cocatalysts such as Pt and RuO2 has
been employed to reduce surface recombination.
Layered perovskites oxides of (110), (100), and (111)
families with formulas of (An+1BnO3n+3) and (AnBnO3n+2),
(Bi2O2)(An−1BnO3n+1) (Aurivillius phase, AL), An+1BnO3n+1, or
A′2An−1BnO3n+1 (Ruddlesden−Popper phase, RP) and
A′[An−1BnO3n+1] (Dion−Jacobson phase, DJ), where n
represents the number of BO6 octahedra, arranged perpendic-
ular to the layers provide better surface area, interlayer space as
reaction sites, and charge carrier mobility, which make them
promising candidates for photocatalysis.438−440
Ruddlesden−
Popper type perovskite phases and layered Dion−Jacobson
type perovskite phases are the most promising perovskite
oxides for photocatalysis. For example, Sr2.7−xCaxLn0.3Fe2O7−δ
with x = 0 and 0.3 and Ln = La and Nd has demonstrated
excellent visible light assisted MB degradation.441
Enticingly,
due to the possibility of exfoliation of 2D sheets, layered
perovskites can be used for making a 2D/2D heterojunc-
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![for the MoS2/mpg-CN heterojunction and decreased charge
transfer resistance compared to mpg-CN. As a photocatalytic
material for HER, 0.5 wt % MoS2/mpg-CN vdW hybrid (20.6
μmol h−1
) outperforms over 0.5 wt % Pt/mpg-CN (4.8 μmol
H2 h−1
), suggesting the possibility of exclusion of expensive
noble metal catalysts. In a study by Yuan et al., g-C3N4
exfoliated in NMP was hydrothermally reacted with
ammonium tetrathiomolybdate [(NH4)2MoS4] that resulted
in the formation of a 2D/2D MoS2/g-C3N4 heterojunction
(Figure 19).508
The well contacted g-C3N4 and MoS2 were
visible in the HR-TEM images along with the AFM images
(Figure 19a,b). The photocatalytic activity of 2D/2D MoS2/g-
C3N4 catalysts was compared with 3% Pt/g-C3N4 benchmark
catalyst and demonstrated that interfacial contact of g-C3N4
with MoS2 affords more efficient charge separation. The TEM
images of 3% Pt/g-C3N4 reveal homogeneously dispersed Pt
nanoparticles on the surface of g-C3N4 (Figure 19a).
Interestingly, when tested for the photocatalytic H2 evolution,
the 2D/2D MoS2/g-C3N4 displayed an excellent H2 evolution
rate (1155 μmol g−1
h−1
) which was even higher than that of
the 3% Pt/g-C3N4 benchmark photocatalysts (791 μmol g−1
h−1
). Additionally, experiments reveal that the presence of g-
C3N4 in the form of 2D sheets was crucial, and a very poor H2
evolution rate was obtained for MoS2 and bulk g-C3N4. The
gradually decreasing lifetime after adding MoS2 shows better
charge separation, while band structures calculated from the
Mott−Schottky and Tauc plots reveal the formation of a type-I
heterojunction (Figure 19c−e). In a recent report, 2H MoS2
modified g-C3N4(MoS2/g-C3N4) 2D/2D vdW heterojunction
was synthesized using 12-phosphomolybdic acid
(H3PMo12O40, PMA) and thioacetamide (TAA) as Mo and
sulfur sources.509
The MoS2/g-C3N4 catalyst displayed a
significant H2 production rate (1497 μmol gcat
−1
h−1
) with
an associated apparent QY of 3.3% at 410 nm irradiation. The
charge carrier density (Nd) of MoS2/g-C3N4 samples was
found in the range of 7.80 × 1017
to 5.13 × 1018
cm−3
, much
larger than pristine g-C3N4 (7.10 × 1017
cm−3
), suggesting a
better establishment of heterojunctions (Figure 19f). Even a
ternary heterojunction constituted of g-C3N4-5%/MoS2/
graphene was also made, which exhibited an RhB degradation
rate of 95% under 20 min.510
7.2. Carbon Nitride−WS2. Another important layered
TMDC of the group-VI family is tungsten disulfide (WS2),
which shows an indirect (1.4 eV) to direct (2.0 eV) band gap
transition when bulk materials are transformed into monolayer
sheets.511,512
Like MoS2, WS2 also exists in a crystalline form
called 2H-WS2 and octahedral 1T WS2. Additionally, 1T WS2
has a metallic nature and can be synthesized by lithium
exfoliation. Exceeding over MoS2, the intrinsic electrical
conductivity of WS2 is higher than that of MoS2, which
makes it a suitable cocatalyst candidate. Indeed, numerous
reports are available on the use of nanostructured WS2 as a
cocatalyst and sensitizer in photocatalysis.513−515
Exfoliation of
bulk WS2 sheets via lithium intercalation led to phase
Figure 19. (a) TEM images of the 3% Pt/g-C3N4 nanosheets photocatalyst. (b) HRTEM image of 0.75% MoS2/g-C3N4 nanosheets composite. (c
and d) Schematic diagrams of 0D-2D Pt/g-C3N4 nanosheets photocatalysts and 2D-2D MoS2/g-C3N4 nanosheets photocatalysts. (e) Time-
resolved fluorescence spectra of different MoS2/g-C3N4 nanosheets photocatalysts loading with various amounts of MoS2. (f) Schematic energy-
level diagrams of MoS2 and g-C3N4 in comparison with the H+
/H2 and O2/H2O redox potentials. Reprinted with permission from ref 508.
Copyright 2019 Elsevier.
Chemistry of Materials pubs.acs.org/cm Review
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![FeS2 is plagued by prodigious charge recombination in grain
boundaries and surface defects. Compared with metal sulfides,
metal selenides are advantageous as they have a narrow band
gap and improved carrier mobility. Although S and Se are the
members of the same periodic group and have almost similar
chemical properties, the bond strength of Se−H (276 kJ/mol)
is significantly lower than that of S−H (363 kJ/mol), leading
to better adsorption−desorption of the proton, which is
essential to facilitate the product desorption from catalytic
sites.547
FeSe2 exists in two polymorphic forms (orthorhombic
marcasite and cubic pyrite), possessing indirect band gaps of
0.86 and 0.67 eV.548
FeSe2 exhibits an excellent conductivity
(resistivity ≈10−3
Ω·cm), populated surface iron atoms, low
toxicity, and benign nature and is favorable for the water
splitting due to the presence of abundant [Fe−Fe] hydro-
genase type active centers accelerating proton adsorption and
H2O2 decomposition.549,550
Even with such excellent proper-
ties, photocatalytic applications of FeSe2 are sparse, and most
of the applications of FeSe2 are limited to solar cells and the
sodium-ion battery (SIB).551−553,553
The 0D/1D hybrid FeSe2
and ZnSe demonstrated increased photosplitting of water due
to better charge separation in the type-I heterojunction.554
Other heterojunctions such as 1D/2D FeSe2/MoSe2 and FeSe2
nanodendrites decorated on GO and g-C3N4 have been
reported555,556
Remarkable results were obtained by the marriage of 2D
FeSe2 and 2D sheets of g-C3N4 (CNNS) (Figure 21).557
The
synthesis of 2D/2D FeSe2/CNNS was achieved via the
formation of g-C3N4 sheets followed by in situ growth of
FeSe2 using Fe(acac)3 and Se precursors in the presence of 1-
octadecene (ODE) and oleylamine (OLA) (Figure 21a). The
close 2D/2D face-to-face contact between FeSe2 and CNNS
was evident from the TEM and AFM images. The optimum
photosplitting of water was observed for the 15% FeSe2/CNNS
reaching a value of 1655.6 μmol g−1
h−1
, almost 2.65 times that
of pure g-C3N4 in the presence of Na2S/Na2SO3 and solar
simulated light (Figure 21b,c). Further, FeSe2/CNNS also
afforded enticing removal efficiencies of 92.6% for Cr(VI) and
99.8% for MB within 120 min while pristine g-C3N4 affords
only 44.7% Cr(VI) and 66.1% MB removal efficiencies under
identical conditions (Figure 21d). Interestingly, liquid
chromatography−mass spectroscopy (LC-MS) demonstrated
the ring-opening degradation followed by mineralization. The
wide visible absorption extended up to 1200 nm, decreased the
PL lifetime, decreased charge transfer resistance, and improved
the transient photocurrent compared to pristine materials,
validating better charge separation in the FeSe2/CNNS
composite structure. Photocatalytic experiments in the
presence of scavengers and radical trapping agent DMPO
demonstrated that •
OH radicals were responsible for the
degradation activity. The validation of the origin of •
OH
radicals from the derivatization of the superoxide anion radical
(O2
•−
) via H2O2 intermediate was done by the DPD-POD
method, which showed increased •
OH radical concentration
during reaction using UV−vis spectroscopy.
7.4. Carbon Nitride−PtS2. Noble metal dichalcogenides
(MX2, M = Pt, Pd, Ir, Re, etc., X = S, Se, Te) are known to
possess photocatalytic activity for a long time.558−563
However,
their use is limited due to the cost issue and small band gap.
Recently, some excellent reports on 2D MX2 have emerged
and rejuvenated the field. Group 10 dichalcogenides such as
PtS2, PtSe2, and PtTe2 have shown great promise due to their
superior properties.564,565
For example, platinum disulfide
(PtS2), a layered material, has displayed high carrier mobility
(3500 cm2
V−1
S−1
) even larger than that of phosphorene
(1000 cm2
V−1
S−1
), which makes it suitable for various
applications from photocatalysis to fast-moving electronic
applications including gas sensors, field-effect transistors
(FET), etc.566
DFT calculation reveals that monolayer PtS2
Figure 21. (a) Schematic image for the synthesis procedures of 2D/2D FeSe2/CNNS interplane heterostructures. Photocatalytic H2 evolution
curves (b) and rates (c) for pure g-C3N4, FeSe2, and various FeSe2/CNNS heterostructures. The photocatalytic MB and Cr(VI) degradation
performance of pure (d) 15% FeSe2/CNNS. Reprinted with permission from ref 557. Copyright 2020 Elsevier.
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![To further improve the performance of the g-C3N4@
ZnIn2S4 vdW heterostructure, S vacancies were introduced in
ZnIn2S4 sheets ,which can ameliorate the light absorption,
lifetime of charge carriers, and also charge kinetics between
two semiconductors (Figure 23, Table 6).602
To attain this
goal, the ZnIn2S4/g-C3N4 (ZIS-S/CN) vdW heterojunction
was synthesized via a calcination−solvothermal method using
CN sheets followed by high-temperature growth of ZIS-S. The
intimate contact with ZIS-S and CN was evident from
HRTEM, AFM, and elemental mapping. Due to improved
UV−vis absorption and increased PL lifetime, the ZIS-S/CN
displayed an increased photocurrent response compared to the
pristine components such as CN and ZIS-S (Figure 23a,d).
Electrostatic potential measurement using DFT calculations
demonstrated that the work functions of g-C3N4 and ZIS-S
were 4.70 and 6.03 eV, which implies that the Fermi level of g-
C3N4 is higher than that of ZIS-S and electrons should flow
from CN to ZIS-S (Figure 23b,c). The charge density
difference of 30ZIS-S/CN vdW shows that the electronic
charge centered on the surface of ZIS-S was primarily derived
from g-C3N4 (Figure 23e). The charge redistribution in the
2D/2D heterojunction contact mainly focused on the 2D/2D
interfaces and accumulated near the ZIS-S monolayer, which
can efficiently annihilate holes that accumulate near the CN
monolayer (Figure 23f). This results in the formation of a bias-
less in-built electric field which facilitates better charge
separation.
Apart from common layered chalcogenides such as WS2 and
MoS2, various other chalcogenides including SnS2,605−608
Sn2S3,609
TaS2,610
Bi2Se3,611
NiS,612
CdS,613
CuInS2,614
MnIn2S4,615
Cu2WS4,616
NiCo2S4,617
etc. have been reported
for making 2D/2D photocatalysts.
8. CARBON NITRIDE−BISMUTH OXYHALIDE 2D/2D
vdW STRUCTURES
8.1. Carbon Nitride−BiOX. Bismuth-based photocatalysts
(fractional, binary/ternary oxides) have been proven as
excellent photocatalytic materials due to their unique
electronic and structural properties and visible light absorb-
ance.618−620
Most of the oxide-based bismuth catalysts such as
BiVO4, Bi2MoO6, BiPO4, Bi2W2O6, etc. have displayed poor
visible light absorption limited to the blue region, compromis-
ing the photocatalytic performances.621
Additionally, the
fundamental problems of low quantum efficiency and lack of
better charge transport properties are key limiting factors.
Bismuth oxyhalides (BiOX; X = Cl, Br, and I) constituted of
elements from the main group family (V−VI−VII) with a
tetragonal matlockite configuration (PbFCl-type) are becom-
ing a rising star in the photocatalysis field due to their
astounding visible absorption with a band gap in the range of
1.7−3.2 eV, inert nature, easy processing, and corrosion
resistance.622
BiOX is constituted of [Bi2O2] slabs interleaved
with double halogen slabs giving rise to a layered structure.
The interlayer atoms in the BiOX are connected through
strong covalent bonding while layers remain bounded together
through weak vdW interaction. Due to the specific crystalline
structure and atomic polarization, an internal static electric
field exists perpendicular to the [Bi2O2] and [X] slices,
resulting in effective charge separation. The band gap of BiOX
is highly dependent on the types of halogen atoms, i.e., the
band gaps for BiOCl, BiOBr, and BiOI were found to be ∼3.3,
2.7, and 1.8 eV, respectively. BiOF, due to its extremely high
band gap (3.64 eV), is usually excluded from the general
notion of the BiOX family.623
BiOX based compounds, either as a stand-alone catalyst
(except BiOI) or in the form of a heterojunction, have been
widely used for numerous photocatalytic applications such as
water splitting, dye degradation, CO2 reduction, N2 reduction
reactions, etc.621,624,625
Like chalcogenides, the inherent trap
assisted recombination reduces the performance of these
catalysts. Various surface passivation approaches such as
decorating with alkyl chains, carbon quantum dots, adding
metal/metal oxides nanoparticles, and heterojunction for-
mation with various semiconductors such as TiO2, BiVO4,
Sn3O4, Bi2O2CO3, etc. have been employed to improve the
performance of these materials.626−631
Due to the layered
structure of BiOX with a residual surface positive charge and
weak p-type nature, g-C3N4 is an ideal contender to make a p−
n type vdW heterojunction.632,633
The effective interaction
between layered surfaces and the presence of a built-in
electrical field after Fermi level equilibration can facilitate
better charge separation.634−636
The inherent low band gap of BiOI associated with less
negative CB restricts its usage as a stand-alone catalyst which
again necessitates the formation of a heterojunction. Alam et
al. have synthesized BiOI heterojunctions with few-layered g-
C3N4 and F-doped and Cl-intercalated g-C3N4 (CNFCl) via an
in situ approach displaying an enhanced photoelectrochemical
water splitting performance reaching a photocurrent density of
0.70 mA cm−2
and 1.3 mA cm−2
, respectively, under AM 1.5G
solar simulated light.44,637
Kelvin probe force microscopy
(KPFM) reveals better charge carrier generation and
separation in the BiOI/carbon nitride heterojunctions. Further,
after the formation of the heterojunction, the Fermi level of
BiOI was uplifted, and then g-C3N4 facilitates the migration of
electrons on conjugated carbon nitride sheets.
Most of the reported BiOI/g-C3N4 heterojunctions have
displayed 3D platelets, microspheres, and flower-like morphol-
ogies due to the uncontrolled growth of BiOX lattices on g-
C3N4.638
The 3D structures limit catalytic performance due to
hindered active sites, reduced active surface area, and
minimum interfacial contact between two catalyst components.
To surmount the drawback of unfavorable redox potential and
stability of BiOI and BiOBr, the BiOCl with a wide indirect
band gap and relatively more resilient nature seems to be a
good replacement. Introducing the oxygen vacancies or Bi(0)
doping in the BiOCl nanosheets was found to improve the
photocatalytic performance significantly due to increased
visible absorption and formation of subgap energy levels.639,640
The performance can be further improved by making a 2D/2D
vdW heterojunction.641
Wang et al. synthesized an oxygen
vacancy-rich ultrathin g-C3N4/BiOCl 2D/2D heterojunction
using polyvinylpyrrolidone (PVP) which displayed excellent
photocatalytic degradation performance for 4-chlorophenol (4-
CP) and bisphenol A (BPA) degradation (Figure 24).642
The
morphological characterization using HR-TEM, EDS mapping,
and AFM confirm intimate contact and formation of the 2D/
2D structure (Figure 24a−c). The O1 XPS spectra displayed
an increased intensity of the O2 signature peak at 531.3 eV
assigned to increased vacancies in the 2D/2D structure. ESR
spectra of 50CN-50BC (prepared using PVP) compared to
50CN-50BC-P (without PVP) displayed increased ESR signals
verifying the presence of oxygen vacancies. To deduce the
mechanism of enhanced photoactivity, a scavenger test using
O2
•−
(ascorbic acid, AA), holes (sodium oxalate, SO), and
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![Table
6.
2D/2D
Carbon
Nitride−Chalcogenide
Based
Heterojunction
Photocatalysts
photocatalyst
synthesis
application
light
source
AQY/STH
remarks
ref
Chalcogenides
Metallic
1T-MoS
2
/
monolayer
O-g-C
3
N
4
Hydrothermal
treatment
of
O-g-C
3
N
4
and
MoCl
5
at
200
°C
for
24
h.
Photocatalytic
H
2
evolution
300
W
Xe
lamp
(λ
≥
420
nm)
∼7.11%
(420
nm)
H
2
1841.72
μmol
g
−1
h
−1
,
∼4
times
of
Pt/O-g-C
3
N
4
(480.15
μmol
g
−1
h
−1
)
499
MoS
2
/carbonyl
linked
g-C
3
N
4
(MoS
2
/CO-
C
3
N
4
)
CO-C
3
N
4
was
mixed
with
the
MoS
2
solution
and
pumped
at
ca.
−100
kPa
for
15
min
under
simulated
sunlight
and
vacuum
dried.
Photocatalytic
H
2
evolution
200
W
Xe
lamp
or
LED
lamps
-
H
2
823.4
μmol
g
−1
h
−1
(10%
MoS
2
/CO-C
3
N
4
)
500
Carbon
nitride/MoS
2
MoS
2
and
SCN
were
sonicated
in
anhydrous
ethanol
for
2
h
followed
by
stirring
for
10
h
and
finally
heating
under
N
2
at
300
°C
for
1
h.
Photocatalytic
H
2
evolution
300
W
Xe
lamp
(λ
≥
420
nm)
5.7%
(430
nm)
H
2
2120
μmol
g
−1
h
−1
(MCN-3)
502
PCN
(with
Pt)11
μmol
g
−1
h
−1
MoS
2
/g-C
3
N
4
nano-
flowers
Ammonium
molybdate
tetrahydrate
and
thiourea
were
added
to
a
g-C
3
N
4
dispersion
followed
by
microwave
treatment
at
180
°C
for
30
min.
Photocatalytic
deg-
radation
of
MB
and
fipronil
Visible
light
-
MoS
2
/g-C
3
N
4
∼94%
MB
and
77%
fipronil
degrada-
tion
in
60
min
503
g-C
3
N
4
-Ni
2
P-MoS
2
g-C
3
N
4
-1%Ni
2
P
and
MoS
2
were
dispersed
together
via
ultrasonication
and
stirring.
Photocatalytic
H
2
evolution
300
W
Xe
lamp
(λ
≥
400
nm)
1.45%
(405
nm)
H
2
g-C
3
N
4
-1%Ni
2
P-1.5%MoS
2
2.47
and
5.15
times
of
g-C
3
N
4
-1.5%MoS
2
and
g-C
3
N
4
-1%Ni
2
P
504
g-C
3
N
4
-MoS
2
-M(OH)
x
g-C
3
N
4
-MoS
2
M(NO
3
)
x
·6H
2
O
were
dissolved
in
deionized
water
by
ultrasonication,
and
aqueous
ammonia
was
added
dropwise.
Finally,
the
resultant
sample
was
obtained
after
evaporation
and
drying
in
an
oven
at
80
°C
for
12
h.
Photocatalytic
H
2
evolution
300
W
Xe
arc
lamp
6.4%
(420
±
8
nm)
H
2
889.4
μmol
g
−1
h
−1
(g-C
3
N
4
-MoS
2
-Ni(OH)
x
)
506
g-C
3
N
4
9.3
μmol
g
−1
h
−1
MoS
2
/g-C
3
N
4
Solvent-thermal
method:
g-C
3
N
4
nanosheets
and
[(NH
4
)
2
MoS
4
]
dissolved
in
DMF
were
hydrothermally
treated
at
210
°C
for
24
h.
Photocatalytic
H
2
evolution
300
W
Xe
lamp
(λ
≥
420
nm)
6.8%
(420
nm)
H
2
1155
μmol
g
−1
h
−1
(MoS
2
/g-C
3
N
4
)
508
g-C
3
N
4
25
μmol
g
−1
h
−1
MoS
2
-modified
graph-
itic
carbon
nitride
(MoS
2
/g-C
3
N
4
)
The
exfoliated
solution
of
g-C
3
N
4
sheets
and
MoS
2
NSs
were
mixed
and
sonicated
for
another
2
h.
Photocatalytic
H
2
evolution
300
W
Xe
lamp
(λ
≥
360
nm)
3.3%
(410
nm)
H
2
1497
μmol
g
−1
h
−1
(MSCN-3)
509
Metallic
1T-WS
2
/2D-
C
3
N
4
Grinding
method:
1T-WS
2
was
dispersed
in
hexane
followed
by
the
addition
of
2D-C
3
N
4
and
1T-WS
2
followed
by
grinding.
Photocatalytic
H
2
evolution
300
W
Xe
lamp
(λ
≥
420
nm)
1.12%
(420
nm)
H
2
331.09
μmol
g
−1
h
−1
(1T-WS
2
/2D-C
3
N
4
with
64.1%
1T
phase)
515
∼43.3
times
bare
2D-C
3
N
4
CdS/WS
2
/CN
Simple
mixing
procedure:
CN
and
WS
2
nanosheets
were
stirred
together
for
12
h.
The
obtained
WS
2
/CN
was
immersed
in
Cd(CH
3
COO)
2
followed
by
immersion
in
Na
2
S
to
deposit
CdS.
Photocatalytic
H
2
evolution
200
W
Hg
lamp
(λ
≥
420
nm)
5.4%
(400
nm)
H
2
1174.5
μmol
g
−1
h
−1
(CdS/WS
2
/CN)
540
∼67
times
CN
(17.2
μmol
g
−1
h
−1
)
FeSe
2
/g-C
3
N
4
Fe(acac)
3
,
oleic
acid
(OA),
1-octadecene
(ODE),
and
oleylamine
(OLA)
were
mixed
ultrasonically
under
a
N
2
atmosphere
followed
by
the
addition
of
g-C
3
N
4
nanosheets
and
heated
to
175
°C.
Finally,
OLA
solution
containing
Se
powder
was
injected
to
form
2D/2D
FeSe
2
/CN
NS
heterostructures.
Photocatalytic
H
2
evolution
and
re-
moval
of
MB
and
Cr(VI)
300
W
Xe
lamp
-
H
2
1655.6
μmol
g
−1
h
−1
(FeSe
2
/CN
NS
)
557
∼2.65
times
pure
g-C
3
N
4
(623.7
μmol
g
−1
h
−1
)
FeSe
2
/CN
NS
92.6%
Cr(VI)
removal
g-C
3
N
4
44.7%
Cr(VI)
removal
FeSe
2
/CN
NS
99.8%
MB
degradation
g-C
3
N
4
66.1%
MB
degradation
PtS
2
/MCN
MCN
and
PtS
2
nanosheets
(PtS
2
-8000)
were
stirred
together
for
6
h.
Photocatalytic
H
2
evolution
300
W
Xe
lamp
(λ
≥
400
nm)
1.16%
(405
nm)
H
2
1168
μmol
g
−1
(PtS
2
(U)/MCN)
578
ZnIn
2
S
4
/protonated
g-
C
3
N
4
ZnIn
2
S
4
and
pCN
were
dispersed
in
DI
water
and
mixed
with
ultrasonication.
Photocatalytic
H
2
evolution
and
deg-
300
W
Xe
lamp
(λ
0.92%
(400
nm)
H
2
8601.16
μmol
g
−1
h
−1
(ZnIn
2
S
4
/pCN)
599
∼108
times
of
pCN
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![Table
7.
2D/2D
Carbon
Nitride-Bismuth
Oxyhalides
Based
Heterojunction
Photocatalysts
photocatalyst
synthesis
application
light
source
AQY/
STH
remarks
ref
Bismuth
Oxyhalides
g-C
3
N
4
/BiOBr
Solvothermal
route:
g-C
3
N
4
,
Bi(NO
3
)
3
·5H
2
O,
and
CTAB
solution
in
ethylene
glycol
were
treated
in
a
Teflon-lined
autoclave,
at
160
°C
for
12
h.
Photocatalytic
degradation
of
RhB
500
W
Xe
lamp
(λ
>
400
nm)
-
2.0%
g-C
3
N
4
/BiOBr97.9%
of
RhB
degradation
after
150
min
633
C
3
N
4
/BiOBr
Bi(NO)
3
·5H
2
O,
KBr,
and
Pg-C
3
N
4
were
dispersed
in
H
2
O
and
EG
and
hydrothermally
treated
at
110
°C
for
10
h.
Photocatalytic
degradation
of
MB
50
W
410
nm
LED
light
-
20%Pg-C
3
N
4
/BiOBr90%
MB
degradation
in
40
min
634
pure
g-C
3
N
4
40%
MB
degra-
dation
in
40
min
g-C
3
N
4
/BiOI
Bi(NO
3
)
3
·5H
2
O,
Pg-C
3
N
4
,
and
KI
were
hydrothermally
treated
at
120
°C
for
6
h.
Photocatalytic
degradation
of
MB
50
W
410
nm
LED
light
-
degradation
rate0.01596
min
−1
(Pg-C
3
N
4
/BiOI)
635
∼5.7
times
of
g-C
3
N
4
(0.0028
min
−1
)
BiOBr/graphitic
C
3
N
4
(BiOBr/CNNS)
Simple
reflux
process:
Bi(NO
3
)
3
·5H
2
O
and
KBr
in
EG
and
CNNS
in
DI
water
were
mixed
and
refluxed
at
80
°C
under
vigorous
stirring
for
2
h.
Photocatalytic
degradation
of
RhB
and
BPA
300
W
Xe
lamp
(λ
≥
420
nm)
-
BiOBr/CNNS88.5%
TOC
removal
after
50
min
636
BiOCl/C
3
N
4
C
3
N
4
nanosheets
and
BiOCl
nanoplates
were
mixed,
and
the
obtained
mixture
was
calcined
at
250
°C
for
3
h.
Photocatalytic
degradation
of
MO
300
W
Xe
lamp
(λ
≥
420
nm)
-
BOC/CN-0.7−84.28%
MO
degradation
after
180
min
641
C
3
N
4
22.49%
MO
degradation
after
180
min
BiOCl-g-C
3
N
4
g-C
3
N
4
nanosheets,
Bi(NO
3
)
3
·5H
2
O,
PVP,
and
glycerol
and
NaCl
were
hydrothermally
treated
at
160
°C
for
6
h.
Photocatalytic
H
2
evolution
300
W
Xe
lamp
(λ
≥
420
nm)
-
50CN-50BC95%
4-CP
degra-
dation
efficiency
in
120
min
642
pure
BC22.7%
4-CP
degra-
dation
efficiency
in
120
min
g-C
3
N
4
/Bi
12
O
17
Cl
2
(PGCN/
BOC)
BiCl
3
solution
in
ethanol
was
added
to
PGCN
and
was
and
calcined
at
250
°C,
350
°C,
450
°C,
and
550
°C
for
2
h.
Photocatalytic
CO
2
reduction
Visible
light
(λ
≥
420
nm)
-
CH
4
24.4
μmmol
g
−1
h
−1
(PGCN/BOC)
662
PGCN
∼
negligible
Bi
3
O
4
Cl/g-C
3
N
4
Solid-phase
calcination
method:
Bi
3
O
4
Cl
and
g-C
3
N
4
were
mixed
in
an
agate
mortar,
and
the
obtained
powder
was
calcined
at
400
°C
for
2
h.
Photocatalytic
degradation
of
TC
and
RhB
and
reduction
of
Cr(VI)
250
W
Xe
lamp
(λ
≥
420
nm)
-
Bi
3
O
4
Cl/g-C
3
N
4
98.3%
RhB
degradation
663
Bi
3
O
4
Cl/g-C
3
N
4
∼
76%
TC
degradation
Bi
3
O
4
Cl/g-C
3
N
4
75.7%
Cr(VI)
removal
efficiency
∼2.86
times
of
g-C
3
N
4
26.4%
Cr(VI)
removal
efficiency
g-C
3
N
4
/Bi
12
O
17
Cl
2
g-C
3
N
4
and
BiCl
3
were
dissolved
in
ethanol
followed
by
the
addition
of
NaOH
and
stirred.
Photocatalytic
degradation
of
RhB
and
MO
300
W
Xe
lamp
(λ
≥
400
nm)
-
g-C
3
N
4
/Bi
12
O
17
Cl
2
85%
RhB
degradation
in
the
120
min
664
g-C
3
N
4
76%
RhB
degradation
in
the
120
min
g-C
3
N
4
/Bi
4
O
5
Br
2
Bi(NO
3
)
3
·5H
2
O,
[C
16
mim]Br,
and
2D
graphene-like
g-C
3
N
4
were
dissolved
in
mannitol
aqueous
solution
followed
by
the
addition
of
NaOH
and
hydrothermally
treated
at
140
°C
for
24
h.
Photocatalytic
degradation
of
ci-
profloxacin
(CIP)
and
RhB
500
W
Xe
lamp
-
1
wt
%
g-C
3
N
4
/Bi
4
O
5
Br
2
50%
CIP
degradation
in
30
min
665
Bi
4
O
5
Br
2
30%
CIP
degrada-
tion
Carbon-doped
carbon
nitride/
Bi
12
O
17
Cl
2
(CCN/
Bi
12
O
17
C
l2
)
In
situ
method:
CCN
and
Bi
12
O
17
Cl
2
were
mixed
via
sonication
for
another
1
h
followed
by
stirring
for
12
h.
The
obtained
samples
were
further
treated
at
120
°C
for
2
h.
Photocatalytic
degradation
of
TC
300
W
Xe
lamp
(λ
≥
420
nm)
-
CCN/Bi
12
O
17
Cl
2
94%
of
TC
was
removed
in
1
h
666
CCN82%
of
TC
was
removed
in
1
h
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![Table
10.
2D/2D
Carbon
Nitride−2D
Polymers-Based
Heterojunction
Photocatalysts
photocatalyst
synthesis
application
light
source
AQY/
STH
remarks
ref
2D
Polymers
2D/2D
graphitic
carbon
nitride
nano-
sheet/carbonized
polydopamine
(CNNS/CPDA)
Thermal
annealing
of
CN
and
PANI
precursors
RhB
degradation
visible
light
(λ
≥
420
nm)
-
CNNS/CPDA-298.2%
RhB
degradation
within
60
min.
790
Polydopamine/graphitic
carbon
nitride
PDA/g-C
3
N
4
Dopamine
hydrochloride
was
added
to
the
aqueous
dispersion
of
g-C
3
N
4
sheets
followed
by
the
addition
of
tris-HCl
solution
and
adjustment
of
pH
to
8.5
by
using
1
M
NaOH
solution
and
vigorous
stirring
at
60
°C
for
24
h.
Photocatalytic
degra-
dation
of
MB
500
Xe
lamp
with
a
cutoff
filter
-
PDA/g-C
3
N
4
98.84%
MB
degradation
791
∼4
times
higher
than
pure
g-C
3
N
4
Imine
linked
COF/g-C
3
N
4
nanosheets
(CNS-COF)
In
situ
reaction
of
4,4′,4″-(1,3,5-triazine-2,4,6-triyl)
trianiline
(TTA)
and
1,3,5-triformylphloroglucinol
(TP)
in
the
presence
of
CNS
Photocatalytic
H
2
evo-
lution
300
W
Xe
(λ
>
420
nm)
31.8%
(425
nm)
H
2
9.1
mmol
h
−1
g
−1
(CNS-COF)
802
CN1.2
mmol
h
−1
g
−1
Carboxyl
rich
CTF
nanosheets
and
graphitic
carbon
nitride
nanosheets
(CTFNS/CNNS)
H
2
SO
4
assisted
exfoliation
followed
by
electrostatic
assembly
Photocatalytic
sulfa-
methazine
(SMT)
degradation
500
W
Xe
lamp
-
5%
CTFNS/CNNSof
94.9%
SMT
degradation
in
180
min
804
Benzo[ghi]perylenetriimide/graphitic
carbon
nitride
(BPTI/g-C
3
N
4
)
Solution
phase
self-assembly
of
BPDI
and
g-C
3
N
4
in
quinoline
for
8
h
at
150
°C.
RhB
degradation
Visible
light
(λ>
420
nm).
-
1:3
BPTI/g-C
3
N
4
∼
89%
RhB
degradation
809
∼36%
is
higher
than
that
of
g-C
3
N
4
PI/g-C
3
N
4
Sonication
thermal
approach
Degradation
of
2,4-di-
chlorophenol
300
W
Xe
lamp
-
30%
PI/g-C
3
N
4
∼99%
DCP
degradation
in
4
h
820
∼3.8
times
of
pristine
g-
C
3
N
4
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![between polymer semiconductors and carbon nitrides can be
realized for mutually benefitted interaction including band
edge modulation, enhance visible absorption, and charge
separation (Figure 33). Cao et al. demonstrated the synthesis
of an all organic 2D/2D heterojunction between amine-
functionalized graphitic carbon nitride (GCN) nanosheets
(CNNS) and carboxyl rich CTF nanosheets (CTFNS) via
electrostatic interaction.804
Acid-assisted exfoliation of CTFs
not only provides CTFNS but also overcomes the size
requirement for efficient interaction, resulting in improved
photocurrent generation and 95.8% removal efficiency for
sulfamethazine. Isolated heptazine or triazine-based carbon
nitride struggle with the issue of charge separation due to the
absence of charge collection sites. Coupling triazine and
heptazine units together in the same carbon nitride framework
was found to solve this issue by the formation of a donor−
acceptor network. Zhang et al. displayed that changing the
LiCl/KCl with NaCl/KCl in the ionothermal molten salt
method led to a diversion of the polymerization process due to
the high melting point of the NaCl/KCl mixture and afforded
triazine−heptazine based carbon nitride.805
The HER for CN-
NaK was found to be 278 mmol h−1
with an AQE of 32% (at
420 nm). Molten salt assisted synthesis in the presence of
LiCl/KCl usually produces crystalline polytriazine imides
(PTI) in which two triazine units are connected with the
−NH− group; however, 5-aminotetrazole precursor under
identical conditions leads to formation of polyheptazine imides
(PHI).806
CN is not considered as a good water oxidation
catalyst due to less positive VB, limiting its application in high
oxidation potential demanding reactions such as H2O2
formation. Polydiimides (PDI) synthesized by coupling of
anhydrides and melem/melamine units have more positive
valence bands than have been widely explored for H2O2
generation. Apart from the previously mentioned melem-
PMDA based PDI, several other substituents such as
naphthalene dianhydride, biphenyl tetracarboxylic dianhydride
(BTCDA), perylene dianhydride, mellitic trianhydride with
three coordination sites, melamine, etc. have been used for
PDI synthesis.703,705,807
Taking advantage of the layered
structure of the CN/PDI polymer, a 2D/2D hybrid with
graphene was prepared which can photocatalyze water
oxidation to H2O2 at a high rate, reaching a solar to hydrogen
(STH) efficiency of 0.20%.707
When a high CB position
containing carbon nitride with less positive VB is integrated
with CN/PDI, usually a solid-state Z-scheme is the preferred
mechanism of charge separation, leading to enhanced kinetics
of overall water splitting. For example, Miao et al.
demonstrated a perylenetetracarboxylic diimide (PDI) and
carbon nitride Z-scheme heterojunction that can reach an H2
evolution rate of g-C3N4/PDI (1649.93 μmol g−1
h−1
), which
is 2.03 times higher than that of the g-C3N4 nanosheet (814.03
μmol g−1
h−1
).808
In another work, benzo[ghi]-
perylenetriimide/graphitic carbon nitride (BPTI/g-C3N4)
synthesized by N-amidation reaction displayed enhanced
RhB degradation in a direct scheme mechanism.809
With the
advent of new 2D polymeric semiconductor materials such as
C2N, C3N, C3N2, C3N3, C4N3, C3N5, C3N6, etc., the choices of
fabrication of 2D/2D vdW heterojunctions are expanding
(Figure 33, Table 10).73,810−816
For example, distinct from
traditional six-member ring carbon nitride, a novel five-
member ring (imidazole) containing carbon nitride with
C3N2 stoichiometric composition and C−C bridging coordi-
nation can demonstrate a band gap as low as 0.81 eV and is
employed for PEC biosensing applications.814
Apart from
photo/photoelectrochemical applications, the new band gap
tuned carbon nitride containing 2D/2D heterojunctions will
find applications in other fields including optoelectronic
device, FET, LEDs/OLEDs, organic solar cells, etc. As CTF,
PTI, and PHI iso-element conjugates (CxNy), there exists no
distinct boundary. While generally referring to carbon nitrides,
depending upon their coordination (bridging N in CN, fused
benezenic ring), C/N content, and structural similarity, they
can be categorized as graphene type (low band gap) and
carbon nitride type structures (moderate band gap). For
example, C2N, also called nitrogenated holey graphene, has a
direct band gap of 1.96 eV.812
Theoretical studies using DFT
suggests C2N stacking on carbon nitride sheets forms a direct
scheme type-II heterojunction with suitable band edge
positions for water splitting.817
Sadly, most of such semi-
conductor heterojunctions with g-C3N4 are just reported based
on the theoretical calculation, and more work is needed in this
direction. 2D conjugated metal complex polymers such as a
Schiff base polymer synthesized by reaction of tert-amino
functionalized porphyrin and 2,5-dihydroxyterephthalaldehyde,
which demonstrated almost 10 mA cm−2
current density,
might also be explored for such applications.818
12. MISCELLANEOUS 2D/2D vdW STRUCTURES
Apart from the above-mentioned materials, several other
layered materials such as Ni(OH)2, Mo2N, montmorillonite,
cobalt phosphide (CoP), UNiMOF, MgFe MMO, and In2Se3
MO2C have been reported, forming a 2D/2D vdW
heterojunction with carbon nitrides to improve the photo-
catalytic performance (Table 11).821−829
Among them, the
large band gap hexagonal boron nitride (h-BN) is worth
mentioning.830−832
The h-BN possesses a 2D graphene-like
structure, an excellent chemical stability, a high thermal
conductivity, and a melting point which makes it suitable for
various applications including lubricants to the high surface
area supporting materials. However, due to a significantly high
band gap (5.5 eV), it is among the less explored 2D materials
in photocatalytic application. Though from the point of visible
light collection, h-BN does not fit in visible light mediated
photocatalysis but essentially provides the large oxidation
potential necessary for the oxidation of various recalcitrant
pollutants and water oxidation.833,834
Further, like graphene,
the charge distribution on the h-BN surface can be
manipulated by the formation of a 2D/2D interface. Indeed,
various 2D/2D interfacial catalysts amalgamated with h-BN
and inorganic semiconductors have been reported. BN can
form excellent lattice matched stacking with CN due to
analogous structure followed by favorable charge redistribution
in the close-packed CN-BN heterostructure. Besides band gap
modulation, the high electronegativity of BN compared to CN
facilitates efficient hole collection from the CN to accelerate
rate-limiting oxidation kinetics.835
In the CN-BN 2D/2D
host−guest structure, due to the electron-rich and deficient
pattern, a donor−acceptor relationship can be established
while close 2D/2D interfacial contact will minimize the
recombination losses. In a study, Tu et al. reported the
synthesis of h-BN and g-C3N4 heterojunction by a thermal
recrystallization process using diluted aqueous HNO3 at 180
°C in an autoclave.836
Unexpectedly, the visible absorption of
h-BN/g-C3N4 was found to be higher than that of the carbon
nitride, suggesting an electronic charge redistribution. The
band gap with 40% BN containing material was shifted to 2.44
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Boosting Photocatalytic Activity Using Carbon Nitride Based 2D/2D van der Waals Heterojunctions
- 1. Boosting Photocatalytic Activity Using Carbon Nitride Based 2D/2D van der Waals Heterojunctions Pawan Kumar,* Devika Laishram, Rakesh K. Sharma, Ajayan Vinu,* Jinguang Hu,* and Md. Golam Kibria* Cite This: https://doi.org/10.1021/acs.chemmater.1c03166 Read Online ACCESS Metrics & More Article Recommendations ABSTRACT: The surging demand for energy and staggering pollutants in the environment have geared the scientific community to explore sustainable pathways that are economically feasible and environmentally compelling. In this context, harnessing solar energy using semiconductor materials to generate charge pairs to drive photoredox reactions has been envisioned as a futuristic approach. Numerous inorganic crystals with promising nanoregime properties investigated in the past decade have yet to demonstrate practical application due to limited photon absorption and sluggish charge separation kinetics. Two- dimensional semiconductors with tunable optical and electronic properties and quasi-resistance-free lateral charge transfer mechanisms have shown great promise in photocatalysis. Polymeric graphitic carbon nitride (g-C3N4) is among the most promising candidates due to fine-tuned band edges and the feasibility of optimizing the optical properties via materials genomics. Constructing a two-dimensional (2D)/2D van der Waals (vdW) heterojunction by allies of 2D carbon nitride sheets and other 2D semiconductors has demonstrated enhanced charge separation with improved visible photon absorption, and the performance is not restricted by the lattice matching of constituting materials. With the advent of new 2D semiconductors over the recent past, the 2D/2D heterojunction assemblies are gaining momentum to design high performance photocatalysts for numerous applications. This review aims to highlight recent advancements and key understanding in carbon nitride based 2D/2D heterojunctions and their applications in photocatalysis, including small molecules activation, conversion, and degradations. We conclude with a forward-looking perspective discussing the key challenges and opportunity areas for future research. 1. INTRODUCTION The access to clean energy and per capita energy consumption is an archetype of societal and scientific progress and directly related to human living standards and economic prosperity.1,2 In the year 2019, the total world energy consumption has been estimated to be ≈14 500 Mtoe.3 Unfortunately, a significant fraction of world energy is exploited from fossil fuels that have skyrocketed global CO2 concentration to a catastrophic level of 420 ppm (May 2021), a significantly higher number than the preindustrial era.4 High per capita energy consumption is also responsible for the deteriorating environment and climate change.5 The United States alone, which has only 5% of the world population, consumes 20% of the world energy and emits 6.5 billion metric tons of CO2e greenhouse gases.6,7 Irresponsible industrializations, rapid urbanization, and abusive exploitation of natural resources have adversely affected earth conditions of which water pollution is most severe. Almost 80% of the world’s wastewater (34 billion gallons of wastewater per day) is dumped in water bodies without any treatment.8 According to the United Nations’ World Water Development Report 2018, the demand for clean water is expected to increase by nearly one-third by 2050.9 In addition to recalcitrant pollutants such as pesticides, herbicides, fungicides, insecticides, antibiotics, heavy metals, etc., colored dye from textile industries is plaguing the water bodies due to shrinking penetration depth, leading to eutrophication and death of aquatic flora and fauna.10,11 The impact of climate change and pollution is discernible from the global warming and extreme weather events such as unusual melting of icecaps, excessive rain, etc.12−14 To limit the global temperature rise to below 1.5 °C, as suggested in the Paris agreement, at least a 7% emission reduction per year is needed.15 To foster alternative energy usage, governments are adopting policies and subsidizing Received: September 14, 2021 Revised: November 2, 2021 Review pubs.acs.org/cm © XXXX American Chemical Society A https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX Downloaded via UNIV OF ALBERTA on November 30, 2021 at 08:24:32 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
- 2. technologies that make a minimum adverse impact on the environment.16,17 Among various energy sources, inexhaustible and clean solar energy is the most abundant, fulfilling all our future energy demands (1.7 × 1022 J energy is being dissipated on the earth’s surface in 1.5 days, which is equivalent to 3 trillion barrels of total oil resources found on Earth).18 Solar energy is expected to contribute significantly and is projected to reach up to 1200 GW by the end of 2024.19 Among various solar energy harvesting technologies, photovoltaic cells are at the forefront, which can transform solar energy into electrical energy at an efficiency of ∼22.5% (47.1% in multijunction PV cells under concentrated solar light) and is significantly higher than natural photosynthesis (0.5−1% in most plants and up to 5% in some algae).18,20−22 However, intermittency, unequal insolation in a different part of the world, associated energy storage and transportation issues, and longer payback time are some challenges for widespread implementation.23 Artificial photosynthesis using sunlight to energize electrons (and holes) in photocatalysts and their subsequent storage in the chemicals bonds to convert CO2 into hydrocarbons and water into hydrogen is a preeminent way to capture and utilize sunlight.24,25 Conventional thermochemical conversions of CO2 into hydrocarbons (Fischer−Tropsch) and value-added chemicals such as ethylene glycol and ethylene carbonates relies on expensive catalysts and an energy carrier (usually hydrogen) and requires elevated temperature and pressure.26,27 Photocatalysis provides a low energy route using photo- catalysts and sunlight to transform extremely stable CO2 (ΔHf −394 kJ/mol) into useful chemicals.28 H2 is an attractive clean energy fuel due to its high energy density (120 kJ/g), used as a fuel for fuel cells to produce electricity, and water is the sole byproduct. Additionally, hydrogen works as a high-energy carrier that can transform CO2/CO into hydrocarbons addressing challenges associated with voluminous storage and explosion hazards and avails easy transportation.29−31 Research efforts in past decades on semiconductors were mostly focused on the heterogeneous inorganic crystalline materials and homogeneous catalysts, which led to activity only up to the micromole to millimole regime due to the lack of sufficient photon absorption in the visible region, fast charge recombination, and nonresiliency.32−36 Even though a catalyst can harvest visible photons, a major fraction of generated charge carriers get recombined before any fruitful reactions. The estimated time for a photon to absorb on a nanocrystal under solar irradiation with a flux density of 2000 μmol s−1 m−2 is approximately 4 ms while the time taken in recombination is on the order of microseconds. Therefore, driving reactions that involve multiple electrons, such as water splitting and CO2 reduction (4 for H2 and 8 for CH4), becomes challenging.37,38 Various strategies such as surface modification with cocatalysts, increasing performance by plasmonic materials, organic 0D−2D sensitizers, quantum dots, and heterojunctions formation have been suggested to improve the visible light absorption and charge separa- tions.39−42 The formation of a heterojunction of carbon nitride with other semiconductor materials provides an opportunity to synchronize the wide fraction of the solar spectrum and charge separation requirement.43 Various types of heterojunctions can be realized, such as type-I (straddling), type-II (staggered or direct scheme), type- III (broken), Z-scheme, and S-scheme, based on the band position type of electron flow and nature of conductivity (n- or p-type) (Figure 1).44−46 In a type-I heterojunction, the conduction and valence band of one semiconductor are straddled between the conduction and valence band of the second semiconductor, resulting in the flow of electrons and holes from the high band gap semiconductor to low band gap semiconductor (Figure 1a). The type-II heterojunction facilitates better charge separation and maximum utilization of absorbing photons; however, this configuration relies on a wide band gap semiconductor to meet the criteria of the redox reaction (ECB > 0.00 V and EVB > 1.23 eV), and the opposite flow of electrons and holes further reduces the potential gap (Figure 1b).47 In type-III heterojunctions, the valence and conduction bands of two semiconductors have a significant Figure 1. Types of heterojunctions: (a) Type I (Straddling), (b) Type II (Staggered), (c) Z-scheme, (d) S-scheme, and (e) 0D/2D, 1D/2D, 3D/ 2D, and 2D/2D. Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX B
- 3. energy difference and so are not aligned together and scarcely reported. In contrast, Z-scheme and S-scheme photocatalysts are constituted of two tandem structured photosystems that can meet the requirement of higher wavelength absorption and wideband energy gap of conduction and valence bands; however, the two-photon excitation mechanism reduces half of the efficiency of the system (Figure 1c,d).48,49 The physicochemical/photophysical properties and charge separa- tion mechanism in the semiconductor heterojunction and their advantage over conventional modification approaches such as using electron and hole capturing agents have already been discussed before in other pioneering reviews.40,43,48,50 Theoretically, heterojunction formation seems to solve the problem of light absorption and charge separation; however, practically, these problems persist due to the enormous recombination of charge carriers at the interface of the two semiconductors and long migration distance in the hetero- junction to reach another semiconductor.51 Two-dimensional materials have emerged as a promising material platform that can overcome these issues due to excellent charge carrier mobility, confinement of electrons in a few angstrom thick layers, short diffusion distance, and better absorption even at a low flux density.37 Graphene is the most influential member of the 2D family due to its excellent electron mobility (>50 000 cm2 /(V s) at room temperature).52 Contrary to inorganic nanocrystals, the charge transport along the 2D graphene sheets is almost resistant-free, which provides excellent charge carrier mobility.53,54 However, due to the absence of a band gap, its photocatalytic application is limited to charge capturing agents, macromolecular sensitizers, and redox mediators in heterojunction structures.55 Heteroatom doping such as N, P, and S opens up the band gap in graphene and influences the charge distribution of neighboring carbon atoms, resulting in shifting of the Fermi level above the Dirac point, and graphene behaves as a semiconductor.56,57 However, due to the low doping level and multitudinous nature of doping (pyridinic, pyrrolic, quaternary N’s, etc.), the band gap of graphene is restricted below 1.0 eV, which cannot meet the theoretical band gap requirement of water splitting (1.23 eV) or CO2 reduction (over 1.0 eV).58 Other 2D semiconductors, such as 2D hexagonal boron nitride (hBN), are lagging because of their wide band gap (∼6 eV).59 Beyond hexagonal sp2 carbon-containing graphene, 2D transition metal dichalcogenides (TMDCs) of Group V and VI metals (V, Nb, Ta, Cr, Mo, and W) and chalcogens (i.e., S, Se, and Te) with a layered structure and trigonal prismatic 1H, 1T, and 1T′ phases have shown great promise in the photo- catalysis.60−62 Though the most investigated members such as MoS2 and WS2 possess a low band gap, their inability to catalyze both sides of redox reactions limits their applic- ability.63−65 Again, fast charge recombination coupled with the instability of some chalcogenides remains an evident problem. Recently, metal-free graphitic carbon nitride (g-C3N4, CN, melon) composed of tertiary nitrogen linked tris-s-triazine (heptazine; C6N7) units arranged in a 2D sheets structure has galvanized the photocatalysis field due to its attractive optical and electronic properties.66,67 The graphene-like 2D structure, moderate band gap (2.6−2.7 eV), compelling band positions (ECB: −1.1 and EVB: + 1.6 eV vs NHE) to facilitate oxidation and reduction at their valence and conduction bands (water splitting, CO2 reduction, pollutant oxidation) make them a superlative photocatalyst.68 Unfortunately, bare absorption after the blue region (450 nm), fast charge recombination, hydrogen-bonded sheets, and intricate film formation are major obstacles.69 A plethora of articles reported metal/ nonmetal doping,70,71 insertion of N/C rich units,72 changing of the coordination/bridging pattern,73 self-doping, surface chemical modification using metal complexes,74,75 quantum Figure 2. Face-to-face interaction and vertical charge transport mechanism in the 2D/2D vdW heterostructure. (a) 2D/2D vdW heterojunction between two inorganic heterostructures. (b) Unidirectional charge flow in inorganic/graphene-based 2D/2D vdW heterojunctions. (c) 2D/2D carbon nitride/inorganic semiconductors vdW heterostructure. (d) 2D/2D interfacial vdW heterojunction between carbon nitrides. (e) Comparison between conventional and flexible vertical FET on a flexible plastic substrate. Reprinted with permission from ref 105. Copyright 2016 Macmillan Publishers Limited (Springer Nature). Reprinted with permission from ref 121. Copyright 2014 American Chemical Society. (f) Quasi- particle band gap values and types for various 2D vertical heterostructures composed of TMDC, obtained using DFT−PBE calculations and displayed as a heatmap. Reprinted with permission from ref 122 by Chaves et al. under the terms of the Creative Commons Attribution 4.0 International License (CC BY) (https://creativecommons.org/licenses/by/4.0/). Copyright 2020 Chaves et al. Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX C
- 4. dots, heterojunction formations,76 etc. to improve the visible absorption of the carbon nitride in longer wavelengths.68,77 During the synthesis of bulk carbon nitride from its precursor, several uncondensed fragments with terminated NH/NH2 promote intersheet hydrogen bonding, leading to low crystallinity.78 Further, these regions also act as localized charge recombination centers. Transformation of bulk g-C3N4 into single to few-layered sheets by breaking hydrogen bonding has been suggested as a viable approach to alleviating these problems.79,80 Many approaches such as solvent assisted exfoliation (in water, IPA, butanol, DMF, NMP),81 thermal exfoliation, chemical exfoliation using harsh chemicals such as LiCl,82 HNO3,83−85 H2SO4,86 KOH,87 KMnO4+H2SO4/ H3PO4,88 and even altering/adding precursors during syn- thesis89,90 have been reported to yield few/monolayered carbon nitride sheets.84,91 However, these processes lead to poor crystallinity (periodicity), interfering with charge migration distance.92−94 Further, due to the confinement effect in monolayered sheets, visible photon absorption is compromised, which further intensifies the problem of charge recombination. The crystallinity of both triazine and heptazine- based carbon nitride can be improved by molten salt-assisted synthesis using KCl and LiCl. Many other approaches such as breaking of hydrogen bonding, improving polymerization degree, using hydrogen-bonded precursors with planar structure, etc. have also been investigated to improve the performance of heptazine/triazine-based networks.95 Depending upon the morphology and interfacial interaction between various semiconductors and 2D materials, mainly four types of heterojunctions can be realized, namely, 0D/2D, 1D/ 2D, 2D/2D, and 3D/2D (Figure 1e).96−99 Since 0D spherical morphology has a low surface area, point interaction with the 2D materials makes the effective interaction poor to achieve facile charge migration. On the other hand, the 1D configuration provides the advantage of high surface, direc- tional charge transport, and reduced recombination losses due to a less populated bulk phenomenon; however, the interfacial contact between the 1D and the 2D structure remains confined to the stem diameter of the 1D structure.100 Though 3D morphology provides relatively improved interaction, the bulk recombination in the 3D structure remains prevalent.101 In recent years, 2D/2D heterojunctions constituted via the face- to-face interaction of two semiconductors are gaining popular- ity due to maximum charge separation between two semi- conductors interfaces (Figure 1e).102−104 Additionally, a distinct interfacial charge separation mechanism prevents recombination due to short diffusion length (few Å ≈ equal to interplanar distance).37 In lateral/bulk 3D heterojunctions, a significant fraction of carriers get recombined at the materials heterointerface. The vertical charge transport mechanism in the 2D/2D heterojunction ensures minimum migration length for the majority and minority charge carriers for efficient collection of photogenerated charge by the second semi- conductor before the annihilation (Figure 2).105−107 The constitution of the 2D/2D heterostructure also provides maximum accessibility to active sites for reactant molecules to adsorb and react on the semiconductor surface.108 Several 2D/2D heterojunctions designed from inorganic 2D semi- conductors have been reported previously for various applications, including photocatalysis.109−115 For the inorganic 2D/2D heterojunction, epitaxial matching is an indispensable criterion for efficient charge transfer between two semi- conducting materials (Figure 2a). The lattice mismatch limits the choice of available materials (lattice mismatch < 10%) and compromises the quantum efficiency due to populated recombination at the interface.116,117 Several approaches such as chemical vapor deposition (CVD), molecular beam epitaxy (MBE), and pulsed laser deposition (PLD) have been devised for layer-by-layer growth of the 2D/2D heterojunction; however, such processes are slow/energy intensive and require sophisticated tools.118,119 Furthermore, stiff strenuous nano- architecture impedes their applications in flexible devices. In contrast, the 2D/2D heterojunction of layered carbonaceous materials (especially graphene and carbon nitride) and inorganic 2D semiconductors does not require lattice matching and can remain in contact due to van der Waals interaction (Figure 2b−d).76,103,120 When two 2D materials are in close contact with weak van der Waal forces, they are generally referred to as 2D/2D vdW heterostructure. Graphene based 2D/2D vdW heterostructures have been widely investigated for electronics/optoelectronics such as FETs. Due to the flexible and conductive nature of graphene, it can overcome the issues of broken junction in conventional inorganic semiconductor-based devices to fabricate foldable devices on a plastic substrate (Figure 2e).105 Although zero-band gap and graphene enhanced photocatalytic activity due to better charge collection on conductive sheets, it however does not contribute further because of their inability to produce electron−hole pairs. On the other hand, carbon nitride due to moderated band gap, conjugated nature, and analogous graphenic structure is an ideal candidate for making 2D/2D hetero- junctions. The 2D/2D heterojunction of 2D carbon nitrides with inorganic/organic 2D semiconductors demonstrated excellent potential for photocatalytic and electronic applications. The weak van der Waals interactions between carbon nitride sheets and 2D semiconductors overcome the conventional lattice matching constraint.105,123−125 Beyond the advantage of 2D/ 2D vdW heterojunctions in better charge separation, the 2D/ 2D architecture also influences the charge distribution on two semiconductor sheets resulting in band gap modulation. As depicted in Figure 2f, by choosing an appropriate combination of semiconductors and controlling the number of layers, the band gap can be tuned to get the desired products.122,126,127 The ability of carbon nitride to form a strain-free heterojunction with other 2D semiconductors allows vast permutation to fabricate many 2D/2D heterostructures.54,128 Layered double hydroxides (LDHs), 2D oxides, oxynitrides, sulfides, and mixed oxides including perovskite oxides have been explored to form a 2D/2D heterojunction assembly with carbon nitrides for various applications such as photocatalytic hydrogen evolution, CO2 reduction, pollutants, antibiotics, NO2 degradations, etc.129,130 The formation of a p−n type heterojunction by coupling of n-type carbon nitride with p- type semiconductors such as bismuth oxyhalides (BiOX: X = Cl, Br, I), perovskite oxides, phosphorene, etc. is particularly appealing because of the dual advantage of better face to face interaction and innate charge separation in the built-in electric field.131 In recent years, new 2D materials with promising properties are emerging such as conductive/semiconductor MXenes, low band gap phosphorene (P), borophene (B), stanene (Sn), tellurene (Te), silicene (Si), bismuthine (Bi), arsenene (As), antimonene (Sb), etc. which further ameliorate the scope of carbon nitride-based 2D/2D heterojunc- tions.132−137 The possibility of chemical structure manipu- lation and decoration with single atoms (single atom catalysts, Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. 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- 5. SACs) of carbon nitride sheets provide an opportunity to facilitate selective adsorption of a specific substrate on the surface to achieve excellent product selectivity.138−143 The chemically modified carbon nitrides with a differential band gap can make a metal free n−n isotype heterojunction. The doping of carbon nitride with electron deficient elements/units (such as boron) can shift the Fermi level to transform n-type carbon nitride to p-type carbon nitride, suitable to fabricate a p−n isotype heterojunction. The research on 2D/2D carbon nitride-based vdW heterojunction materials is gaining momentum. Several reports have emerged in recent years demonstrating excellent photo- catalytic performance over conventional heterojunction photo- catalysts and other applications. This review article focuses on photocatalytic processes for clean energy production and pollutants degradation using 2D/2D carbon nitride vdW heterostructures (Figure 3). As of today, no comprehensive review has been reported explicitly focusing on the carbon nitride-based 2D/2D vdW heterostructure. This review compiles the research work done in the field in the past four years and emphasizes various synthetic protocols such as solvent and chemical exfoliation, in situ approach, and electrostatic interaction. The 2D/2D vdW heterojunction of g-C3N4 with 2D materials such as elemental 2D materials (black P, red P, antimonene), MXenes, metal oxides (TiO2, MnO2, WO3, ZnV2O6), transition and noble metal chalcoge- nides (MoS2, WS2, FeSe2, ZnIn2S4, PtS2), bismuth oxyhalides, perovskite oxides, LDHs, etc. has been thoroughly investigated. Additionally, a broad section dedicated to isotropic hetero- junctions has been added, which are rarely discussed in any report. An implanted carbon-containing heterostructure is a new subdiscipline of the field. Additionally, a section demonstrating various 2D polymer semiconductors which can be used for the 2D/2D interfacial junctions has also been canvassed. Finally, a comparison between vdW and lateral 2D/ 2D heterostructures has been made. Among photocatalytic applications, we have thoroughly revisited hydrogen evolution, CO2 reduction, and pollutant degradations. The present review bridges the gap as it highlights the recent research work done in the field of the 2D/2D carbon nitride-based heterojunction. In the following sections, we have focused our discussion on carbon nitride-based 2D/2D heterojunction with a wide variety of inorganic and allotropic 2D semiconductors. The photophysical properties of each material amalgamating with carbon nitride on the 2D/2D heterojunction are also discussed in detail. We conclude with forward-looking perspectives and rational design of 2D/2D configuration to develop a universal catalyst with complete visible photon absorption and efficient charge separation. 2. CARBON NITRIDE−ELEMENTAL 2D MATERIAL vdW STRUCTURES 2.1. Carbon Nitride−Black Phosphorus (BP)/Phos- phorene. Black phosphorus (BP), a stable elemental allotrope of phosphorus (compared to red and white phosphorus), has attracted significant interest in the materials science community due to its low direct band gap (0.3 eV) with tunability depending on numbers of layers, 2D graphite type layered structure, and excellent field-effect mobility (∼1000 cm2 /(V s) at room temperature).144 The high-temperature synthesis at 873 K (in the presence of gold, tin, and tin(IV) iodide)145,146 usually led to rhombohedral forms with Figure 3. Schematic illustration of carbon nitride-based 2D/2D vdW heterojunctions with various 2D materials. Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX E
- 6. directional ductility along with the sheet and large in-plane anisotropy due to its puckered atomic structure (dichro- ism).147 The exfoliation of black phosphorus in aprotic and polar solvents such as acetone,148 chloroform,149 ethanol,150 isopropyl alcohol (IPA),150 dimethylformamide (DMF),151 dimethyl sulfoxide (DMSO),152 N-methyl 2-pyrrolidone (NMP),153 and N-cyclohexyl 2-pyrrolidone (CHP)154 offers single-layered sp3 hybridized 2D sheets of phosphorene with a band gap of ∼2.1 eV and theoretical electron mobility of 10 000−26 000 cm2 V−1 s−1 .155 Unlike MoS2 and WS2, the transition of bulk BP into phosphorene does not lead to indirect-to-direct band gap transition while going from bulk to monolayer, which provides flexibility to use band gap tuned fragments (depending on the numbers of layers) for photo- catalysis.156 Due to its wide absorption profile extended up to the NIR region and p-type conductivity with high hole mobility (105 cm2 /(V·s)),157 BP is an excellent material for making a heterojunction with n-type materials.147 Unfortu- nately, under air and moisture phosphorene forms strong P−O and PO dangling bonds with oxygens to form indirect band gap nonstoichiometric oxides POx, which ultimately degrades via the formation of phosphoric acid. Since the first few reports on the potential of BP to enhance dye degradation and water splitting in black−red phosphorus heterostructure158 and Ag/BP nanohybrids,159 numerous reports on BP have emerged.160−163 DFT calculations reveal the valence band position (0.21 eV) of phosphorene does not meet the criteria of water oxidation (H2O/O2; +1.23 eV vs NHE at pH 0) while the conduction band (−0.56 eV) is well suited for the proton reduction to hydrogen (H+ /H2; 0.0 eV vs NHE at pH 0), making it a good hydrogen evolution catalyst in the presence of a sacrificial donor.164 However, switching the pH of the solution to 8.0 was found to shift the valence band toward the positive side, facilitating both an oxidation and a reduction reaction for overall water splitting.165 Additionally, it was predicted that the edge decoration of BP with pseudohalogens such as nitrile (CN) and cyanate (OCN) can also tune the band position to facilitate water splitting.166 To sustain the water-splitting process and prevent fast oxidation, the use of alkali and tedious chemical functionaliza- tion is undesirable. To overcome the stability and band alignment issues without compromising the visible absorption, heterostructures of BP with a wide variety of materials such as graphene, TiO2, WS2, BiVO4, MoS2, and ZIF-8 and plasmonic metals such as Ag and Au have been fabicated.167,168 However, carbon nitride remains one of the best candidates to make a heterostructure with BP due to its appropriate band structure and 2D Figure 4. (a) HAADF-STEM image. EDX elemental mapping of (b) N and (c) P and (d) overlay of HAADF-STEM of N (green) and P (red) elements of BP/CN. (e) UV−vis diffuse reflectance spectra of CN, BP, and BP/CN. (f) Photocatalytic H2 evolution from water containing methanol (20 vol %) on different catalysts under visible light (>420 nm) irradiation. (g) Effect of BP:CN ratio in BP/CN on photocatalytic H2 evolution rate under visible light irradiation for 3 h. (h) Photocatalytic H2 evolution from BP/CN with >780 nm light irradiation. (i) Proposed mechanism for the visible and NIR light-activated photocatalytic H2 evolution using BP/CN in the presence of methanol. Reprinted with permission from ref 178. Copyright 2017 American Chemical Society. Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX F
- 7. nature.169 Many reports demonstrate the harsh sonication of BP in NMP/aprotic solvents/deoxygenated water to form BP quantum dots followed by mixing with carbon nitride to form 0D/1D hybrids.170−174 Transformation of BP in quantum dots reduces the absorption profile and compromises the potential of BP to harvest NIR photons.175 Though quantum dots are still visiblly active, the processability, leaching, and presence of plenty of oxygen functionality (e.g., P−O, PO) presents stability and resiliency-related challenges.176 Zheng et al. demonstrated that in situ exfoliation of bulk BP and g-C3N4 powders in NMP is a compelling approach to produce BP sheets supported on carbon nitride sheets.177 The developed catalyst with 10% BP (10%BP/CN; average thickness of 4.2 ± 1.0 nm) displayed the maximum visible light degradation efficiency toward the rhodamine B (RhB) and H2O2 production. Although the material displayed absorption as far as up to 800 nm, only visible light from the solar simulator (λ > 420 nm) was employed that can achieve a 98% RhB degradation efficiency within 15 min and 540 μmol g−1 H2O2 generation after 1 h. The mechanism evaluation using radical scavengers such as 1,4-benzoquinone (BQ as a • O2 − scavenger), KI (h+ and • OH radical scavenger), and isopropanol (IPA as a • OH radical scavenger) followed by radical trapping using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and measuring EPR demonstrate that superoxide radicals (quartet for DMPO−• O2 − adduct 1:1:1:1) were the main reactive oxygen species. To further explore the potential of BP and g-C3N4 (CN) hybrid to harvest NIR light, Zhu and co-workers synthesized BP/CN hybrid via sonication and stirring of bulk BP and CN in NMP.178 HR-TEM and HAADF-STEM images display amalgamated BP and CN sheets in the hybrid (Figure 4a−d). DFT calculations on the BP/CN structure suggested that P atoms are situated in the interstitial sites of CN through P−N coordinate bond and connected to two adjacent pyridinic-N atoms from two separate triazine units to form a P−N3C2 ring. FTIR spectra of BP/CN exhibited a signature peak at around 960 cm−1 due to P−N stretch and further demonstrates a strong interfacial interaction between BP and CN. Usually, the formation of a heterojunction proceeds through charge migration and Fermi level alignment, which can be observed via a change in binding energy and shifting of valence band positions. The P 2p XPS spectra of BP/CN displaying a downshifting of the binding energy by a factor of 0.1 eV relative to pristine BP along with observation of a new peak at 133.2 eV (due to P−N of P3N5) further validate the formation of the chemically stable BP/CN hybrid. A similar pattern was also observed in UPS spectra, where the onset potential of BP/ Figure 5. (a) XANES P K-edge of FP and 1.8PCN. (b) XANES N K-edge of 0.0PCN and 1.8PCN. (c) Photocatalytic H2-production activities of 0.0PCN, 0.6PCN, 1.8PCN, 4.0PCN, 9.5PCN, and 1.8Pt-CN in 18 vol % lactic acid aqueous solution under visible-light irradiation (λ > 400 nm). (d) UV−vis diffuse reflectance spectra of 0.0PCN, 0.6PCN, 1.8PCN, 4.0PCN, and 9.5PCN. The insets show the colors of these samples, (e) Steady-state SPV spectra of 0.0PCN and 1.8PCN. (f) Transient-state SPV spectra of 0.0PCN and 1.8PCN. (g) Charge separation and transfer in the FP/CNS system under visible-light irradiation (λ > 400 nm). (h) Schematic illustration of photocatalytic H2 production in the FP/CNS system under visible-light irradiation (λ > 400 nm). The red, green, gray, blue, and black spheres denote H+ , H, C, N, and P atoms, respectively. Reprinted with permission from ref 180. Copyright 2018 Wiley-VCH. Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX G
- 8. CN was slightly shifted. The solid evidence of a P−N coordinate bond between two 2D heterostructures comes from solid-state 31 P NMR, which displayed the main peak at 17.1 ppm for P atoms in a BP sheet scaffold and showed two additional signals at −0.2 and 5.4 ppm due to P−N coordination. The UV−vis profile of BP showed absorption up to NIR (absorption edge at 1740 nm) (Figure 4e). The UV−vis of BP/CN after subtracting the absorbance of CN and BP evidenced NIR absorption of the materials. When tested for the hydrogen evolution in the presence of methanol as a sacrificial donor, the BP/CN in 1:4 weight ratio demonstrated a hydrogen evolution rate of 427 μmol g−1 h−1 under visible light irradiation (λ > 420 nm), while under identical conditions CN and BP show negligible hydrogen evolution (Figure 4f,g). Interestingly, under NIR illumination (>780 nm, 780−1800 nm, and 808 nm laser), a fair hydrogen evolution rate was maintained for BP/CN only, and no product was observed for pristine BP and CN (Figure 4h). Time-resolved diffuse reflectance (TDR) spectroscopy using 780 nm laser revealed that BP/CN heterostructure displayed a much longer lifetime (44 ps) compared to pristine BP (0.8 ps), suggesting the P−N coordinate bond at the interface acts as the trap site to facilitate hydrogen generation. Under visible irradiation, most of the charge is generated from the CN, and a small band gap of BP established a straddling gap (type-I heterojunction) with carbon nitride (Figure 4i). Mechanochemical synthesis using ball milling of red phosphorus (RP) to form BP followed by ball milling with carbon nitride was also found to form 2D−2D BP/CN heterostructure.179 The assembled BP/CN hybrid demonstrated a hydrogen evolution rate (786 μmol h−1 g−1 ) and RhB degradation (complete degradation within 25 min) even under the weak intensity of LED (440−445 nm) and was comparable to BP/CN hybrid realized using a solvent assisted exfoliation. This synthetic approach provides an opportunity for scalable and low-cost production of photocatalysts. The solvent exfoliation of bulk BP under strong sonication and air leads to oxidation and nonuniform fragmentation of BP sheets, resulting in the formation of quantum dots with low photocatalytic performance. Ran et al. devised a method to exfoliate bulk BP in ethanol under intermittent low sonication power, inert atmosphere, and low temperature.180 The 2D/2D vdW heterojunction of few-layered phosphorene (FP; thick- ness ≈ 5−6 nm, lateral sizes ≈ 100−450 nm) and g-C3N4 nanosheet (CNS; prepared by thermal annealing of bulk g- C3N4 powder at 500 °C) denoted as PCN were realized by mixing both components in a mortar under an inert environment. The shifting of P 2p XPS signals (≈0.8 eV) and XANES P K-edge of PCN toward a lower binding energy value suggest electron migration from CNS to FP to form a p− n type heterojunction (Figure 5a). At the same time, the C K- edge and N K-edge exhibited a positive shift due to the n-type conductivity of g-C3N4, which concomitantly transfers electrons to electron deficient FP in the heterojunction (Figure 5b). This assumption was confirmed by DFT calculations which reveal the work functions (Φ) of g-C3N4 and phosphorene were 4.69 and 5.01 eV, respectively. Due to the higher Fermi level (Evac- Φ) of g-C3N4, the electrons are expected to migrate from g-C3N4 to FP. The 2D/2D FP/CNS vdW heterojunction with 1.8 wt % of FP displayed an H2 evolution rate of 571 μmol h−1 g−1 , which was higher than 1.8 Figure 6. (a) Schematic illustration of the preparation of BP nanosheets with the NMP ice-assisted exfoliation method. (b) Tapping mode AFM topographical image of few-layer of BP nanosheets. (c) Height profiles of BP nanosheets along the blue line 1 and green line 2 in part (b). (d) Distribution of BP layers calculated from the height profiles of 150 BP nanosheets in AFM images. (e) XRD patterns of bulk BP, BP nanosheets, g- C3N4, and BP/g-C3N4 samples. The inset shows the amplification of XRD patterns of bulk BP and BP nanosheets in the lower-angle range, which is marked by the dashed rectangle (f). Theoretical Tauc-plot curves of BP with different layer numbers (1−4 and 6 layers). (h) Photocatalytic H2 evolution rate achieved in the presence of BP (orange), g-C3N4 (blue), 3 wt % BP/g-C3N4 (red), 10 wt % BP/g-C3N4 (green), and 15 wt % BP/g- C3N4 (purple) photocatalysts under λ > 420 nm light irradiations. Schematic energy diagram of BP/g-C3N4 photocatalyst and proposed possible mechanism for the photocatalytic H2 evolution under (h) λ > 420 nm and (i) λ > 475 nm light irradiation. Reprinted with permission from ref 182. Copyright 2019 Wiley-VCH. Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX H
- 9. wt % Pt loaded CNS (1.8 Pt-CN, 548 μmol h−1 g−1 ) and the highest among metal-free catalysts (Figure 5c). Even though with the addition of FP the visible light absorption gradually increased, the photoactivity decreased beyond 1.8 wt % due to occupancy of active sites and shielded light absorption by the phosphorene (Figure 5d). Steady-state surface photovoltage (SPV) spectra and transient-state SPV (TSSPV) spectra of 0.0PCN and 1.8PCN demonstrated a higher photovoltage on the surface of 1.8PCN, corroborating the fact that the presence of FP can efficiently promote the dissociation of photoinduced charge carriers in CNS (Figure 5e,f). The effective interfacial contact and straddled type-I heterojunction in 2D/2D PCN vdW heterostructure offered better charge separation to accelerate catalytic performance (Figure 5g,h). The conventional approach for phosphorene synthesis from BP using solvent-assisted exfoliations is challenging due to long hours of synthesis of poor-quality sheets. As van der Waals interaction between P atoms in BP is much stronger than graphene and other 2D structures, a strong sonication power and longer hours are usually needed, which break the sheets and reduce their lateral size as well.181 Zhang et al. present an intelligent idea of ice-assisted exfoliation to reduce the processing time and increase phosphorene yield.182 In this synthesis, a dispersion of bulk BP in NMP was frozen using liquid N2 (Figure 6a). Due to the growth of NMP crystals in between BP sheets, the van der Waals interaction gradually weakens, and mild sonication in the next step can easily exfoliate the BP sheets. By employing this approach, the total time required for the sonication can be reduced up to 2 h at 70 W sonication power with a significantly high 75% yield. The obtained BP sheets have excellent uniformity with a thickness distribution of 93% (mean numbers of layer = 5.9 ± 1.5) and a lateral size of 50 nm to 3 μm (Figure 6b−d). When coupled with g-C3N4 in IPA, a well-designed 2D/2D heterojunction was established, evident from TEM and STEM images. Interestingly, no XPS peak corresponding to oxidized PXOy species was detected in pristine BP or BP/g-C3N4 ascribed to nonoxidative exfoliation of BP. However, the P 2p XPS signal was shifted to lower binding energy due to charge transfer from g-C3N4 to BP. After ice-assisted exfoliation, the XRD peak at 16.95° for bulk BP was downshifted to 15.89°, corresponding to interplanar distances of 5.2 and 5.6 Å, respectively, substantiating that intercalation of NMP molecules assists exfoliation of BP (Figure 6e). Due to the formation of few- layered sheets, the BP displayed a band gap of ≈1.39 eV also verified with DFT while the composite still has excellent visible (band edge at 474 nm) to NIR absorption (band tail) (Figure 6f). Using 3% BP/g-C3N4 as a photocatalyst under visible light (λ>420 nm), the H2 generation rate was found to be 384.17 μmol g−1 h−1 , almost 7 and 4.5 times those obtained from pristine BP (54.88 μmol g−1 h−1 ) and g-C3N4 (86.23 μmol g−1 h−1 ) (Figure 6g). The introduction of g-C3N4 in the BP/g- C3N4 heterostructure not only improves the photocatalytic performance but also strengthens the stability of the material. This was evident from P 2p XPS spectra of BP and BP/g-C3N4 after a long reaction time, showing that 21.6 and 7.5 atm % of P transformed to PXOy, respectively. The excitation wavelength above 475 nm does not yield any product for pristine BP, g- C3N4, and 3% BP/g-C3N4 while the 10% BP/g-C3N4 vdW heterojunction still affords significant hydrogen (143.47 μmol g−1 h−1 ), demonstrating that at high wavelength excitation and optimum phosphorus contents only BP contributes to the water reduction reaction while CN facilitates charge separation. The small semicircle in the Nyquist plot quenched PL intensity and shorter average PL lifetime (486 ± 5 ns) of BP/g-C3N4 compared to its constituting components validates better charge separation in the vdW heterostructure. Valence band edge calculation by UPS followed by determination of CB using optical band gap demonstrates that the CB of BP is more positive (−0.60 V) than that of g-C3N4 (−1.18 V). So, electrons can be easily transferred from CB of g-C3N4 to BP and reducing protons (H+ /H2; 0.00 eV vs RHE at pH 0) (Figure 6h,i). 2.2. Carbon Nitride−Red Phosphorus (RP). Another stable allotrope of phosphorus named red phosphorus (RP) is emerging as a new photocatalytic material due to its metal-free earth-abundant nature and well-tuned band positions with visible absorption onset extended to 700 nm.183−187 Among four electronic structures (amorphous, Hittorf, fibrous, tubular), Hittorf’s and fibrous phosphorus are important, consisting of polymeric tubular repeating units with a pentagonal cross-section. Due to its more reductive CB for water reduction and sufficient positive VB, RP is a winning candidate among the phosphorus family. Fibrous red P (1.7 eV) demonstrated the optimum photocatalytic performance with the highest reported H2 evolution record among the elemental photocatalysts such as silicon, boron, and sulfur.184,188 However, in the viewpoint of 2D structure, Hittorf’s phosphorus in which double tube layers are stacked in the c direction and held together via vdW forces is important, can attain a layered structure, and can be exfoliated in few layers to monolayers.189−191 The binding energy to exfoliate bulk Hittorf’s phosphorus to single layer Hittorf’s phosphorene is 0.35 J m−2 , which is significantly lower than that of BP (0.40 J m−2 ).192 Regrettably, Hittorf’s phosphorene has a theoretical direct band gap (2.52 eV) while Hittorf’s phosphorus exhibits an indirect band gap (2.17 eV) which makes exfoliation or fabrication of single to few layers a desirable step. Although the theoretical mobility of Hittorf’s phosphorene is 3000−7000 cm2 V−1 s−1 , which is comparable to that of black phosphorene (10000 cm2 V−1 s−1 ), the experimental performance is far too low due to prodigious stacking.144,192−194 Recently, research has been intensified to develop RP based photocatalytic materials using various approaches such as hydrothermal, high-temperature vapor deposition, ball milling, etc.195 Though RP alone is an excellent photocatalytic material, its performance is confined due to sluggish charge mobility both in few-layered and in the bulk form. Heterojunction formation with other semiconductors such as TiO2, graphene oxide, MOF, CdS, ZnO, etc. has been identified as a promising approach to enhance photocatalytic performance.196−203 For example, RP was deposited on the TiO2 nanofibers by the vapor deposition method and displayed excellent performance for H2 evolution from pure water.204 The heterojunction of 2D BP with 2D RP in Z scheme or type-I configuration is an emerging approach to make an all-inorganic heterojunc- tion.158,205 Liu et al. demonstrated that BP/RP 2D/2D Z- scheme catalyst can self-sustain the water splitting performance without any sacrificial donor.206 However, the reaction rate and the product yield remain low in these approaches. Coupling of RP with g-C3N4 has been found to increase the photocatalytic performance due to synergistic absorption, trap passivation, increased mobility, and better charge separa- tion.207−209 Jing et al. demonstrated that the introduction of ultrasmall RP particles in the g-C3N4 scaffold can minimize the number of defects in the g-C3N4 structure due to the formation Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX I
- 10. of P−C and P−N bonds, resulting in record hydrogen evolution performance than previously reported for carbon nitride−phosphorus based systems (2565 μmol g−1 h−1 ).210 In another report, [001]-oriented Hittorf’s phosphorus (HP) nanorods were fabricated on g-C3N4 using vapor deposition approach reaching a H2 evolution rate (HER) of 33.2 μmol h−1 from pure water.211 The 2D/2D vdW heterojunction of RP and g-C3N4 is particularly important as it can overcome the shortcomings of other dimensionalities due to intimate electronic contact between two semiconductor surfaces. Wang and coauthors synthesized 2D/2D RP/CN vdW catalysts via low temperature (300 °C) in situ phosphorizations on CN sheets (prepared via hydrothermal exfoliation of bulk CN in NH4OH) (Figure 7a).212 The thermal decomposition of NaH2PO2 followed by doping and deposition leads to deposition of a uniformly thick RP layer (∼4.6 nm) on CN (Figure 7b,c). The HR-TEM and corresponding EDX elemental mapping display the presence of RP nanosheet fragments on the surface of CN. The XRD peak of CN was significantly suppressed and slightly shifted after the formation of the heterostructure, suggesting dense surface coverage via RP. The RP/CN vdW showed a broad visible absorption extended up to NIR while XPS peaks in the C 1s and N 1s region were shifted toward a positive value, suggesting better intimate contact and charge transfer (Figure 7d). The conduction and the valence band positions of CN and RP determined using the Mott−Schottky plot were −0.96 and 1.83 vs RHE and −0.22 and 1.87 V, respectively. In comparison to constituting elements and physical mixture of RP and CN, the RP/CN vdW heterostructure displayed an exception with the enhancement of H2 evolution, reaching a value of 367.0 μmol g−1 h−1 (Figure 7e). Further, the RP/CN displayed activity in all visible light ranges up to 620 nm with long hour stability (Figure 7f). However, the stability of the catalyst was reduced after a long run at high temperatures due to the formation of H2O2 instead of O2, oxidizing the RP surface. To validate this hypothesis, when MnO2 was introduced into the reaction system, the O2 evolution rate was increased due to the decomposition of H2O2 in the presence of MnO2 (Figure 7g). A significant PL quenching and decreased charge transfer resistance suggest a better charge separation in the RP/CN vdW heterostructure. Due to more negative CB and VB of CN, a type II (staggered) heterojunction was formed where the electron flows from CB of CN to RP, and the holes move in the opposite direction, leading to better carriers separation (Figure 7h). 2.3. Carbon Nitride−Antimonene. Antimonene (Sb) is a relatively newly discovered 2D elemental semiconductor that gained significant attention due to its remarkable electronic and optical properties.213,214 The 2D structure of antimonene was first predicted theoretically in 2015 by Zhang et al. along with arsenene (As).215 However, the experimental synthesis of Sb was only realized in 2016 by Gibaja et al. by liquid-phase exfoliation of Sb crystal in a water/ethanol mixture.216 After that, several procedures to exfoliate Sb have been developed, including mechanochemical and sonochemical methods.217,218 Additionally, high-quality Sb up to a single atom thickness can be synthesized using epitaxial growth, which includes van der Waals epitaxy and molecular beam epitaxy (MBE).219 Employing harsh sonication conditions, using NMP, polyols such as PEG, and surfactants, well stabilized antimonene quantum dots can be isolated.220−222 Van der Waals epitaxial Figure 7. (a) Schematic diagram for fabricating the 2D/2D RP/CN heterostructure. (b) AFM image of RP/CN. (c) Corresponding height profiles along the denoted lines. (d) UV−vis spectra for CN nanosheets, RP/CN, and commercial RP (inset, the digital photographs). (e) H2 evolution rates of as-prepared samples under full arc irradiation. (f) H2 evolution rates of RP/CN under the incident light with varied wavelength ranges. (g) H2 and O2 production over time after the light was turned off (MnO2 added into the reaction suspension after the first 2 h). (h) Schematic diagram of a photocatalytic process for RP/CN. Reprinted with permission from ref 212. Copyright 2020 American Chemical Society. Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX J
- 11. growth, which involves the heating of the substrate in one zone and Ar/H2 assisted deposition to the relatively cooler substrate, is particularly important due to easy fabrication, easy deposition on any substrate, etc. Belonging from the same phosphorus VA group, Sb possesses the same allotropic structure as BP with a theoretically predicted band gap of 1.2 eV. Contrary to BP, the Sb is relatively stable under oxygen and atmospheric conditions, making them a suitable candidate for several applications. As it exhibits a small tunable band gap (0 to 2.28 eV) and high carrier mobility, it is complementing graphene for optoelectronic applications, including the hole transport layer in solar cells, thermophotovoltaic devices, electrocatalysis (CO2 reduction reaction to formates), energy storage devices, photodetectors, etc.218,223−226 However, due to the small band gap, the reports on the experimental photocatalytic application of antimonene are sparse and limited.227 Ji et al. synthesized an Sb and BP hybrid nanosheet (HNSs) based Z-scheme artificial photosynthetic system for the reduction of CO2.228 In this system Cp*Rh(phen)Cl was used as an electron shuttle with PEI-PEG-C18-M as a “double-side tap” in the presence of NAD(H+ ) and enzymes. Until now, only one report by Barrio et al. had existed on the heterojunction of Sb and g-C3N4.229 In this work, the authors used 2D sheets of g-C3N4 and few-layered flakes of Sb (CNSbx) to fabricate a 2D/2D vdW heterojunction (Figure 8, Table 1). The AFM images of CNSbx displayed average lateral dimensions of ≈1 μm and an average thickness of ≈5 nm (Figure 8a). Characteristic XRD peaks of g-C3N4 were gradually decreased after the formation of a heterojunction while absorbance of CNSb was increased, extending up to 800 nm (Figure 8b). At the same time the PL intensity was quenched for the CNSbx composites, demonstrating reduced radiative recombination (Figure 8c). The calculation of band energies using the Mott−Schottky measurement confirmed that the CB of CN was situated at −0.98 V while for Sb it was at −0.91 V vs NHE. Thus, the electron can move to Sb without any applied field (Figure 8d and schematic illustration). Photocatalytic testing of CNSb0.25 catalysts using RhB as a model pollutant offered a complete degradation within 20 min (Figure 8e). The improved activity was due to better charge injection from CN to Sb, which forms a type-I heterojunction with CN. Elucidation of the reaction mecha- nism using triethanolamine (TEOA) as a hole scavenger and AgNO3 as an electron scavenger demonstrates that N- deethylation of RhB to the N-deethylated noncolored compound is catalyzed via photogenerated holes in the CB of g-C3N4. 3. CARBON NITRIDE-MXENES MXene is a family of transition metal carbides, nitrides, and carbo-nitrides having layered 2D structures and the general formula Mn+1XnTx, where n is 1−3, M is a transition metal (Ti, Cr, Nb, Sc, Mo, etc.), X is carbon/nitrogen, Tx is a surface- oriented functional group (−OH, −O, and −F), their numbers per empirical units. In 2011, Gogotsi et al. discovered Ti3C2Tx MXenes with attractive properties such as high chemical stability, hydrophilicity, and good electrical conductivity.230 MXenes are 2D materials prepared by etching the A (aluminum) layers from the MAX phase, where M is a transition metal, A is an A-group element such as Al, and X is the C or N element. Since then, they have been of great interest for many applications such as energy storage, catalysis, and biomedicine.231−234 Additionally, theoretical studies have shown that MXenes have near-zero Gibb’s free energy with a low Fermi energy, which is excellent for application as an electrocatalyst in HER and OER reactions.235 The broad absorption of MXenes until the NIR region and the ability to promote charge transfer by accepting electrons make them a very promising material. Despite all the remarkable properties, MXene has a few shortcomings such as low work function, limited thermal stability, and highly exposed metal atoms on the surface.236 The conductive nature of MXene prevented its exploration in photocatalysis until 2014, when the first report appeared, displaying Ti3C2Tx has superb MB adsorption and degradation Figure 8. (a) AFM image of two flakes of the CNSb heterostructure with their topographic profiles. (b) UV−vis absorbance spectra. (c) Photoluminescence spectra of CNSbx heterostructures. (d) Derived band structure of the CNSbx heterostructure. (e) RhB degradation curves for CNSbx heterostructures. Reprinted with permission from ref 229. Copyright 2018 Wiley-VCH. Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX K
- 13. ability.237 The photocatalytic effect was assumed due to the presence of titanium hydroxide and/or TiO2. Since then, a plethora of reports has been published using MXene as charge transporters, cocatalysts, or modified MXenes to generate photoactive centers.238−241 Further, the formation of the MXene nanostructure with other semiconductors has been widely explored for all sorts of photocatalytic reactions such as water splitting, volatile organic chemicals degradation, CO2 reduction, N2 reduction reactions, etc.238,242−244 Several variants of MXene have been discovered in the past few years such as Mo2CTx, Zr3C2, Hf3C2, and double-M MXenes (Mo2Ti2C3Tx, Cr2TiC2Tx, and Mo2TiC2Tx), showing ad- vanced physicochemical properties.245−247 The introduction of semiconductive properties in MXenes and the possibility of fabrication of single-atom catalysts (SACs) open the gate to developing profoundly active and product selective cata- lysts.248−254 The formation of a heterostructure by coupling of 2D MXene with another 2D material such as g-C3N4 widens the possibility of further enhancing the activity and efficiency as a photocatalyst.255−257 The presence of plenty of surface functional groups such as −OH, −O, and −F provides a growing/interacting platform for other semiconductors with- out compromising electronic mobility.258 Due to its inherent architecture, this layered heterostructure will ensure intimate interfacial contact, promote fast separation, and prolong the lifetime of the induced charge carriers and greater exposed active sites. For example, Lin et al. used Ti3C2 MXene as an electron acceptor and O-doped g-C3N4 as a visible absorbing semiconductor to design a 2D/2D Schottky junction and observed improved H2 production.259 Yang et al. synthesized an ultrathin Ti3C2 MXene and g-C3N42D/2D heterojunction via calcination of an MXene and g-C3N4 mixture, as shown in Figure 9a.260 The urea molecules are well adsorbed on the surface of the exfoliated Ti3C2, which, after calcination at high temperature, forms ultrathin nanosheets of g-C3N4 over MXene. The photoactivity was estimated by subjecting the synthesized material to photoreduction of CO2 and OER (Figure 9b). Interestingly, both MXene and pure g-C3N4 showed almost no activity for the former but indicated an improved activity with increasing MXene content to an optimum level. Additionally, the optimized 10TC indicated stability up to 5 cycles when subjected to OER (Figure 9c). Furthermore, using isotopes of carbon (13 C and 12 C) and gas chromatography− mass spectrometry (GC−MS), it was confirmed that the produced products were originated from the photoreduction of CO2 (Figure 9d). The electronic band structure revealed that due to intimate contact between g-C3N4 and Ti3C2 MXene, Ef was organized to equilibrium with Ef,equ = −0.95 V. The final equilibrium was brought about by a positive shifting of g-C3N4 and a negative shifting of Ti3C2 MXene. Thus, the remarkable photoactivity shown can be due to fast transfer and extraordinary capture of the photogenerated electrons to reduce CO2. Various other modifications on MXene or the g- C3N4 were carried out for more enhancement. For example, g- C3N4 was functionalized by protonation. This results in protonated g-C3N4 that is positively charged with a hyped ionic conductivity and electronic band gap shift.261,262 Other modifications include growing metal oxides such as TiO2 with g-C3N4 and then integrating with MXene via an electrostatic interaction, creating a 2D/2D vdW heterostructure for convenient electron transfer and good interfacial contact. Besides, black phosphorus quantum dots (BQs) with high absorption coefficients, tunable band gap, high hole mobility, and excellent quantum confinement effects have also been employed alongside g-C3N4 nanosheets.263,264 However, MXene is used as an intermediate to fast track transmission of photogenerated charges and also to overcome the shortcomings of BQ/g-C3N4 such as low interfacial contact and low charge carrier mobility.265 Modifications in MXene such as creating 3-D hollow morphological structure and oxygen vacancies (OV) are also done for superior perform- ance.266,267 For example, Tahir et al. not only observed that the Figure 9. Schematic illustration for the fabrication process of the ultrathin 2D/2D Ti3C2/g-C3N4 nanosheets heterojunction. (b) Photocatalytic CO2 reduction performance of as-prepared samples. (c) Cycling tests over the 10TC sample. (d) GC−MS analysis of products from photoreduction of CO2 over 10TC using labeled 12 CO2 and 13 CO2 as the carbon sources. Reprinted with permission from ref 260. Copyright 2020 Elsevier. Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX M
- 14. creation of oxygen vacant sites in MXene enhances the optical absorption and charge transportation but also found that, due to high electron conductivity, the synthesized material showed promising results for selective CO2 methanation.267 Other variations in MXene such as delamination (d-Ti3C2) are performed to provide a homogeneous and uniform distribution while making composites with g-C3N4.268 The thermal treatment of d-Ti3C2 and g-C3N4 generates ternary Ti3C2/TiO2/g-C3N4 nanocomposites where Ti3C2 and g-C3N4 were glued together with TiO2 nanoparticles derived from partial degradation of MXene. The introduction of nano- particles in between 2D/2D interfaces ensures better electron− hole separation and physicochemical stability. As can be seen in Figure 10a (Table 2), the TEM image of d-Ti3C2 and TiO2/ g-C3N4 mixed in a 4:1 ratio and calcined at 350 °C for 1 h (4- 1-350-1) has an ultrathin layer which acts as a support and also facilitates fast charge transfer. The elemental composition, SAED pattern, and d-spacings projected intimate contact between TiO2 and d-Ti3C2 and g-C3N4 (Figure 10b,c). The increased visible absorption and photocurrent density justify the excellent charge generation and subsequent separation. Photocatalytic water splitting reaction revealed that the sample 4-1-350-1, prepared by using g-C3N4/d-Ti3C2 in a mass ratio of 4:1 and calcined at 350 °C for 1 h, gave a maximum H2 evolution (324.2 μmol) after 4 h (1.62 mmol h−1 g−1 ) with an AQE of 4.16% at 420 nm (Figure 10d,e). The cyclic run using the optimized 4-1-350-1 sample demonstrated high stability up to 12 h with maximum H2 evolution (302.7 μmol). This significant enhancement of photosplitting of water can be ascribed to an appropriate band alignment between partially oxidized d-Ti3C2 and g-C3N4, forming a type-II heterojunction whereby an excellent system of charge transfer occurs along as a partially oxidized d-Ti3C2 electron trap, preventing the charge recombination (Figure 10f,g). 4. CARBON NITRIDE−METAL OXIDE 2D/2D vdW STRUCTURES Metal oxides showcase strong photocatalytic activities and have been exhaustively investigated in the past few decades.35,270,271 This results from the ability to grow and develop nanomateri- als with a certain type of structure, orientation, and morphology that can improve catalytic performance.272 Metal oxides such as TiO2, CeO2, ZnO, WO3, and Fe2O3 exhibit properties such as wide band gaps enabling photon absorption, the formation of charge carriers that have the potential to oxidize and/or reduce on the surface, and the ability to perform charge separation.273−275 Additionally, these metal oxides are extensively used due to their stability, wide abundance, and biocompatibility.276 Some of these transition metal oxides such as TiO2 and ZnO have electronic structures that are either completely occupied d10 or empty d0 orbitals. Electronic excitation of charges after absorbing photons occurs from the valence band to the conduction band, formed from metal 3d or 4s and oxygen 2p. Consequently, these metal oxides show potential for photocatalytic applications due to this excitation and separation of charges.277 However, these materials suffer from low efficiency due to high recombination and absorption only in the UV region of the solar spectrum.50 Moreover, using only a single component restricts its application due to difficulties in simultaneously Figure 10. (a) TEM image. (b) Drift-corrected spectrum and elemental mapping. (c) HR-TEM and SAED images of 4-1-350-1. (d) Photocatalytic H2 evolution rate of the samples with different g-C3N4/d-Ti3C2 mass ratio. (e) Comparison of the photocatalytic H2 evolution rate of all the samples. (f) Band structure alignments of partially oxidized Ti3C2, pristine g-C3N4, and composite 4-1-350-1. (g) Simulated microstructure of the composite sample 4-1-350-1. Reprinted with permission from ref 268. Copyright 2018 Wiley-VCH. Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX N
- 16. obtaining a strong visible light response and good redox property. g-C3N4 due to sp2 hybridized C and N content forms conjugated planes, increasing the electrical conductivity, stability, and small band gap (2.7 eV).278 However, g-C3N4 individually is moderately performing, and therefore, the use of metal oxides and g-C3N4 provides a favorable connection. The 2D/2D structure displays strong redox potential with active oxidation and reduction sites with well-separated charge carriers.279 Furthermore, fabrication of such a 2D/2D vdW heterostructure is effective and advantageous compared to other hybrid features. It eliminates complications such as defects arising from point-to-point or point-to-face contact, light-shielding due to thick material, and long charge-transfer distance. For example, the construction of Z-scheme and S- scheme type band structure improves the efficiency of a catalyst by providing a seamless contact, unique morphological features, and proper band alignment, favoring the reaction mechanism.81,280 Therefore, it can be inferred that the construction of a 2D/2D heterojunction is a smart way to improve photocatalytic performance by matching the band energies of different semiconductors.281−283 Some of the 2D/ 2D heterojunctions of various metal oxides and carbon nitride are discussed in this section. 4.1. Carbon Nitride-WO3. Tungsten trioxide (WO3) has a narrow band gap of 2.4 eV and possesses suitable band edge potentials with a deep valence band. It is interesting because of its low cost, facile synthesis, resistance to photocorrosion, and strong stability in an aqueous solution. Therefore, it is one of the many important photocatalysts, especially for O2 evolution reaction and wastewater treatment.284,285 Despite its interest- ing properties, because of the high recombination of photoexcited charge carriers, the use of WO3 is limited. There are various reports on the structural and surface modifications of WO3 for enhancing the photocatalytic activity, such as smaller grain size that improves the charge carrier transport efficiency from the bulk to the surface.286,287 Wicaksana et al. synthesized the crystalline nanostructure of WO3 by a hydrothermal method to improve photoactivity.288 However, intrinsic drawbacks of these metal oxides such as low quantum yield and poorly visible light harvesting still prevail. The deep VB of WO3 restricts its application in the reduction process, while for self-sustained photocatalysis, both oxidation and reduction are required. In this regard, hybrid semiconductor nanocomposites like g-C3N4 with negative CB forming heterojunctions such as S-scheme and Z-schemes are highly attractive due to the ease of transfer of light-induced charge carriers and fulfillment of the wide potential require- ment.40,280,289−291 Yang et al. constructed an ultrathin WO3. H2O/g-C3N4 nanosheets are based on direct Z scheme vdW heterojunctions for efficient water splitting.292 The conduction band minimum (CBM) of WO3 in the designed 2D/2D system is 0.5 eV higher than the valence band maximum (VBM) of g-C3N4, resulting in fast recombination of electrons from CBM of WO3 with holes from the VBM of g-C3N4. Consequently, more holes in WO3 VBM and more electrons in g-C3N4 are retained, leading to higher photocatalytic activity. Additionally, there is accelerated transportation of visible-light- induced charge carriers and strong absorption in the visible region. Liu et al. adopted a similar direct Z scheme of 2D/2D WO3/g-C3N4 for H2 production via additional modifications such as loading Pt in g-C3N4 and WO3 nanosheets for hydrogen generation.293 The synergistic and strong affinity between the coupled nanosheets exhibited a higher number of coordinated surface atoms boosting the H2 production (862 μmol h−1 ).280 For a similar application of photosplitting of water to H2, Fu et al. synthesized a WO3/g-C3N4 heterostructure.294 A high and opposite zeta potential is observed, which is indicative of the strong Coulombic electrostatic attraction between 2D/2D WO3 and g-C3N4 nanosheets. The formation of the 2D/2D nanosheets is shown in the schematic representation in Figure 11a. Theoretical DFT calculation of the designed system showed a higher work function (WF) of WO3, which is indicative of charge transfer between the nanosheets (Figure 11b,c). Such a phenomenon results in a built-in electric field on the interface that significantly boosts the charge transfer Figure 11. (a) Formation schematic diagram of 2D/2D WO3/g-C3N4 heterojunctions by Coulomb electrostatic interaction. Electrostatic potentials of (b) WO3 (001) surface and (c) g-C3N4 (001) surface. Insets show the structural models of the materials for DFT calculation. (d) Work functions of g-C3N4 and WO3 before contact. (e) Internal electric field and band edge bending at the interface of WO3/g-C3N4 after contact. (f) S- scheme charge transfer mechanism between WO3 and g-C3N4 under light irradiation. Reprinted with permission from ref 294. Copyright 2019 Elsevier. Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX P
- 17. efficiency. The close vicinity of WO3 with higher WF (6.23 eV) and g-C3N4 with smaller WF (4.18 eV) creates a spontaneous transfer of electrons from the former to the latter until a Fermi- level equilibrium was reached. This results in band bending due to the gain and loss of electrons in WO3 (downward) and g-C3N4 (upward), respectively (Figure 11d,e). Due to the S- scheme heterojunction, “useless” electrons and holes from the CB of WO3 and VB of g-C3N4 get eliminated through recombination holding “useful” electrons (CB of g-C3N4) and holes (VB of WO3) (Figure 11f). When employed as a photocatalyst for H2 production, the constructed S-scheme 2D/2D WO3/g-C3N4 heterojunction showed a remarkable performance 1.7 times higher than pristine g-C3N4. This enhanced efficiency can be attributed to the construction of a close contact step-scheme designed to remove “useless” charge carriers via a recombination process, leaving behind the “useful” electrons and holes for an excellent oxidation/ reduction system. 4.2. Carbon Nitride-TiO2. TiO2 is an extensively and exhaustively used wide band gap semiconductor in photo- catalytic and photoelectrochemical applications due to its excellent chemical stability, nontoxicity, wide abundance, and low cost.295−297 However, the device’s efficiency is limited because of its wide band gap nature, leading to underutilization of light resources and a high recombination rate. Various strategies have been adopted to overcome these issues, such as metal/nonmetal doping (Ag, Au, Ru, Cu, N, P, S, F, etc.), surface area modification, sensitization with organic and inorganic molecules, and fabrication of 1D, 2D, and 3D nanostructures.298−302 Building the heterojunction of two nanomaterials is one of the most pragmatic approaches for overcoming these draw- backs as it integrates the merits of the individual component.303,304 For instance, He et al. designed a 2D/2D vdW heterojunction core−shell of TiO2/C3N4 and electro- statically integrated MXene quantum dots (TCQD) for photoreduction of CO2 (Figure 12a).269 The XPS spectra elucidate the change in the concentration of electrons with a shift in C 1s and N 1s binding energies toward lower energy after UV illumination indicating an accumulation of electrons in CN and depletion in TiO2(T) (Figure 12b,c). The designed S-scheme heterojunction between the TiO2 and C3N4 provided strong redox capacity and an efficient transport channel for light-induced charges, whereas a Schottky heterojunction of C3N4 with TCQD provided a pathway for electron transport, thereby creating a spatial separation of charge carriers. The state of the electronic bands before the formation of the heterojunction revealed higher Fermi-level energy for CN compared to T and TCQD, which upon contact forms an equilibrium state due to spontaneous transfer of charges from CN to T and TCQD (Figure 12d,e). This phenomenon is accompanied by band bending and the creation of an internal electric field (IEF) at the interfaces. However, under solar irradiation, the light-induced separation of charges occurs in both T and CN (Figure 12f). The electrons at the CB of T combined with holes from VB of CN leave behind electrons in the CB of CN, which migrates to the TCQD to reduce CO2 to useful hydrocarbon fuels. The multijunction system enhanced the photocatalytic activity through an efficient system of charge separation and transfer, and also an intimate contact of C3N4 with TCQD having Figure 12. (a) Schematic of the synthesis of ultrathin TCQD anchored TiO2/C3N4 core−shell nanosheets. Comparison of (b) C 1s and (c) N 1s XPS spectra for T, CN, T−CN, and T−CN−TC in the dark or under 365 nm LED irradiation. The S-scheme heterojunction of TiO2/C3N4/Ti3C2 quantum dots: (d) before contact, (e) after contact, and (f) after contact upon irradiation and charge migration and separation. Reprinted with permission from ref 269. Copyright 2020 Elsevier. Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX Q
- 18. −NH2 terminal groups acts as an active site for better catalytic reactions. A similar study demonstrates ultrathin g-C3N4 in a face-to- face interfacial sandwiched with anatase TiO2 nanosheets. Bronze type phase TiO2 (TCN-B-x) containing excessive electron trapping Ti3+ sites demonstrate a low photocatalytic performance. Optimizing to a favorable amount of Ti3+ content is beneficial as it helps in electron hopping. This attracted Gu et al. to perform an air annealing process to remove the vacant oxygen sites.305 This heterojunction with high energy (010) facet exposed to TiO2 with g-C3N4 was able to increase visible light absorption and curb electron−hole recombination by promoting better charge separation. Figure 13a gives insight into the synthetic process, whereby positively charged g-C3N4 and TiO2 precursors were hydrothermally treated to form initially TCN-B-x, which on further annealing in the air gives TCN-A-x following a phase conversion to anatase. Annealing in the air provided required oxygen to vacant oxygen sites and perhaps inhibited the aggregation to form ultrathin nanosheets. An inverse micelle is formed by the ethylene glycol (EG) and another surfactant where the hydrophilic part traps the protonated g-C3N4 forming a sandwich-like structure. The AFM image confirmed the formation of ultrathin nanosheets of 1.4 nm attached to larger 3 nm nanosheets corresponding to g- C3N4. The photocatalytic activity was evaluated in a dye degradation experiment using methyl orange (MO) as a model compound. The designed heterojunction of TCN-A-x performed exceedingly well, degrading 98% of MO in 15 min compared to using a single component such as TiO2 and g-C3N4. Further, the material was subjected to photocatalytic H2 production with an observed higher yield reaching up to 91 060 μmol/g in 5 h, as shown (Figure 13b). The role of each of TiO2 and g- C3N4 nanosheet during the process of wavelength-dependent H2 production was also investigated. The quantum yields at 365 and 380 nm were found to be 5.1% and 5.3%, respectively, which is lower than the individual nanosheets, strongly indicating the upsides of forming a heterojunction. The band gap and reaction mechanism is shown in Figure 13c with favorable thermodynamics and potentials for dye degradation and H2 production. In TCN-A, photoexcited electrons get injected from the CB of g-C3N4 to the CB of TiO2-A, forming superoxide radicals with high oxidizing power on the surface. Similarly, holes migrate from the VB of TiO2-A to the VB of g- C3N4 to oxidize organic pollutants into degradation by- products. The system of charge transfer is efficient in the bicomponent face-to-face heterojunction, reducing the travel path for electrons, thereby increasing the electron lifetime. Comparable work in building the heterojunction of TiO2 and g-C3N4 has been reported for photocatalytic applications under LED illumination and photoelectrochemical applica- tion.306,307308 4.3. Carbon Nitride−MnO2. Wide band gap transition metal oxides (TMOs) with completely filled and empty d- orbitals such as ZnO and TiO2 are preferably used as a photocatalyst. These materials have certain shortcomings as they are active mostly to UV irradiation which thus reduces their practical usage. For this reason, partially filled d-level TMOs such as MnO2 are attractive due to the possibility of light absorption following a d−d transition.277,309,310 Unfortu- nately, the d-electrons do not migrate to the interface/surface as it stays confined in the metal ion resulting in recombination. As a result, such metal oxides showed the inability to sufficiently generate electron−hole pairs and therefore are Figure 13. Schematic illustration of the preparation process of (a) 2D TCN-A-x nanosheets. TCN-B-x containing bronze-type TiO2 was attained after hydrothermal sandwich assembly and was converted to TCN-A-x containing anatase-type TiO2 through a controlled air-annealing (O2- insertion) treatment. (b) Photocatalytic H2 evolution activity of TCN-A nanosheets, g-C3N4 nanosheets, and TCN-A-x samples with various g- C3N4 contents. Proposed band gap structure and photocatalytic mechanism for (c) photogeneration of H2 over 2D TCN-A-70 nanosheets and TiO2-A nanosheet photocatalysts under UV−vis light irradiation. Reprinted with permission from ref 305. Copyright 2017 American Chemical Society. Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX R
- 19. infrequently used. To overcome these shortcomings, certain modifications to its crystal structure are required.311 MnO2 with partially filled d5 configuration and other attractive properties such as narrow band gap, stability, low cost, wide abundance, and environmental friendliness makes it promising for use in photocatalytic applications.312 Of the many crystal structures, layered δ-MnO2 with multiple oxygen vacancies shows higher catalytic activity. Additionally, it utilizes the visible spectrum for better absorption due to its narrow band gap. However, to improve the efficiency of photocatalytic devices, the construction of a 2D/2D architecture can create the possibility for aligning suitably the band structures for proper channeling of charge carriers.313 Xia et al. synthesized 2D/2D nanocomposite of g-C3N4/MnO2 by in situ depositing MnO2 on exfoliated g-C3N4 in a solution.314 The TEM images of g- C3N4/MnO2 display a close integration of the nanosheets, which revealed the formation of a heterojunction with the observed lattice fringes corresponding to δ-MnO2 in (Figure 14a). The UV−vis spectra further substantiate the formation of a heterojunction by an observed optical absorption well beyond the UV region, extending to the visible region (Figure 14b). The photocatalytic activity of the heterojunction nanocomposite was tested against the single-component material for degradation of rhodamine B (RhB) and phenol in an aqueous solution, as shown in Figure 14c,d. RhB was efficiently degraded up to 91.3% within 60 min by the nanocomposite of g-C3N4/MnO2 with a higher apparent reaction rate compared to individual MnO2 and g-C3N4. The photocatalytic efficiency of phenol removal was significantly higher for g-C3N4/MnO2, reaching 73.6% at 180 min of illumination compared to 12.3% and 35.4% using g-C3N4 and MnO2, respectively. Work functions relative to the vacuum level were calculated to be 6.8 and 4.5 eV for MnO2 (001) and g-C3N4 (001), respectively (Figure 14e). Therefore, at the heterojunction near the interface, the g-C3N4 from where the electrons flow is more positively charged with respect to MnO2, which is slightly negatively charged. The charge deficiency model of the nanocomposite in Figure 14f depicts the different regions of electronic charge accumulation and depletion represented in cyan and yellow, respectively, showing the flow of electrons due to the formation of the g-C3N4/MnO2 heterojunction. Based on the Mott−Schottky results and the valence band XPS spectra in Figure 14g, a band structure showing the mechanism of photocatalysis is proposed. The Mott−Schottky analysis revealed positive slopes for both g-C3N4 and MnO2, indicating an n-type semiconductor behavior. The observed flat band potential in such a case can be assumed to be the levels of conduction bands with −1.61 and 1.22 V correspondingly for g-C3N4 and MnO2. Subsequently, the valence band positions were measured from the XPS valence band spectra as 1.81 and 3.26 V for g-C3N4 and MnO2. The assimilated results revealed a Z-scheme type of heterostructure formed between the two nanosheets, showing better utilization of the charge carriers and significantly improving the photocatalytic efficiency. Similar 2D/2D heterojunction synthesis of MnO2/g-C3N4 has been reported by the in situ redox reaction of Mn precursors on the surface of g-C3N4 for CO2 photo- reduction.315 Other modifications such as CNT comodified g-C3N4 have been used with MnO2 for water splitting application.316 In such a case, CNTs with higher electron capture capacity are expected to increase the electron transfer to a higher activity surface. 4.4. Carbon Nitride−Fe2O3. Hematite (α-Fe2O3) is an iron oxide semiconductor material having band gap energy in the range 1.9−2.2 eV, absorbing a broad range of the solar spectrum, and, therefore, is favorable for various photocatalytic Figure 14. (a) TEM image of the g-C3N4/MnO2 nanocomposite. (b) UV−vis spectra of g-C3N4, MnO2, and g-C3N4/MnO2 samples. Photocatalytic degradation rate of (c) RhB, (d) phenol over g-C3N4, MnO2, and the g-C3N4/MnO2 nanocomposite. (e) Calculated electrostatic potentials for g-C3N4 and MnO2 nanosheets, respectively. (f) Charge density difference model of the g-C3N4/MnO2 nanocomposite. The isosurface is 0.0004 eV Å−3 . (g) XPS valence band spectra of MnO2 and g-C3N4 nanosheets. Reprinted with permission from ref 314. Copyright 2018 American Chemical Society. Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX S
- 20. reactions.317,318 It is one of the most abundantly found materials and has been considerably explored due to its excellent chemical stability, low cost, and abundance. It is used in a variety of applications, including solar cells,319 batteries,320 photoelectrochemical water splitting, etc. Unfortunately, Fe2O3 is an indirect band gap semiconductor and has a low absorption coefficient, small hole diffusion length (2 to 4 nm), unfavorable CB position for reduction of water to hydrogen, and short excitation lifetime (1 ps).321,322 These dictate modification in pure hematite to remove the inefficiencies and reduce other intrinsic limitations to improve the material and device performance. Morphological and structural nanoarchitectures such as 1D, 2D, and 3D have been explored for enhanced photoactivity by exposing the reactive facets.323−326 Other modifications in the form of doping with various elements in the Fe2O3 lattice have been reported.327,328 For example, Cesar et al. doped Si on Fe2O3 to get (001) oriented nanoleaflets grown normal to the substrate for an enhanced solar to chemical conversion.329 She et al. constructed a Z- scheme heterojunction of Fe2O3 and ultrathin 2D nanosheets of the g-C3N4 photocatalyst for the H2 evolution reaction.330 They used a simple one-step method to mix colloidal α-Fe2O3 with melamine and annealing at 550 °C to form a hybrid composite α-Fe2O3/g-C3N4 followed by subsequent calcina- tion to transform multilayer to ultrathin g-C3N4 (Figure 15a). The formation of ultrathin mono- and bilayers of g-C3N4 was confirmed through AFM measurements (Figure 15b). Addi- tionally, hexagonal morphological Fe2O3 nanostructures with a lateral size around 210 nm and thickness of 15 nm were observed in the TEM image (Figure 15c). A sharp interface observed in the HRTEM suggests a successful formation of the Fe2O3/2D g-C3N4 heterojunction (Figure 15d). α-Fe2O3/2D g-C3N4 showed 8.95 times higher H2 production than α- Fe2O3/ML g-C3N4 hybrids. Additionally, it also showed a better turnover frequency (TOF) compared to an individual component such as monolayer (ML) g-C3N4 and 2D g-C3N4 (Figure 15e). Furthermore, the external quantum efficiency (EQE) at λ = 420 nm was calculated to be 44.35% higher than others previously reported for g-C3N4 based photocatalytic systems (Figure 15f). The proposed band energy diagram in Figure 15g depicts the charge transfer and migration in the constructed hybrid α-Fe2O3/2D g-C3N4 system. The compact and intimate interface between the composites easily transports the electrons created from photoexcited α- Fe2O3 combined with VB holes in the g-C3N4. This leaves the electrons jumping to the CB of g-C3N4, which can migrate onto the surface for participating in reactions. Similarly, holes in the valence band of α-Fe2O3 are actively available for oxidation reactions. The process trails a Z scheme and subdues the electron−hole recombination in both α-Fe2O3 and 2D g- C3N4, maximizing the utility of the charge carriers in both the contributing composites. The direct and tight contact Z- scheme formation of the composites result in eliminating the shuttle-mediator redox reactions that showed improved quantum efficiency superior to previously reported single component g-C3N4 and metal oxides. Similar work has been reported with variations. For example, Xu et al. reported the construction of 2D/2D α-Fe2O3/g-C3N4 by adopting an electrostatic self-assembly process exploiting the strong interaction between the participating materials.331 Furthermore, other approaches such as ultrasonic-assisted Figure 15. (a) Scheme of the proposed synthetic route to produce the α-Fe2O3/2D g-C3N4 hybrids. The presence of iron oxide is essential to originate 2D structures. (b) AFM of pure 2D g-C3N4, obtained after etching away α-Fe2O3 using HCl. Scale bar: 1 μm. (c) TEM image of an α- Fe2O3 nanosheet. Scale bar: 20 nm. (d) HRTEM image of α-Fe2O3/2D g-C3N4 (3.8%) hybrid. Scale bar: 5 nm. (e) Turnover frequency of different materials. (f) Wavelength dependence of external quantum efficiency for α-Fe2O3/2D g-C3N4 hybrid. (g) Energy band diagram of the Z-scheme mechanism in α-Fe2O3/2D g-C3N4 hybrids at pH = 0. Reprinted with permission from ref 330. Copyright 2017 Wiley VCH. Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX T
- 21. preparation methods have been used to synthesize a similar configuration of 2D α-Fe2O3/g-C3N4.332 4.5. Carbon Nitride−ZnV2O6. Zinc oxide ZnO, an excellent photocatalyst, is restricted from functional application due to its wide band gap (3.3. eV), fast recombination rate, and photocorrosion.333,334 Another important oxide semiconductor material, vanadium oxide (V2O5), with a band gap of ∼2.4 eV has also been explored for many photocatalytic applications; however, its performance is tormented by easy dissolution in aqueous solution and causes secondary pollution.335,336 Coupling of ZnO and V2O5 has been found to show synergistic benefits due to stabilization of V2O5, and the less negative CB of ZnO can accept electrons from V2O5, leading better charge separation.337−339 Unfortunately, interfacial recombination, charge carrier energy loss in a type-I heterojunction due to opposite migration, and limited hole mobility are still challenging issues. Using mixed metal oxides such as binary metal vanadates is more appealing to amalgamate the properties of two catalytic components to reach a narrow band gap, resilient chemical nature, better charge mobilities, etc.340−342 Zinc vanadium oxide ZnV2O4 is an emerging photocatalytic material because of its low band gap, photostable nature, and intriguing structural change at low temperatures. This allowed the fabrication of ZnV2O4 with various morphological structures such as hollow spheres, nanosheets, clawlike hollow structures, etc.343−348 Interestingly, DFT calculations revealed that, compared to electron migration in the CB, the hole transfer to the VB is faster, removing the bottleneck of the low oxidation rate at the valence band.344 ZnV2O4 either in a standalone349 form or as a heterojunction composite has been widely investigated for various photoredox reactions. The intriguing structural flexibility of ZnV2O4 can be harvested to design new more efficient light-harvesting systems. The 2D ZnV2O4 nanosheet structure with high specific surface area and active centers has been effectively exploited to form a 2D/2D heterojunction such as ZnV2O4/rGO,350,351 ZnV2O4/V2O5,352 etc. 2D/2D vdW heterostructures of ZnV2O6 with g-C3N4 have shown encouraging performance, which is worth further investigations. The workgroup of Tahir et al. constructed a 2D/2D ZnV2O6/pCN vdW heterostructure for the reduction of CO2 to CH3OH, CO, and CH4 (Figure 16, Table 3).353 For the fabrication of heterojunctions, g-C3N4 nanosheets were first protonated with nitric acid, and then ZnV2O6 nanosheets were hydrothermally grown on protonated g-C3N4 (pCN) sheets. In the process, the positive charge on protonated pCN provides reaction sites to form self-assembled 2D/2D ZnV2O6/pCN architecture. ZnV2O6/pCN exhibited excellent visible absorp- tion up to 800 nm. ZnV2O6 displayed a selective CH3OH yield in liquid phase reaction compared to g-C3N4 and pCN which was almost doubled for the ZnV2O6/pCN vdW hetero- structure reaching a maximum value of 776 μmol g-cat−1 h−1 after 4 h with a quantum yield of 0.081 (Figure 16a−c). Further, when the reaction was pursued in the gas phase, CO was the main reaction product along with a small amount of CH3OH and CH4. The improved performance of 2D/2D ZnV2O6/pCN was originated from the better face-to-face interaction, and protonated sites provide high-speed charge transfer nanochannels for effective charge separation (Figure 16d). Further, pCN in the established heterojunction can transfer electrons to the CB of ZnV2O6 due to the presence of proton centers which serve as a trap center to facilitate charge transportation (Figure 16e). The same group has reported that when reduced graphene oxide is introduced in the system, the charge transfer mechanism was changed from type-I to Z- scheme. In the ternary ZnV2O6/RGO/g-C3N4 Z-scheme heterojunction, graphene served as an electron mediator and facilitated the efficient transfer of photogenerated electrons on the CB of ZnV2O6 to the VB of pCN.354 Interestingly, the ZnV2O6/RGO/g-C3N4 heterostructure displayed a quantum Figure 16. (a) Yield of methanol for different reaction mediums. (b) Effect of types of photocatalysts on the yield of methanol. (c) Yield of methanol over various photocatalysts: reaction parameters (room temperature, atmospheric pressure, feed flow rate 20 mL/min and irradiation time 2 h). Schematic illustration of contact interfaces for (d) 2D/2D heterojunction and (g) 2D/2D heterojunction with protonation (HNO3) as a mediator. (e) Schematic diagram of the separation and transfer of photogenerated charges in ZnV2O6/pCN composite under visible light irradiation. Reprinted with permission from ref 353. Copyright 2019 Elsevier. Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX U
- 24. yield of 0.2830, which was 3.5 times higher than that of the binary ZnV2O6/pCN system (0.081). 5. CARBON NITRIDE−LAYERED DOUBLE HYDROXIDES (LDHS) 2D/2D vdW STRUCTURES Layered double hydroxides (LDHs), also called hydrotalcite- like materials (structural similarity with [Mg6Al2(OH)16]CO3· 4H2O), or anionic clays is a class of layered materials with a chemical formula [M1−x 2+ Mx 3+ (OH)2]2x+ (An− )x/n·yH2O, where M2+ is divalent metal cations (Mg2+ , Zn2+ , Co2+ , Mn2+ , Ni2+ , or Ca2+ ) and M3+ is trivalent metal cations (Fe3+ , Cr3+ , and Al3+ , etc.) occupying octahedral positions within the hydroxide layers, An− is the nonframework exchangeable interlayer n- valent anions with highly chemical reactivity (Cl− , ClO4 − , NO3 − , CO3 2− , SO4 2− , etc.), and x is the molar ratio of M2+ / (M2+ + M3+ ).355−358 Due to its layered structure, several guest molecules have been intercalated in between galleries of LDHs for various applications. The presence of a basic site on LDHs makes them suitable catalysts to promote base-catalyzed reactions without using harsh alkaline conditions.359,360 For example, La-doped Ca−Mg−Al layered double hydroxide (La- CaMgAl-LDH) can catalyze base-free aerobic oxidation of HMF to FDCA in water.361 In another example, cobalt phthalocyanine-assisted oxidation of thiols to disulfide usually takes place under alkaline conditions. However, when a magnetically recyclable MgAl-LDHs tethered phthalocyanine was used, oxidation can occur without using an alkali.362 Interestingly, in the recent years, several visible light active LDHs have been synthesized by changing the metal combination (Zn/Cr, Mg/Cr, Zn/Fe, Mg/Fe, Cu/Cr, Co/ Cr), doping (Cu, Ni, Zn), and intercalated ions in between brucite layers. The simplicity of LDH synthesis by coprecipitation provides an opportunity to play with compositions, and many fractional composition LDHs have been reported for the photocatalytic applications, especially CO2 reduction due to the weak acidic nature of CO2.363,364 Izumi and co-workers have reported several novel LDHs such as [Zn3Ga(OH)8]+ 2[Cu(OH)4]2− ·mH2O, [Zn1.5Cu1.5Ga- (OH)8]2 + [Cu(OH)4]2− ·mH2O, Zn−Al LDH, etc. for photo- conversion of CO2 to value-added chemicals.365−368 In LDHs, brucite layers remain together due to attraction between the positively charged brucite layers and the negatively charged interlayers of H-bonded metal hydroxide and the oxygen atoms of the intercalated anions. Through strong force between the LDH layer, methods such as intercalation with small molecules (formamide, glycine) followed by sonication, anion exchange, plasma-induced exfoliation, etc. have been developed to convert the LDHs into 2D sheets through delamination.369−372 These 2D sheets, due to high surface area, rich active sites, and excellent visible absorption, can induce various photochemical reactions. The face-to-face interaction of LDHs with carbon nitride to make the vdW heterojunction has been found to boost photo- catalytic performance.373−375 The potential of such a combination has already been traversed by Song et al.129 Within a short span, a few more reports have emerged on the 2D/2D heterojunction/close contact of carbon nitrides and LDHs.376−387 Among various LDHs, bimetallic NiAl-LDH has been proven as the most efficient catalyst due to its excellent visible absorption profile originating from ligand-to-metal charge transfer (LMCT)(O → Ni2+ ).388,389 However, d−d transitions of the Ni2+ interelectronic excitation pathway hinder the photocatalytic performance due to reduced carrier effi- ciency.390,391 The charge carrier separation can be improved by making heterojunctions, i.e., the coupling of n-type NiAl- LDHs with p-type CuFe2O4 can achieve an excellent charge separation and produce H2 (345.76 μmol h−1 ) almost 7 times the values of pristine NiAl-LDHs.392 Similarly, the coupling of NiAl-LDHs with β-In2S3 leads to an increment of CO2 reduction performance. Interestingly, g-C3N4 can make a 2D/2D interface with sharp edges even without converting LDHs into single-layered sheets.379,393−395 To crop the tunability of carbon nitride to make a heterojunction, Tonda et al. synthesized a g-C3N4/NiAl- LDH nanocomposite via in situ hydrothermal depositions of Figure 17. (a) Schematic illustration of the synthesis process of g-C3N4/NiAl-LDH hybrid heterojunctions. TEM images of (b) NiAl-LDH, (c, d) CNLDH-10, and (e) HRTEM image of the CNLDH-10 heterojunction. (f) UV−vis DRS of g-C3N4, NiAl-LDH, and g-C3N4/NiAl-LDH heterojunction samples. (g) Schematic illustration of the proposed mechanism for CO2 photoreduction in the g-C3N4/NiAl-LDH heterojunctions. Reprinted with permission from ref 396. Copyright 2018 American Chemical Society. Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX X
- 26. NiAl-LDHs on g-C3N4 sheets (Figure 17a, Table 4). Without using any g-C3N4, the pristine NiAl-LDHs displayed flower- like morphology with sharp edges (Figure 17b).396 However, this morphology was disappeared when g-C3N4 was used during synthesis and 2D sheets of LDHs were grown on the surface of g-C3N4, which suggests a strong interaction between g-C3N4 and NiAl-LDHs (Figure 17c−e). The UV−vis bands of LDHs were slightly blue-shifted in the 2D/2D g-C3N4/NiAl- LDHs, while the materials still have a strong visible absorption profile up to 800 nm (Figure 17f). The photocatalytic application in CO2 reduction using the CNLDH-10 sample with 10% LDH was found to be optimum and afforded 8.2 μmol g−1 h−1 of CO, which was more than 5 times compared to g-C3N4 (1.56 μmol g−1 h−1 ) (Figure 17g). In the overall water splitting reaction, the kinetics of water oxidation remains a rate-determining step due to the requirement of multiple electron transfer steps followed by O−H bond cleavage and O−O bond formation. Since carbon nitride has a poor oxidizing valence band, the amalgamation of CN with water oxidation catalysts is an appropriate approach to enhance reaction kinetics.397 Cobalt-based catalysts have demonstrated promising oxygen evolution performance, which makes them an ideal candidate for the formation of 2D/2D heterojunctions with carbon nitride.398−401 Interestingly, the growth of cobalt hydroxide (Co(OH)2) in the presence of carbon nitride sheets can lead to the formation of a layered structure. For example, Zhang et al. have fabricated a Co(OH)2/g-C3N4 heterojunction by using a cobalt nitrate precursor in the presence of NH3, which demonstrated enhanced visible absorption.402 The close contact of Co(OH)2 and g-C3N4 in 2D/2D fashion led to better charge migration that was evident from the reduced semicircle diameter in the EIS Nyquist plot and PL quenching. Under optimized conditions of 3 wt % Co(OH)2 loading and AgNO3 as an electron acceptor, the oxygen evolution rate was 27.4 μmol h−1 using 300 W solar simulated light. The cobalt-based LDHs with Ni counterparts were also found to be good water oxidation catalysts (WOCs).403,404 Zhang et al. synthesized a 2D/2D heterostructure of Ni−Co LDHs (NixCo3−x LDHs; Ni2+ /Co2+ = 0, 1, 1.5, 2) and carbon nitride nanosheets (CNU) by a simple ultrasonication approach while keeping the ratio of Ni−Co LDHs in the range of 1−5 wt %.405 The diminished PL intensity and enhanced photocurrent density in PEC measurement suggest better charge separation in the Ni− Co LDHs/CNU heterojunction. When used as a water oxidation photocatalyst, the NiCo2 LDHS/CN catalysts with 1:2 Ni/Co stoichiometric composition and 3 wt % concentration demonstrated the highest O2 evolution rate (26.7 μmol h−1 ), which was 6.5 times higher than pure carbon nitride sheets. 6. CARBON NITRIDE−PEROVSKITE OXIDE 2D/2D vdW STRUCTURES Perovskite materials with a general formula ABX3 or A2BX4 are constituted of A and B cations coordinated to an X anion. In this structure, A and B cations have 6- and 12-fold coordination, surrounded by an octahedron of the X anions (usually oxygen).406,407 Due to the stable structure of the perovskite lattice (except halide perovskites), more than 90% of metal elements have been successfully introduced into the perovskite lattice.408 When X is a halogen (F, Cl, Br, or I) with a monovalent (Cs+ , CH3NH3 + , formamidinium(HC(NH2)2 + )) and a divalent cation (Pb2+ , Ge2+ , Sn2+ ) present in the A and B sites, it is referred to as a halide perovskite.409 Lead halide- based perovskites have been widely explored for photovoltaics reaching an ∼25.2% efficiency in a monolithic cell nearing the Shockley-Queisser limit of 31.4% due to their excellent visible absorption, long carrier migration length, easy processability, etc.410 Halide perovskites have also been used as the photocatalysts for the various photocatalytic applications such as CO2 reduction, organic synthesis, photoelectrochem- ical synthesis, etc.411−413 Unfortunately, halide perovskites suffer from a trap assisted recombination and structural stability issue under the ambient conditions such as air and moisture which further deepen becaise of the toxicity of lead.414,415 Some lead-free perovskites such as Cs2AgBiBr6 have been recently reported with enhanced stability and photo- catalytic performance, but still, the stability is not satisfactory under operating reaction conditions.412 Several attempts to stabilize halide perovskites such as surface passivation, elemental doping, and alloying have been employed; however, the problem of stability persists. Recently, perovskite nano- crystal encasing inside of the inorganic shell such as silica, metal oxides, or wrapping with various types of 2D material such as graphene, g-C3N4, has been identified to prevent degradation.416−418 However, such approaches inevitably reduce visible absorption and enhance surface recombination. On the other hand, perovskite oxides with a general formula ABO3, due to their extreme stability, excellent visible absorption (narrow band gap), and the possibility of multitudinous elemental combinations are gaining popularity in electrocatalysis, catalysis, solid oxides fuel cells (SOFCs), and photocatalysis.34,419−423 Several bi/trimetallic perovskite materials possessing BO6 type octahedron, such as SrTiO3, BaTi4O9, CaxTiyO3, CoTiO3, LaMnO3, LaCoO3, K3Ta3Si2O13, LaNiO3, SrVO3, PbZrO3, Na2Ti6O13, SrNbO3, K2Ti4O9, K4Nb6O17, and K2La2Ti3O10, have been explored in recent years for photocatalysis.424−432 Double perovskites (A2B′B″O6) are another subclass of perovskites that have shown great promise for photocatalytic applications, especially fuel cells and water splitting.433−435 Several double perovskite oxides with stoichiometric and nonstoichiometric composi- t i o n s s u c h a s S r 2 N i W O 6 , Sr2Sc0.125Ti0.875O4Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF), etc. have been successfully employed in such applications.436,437 Though an excellent visible absorber, the main challenge associated with perovskite oxides is their low charge migration distance which makes bulk recombination very prominent. The use of reduction and oxidation cocatalysts such as Pt and RuO2 has been employed to reduce surface recombination. Layered perovskites oxides of (110), (100), and (111) families with formulas of (An+1BnO3n+3) and (AnBnO3n+2), (Bi2O2)(An−1BnO3n+1) (Aurivillius phase, AL), An+1BnO3n+1, or A′2An−1BnO3n+1 (Ruddlesden−Popper phase, RP) and A′[An−1BnO3n+1] (Dion−Jacobson phase, DJ), where n represents the number of BO6 octahedra, arranged perpendic- ular to the layers provide better surface area, interlayer space as reaction sites, and charge carrier mobility, which make them promising candidates for photocatalysis.438−440 Ruddlesden− Popper type perovskite phases and layered Dion−Jacobson type perovskite phases are the most promising perovskite oxides for photocatalysis. For example, Sr2.7−xCaxLn0.3Fe2O7−δ with x = 0 and 0.3 and Ln = La and Nd has demonstrated excellent visible light assisted MB degradation.441 Enticingly, due to the possibility of exfoliation of 2D sheets, layered perovskites can be used for making a 2D/2D heterojunc- Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX Z
- 27. tion.442,443 Various 2D/2D heterojunctions of layered perov- skite oxides such as BiOCl/K+ Ca2Nb3O10 − Z-scheme hetero- structure (tetracycline TC; degradation),444 WO3/ K+ Ca2Nb3O10 − (TC degradation),445 Bi6Fe2Ti3O18−BiOBr (oxygen evolution),446 HSr2Nb3O10/CdS (H2 evolution),447 and HSr2Nb3O10/WO3 (methyl orange degradation)448 were developed. Heterojunctions with a 2D/2D contact of perov- skite oxides and g-C3N4 were demonstrated to improve performance significantly due to accommodating interac- tions.449 When 2D nanosheets of Bi4NbO8Cl were prepared by a molten-salt method coupled with g-C3N4 nanosheets using ball milling and thermal annealing, an improved TC degradation and CO2 reduction were observed.450 Type-II heterojunctions were established between two systems and photodeposition of Pt and MnOx facilitating better charge separation. Recently, Kumar et al. demonstrated a p−n heterojunction of Ba2Ca0.66Nb0.68Fe0.33Co0.33O6−δ (BCNFCo) and carbon nitride (BCNFCo/CN) exfoliated in a dichlor- obenzene and glycerol mixture (10/1, v/v) that can achieve a photocurrent density as high as 1.5 mA cm−2 under solar simulated light.451 The low band gap of BCNFCo coupled with intimate contact with exfoliated carbon nitride sheets was attributed for better visible absorption and concomitant capture via carbon nitride. In another work, KCa2Nb3O10 (KCNO), a member of the Dion−Jacobson phases layered perovskite, was exfoliated using HCNO and TBAOH (tetrabutylammonium hydroxide) to form ultrathin K+ Ca2Nb3O10 − nanosheets followed by fabrication of a 2D/2D hybrid structure with g-C3N4 via a hydrothermal approach.452 The afforded g-C3N4/ K+ Ca2Nb3O10 − nanojunction displayed improved amperomet- ric photocurrent density and TC degradation (81% in 90 min) due to better charge separation. In a recent report, HCa2Nb3O10 nanosheets synthesized by ion exchange and solvent exfoliation methods were in situ grown on g-C3N4 using a dicyandiamide precursor at high temperature to make a 2D/2D HCa2Nb3O10/g-C3N4 heterojunction (HCNO/CN) (Figure 18a, Table 5).453 The HR-TEM images of g-C3N4 sheets displayed graphene sheet-like morphology while HCNO/CN displayed an imminent contact between two materials along with a visible lattice fringe of HCNO in the TEM images (Figure 18b−d). The absorption spectra show visible to NIR absorption of the materials with excellent charge separation evident from PL spectra. When tested for the hydrogen evolution in the presence of Pt and TEOA as cocatalyst and hole scavengers, respectively, a H2 evolution rate of 794 μmol g−1 h−1 was obtained. The obtained activity was almost 4.5 times that of pristine g-C3N4. It was suggested that in a type-II heterojunction electrons were transferred to HCa2Nb3O10 and subsequently to Pt where hydrogen evolution takes place (Figure 18e). 7. CARBON NITRIDE−CHALCOGENIDE 2D/2D vdW STRUCTURES Transition metal dichalcogenides (TMDCs) constituted of hexagonal layers of metal atoms (M) sandwiched between two chalcogen layers due to their small tunable band gap, crystal structure, and excellent electronic properties have been widely explored for photocatalytic and other applications that rely on small band gap semiconductors. Depending upon the nature of the metal and chalcogens, the properties of TMDCs vary from insulator (HfS2), semiconductor (MoS2, WSe2), and semi- metallic (WTe2) to metallic (TiS2). Additionally, composition and crystalline structure also govern the conductivity (n- or p- type), band gap, and metallic transition. For example, MoS2 is a p-type semiconductor and exists in two forms: (1) prismatic Figure 18. (a) Illustration of the preparation process of HCNO/CN composites. (b) TEM image of CN. (c, d) HRTEM images of 1.0-HCNO/ CN. (e) Schematic diagram of photocatalytic H2 evolution over HCNO/CN under visible-light irradiation (λ > 420 nm). Reprinted with permission from ref 453. Copyright 2020 Royal Society of Chemistry. Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX AA
- 29. trigonal 2H phase with semiconducting properties and (1) octahedral 1T phase with metallic properties. Additionally, the transformation of bulk indirect band gap 2H-MoS2 into a monolayer 1T phase switches the electronic band structure into a direct band gap. Layered transition metal with high effective surface area, numerous exposed active sites, available surface for vdW interaction, manipulatable band structure, and ease of synthesis by numerous chemical and physical methods are considered efficient photocatalytic materials. A typical example is monolayer SnS2, which yielded a photocurrent density of 2.75 mA cm−2 at 1.0 V, nearly 72 times larger than that of bulk SnS2, proven in theory and experiment.454 TMDC 2D sheets have been widely used in photocatalysis as a standalone catalyst, heterojunctions, or cocatalyst to improve the performance of wide band gap semiconductors.455 TMDCs of groups IV−VI in 0D, 1D, 2D, and 3D morphology such as MoS2, WS2, TiS2, MoSe2, and WSe2 have been extensively explored due to their excellent optical, electronic, and catalytic activities (unusual catalytic activity at the edges), crystalline structure, layer dependent metal to insulator transition, etc. Cadmium sulfide (CdS), the most investigated chalcogenide with a low band gap (∼2.42 eV) and excellent electronic mobility, suffers from the drawback of extreme photocorrosion.456−458 To stabilize and modify the optical properties, a solid solution and homojunction of CdS with ZnS with a fractional composition (Cd(1−x)ZnxS) have been reported.459,460 Ternary and multinary chalcogenides such as I-III-VI2, I2-II-IV-VI4, Cu2MoS4, Cu2MoSe4, Cu2WS4, AgGaS2, LiAlS2, LiGaSe2, Cu2FeSnS4, Cu2NiSnS4, Cu2ZnSnS4, and Dy4S4Te3 prevail upon binary TMDCs because of their tunability, choice of several atomic combinations, suitable band gap, and easy fabrication via solution processing/ hydrothermal/solid-state synthesis in the 2D structures.461−465 Further, heterojunction formation with numerous low and high band gap semiconductors in different morphological structures has been utilized. Astonishingly high numbers of 2D/2D heterojunction are reported using metal chalcogenides as they can easily attain epitaxial or nonepitaxial 2D growth on the different 2D materials.466−471 Interestingly, small 2D conjugated dye molecules, aromatics, and polymers such as phthalocyanine and pentacene are also found to make a 2D/ 2D structure with TMDCs.468,472,473 New 2D organic semiconductors with long-range ordering such as C2N, C3N, and C4N3 are the potential candidates to make a 2D/2D vdW heterostructure as they possess exceptional optical and chemical properties with an electron-rich conjugated surface to interact effectively with TMDCs.474−476 C2N with a graphene-like hexagonal framework with a small cavity constituted of carbons and nitrogens provides faster transport of the charge carrier but behaves like a semiconductor and has plenty of sites for the reactions.477,478 In 2D/2D C2N/ TMDCs, heterojunctions such as C2N/MoS2 and C2N/WS2, the deep valence band of C2N was usually exploited for oxidation while coupled TMDCs with negative CB were used for reduction reactions.479−483 These 2D organic semi- conductors are usually synthesized in milligram scale using sophisticated chemicals and possess only reductive or oxidative bands, so they do not fulfill the criteria of scalable photocatalysis. In contrast, 2D g-C3N4 and their conjugates with certain doping can be produced at a large scale to make cheap and resilient photocatalysts for real applications. In the next sections, the 2D/2D vdW heterostructure constituted of binary, ternary, and noble metal-based chalcogenides will be discussed with a few representative examples. 7.1. Carbon Nitride−MoS2. Natural MoS2 is an indirect band gap (1.29 eV) semiconductor that exists as hexagonal form 2H-MoS2 (Hhexagonal symmetry) and has been the most studied chalcogenide due to an earth-abundant nature and unique electronic, optical, and magnetic proper- ties.63,484,485 Due to its excellent properties, MoS2 has potential applications in electronics, optoelectronics, and energy applications, including water splitting. The catalytic activity of MoS2 arises from sharp edges constituted of S atoms and defects while the basal plane is catalytically inert.486 However, the surface energy at the basal plane is almost two times lower than that at the edge, which allows MoS2 growth along the basal plane and also provides a platform for epitaxial and nonepitaxial growth of other semiconductors to make heterojunctions.487 The edges are usually transformed into an unstable sulfide state (Mo−S−O links) during photo- catalytic reactions and reducing the photocatalytic perform- ance.488 The activity of MoS2 can be increased by the transformation of the 2H crystal phase into more conductive 1T-MoS2 (T stands for trigonal symmetry with octahedral Mo−S coordination and zigzag (Mo)n chains)) phase, which has a high electron conductivity to the active sites.489,490 Due to their extreme metallic nature, 1T-MoS2 has been used as a cocatalyst in photocatalytic applications.491,492 Unfortunately, metallic 1T-MoS2 remain stable only in the presence of excess negative charge on the MoS2 sheets. An intercalated alkali metal such as Li, Na, etc. has been used to provide electrons; however, the afforded structures are extremely air-sensitive, and removal of alkali atoms leads to phase reversal under mild conditions.493,494 Use of organic cations such as alkylammo- nium cations, imidazolium, etc. has been used to stabilize the 1T-MoS2 sheets.495−497 Recently, some reports demonstrate the stabilization of 1T- MoS2 sheets on the surface of g-C3N4 to form 2D/2D heterostructures.498 For example, the 2D 1T-MoS2 hetero- junction with 2D sheets of oxygen doped carbon nitride (O-g- C3N4) was prepared via a hydrothermal approach and demonstrated improved photocurrent generation and H2 evolution rate in the presence of a triethanolamine sacrificial donor.499 The metallic component in heterojunctions contrib- utes only to charge separation, while their negligible contribution in absorbance and photocarrier generation coupled with the inappropriate band structure of the single semiconductor are some undesirable attributes. So, the semiconductive form 2H-MoS2 is more appropriate to fabricate 2D/2D heterostructures. Several 2D/2D hetero- structures using the 2H-MoS2 phase and g-C3N4 (modified/ unmodified) have been realized for the photocatalytic performance enhancement.500−504 MoS2 demonstrated excellent HER activity due to S terminated edge and S−Mo−S layered structure analogous to carbon nitride that minimizes lattice mismatch in the 2D/ 2D heterostructure resulting in improved charge separa- tion.505,506 The pioneering work by Hou et al. demonstrated lateral growth of MoS2 sheets on g-C3N4 using a (NH4)2MoS4 precursor followed by sulfidation with H2S gas at 350 °C.507 The TEM images demonstrated 2−3 layered thick MoS2 grown on the surface of carbon nitride, while XPS spectra confirm the presence of populated Mo4+ and S2− in a well- constituted MoS2 structure. The electrochemical HER polar- ization curve in Na2SO4 displayed an enhanced current density Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX AC
- 30. for the MoS2/mpg-CN heterojunction and decreased charge transfer resistance compared to mpg-CN. As a photocatalytic material for HER, 0.5 wt % MoS2/mpg-CN vdW hybrid (20.6 μmol h−1 ) outperforms over 0.5 wt % Pt/mpg-CN (4.8 μmol H2 h−1 ), suggesting the possibility of exclusion of expensive noble metal catalysts. In a study by Yuan et al., g-C3N4 exfoliated in NMP was hydrothermally reacted with ammonium tetrathiomolybdate [(NH4)2MoS4] that resulted in the formation of a 2D/2D MoS2/g-C3N4 heterojunction (Figure 19).508 The well contacted g-C3N4 and MoS2 were visible in the HR-TEM images along with the AFM images (Figure 19a,b). The photocatalytic activity of 2D/2D MoS2/g- C3N4 catalysts was compared with 3% Pt/g-C3N4 benchmark catalyst and demonstrated that interfacial contact of g-C3N4 with MoS2 affords more efficient charge separation. The TEM images of 3% Pt/g-C3N4 reveal homogeneously dispersed Pt nanoparticles on the surface of g-C3N4 (Figure 19a). Interestingly, when tested for the photocatalytic H2 evolution, the 2D/2D MoS2/g-C3N4 displayed an excellent H2 evolution rate (1155 μmol g−1 h−1 ) which was even higher than that of the 3% Pt/g-C3N4 benchmark photocatalysts (791 μmol g−1 h−1 ). Additionally, experiments reveal that the presence of g- C3N4 in the form of 2D sheets was crucial, and a very poor H2 evolution rate was obtained for MoS2 and bulk g-C3N4. The gradually decreasing lifetime after adding MoS2 shows better charge separation, while band structures calculated from the Mott−Schottky and Tauc plots reveal the formation of a type-I heterojunction (Figure 19c−e). In a recent report, 2H MoS2 modified g-C3N4(MoS2/g-C3N4) 2D/2D vdW heterojunction was synthesized using 12-phosphomolybdic acid (H3PMo12O40, PMA) and thioacetamide (TAA) as Mo and sulfur sources.509 The MoS2/g-C3N4 catalyst displayed a significant H2 production rate (1497 μmol gcat −1 h−1 ) with an associated apparent QY of 3.3% at 410 nm irradiation. The charge carrier density (Nd) of MoS2/g-C3N4 samples was found in the range of 7.80 × 1017 to 5.13 × 1018 cm−3 , much larger than pristine g-C3N4 (7.10 × 1017 cm−3 ), suggesting a better establishment of heterojunctions (Figure 19f). Even a ternary heterojunction constituted of g-C3N4-5%/MoS2/ graphene was also made, which exhibited an RhB degradation rate of 95% under 20 min.510 7.2. Carbon Nitride−WS2. Another important layered TMDC of the group-VI family is tungsten disulfide (WS2), which shows an indirect (1.4 eV) to direct (2.0 eV) band gap transition when bulk materials are transformed into monolayer sheets.511,512 Like MoS2, WS2 also exists in a crystalline form called 2H-WS2 and octahedral 1T WS2. Additionally, 1T WS2 has a metallic nature and can be synthesized by lithium exfoliation. Exceeding over MoS2, the intrinsic electrical conductivity of WS2 is higher than that of MoS2, which makes it a suitable cocatalyst candidate. Indeed, numerous reports are available on the use of nanostructured WS2 as a cocatalyst and sensitizer in photocatalysis.513−515 Exfoliation of bulk WS2 sheets via lithium intercalation led to phase Figure 19. (a) TEM images of the 3% Pt/g-C3N4 nanosheets photocatalyst. (b) HRTEM image of 0.75% MoS2/g-C3N4 nanosheets composite. (c and d) Schematic diagrams of 0D-2D Pt/g-C3N4 nanosheets photocatalysts and 2D-2D MoS2/g-C3N4 nanosheets photocatalysts. (e) Time- resolved fluorescence spectra of different MoS2/g-C3N4 nanosheets photocatalysts loading with various amounts of MoS2. (f) Schematic energy- level diagrams of MoS2 and g-C3N4 in comparison with the H+ /H2 and O2/H2O redox potentials. Reprinted with permission from ref 508. Copyright 2019 Elsevier. Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX AD
- 31. conversion (2H to the metallic 1T). The reversion of the 1T phase to 2H phase requires further annealing, which introduces plenty of defect states and a charge recombination center, which is undesirable for photocatalysis. High-quality 2H WS2 sheets without any 2H to 1T phase transition can be synthesized by micromechanical exfoliation; however, the yield of sheets remains too low for scalable production.516 Xu et al. developed a scalable method to produce 2H WS2 sheets in 18−22% yield, which involves preintercalation of a stoichiometric amount of lithium ions followed by exfoliation in sodium chlorate/water solution.517 Interestingly, when coupled with CdS nanorods, the WS2/CdS NRs hybrid displayed an impressive 26-fold increment of the H2 evolution rate with an associated AQE of 67% at 420 nm. Additionally, stand-alone 2H WS2 nanosheets decorated with Pd nanoparticles can promote Suzuki coupling under visible light with a turnover frequency as high as 1244 h−1 .518 However, for water splitting, CO2 reduction and degradation of pollutant high redox potential are needed and it became essential to integrate WS2 with other semiconductors to meet the minimum energy (1.23 eV) requirement. Several heterojunctions using semiconductive WS2, such as WS2/ TiO2,519 WS2/Zn2InS4,520 WS2/BiOBr,521 WS2/Bi2O2CO3,522 and CdS/WS2,523,524 in various morphological forms have also been fabricated as they not only improve the visible absorption and the charge separation but also act as a cocatalyst.525 To garner the superior properties of such as 2D structure, high surface area, excellent electronic mobility, and suitable low band gap 2D/2D vdW heterostructures such as WS2/TiO2,526 MoS2/WS2,527,528 WS2/CdS,529 Bi2WO6/WS2−x,530 and WS2/ ZnO531 have also been developed. Although many WS2/g-C3N4 hybrids have been developed, which showed increased photocatalytic performance, the formation of 2D/2D contact remains ambiguous in many of them and only a few demonstrated well-constructed hetero- junctions between 2D WS2 and g-C3N4 nanosheets.532−537 Recently, Li et al. demonstrated the synthesis of a 2D/2D Pg- C3N4/WS2 by the self-assembly of protonated g-C3N4 and WS2 in sensing applications, reaching a fabulous detection limit of 3.8 pM for 5-formylcytosine.538 In another study, to accelerate the charge transport between WS2 and g-C3N4 nanosheets, a g- C3N4/WS22D/2D architecture bridged with Ag was prepared by sequential deposition of constituting components.539 The WS2/Ag/g-C3N4 displayed improved NO removal and H2 production because the increased interlayer spacing reactant can access large numbers of active sites while Ag promotes a better charge separation. Similarly, CdS nanoparticle decorated WS2/g-C3N4 2D/2D vdW heterostructures (CdS/WS2/CN) were prepared by sequential deposition of Cd2+ and S2− on WS2/CN (Figure 20).540 The HRTEM images show intimated contact between NMP assisted exfoliated CN and WS2 nanosheets, and spherical CdS particles were sandwiched in a WS2/CN hybrid (Figure 20a,b). The CdS/WS2/CN hybrid displayed an H2 evolution rate of 1174.5 μmol g−1 h−1 , which was 67 times higher than that of CN (Figure 20c). The corresponding quantum efficiency was calculated to be 5.4% at 400 nm. The enhanced activity was assumed to be due to better electron transfer from CN and CdS to WS2 while hole transfer occurred from CdS/WS2 to CN (Figure 20d). 7.3. Carbon Nitride−FeSe2. Iron-based binary chalcoge- nides such as iron pyrite (FeS2; iron disulfide) and FeSe2 can absorb a major fraction of electromagnetic radiation in the UV−visible to NIR region and thus found applications in many photocatalytic and electronic applications.541−544 The repre- sentative member FeS2 has an indirect band gap of ≈1.0 eV (2- fold high photon absorption coefficient, 105 cm−1 more than silicon), high carrier mobility, and a theoretical power conversion efficiency of 28% and can be easily synthesized using earth-abundant chemicals like iron (∼5% of the earth’s crust) and sulfur (0.042% of the earth’s crust) sources such as sulfur powder, Na2S, thiourea, thioacetamide, etc.545,546 FeS2 has been widely investigated for solar cell application and heterojunction photocatalysis with numerous inorganic and organic semiconductors. Unfortunately, the performance of Figure 20. (a, b) TEM images of CdS/WS2/CN. (c) Average photocatalytic H2 production rates over different samples under visible-light irradiation (λ > 420 nm). (d) Photocatalytic mechanism in pathway III and the band positions of the samples, together with O2/• O2 − , • OH/H2O, H+ /H2, and OH− /• OH redox potentials. Reprinted with permission from ref 540. Copyright 2018 Wiley-VCH. Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX AE
- 32. FeS2 is plagued by prodigious charge recombination in grain boundaries and surface defects. Compared with metal sulfides, metal selenides are advantageous as they have a narrow band gap and improved carrier mobility. Although S and Se are the members of the same periodic group and have almost similar chemical properties, the bond strength of Se−H (276 kJ/mol) is significantly lower than that of S−H (363 kJ/mol), leading to better adsorption−desorption of the proton, which is essential to facilitate the product desorption from catalytic sites.547 FeSe2 exists in two polymorphic forms (orthorhombic marcasite and cubic pyrite), possessing indirect band gaps of 0.86 and 0.67 eV.548 FeSe2 exhibits an excellent conductivity (resistivity ≈10−3 Ω·cm), populated surface iron atoms, low toxicity, and benign nature and is favorable for the water splitting due to the presence of abundant [Fe−Fe] hydro- genase type active centers accelerating proton adsorption and H2O2 decomposition.549,550 Even with such excellent proper- ties, photocatalytic applications of FeSe2 are sparse, and most of the applications of FeSe2 are limited to solar cells and the sodium-ion battery (SIB).551−553,553 The 0D/1D hybrid FeSe2 and ZnSe demonstrated increased photosplitting of water due to better charge separation in the type-I heterojunction.554 Other heterojunctions such as 1D/2D FeSe2/MoSe2 and FeSe2 nanodendrites decorated on GO and g-C3N4 have been reported555,556 Remarkable results were obtained by the marriage of 2D FeSe2 and 2D sheets of g-C3N4 (CNNS) (Figure 21).557 The synthesis of 2D/2D FeSe2/CNNS was achieved via the formation of g-C3N4 sheets followed by in situ growth of FeSe2 using Fe(acac)3 and Se precursors in the presence of 1- octadecene (ODE) and oleylamine (OLA) (Figure 21a). The close 2D/2D face-to-face contact between FeSe2 and CNNS was evident from the TEM and AFM images. The optimum photosplitting of water was observed for the 15% FeSe2/CNNS reaching a value of 1655.6 μmol g−1 h−1 , almost 2.65 times that of pure g-C3N4 in the presence of Na2S/Na2SO3 and solar simulated light (Figure 21b,c). Further, FeSe2/CNNS also afforded enticing removal efficiencies of 92.6% for Cr(VI) and 99.8% for MB within 120 min while pristine g-C3N4 affords only 44.7% Cr(VI) and 66.1% MB removal efficiencies under identical conditions (Figure 21d). Interestingly, liquid chromatography−mass spectroscopy (LC-MS) demonstrated the ring-opening degradation followed by mineralization. The wide visible absorption extended up to 1200 nm, decreased the PL lifetime, decreased charge transfer resistance, and improved the transient photocurrent compared to pristine materials, validating better charge separation in the FeSe2/CNNS composite structure. Photocatalytic experiments in the presence of scavengers and radical trapping agent DMPO demonstrated that • OH radicals were responsible for the degradation activity. The validation of the origin of • OH radicals from the derivatization of the superoxide anion radical (O2 •− ) via H2O2 intermediate was done by the DPD-POD method, which showed increased • OH radical concentration during reaction using UV−vis spectroscopy. 7.4. Carbon Nitride−PtS2. Noble metal dichalcogenides (MX2, M = Pt, Pd, Ir, Re, etc., X = S, Se, Te) are known to possess photocatalytic activity for a long time.558−563 However, their use is limited due to the cost issue and small band gap. Recently, some excellent reports on 2D MX2 have emerged and rejuvenated the field. Group 10 dichalcogenides such as PtS2, PtSe2, and PtTe2 have shown great promise due to their superior properties.564,565 For example, platinum disulfide (PtS2), a layered material, has displayed high carrier mobility (3500 cm2 V−1 S−1 ) even larger than that of phosphorene (1000 cm2 V−1 S−1 ), which makes it suitable for various applications from photocatalysis to fast-moving electronic applications including gas sensors, field-effect transistors (FET), etc.566 DFT calculation reveals that monolayer PtS2 Figure 21. (a) Schematic image for the synthesis procedures of 2D/2D FeSe2/CNNS interplane heterostructures. Photocatalytic H2 evolution curves (b) and rates (c) for pure g-C3N4, FeSe2, and various FeSe2/CNNS heterostructures. The photocatalytic MB and Cr(VI) degradation performance of pure (d) 15% FeSe2/CNNS. Reprinted with permission from ref 557. Copyright 2020 Elsevier. Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX AF
- 33. is a semiconductor with an indirect energy gap between 1.60 and 1.80 eV, while bulk PtS2 reveals band gaps of 0.95 and 0.87 eV. PtS2 and PtSe2 in the forms of nanoparticles and nanosheets can be synthesized via various routes such as high-pressure synthesis, chemical vapor transport, liquid-phase exfoliation, etc. Ajibade et al. demonstrated the synthesis of PtS2 nanoparticles using a bis(morpholinyl-4-carbodithioato)- platinum(II) thermalization, which showed a visible light degradation of MB.567 Due to its layered nature, PtS2 is an ideal candidate for the fabrication of heterojunction with various semiconductors. For instance, DFT calculation suggests that PtS2 and arsenene can make a 2D/2D Z-scheme heterojunction due to the epitaxial matching with the only mismatch of less than 2%. The calculated electrostatic potential unveils the potential difference can make a built-in electric field to make charge transfer feasible. To date, vdW PbI2/PtS2,568 PtS2/MoS2,569,570 MoSe2/PtS2,571 graphene/PtS2,572 PtS2/ InSe,573 PtS2/PtSe2,574 etc. have demonstrated excellent electronic and optical properties for photodetectors, solar cells, FET, and other optoelectronics. A very recent theoretical report demonstrated that PtS2 is an ideal substrate that can stabilize single atoms to make a single-atom catalyst (SACs). Among 15 kinds of possible SACs (Ti, V, Cr, Mn, Fe, Co, Ni, Zr, Nb, Mo, Rh, Ru, Pd, Ir, and Pt), Ru SAC-PtS2 can optimally catalyze the N2 reduction reaction (NRR).575 PtS2 exists in two forms, 1T PtX2 and 3R PtX2. Villaos et al. calculated the lattice constant band gap energy and concluded that octahedral 1T PtX2 is the most stable form of bulk PtX2 structure, which was consistent with experiment evi- dence.558,576,577 Bulk PtS2 can be transformed into 2D sheets by taking advantage of weak vdW interaction between S−Pt−S bonded PtS2 sheets, which can be overcome by mild sonication conditions. Liu et al. synthesized defect-rich PtS2 by chemical vapor transport (CVT) followed by the cryo-mediated liquid- phase exfoliation (LPE) method (Figure 22).578 The treatment with liquid N2 and subsequent ultrasonication in isopropanol/ H2O afforded high-quality defect-rich sheets (Figure 22a). The PtS2 sheets were coupled with liquid exfoliated mesoporous carbon nitride (MCN) sheets. The decrement of the (001) and (002) peaks for c-axis orientation along with TEM images and SAED suggests a successful exfoliation of sheets (Figure 22b−e). Blue and red Raman shifts of E1 g and A1g phonon modes integrated with decreasing peak intensity also demonstrate a transformation of bulk PtS2 in nanosheets (Figure 22f). The average thickness of sheets was found to be 1.18 nm (equal to a two-unit-cell PtS2 slab), while the interplanar spacing and dihedral angle were calculated to be 0.315 nm and 60°, respectively. The presence of a defect-rich state was evident from the TEM images and was further confirmed from the EPR signal at the g-value of 2.006 due to the presence of S vacancies. The intimate 2D/2D contact in 1 wt % PtS2(U)/MCN hybrid was visible in HR-TEM images, showing amorphous MCN cemented with crystalline PtS2 (Figure 22g). Optimization of the catalyst demonstrated that PtS2 sheets prepared by centrifugation at 8000 (PtS2-8000) and in a 1 wt % addition showed optimum photocatalytic performance for hydrogen evolution (1168 μmol g−1 ) with an associated quantum efficiency (QE) of 1.16% at 405 nm (Figure 22h). Trapping the radicals with DMPO followed by ESR measurement demonstrated a strong signal of superoxide Figure 22. (a) Schematic illustrating the synthesis procedure of ultrathin PtS2 nanosheets. (b) Enlarged TEM image of the PtS2-8000. (c, d, e) High-resolution TEM image of PtS2-8000; the insets correspond to the enlarged write frames. (f) Raman spectra. (g) HRTEM image of the 1 wt % PtS2(U)/MCN composites. (h) Wavelength dependence of the external quantum efficiency for the PtS2(U)/MCN composites. (i) ESR spectra of DMPO-O2 ̇•− and (j) DMPO-• OH adducts in the systems of pristine MCN and 1 wt % PtS2(U)/MCN before and after visible-light irradiation. Reprinted with permission from ref 578. Copyright 2019 Royal Society of Chemistry. Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX AG
- 34. (O2 •− ) and hydroxyl (• OH) radicals (Figure 22i,j) which were very weak with MCN, clearly demonstrating the synergistic role of PtS2 to facilitate better charge transportation and stabilization. 7.5. Carbon Nitride−ZnIn2S4. Compared to binary metal dichalcogenides, ternary metal dichalcogenides have gained significant interest due to their high photocorrosion resistance, tunable band gap, band positions, low toxicity, and easy synthesis. Among many ternary chalcogenides such as CuGaS2, Zn3In2S6, and CuInS2, zinc indium sulfide (ZnIn2S4) is the most appealing because of the direct band gap (2.06−2.85 eV), layered structure, appropriate thermodynamic potential to meet photocatalytic demand, and facile synthesis from earth- abundant precursors.579 Notably, the CB of ZnIn2S4 with d10 electronic configuration is constituted of the sp orbitals of In3+ , which is favorable for transferring the photogenerated electrons to the surface and thereby enhancing the photo- catalytic performance.580−582 ZnIn2S4 exists in three forms, cubic, hexagonal, and rhombohedral phase and all of them show photoactivity with optimum performance for the hexagonal phase. It has been widely used for numerous photocatalytic and optoelectronic applications such as CO2 reduction, photo-organic transformation, water splitting, pollutant degradation, etc.583,584 The main challenge using ZnIn2S4 is colossal bulk and surface charge recombination. The structural and electronic properties of ZnIn2S4 have been improved via various strategies such as surface area modification, morphological modification (i.e., microsphere, nanobelts, nanowires, and nanotubes), doping with metals/ alkaline metal, etc.585−589 Several types of nanoheterojunctions of ZnIn2S4 with other semiconductors have been reported to reduce charge recombination.590−593 Transformation of ZnIn2S4 into mono- or few-layered sheets can shorten the electron travel pathway and, thus, can reduce the charge recombination rate.594,595 Further, an enticing approach to extend the carrier lifetime of ZnIn2S4 is to construct a face-to- face interacting 2D/2D heterojunction with another semi- conductor/conductor, which can either capture electrons or the hole and promote the charge separation. For example, the 2D/2D heterojunction of CuInS2/ZnIn2S4 can achieve better charge separation that boosted the H2 evolution rate than individual components (CuInS2 and ZnIn2S4).596 Though numerous 2D/2D heterojunctions of ZnIn2S4 with other 2D semiconductors such as CoP597 and ZnO598 have been reported, the tedious fabrication, significant carrier loss at the mismatched lattice interfaces and grain boundaries, low quality of 2D films, and limited charge carrier mobility are still challenges. The fabrication of the 2D/2D vdW heterojunction of ZnIn2S4 with g-C3N4 can solve such issues due to the formation of the nonepitaxial heterojunction and the flexible surface.599−604 The conjugated network of g-C3N4 can provide better charge mobility while the N-rich surface promotes the effective interaction between two surfaces, and well-tuned band edge positions promote both reduction and oxidation reactions. Zhou et al. were able to make a 2D/2D vdW heterojunction between ultrathin polymeric carbon nitride (PCN) and ZnIn2S4 subunits via an in situ self-assembling growth of ZnIn2S4 on thermally produced PCN sheets.600 The intimate junctions between ZnIn2S4 and PCN were confirmed from HRTEM and elemental mapping. The PCN/ZnIn2S4 exhibited a high CO2 uptake (17 cm3 g−1 ) compared to pristine materials and was further used for CO2 photo- reduction. Using the PCN/ZnIn2S4 vdW heterostructure as a photocatalyst, triethanolamine (TEOA) as the electron donor, and Co(bpy)3 2+ as the cocatalyst, the CO formation rate was found to be 44.6 μmol h−1 , which was almost 223 times that of the pristine PCN nanosheets. In another report, 2D g-C3N4 sheets were introduced in precursor solution which led to adsorption of Zn2+ and In3+ ions on the g-C3N4 sheets.601 Finally, the growth of ZnIn2S4 takes place in the presence of thioacetamide as a sulfur source and trisodium citrate dihydrate as a surfactant under hydrothermal conditions. The developed 2D/2D g-C3N4@ZnIn2S4 system displayed a remarkable H2 evolution rate (2.78 mmol g−1 h−1 ), which was much higher than any carbon nitride-based catalyst with or without Pt decoration. Figure 23. (a) UV−vis diffuse reflection spectra of g-C3N4, ZIS-S, 30ZIS/CN, and 30ZIS-S/CN and the band gap of g-C3N4 and ZIS-S (inset). Average potential profiles along Z-axis direction for (b) g-C3N4 and (c) ZIS-S. (d) Time-resolved fluorescence spectra of g-C3N4 and 30ZIS-S/CN. (e) Side-view differential charge density maps of g-C3N4 and ZIS-S. (The yellow and blue regions represent net electron accumulation and depletion, respectively. The gray, purple, yellow, brown, and blue spheres are Zn, In, S, C, and N atoms, respectively.) (f) Planar averaged charge density difference Δρ along the Z-direction for the ZIS-S/CN VDW heterojunction (the inset represents the 3D isosurface of the electron density difference for the ZIS-S/CN). Reprinted with permission from ref 602. Copyright 2020 Elsevier. Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX AH
- 35. To further improve the performance of the g-C3N4@ ZnIn2S4 vdW heterostructure, S vacancies were introduced in ZnIn2S4 sheets ,which can ameliorate the light absorption, lifetime of charge carriers, and also charge kinetics between two semiconductors (Figure 23, Table 6).602 To attain this goal, the ZnIn2S4/g-C3N4 (ZIS-S/CN) vdW heterojunction was synthesized via a calcination−solvothermal method using CN sheets followed by high-temperature growth of ZIS-S. The intimate contact with ZIS-S and CN was evident from HRTEM, AFM, and elemental mapping. Due to improved UV−vis absorption and increased PL lifetime, the ZIS-S/CN displayed an increased photocurrent response compared to the pristine components such as CN and ZIS-S (Figure 23a,d). Electrostatic potential measurement using DFT calculations demonstrated that the work functions of g-C3N4 and ZIS-S were 4.70 and 6.03 eV, which implies that the Fermi level of g- C3N4 is higher than that of ZIS-S and electrons should flow from CN to ZIS-S (Figure 23b,c). The charge density difference of 30ZIS-S/CN vdW shows that the electronic charge centered on the surface of ZIS-S was primarily derived from g-C3N4 (Figure 23e). The charge redistribution in the 2D/2D heterojunction contact mainly focused on the 2D/2D interfaces and accumulated near the ZIS-S monolayer, which can efficiently annihilate holes that accumulate near the CN monolayer (Figure 23f). This results in the formation of a bias- less in-built electric field which facilitates better charge separation. Apart from common layered chalcogenides such as WS2 and MoS2, various other chalcogenides including SnS2,605−608 Sn2S3,609 TaS2,610 Bi2Se3,611 NiS,612 CdS,613 CuInS2,614 MnIn2S4,615 Cu2WS4,616 NiCo2S4,617 etc. have been reported for making 2D/2D photocatalysts. 8. CARBON NITRIDE−BISMUTH OXYHALIDE 2D/2D vdW STRUCTURES 8.1. Carbon Nitride−BiOX. Bismuth-based photocatalysts (fractional, binary/ternary oxides) have been proven as excellent photocatalytic materials due to their unique electronic and structural properties and visible light absorb- ance.618−620 Most of the oxide-based bismuth catalysts such as BiVO4, Bi2MoO6, BiPO4, Bi2W2O6, etc. have displayed poor visible light absorption limited to the blue region, compromis- ing the photocatalytic performances.621 Additionally, the fundamental problems of low quantum efficiency and lack of better charge transport properties are key limiting factors. Bismuth oxyhalides (BiOX; X = Cl, Br, and I) constituted of elements from the main group family (V−VI−VII) with a tetragonal matlockite configuration (PbFCl-type) are becom- ing a rising star in the photocatalysis field due to their astounding visible absorption with a band gap in the range of 1.7−3.2 eV, inert nature, easy processing, and corrosion resistance.622 BiOX is constituted of [Bi2O2] slabs interleaved with double halogen slabs giving rise to a layered structure. The interlayer atoms in the BiOX are connected through strong covalent bonding while layers remain bounded together through weak vdW interaction. Due to the specific crystalline structure and atomic polarization, an internal static electric field exists perpendicular to the [Bi2O2] and [X] slices, resulting in effective charge separation. The band gap of BiOX is highly dependent on the types of halogen atoms, i.e., the band gaps for BiOCl, BiOBr, and BiOI were found to be ∼3.3, 2.7, and 1.8 eV, respectively. BiOF, due to its extremely high band gap (3.64 eV), is usually excluded from the general notion of the BiOX family.623 BiOX based compounds, either as a stand-alone catalyst (except BiOI) or in the form of a heterojunction, have been widely used for numerous photocatalytic applications such as water splitting, dye degradation, CO2 reduction, N2 reduction reactions, etc.621,624,625 Like chalcogenides, the inherent trap assisted recombination reduces the performance of these catalysts. Various surface passivation approaches such as decorating with alkyl chains, carbon quantum dots, adding metal/metal oxides nanoparticles, and heterojunction for- mation with various semiconductors such as TiO2, BiVO4, Sn3O4, Bi2O2CO3, etc. have been employed to improve the performance of these materials.626−631 Due to the layered structure of BiOX with a residual surface positive charge and weak p-type nature, g-C3N4 is an ideal contender to make a p− n type vdW heterojunction.632,633 The effective interaction between layered surfaces and the presence of a built-in electrical field after Fermi level equilibration can facilitate better charge separation.634−636 The inherent low band gap of BiOI associated with less negative CB restricts its usage as a stand-alone catalyst which again necessitates the formation of a heterojunction. Alam et al. have synthesized BiOI heterojunctions with few-layered g- C3N4 and F-doped and Cl-intercalated g-C3N4 (CNFCl) via an in situ approach displaying an enhanced photoelectrochemical water splitting performance reaching a photocurrent density of 0.70 mA cm−2 and 1.3 mA cm−2 , respectively, under AM 1.5G solar simulated light.44,637 Kelvin probe force microscopy (KPFM) reveals better charge carrier generation and separation in the BiOI/carbon nitride heterojunctions. Further, after the formation of the heterojunction, the Fermi level of BiOI was uplifted, and then g-C3N4 facilitates the migration of electrons on conjugated carbon nitride sheets. Most of the reported BiOI/g-C3N4 heterojunctions have displayed 3D platelets, microspheres, and flower-like morphol- ogies due to the uncontrolled growth of BiOX lattices on g- C3N4.638 The 3D structures limit catalytic performance due to hindered active sites, reduced active surface area, and minimum interfacial contact between two catalyst components. To surmount the drawback of unfavorable redox potential and stability of BiOI and BiOBr, the BiOCl with a wide indirect band gap and relatively more resilient nature seems to be a good replacement. Introducing the oxygen vacancies or Bi(0) doping in the BiOCl nanosheets was found to improve the photocatalytic performance significantly due to increased visible absorption and formation of subgap energy levels.639,640 The performance can be further improved by making a 2D/2D vdW heterojunction.641 Wang et al. synthesized an oxygen vacancy-rich ultrathin g-C3N4/BiOCl 2D/2D heterojunction using polyvinylpyrrolidone (PVP) which displayed excellent photocatalytic degradation performance for 4-chlorophenol (4- CP) and bisphenol A (BPA) degradation (Figure 24).642 The morphological characterization using HR-TEM, EDS mapping, and AFM confirm intimate contact and formation of the 2D/ 2D structure (Figure 24a−c). The O1 XPS spectra displayed an increased intensity of the O2 signature peak at 531.3 eV assigned to increased vacancies in the 2D/2D structure. ESR spectra of 50CN-50BC (prepared using PVP) compared to 50CN-50BC-P (without PVP) displayed increased ESR signals verifying the presence of oxygen vacancies. To deduce the mechanism of enhanced photoactivity, a scavenger test using O2 •− (ascorbic acid, AA), holes (sodium oxalate, SO), and Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX AI
- 38. • OH radical (isopropyl alcohol, IPA) scavengers followed by EPR using a DMPO trap agent demonstrated that O2 •− and h+ are the main reactive species facilitating degradation (Figure 24d−f). The more reductive CB of BiOCl than oxygen reduction potential (O2/• O2 − ) (−0.33 eV vs NHE, pH 7) coupled with holes on BiOCl and g-C3N4 facilitated the efficient degradation of pollutants (Figure 24g).643 8.2. Carbon Nitride−BixOyXz. Apart from defect/vacancy creation, increasing the Bi/O ratio of BiOX can improve the band alignment and visible absorption.644−646 For BiOXs, the conduction band comprises Bi 6p orbitals while the valence band comprises O 2p and Xn p-orbitals (n = 3, 4, and 5, for F, Br, Cl, and I). By decreasing the content of halogens in BixOyXz compounds, the valence and conduction bands can be tuned to afford maximum absorption without compromising the reduction and oxidation power.622,647 In most such cases, the CB remains more negative, which makes them a favorable catalytic material for the CO2, N2, oxygen, and proton reduction. A number of Bi rich nonstoichiometric compounds such as Bi12O15Cl6, Bi24O31Br10, Bi3O4Cl, Bi24O31Cl10, Bi4O5Br2, Bi12O17Cl2, etc. have been reported so far, which can be easily synthesized by varying different parameters, including the calcination, solvent adjustment, displacement reaction alkalization, etc.648−650 It is interesting to note that the band gap of BiOCl can be adjusted from 3.64 eV to 2.84, 2.80, 2.36, and 2.08 eV in Bi3O4Cl, Bi24O31Cl10, Bi12O15Cl6, and Bi12O17Cl2, respectively.651,652 Further, the activities of these nonstoichiometric BixOyXz compounds can be improved by the decoration of nanoparticles, heterojunction formation, and coupling with metals complexes (i.e., cobalt phthalocya- nine).653,654 Evident from the band gap, the series Bi12O17Cl2 displays an optimum absorption in the visible region and is widely investigated as a standalone and hybrid heterojunction catalyst for various reactions.655,656 For example, Di et al. prepared defect-rich Bi12O17Cl2 superfine nanotubes with structural distortion for the improved photocatalytic CO2 reduction to CO (48.6 μmol g−1 h−1 in water) without any cocatalyst or sacrificial donor.657 In another work, Zhou et al. synthesized the AgI/Bi12O17Cl2 heterojunction by a hydrothermal− precipitation protocol and demonstrated 7.8 and 35.2 times more activity than pristine Bi12O17Cl2 and BiOCl toward photodegradation of sulfamethazine (SMZ; sulfonamide anti- biotic).658 Because BiOX has a p-type behavior and low conductivity, the formation of a heterojunction using their n- type low halogen counterpart BixOyXz can afford better charge separation in p−n type BiOX/BxOyXz heterojunctions.659,660 Even couplings of two nonstochiometric BxOyXz such as Bi3O4Cl/Bi12O17Cl2 have also been explored to fabricate the n−n type of heterojunction with Z-scheme configuration to improve the performance.661 The heterojunction of BxOyXz can further achieve better performance due to the layered structure and appropriate band position. For instance, the g- C3N4/Bi4O5I2 heterojunction can afford better oxidation and reduction reaction to convert CO2 to CO due to the establishment of the Z-scheme heterojunction in the presence of an I3 − /I− redox mediator. 2D/2D contact and heteroatom doping of g-C3N4 will further boost the performance.662−665 Zhou et al. demonstrated that the in situ fabrication of carbon- doped carbon nitride (CCN) with Bi12O17Cl2 can boost photocatalytic degradation of tetracycline (TC) under visible light irradiation (Figure 25, Table 7).666 An excellent interfacial contact displayed in the TEM image and increased Table 6. continued photocatalyst synthesis application light source AQY/STH remarks ref Chalcogenides CdS/g-C 3 N 4 In situ hydrothermal method: Exfoliated g-C 3 N 4 powder and cadmium sulfate were dispersed in DI water followed by addition of thiourea and hydrothermal treatment at 180 °C for 12 h. Photocatalytic deg- radation of RhB 500 W Xe lamp (λ ≥ 420 nm) - CdS/g-C 3 N 4 95.6% RhB degradation 613 g-C 3 N 4 33.4% RhB degradation CuInS 2 /g-C 3 N 4 CuCl, InCl 3 , sulfur powder, and g-C 3 N 4 were mixed in triethylene glycol and hydrothermally treated at 200 °C for 48 h. Photocatalytic deg- radation of TC 300 W Xe lamp (λ ≥ 420 nm) - ∼83.7% TC degradation 60 min 614 MnIn 2 S 4 /g-C 3 N 4 Hydrothermal route: CN nanosheets, manganese chloride tetrahydrate, indium chloride, and thiourea were treated in a polyphenylene-lined stainless steel autoclave at 240 °C for 24 h. Photocatalytic deg- radation of Tetra- cycline hydro- chloride (TCH) 300 W Xe lamp (λ ≥ 420 nm) - MnISCN-20−100% TCH degradation after 120 min 615 CN60.5% TCH degradation after 120 min Cu 2 WS 4 /g-C 3 N 4 Hydrothermal method: a YC/g-C 3 N 4 sample, PVP, and Cu 2 WS 4 NS mixture was treated hydrothermally at 433.15 K for 6 h. Photocatalytic de- composition of TC and reduction of Cr(VI) 300 W Xe lamp (λ ≥ 420 nm) - Cu 2 WS 4 /g-C 3 N 4 - 80% TC degradation in 120 min 616 g-C 3 N 4 46.7% TC degradation in 120 min, complete Cr(VI) reduction NiCo 2 S 4 NSs/P-g-C 3 N 4 Porous g-C 3 N 4 , NiCl 2 ·6H 2 O, and CoCl 2 ·6H 2 O were dispersed together via ultrasonication followed by the addition of thiosemicarbazide. Finally, the mixture was hydrothermally treated at 180 °C for 12 h. Supercapacitor ap- plications NA - NiCo 2 S 4 NSs/P-g-C 3 N 4 specific capacity (506 C g −1 at 1 A g −1 ) 617 Cycling stability100% capacity retention after 1500 cycles at 3 A g −1 ) Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX AL
- 39. visible absorption were responsible for such improvement (Figure 25a-b). 3D excitation−emission spectra (EEMs) displayed increased fluorescence intensity after 30 to 60 min of visible exposure due to the formation of humic acid. The fulvic acids peak (intermediates) was significantly decreased after 120 min, suggesting complete mineralization of TC during the photocatalytic process (also confirmed by HPLC- MS) (Figure 25c−h). The ESR spin trap experiment revealed the presence of O2 •− and • OH radicals while band structures determined via a combination of Mott−Schottky and Tauc plots suggested a charge migration from CB of Bi12O17Cl2 to CCN and vice versa, leading to increased degradation performance (Figure 25i,j). 9. CARBON NITRIDE−CARBON NITRIDE 2D/2D vdW STRUCTURES 9.1. Carbon Nitride-Doped/Undoped Carbon Nitride. As carbon nitride has limited visible absorption, the doping of carbon nitride with various heteroatoms (P, B, F, I, S, N, C, etc.) and even metals has been widely investigated to improve the visible absorption.70,667,668 Among heteroatom doping, P and O doping has demonstrated the most drastic change in the absorption profile.669 For example, a mere 0.1% P doping in the carbon nitride framework using BMiMPF6 ionic liquid can improve visible absorption throughout the visible range.71 To further synergize doping effects, codoping using B and F, P and F, etc. has also been investigated. For example, recently, Kumar et al. synthesized highly porous P and F codoped carbon nitride with 260.93 m2 g−1 surface area displaying excellent CO2 reduction and catalytic activity for the conversion of cellulosic biomass to furanics.670 Incorporation of a small alkali metal such as potassium in the heptazine-based cavity has also been found to improve visible absorption and photocatalytic performance.671−673 Again, codoping with K and P can ameliorate the performance.674 Indeed, the aforementioned approaches improve the visible absorption profile of blue absorbing g-C3N4, but the problem of fast charge carrier recombination (inter and intrasheets) remains prevalent, resulting in a sluggish activity. The difference in band gap energy and band edge positions of pristine g-C3N4 and doped carbon nitride isotype hetero- junction can afford better interlayer charge separation.675,676 Such isotype heterojunction can be synthesized using two approaches: (1) in situ method creating doped sheets within materials, and (2) mixing doped and nondoped g-C3N4 after synthesis or growth of one on another by annealing. g-C3N4/ doped g-C3N4 type 2D/2D vdW structures can afford better charge separation due to lattice match and differential band structure.677−682 Since inorganic 2D semiconductors are hard to synthesize and in many cases get photobleached under solar excitation, it is desirable to develop a stable heterojunction between carbon nitride and the doped carbon nitride to resolve Figure 24. (a) HRTEM image of 50CN-50BC composite nanosheets and (b) the corresponding FFT image. (c) AFM image of 50CN-50BC composite nanosheets and the corresponding height profile shown in the inset of (c). ESR spectra of (d) DMPO-• O2 − (e) and DMPO-• OH in the presence of 50CN-50BC ultrathin nanosheets under dark and visible light irradiation, respectively. (f) Photocatalytic degradation of 4-CP over 50CN-50BC ultrathin nanosheets under visible light irradiation in the presence of different scavengers. (g) Schematic illustration of the visible light photocatalytic degradation pollutants over OVs-rich ultrathin 50CN-50BC nanosheets. Reprinted with permission from ref 642. Copyright 2018 Elsevier Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX AM
- 40. the stability issue while minimizing synthesis cost.683−685 Qin et al. reported the synthesis of S-doped g-C3N4 and porous g- C3N4 isotype heterojunction via an in situ approach using thiourea as a sulfur source for improved visible light H2 evolution.686 In another work, carbon nitride nanosheets (CNS) were prepared by thermal annealing of trithiocyanuric acid (TCA), and then CN was grown on these sheets by further annealing with dicyandiamide. The CB of bulk CN is much higher than that of CNS, which facilitates better charge separation. As a result, H2 production rate can be increased almost 11-fold in comparison to bulk CN. Thiourea and urea can afford a differential band gap heptazine polymeric carbon nitride, which can facilitate better change separation. For example, Dong et al. synthesized a g-C3N4/g-C3N4 (CN-TU) 2D/2D heterojunction by sequential thermal annealing of urea and thiourea, respectively.687 The inbuilt electric field in the n−n type of heterojunction enhances the charge separation. The CN-TU exhibited a NO removal ratio of 47.6%, which was significantly higher than that of thiourea and urea-based carbon nitride (27.3 and 31.7%). Later, carbon nitride (CN), B-modified graphitic carbon nitride (CNB) (CN-CNB), and g- C3N4/g-C3N4 (derived from urea and thiourea) isotype heterojunctions were also reported with the improved performance.687,688 In a recent work, Zhao et al. reported the synthesis of boron-doped and nitrogen-deficient carbon nitride nanosheets (BDCNN) by rapid heating of carbon nitride nanosheets (CNN) in the presence of sodium borohydride (Figure 26a).689 Due to simultaneous doping and N-deficiency and the introduction of mid gap energy states, the band gap was significantly reduced (2.37 eV), extending the band edge absorption up to NIR region (Figure 26b). Further, the CB and VB band positions of BDCNN were significantly shifted toward the positive side, idealizing it to integrate with the n-type carbon nitride nanosheets (CNN) (Figure 26c). When CNN and BDCNN were combined together in 2D/2D fashion by an electrostatic interaction, a Z- scheme heterointerface was realized, which facilitates better charge separation (Figure 26d,e). The Z-scheme CNN/ BDCNN photocatalysts fabricated by taking a 1:1 ratio of CNN and BDCNN due to the presence of sufficient reduction and oxidation potential was able to split pure water. The resultant yields were 32.94 and 16.42 μmol h−1 H2 and O2, respectively, with AQY of 5.95% at 400 nm. The isotopic labeling experiment using 18 O labeled H2O demonstrated that 97.10% O2 was originated from water splitting and validated true photocatalytic behavior. The condensation polymerization of carbon nitride precursors involves a variety of complex steps, so the band edge position and the band gap of the final carbon nitride materials depend upon the type of precursor.690 Paradoxically, identical precursors annealed at the same temperature but with a different heating rate will afford a different band structure. Among various factors, the C/N content and degree of polymerization are of utmost importance. For example, thermal annealing of urea and DCDA proceeds via massive gaseous mass loss (95 and 40 wt %). And the peeling effect of oxygen functionalities resulting in a slightly higher atomic C/N ratio enhanced the surface area in CN synthesized by urea (UCN) compared to dicyandiamide synthesized CN (D-CN). This anomaly produces a variation in band position, which can be cultivated for the formation of isotype heterojunction. Wang et al. reported the synthesis of an isotype heterojunction by cothermal condensation of urea and DCDA (UDx-CN).691 The thin nanosheets of U-CN can be distinctly identified with wrinkled and relatively dense DCN. The hydrogen evolution rate using UD1-CN prepared by using 1 wt % DCDA was optimal, reaching as high as 553 μmol h−1 g−1 , which is almost 17 and 5 times higher than those of the pristine D-CN and U- Figure 25. (a) HRTEM images of CCN/Bi12O17Cl2. (b) UV−vis absorption spectra of samples. Three-dimensional EEMs of the aqueous solution: (c) Taken from the original solution, (d) collected after 60 min of adsorption in the dark, and (e−h) obtained after irradiation times of 30, 60, 80, and 120 min, respectively. (i, j) Proposed charge separation process in the CCN/Bi12O17Cl2 heterostructures under visible-light irradiation. Reprinted with permission from ref 666. Copyright 2018 American Chemical Society. Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX AN
- 42. CN. Enhanced transient current responses and delayed charge recombination lifetime in TRPL measurement suggest the presence of cumulative charge separation in the UCN−DCN heterojunction. Apart from doping, the band structure of carbon nitride can also be tuned by controlling the degree of polymerization due to its polymeric nature. Zhang et al. have demonstrated that the synthesis of carbon nitride in a sulfur medium (CNS) using trithiocyanuric acid (TCA) precursor can significantly influence the condensation/packing of the heptazine structure, resulting in a tuned electronic band structure.692 Compared to regular pristine CN (ECB: −1.42 V, EVB: +1.28 V vs Ag/AgCl) the CB and VB positions of sulfur-mediated CN were shifted to −1.21 and +1.46 V, respectively, appropriate to fabricate the type-II isotype heterojunction with CN. Indeed, depending on the exposed surface of carbon nitride or doped carbon nitride in the CN/CNS isotype heterojunction, two possible configurations, i.e., CNS-CN (CN serving as the host) and CN-CNS (CNS serving as the host), are possible. The presence of an imperfectly condensed heptazine structure in CN and CNS structure provides plenty of −NH2 terminated sites for the growth of a second carbon/doped nitride. The TEM image of CNS-CN displayed the presence of thick CN and paper-thin CNS sheets in close proximity. The deconvoluted HR-XPS spectra of CNS-CN exhibited two peak components (159.3 and 164.2 eV) that were identical to those of CNS, however, completely different those of from S doped carbon nitride (163.9 and 168.5 eV), validating the self- polymerization of the (TCA) precursor instead of doping in CN. The increased EPR signal in CNS-CN and prolonged PL lifetime of the excited state further confirm better charge migration in the CNS-CN heterojunction. Interestingly, the CNS-CN 2D/2D heterojunction demonstrated enhanced photocurrent generation without any applied bias. The optimized CNS-CN-2 and CN-CNS-2 (2 denotes the amount of DCDA and TCA precursors) catalysts can afford HER rates 11 and 2.3 times higher than that of pristine CN using triethanolamine as a sacrificial donor. Distinct from tris-s- triazine based carbon nitride isotype heterojunctions, the allotropic triazine-based carbon nitride (tri-C3N4) is also a promising candidate to form a vdW heterojunction due to structural similarity (C−N graphitic core) and identical physicochemical properties.693 Zeng et al. prepared a crystalline heterojunction between triazine-based C3N4 and tris-s-triazine based C3N4 (tri/tri-s-tri-C3N4) via a sequential condensation in a LiCl + KCl mixture.694 The close contact between tri-C3N4 and tris-tri-C3N4 was evident from the TEM image, and the surface area was increased up to 79.7 m2 /g. The HER and apparent quantum yield (AQY) of tri/tri-s-tri-C3N4 catalysts were found to be ∼150 μmol h−1 and 12.9% (TEOA was used as the electron donor) 9.2. Carbon Nitride−Metal Doped/Intercalated Car- bon Nitride. Another more promising approach is the incorporation of atoms in between g-C3N4 sheets, pillaring, and incorporation of conjugated linkers. These approaches have been envisioned to improve the performance as such molecules provide interlayer galleries for better charge migration. When g-C3N4 was synthesized using an excess of NH4Cl, the Cl atom gets intercalated between the CN sheets, which behave as a bridge between two CN sheets facilitating Figure 26. (a) Schematic of the synthesis of BDCNN derived from CNN. (b) UV−visible DRS of CNN and BDCNN, where the insets show photographs of the CNN and BDCNN. (c) Band structure alignments for CNN and BDCNN. (d) Schematic of the synthesis of the CNN/ BDCNN heterostructure. (e) Side-view differential charge density map of CNN and BDCNN. The iso-surface value is 0.012 e Å−3 . The yellow and blue regions represent net electron accumulation and depletion, respectively. Reprinted with permission from ref 689 by Zhao et al. under exclusive license to Springer Nature. Copyright 2021 Springer Nature. Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX AP
- 43. better charge separation (H2 evolution and NO removal). The presence of Cu metal and P−Cl codoping can further intensify the photoactivity due to better capturing of transported charges.695,696 At first, such systems seem to be doped carbon nitrides; however, in a strict sense, they are 2D/2D homojunction composites having heteroatom charge mediators in between sheets. In another interesting study, Cui et al. synthesized K and NO3 − intercalated carbon nitride containing K and NO3 − species between the neighboring layers (Figure 27).697 The bioriented channels in CN-KN due to the presence of K and NO3 − species in opposite sheets facilitate better steering of charge flow in opposite directions, overcoming the problem of charge accumulation on one sheet. DFT calculations reveal that the N−O bond in NO3 − was highly stable up to 800 and 900 K, with average distances of 2.23 and 2.36 Å, respectively, which are shorter than the interlayer distance of CN (3.68 Å), verifying that NO3 − can be intercalated between sheets (Figure 27a−e). The CN-KN showed excellent NO degradation compared to other components, including a physical mixture of CN and KNO3, suggesting that the copyrolysis of thiourea and KNO3 is essential for rational intercalation (Figure 27b). Trapping of free radicals using DMPO gave a strong signal of DMPO-O2 •− and DMPO-• OH, suggesting generation of O2 •− radical followed by their reduction to • OH radicals. The increased EPR signal of CN-KN after irradiation under light further demonstrated better charge separation. The calculated electrostatic potentials of CN, CN-K, and CN-KN between adjacent layers demonstrated a significant decrement of the energy barrier for CN-KN (−28.17 eV) compared to CN (−34.16 eV), corroborating the feasibility of better charge separation through interlayer electron delivery channels (Figure 27f,g). Numerous multilayered vdW heterostructures have been investigated theoretically and experimentally. The biggest challenges associated with multilayered vdW heterostructures are poor separation of charge generated in each layer of heterojunction due to unoriented charges flowing in the multilayered structure bonded through weak vdW interac- tions.698,699 Internal vdW heterostructures (IVDWHs) con- taining strong interaction between sheets and charge transport channels can overcome such issues to allow unidirectional interlayer charge flows for enhanced photocatalysis.700,701 Li et al. demonstrated that sandwiching alkali atoms between carbon nitride sheets provides a channel for electron flow, and directionality of charge flow can be maintained by introducing O “adjuster” atoms (Figure 28).702 To realize this goal, a cake model was simulated in which oxygen doped carbon nitride (OCN) and CN sheets were bridged together with K ions as a mediator, followed by a spaced O adjustor in the next layer. The OCN-K-CN IVDWHs were prepared via a copyrolysis of thiourea and K2SO4 by changing the amount of K2SO4. DFT calculation demonstrates that, after incorporation of O- adjustors, the band structure of OCN was favorably changed Figure 27. Crystal structures of CN-KN: (a) time evolution of the N−O length of NO3 − and the fluctuation distance of doped NO3 − in the CN interlayer at 800 and 900 K with an AIMD simulation in 10 ps and the optimized local structures of the individual. (b) Evaluation and analysis of the visible light photocatalytic performance of the as-prepared samples. (c) CN, (d) CN-K, and (e) KNO3 doped CN. All the lengths and energies are given in Å. Gray, blue, purple, and red spheres represent C, N, K, and O atoms, respectively. Ed stands for the doping energy; negative values mean heat release. Analysis of electron mobility. (f) Electrostatic potential. (g) Charge difference distribution between metal atoms and CN layers: charge accumulation is in blue and depletion in yellow. The isosurfaces are set to 0.005 eV Å−3 . Reprinted with permission from ref 697. Copyright 2017 Elsevier. Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX AQ
- 44. (CB and VB became more negative) to achieve better charge separation (Figure 28a−c). Comparison of the electrostatic potential energy using electronic structures shows that the potential energies of the OCN layer and CN sublayer in OCN- K-CN were drastically increased after the introduction of O and K. The O adjustor atoms improve the charge density between sheets and affords a strong van der Waals interaction, which was further reinforced by K atoms (Figure 28d−i). The OCN-K-CN afforded a fast NO degradation in the first 5 min with an excellent 100% activity retention after five recycles (purification efficiency of 45% compared to CN (24%). The EPR signals of the tapped radicals, DMPO-O2 •− , and Figure 28. Schematic illustration of the internal van der Waals heterostructure (IVDWH): (a) “Cake Model” and structure of OCN-K-CN. (b) Calculated total density of states (TDOS) of CN and OCN layers. (c) Band sketch of the OCN-K-CN IVDWHF. Layered electrostatic potential energy for pristine (d) CN and (e) OCN-K-C. Calculated Bader effective charge for (f) pristine CN and (g) OCN-K-CN. Charge density difference of (h) K-CN and (i) OCN-K-CN. Blue, green, red, and gold spheres depict N, C, K, and O atoms. Charge accumulation is labeled in blue and depletion in yellow, and the isosurfaces were both set to 0.005 eV Å−3 for (i) and (j). Reaction process and intermediates: in situ DRIFTS spectra and species evolution of NO adsorption in (j and l) dark and (k and m) oxidation under visible light irradiation on CN and OCN-K-CN. Reprinted with permission from ref 702. Copyright 2018 American Chemical Society. Figure 29. (a) Synthetic route for the ultrathin carbon nitride intraplane implanted with graphited carbon ring domain (CN-GP). (b, c) High- resolution TEM image of CN-GP. The selected area electron diffraction (SAED) patterns of (d) g-C3N4 and (e) CN-GP. AFM images of (f) g- C3N4 and (g) CN-GP. (h) Photocatalytic H2 evolution rates of as-synthesized samples under vis−NIR irradiation. (i) Rates of H2 evolution of the compounds determined, respectively, under irradiation with 700 nm wavelength, 800 nm wavelength, and 900 nm wavelength light. (j) Possible mechanism for the photocatalytic H2 evolution of GP+g-C3N4 and CN-GP, respectively. Reprinted with permission from ref 714. Copyright 2019 American Chemical Society. Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX AR
- 45. DMPO-• OH generated from OCN-K-CN were found to be way higher than that of pristine CN, suggesting better charge separation in OCN-K-CN IVDWHs. Further, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurement of NO degradation under dark and visible light shows the NO2 band at 2091 cm−1 was decreased after visible irradiation and more band for (NO2 − and NO3 − ) were observed due to more prominent oxidation of NO over OCN- K-CN (Figure 28j−m). Even though intersheet charge recombination can be suppressed via the above-mentioned strategies, intrasheet charge recombination within the 2D g-C3N4 sheet poses another challenge. To solve the problem of intrasheet charge recombination, incorporation of electron deficient units such as pyromellitic dianhydride (PMDA), mellitic acid trianhydride (MTA), and biphenyl tetracarboxylic dianhydride (BTCDA) to form polyimides has been suggested.703−705 These electron- deficient units extract some charge from the sheets, preventing faster recombination. Additionally, band positions of the materials can be tuned based on the content of such units. For example, Shiraishi and co-workers demonstrated that the introduction of 51% PMDA units in the g-C3N4/PDI network can change the valence band to 1.86 V Ag/AgCl at pH 6.6 compared to g-C3N4 (1.40 V).706 Because of the deep valence band, the catalyst was very efficient for the product of H2O2. The introduction of the graphene sheet in the g-C3N4/PDI scaffold in the 2D/2D assembly was found to again boost the H2O2 generation rate.707 Additionally, some other approaches such as the introduction of donor−acceptor assemblies,708 polyaromatic units,709,710 increasing π conjugation, addition N and C rich units, etc. have also been used to reduce recombination on the sheets.711,712 Inspired by the success of the g-C3N4/graphene based 2D/ 2D van der Waals structure, researchers endeavor to introduce graphene-like conjugated fragments in the carbon nitride framework to facilitate better charge separation. For example, conjugated carbon rings were introduced in the g-C3N4 framework ((Cring)−C3N4) by thermal annealing of melem and glucose. The resulting (Cring)−C3N4 showed excellent charge separation due to the capture of photogenerated electrons on the sheets by conjugated carbon units and displayed almost 5% quantum efficiency in water splitting.713 Indeed, these structures cannot be considered as van der Waals structures where two fragrants are interacting in a face-to-face manner. However, such structures can be categorized in the class of 2D/2D in-plane heterostructures where two 2D sheets interact laterally. In another work, the workgroup of Song et al. demonstrated the synthesis of an ultrathin carbon nitride intraplane implanted with a graphited carbon ring domain (CN-GP) via thermal polymerization of polyvinyl butyral and a melamine membrane (Figure 29a, Table 8).714 To compare the performance, the CN-GP interplane decorated with graphene (GP + g-C3N4) was also prepared by decorating the graphene sheets on carbon nitride to make a vdW heterosystem. The presence of two-phase (1) g-C3N4 and (2) graphene was observable in HRTEM images, AFM images, and the SAED pattern (Figure 29b−g). Notably, in the photo- catalytic H2 evolution, rates of 560.8, 398.4, and 322.8 μmol g−1 h−1 were observed at 700, 800, and 900 nm irradiation, while under the same conditions GP + g-C3N4 achieved very poor yield (Figure 29h−i). Further, a benchmark apparent quantum efficiency (AQE) of 14.8% at 420 nm was observed, exceeding a previously reported yield on similar kinds of systems ((Cring)−C3N4 5%). The improved activity was observed due to prompt migration of the photogenerated charge from the carbon nitride domain to the graphene domain (Figure 29j). 10. CARBON NITRIDE−CARBON 2D/2D vdW STRUCTURES The effective strategy to increase the performance of g-C3N4 based materials is to integrate with carbon-based materials which not only provide a better alternative to avoid metal- based semiconductors but also enhance the absorption and charge separation.715,716 Carbon-based materials are earth- abundant and cheap, and, depending on the nature and hybridization of carbon in the materials, may be a semi- conductor to the conductor. The improvement of the photocatalytic performance of g-C3N4 using carbon-based materials has been achieved mainly through the junction interaction, cocatalyst effect, surface reconstruction, modifica- tion of local electronic structure, electron sink, etc. With the advent of new carbon-based materials such as graphene, graphene oxide, carbon nanotubes, fullerene, carbon quantum dots (CQDs), graphdiyne, carbon nanofibers, etc., the catalytic and photocatalytic properties of various semiconductors have been dramatically improved.717−719 The syntheses of some specific nanostructured carbon materials is far from large-scale synthesis because they rely on costly and time-consuming methods. For example, high- quality graphene is synthesized by chemical vapor deposition (CVD) of methane at high temperature and pressure in the presence of metal catalysts.720 Nanostructured carbon, which possesses a localized sp2 carbon framework, seems to be a better alternative.721 Carbon-based materials due to the localized conjugated sp2 network facilitate better charge separation as they work as electron capturing agents/electron sinks in photocatalysis. Some carbon materials such as carbon quantum dots (CQDs) and graphene quantum dots (GQDs) due to the presence of a quasi-spherical state have sp3 carbons at the edge, giving semiconductive properties and the opportunity to integrate with carbon nitride-based materials.722 Additionally, the quantum confinement effect, up-conversion, and bright photoluminescence of quantum dots are appealing to harvest a major fraction of light. Indeed, extremely high numbers of reports are available on the use of carbon-based materials coupling with inorganic semiconductors to make 0D/ 2D, 0D/3D, 2D/2D, 2D/3D, and 3D/3D hybrids to improve photocatalytic properties.723−726 Among them, 2D sheets of graphene and graphene oxide have proven to be the most promising due to their 2D nature and excellent optoelectronic properties.727−729 Sticking to the scope of this review, we will revisit the scope of 2D carbon-based materials to integrate with g-C3N4 based semiconductors. 10.1. Carbon Nitride−Graphene. Graphene is the most celebrated member of the carbon family.730,731 Since its discovery in 2004, it has revolutionized the field of 2D materials due to its astonishing thermal, chemical, and optoelectronic properties.732 It is sufficient to see the potential of graphene by its calculated properties such as excellent charge carrier mobility (200 000 cm2 V−1 s−1 ), thermal conductivity (5000 W m−1 K−1 ), electrical conductivity (2000 S m−1 ), extremely high theoretical specific surface area (2630 m2 g−1 ), transparency (97.7%), mechanical strength (Young’s modulus ∼ 1 TPa), excellent environmental compatibility, and adsorption capacity for organic and Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX AS
- 47. inorganic molecules.55 Unfortunately, due to the absence of the band gap in graphene, standalone graphene cannot be used for photocatalytic application. The efforts to open up the band gap in graphene using various approaches such as heteroatom doping, incorporation of molecular units, modification of the edge to use the quantum-confinement effect, etc. have limited utility in photocatalysis due to the small band gap compared to the theoretical water splitting value and essentiality of band edge matching.733−735 High electronic mobility of graphene has been harvested to intensify the charge separation in organic/inorganic semiconductors. The introduction of localized sp3 carbon bonded oxygens via transformation of graphene into graphene oxide (GO) creates a significant band gap to use GO as a photocatalyst for water splitting and CO2 reduction.736−739 However, the electronic mobility is compro- mised, which puts GO in the series of other conventional photocatalysts with sluggish reaction rates.740 As the electronic structure of graphene is very sensitive to its surrounding environment, 2D/2D hybridization of graphene with g-C3N4 was investigated theoretically to open the band gap in graphene and elucidate the charge transfer mechanism between two interfaces. Due to the graphitic structure, g-C3N4 can establish a strong vdW interaction with graphene. The graphene/g-C3N4 interface showed strong interlayer electron coupling, resulting in band gap opening in graphene and increased visible absorption for g-C3N4.741 Inspired from these initial findings, several 2D/2D vdW heterostructures of graphene/g-C3N4 have been reported for various photo- induced reactions.742,743 Apart from conventional thermal annealing or mixing of graphene and g-C3N4, the g-C3N4/ graphene architecture can also be synthesized by using molecular organic frameworks or supramolecular assemblies of g-C3N4 precursors. For example, Ma et al. reported the synthesis of the porous g-C3N4 and N-doped graphene (PCN/ NG) hybrid by ball-milling and annealing of the melamine− urea conjugate and N-doped graphene.744 During the establishment of a 2D/2D heterojunction between graphene and CN, It is not only the graphene whose charge distribution gets redistributed, but the carbon nitride is also influenced proportionally. Graphene and graphitic carbon nitrides share a common hexagonal lattice structure, so an efficient π−π stacking can be realized with entirely new electronic properties. Inspired from the unique π−π stacking interaction in tetrathiafulvalene (TTF) and tetracyanoquinodimethane (TCNQ) (TCNQ-TTF), which gave it a distinct metallic electrical conductance,745 Zhang et al. visualize that the combination of graphene and carbon nitride heterojunction must produce intriguing properties.746 To achieve this goal, they synthesized a 2D/2D vdW heterostructure of CN and rGO by thermal annealing of GO and DCDA. The condensation polymerization of DCDA provides a CN framework and also reduces the GO while protecting rGO oxidation at high temperatures. The signature G-band in the Raman spectra, diminished oxygen bonded peaks, dominant XRD peak at 27.4° for the 002 plane of carbon nitride, and absence of (001) GO peaks combined with TGA and TEM analysis clearly demonstrated the formation of an rGO-doped g-C3N4 structure. In contrast to carbon nitride, which indicated an ambipolar behavior in PEC measurement, the 2D/2D carbon nitride graphene (CNG) prepared in argon and air displayed anodic and cathodic photocurrents, respectively, assigned to n- and p-type nature. Furthermore, at an applied voltage of 0.4 V vs Ag/AgCl, the anodic photocurrent was Table 8. continued photocatalyst synthesis application light source AQY/STH remarks ref Isotype Heterojunctions OCN-K-CN Co-pyrolysis of thiourea and K 2 SO 4 . Thiourea and K 2 SO 4 were calcined at 550 °C for 2 h. NO degradation 150 W tung- sten halo- gen lamp - OCN-K-CN45% NO degradation 702 pristine CN (24%) Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX AU
- 48. 300% higher after doping with rGO. Such behavior was explained based on the close packing in O defect free graphene compared with O-defect rich graphene and Fermi levels of CN and rGO equilibration, resulting in significant variation in the flat band potential under the PEC environment. In comparison to graphene, the oxidized state graphene oxide with plenty of oxygen-carrying functionalities is more attractive to make a heterojunction with carbon nitrides due to better effective interaction and semiconductive na- ture.707,742,747,748 A facile hydrothermal, mixing, or copyrolysis approach can afford the 2D/2D composite of GO and g-C3N4. Figure 30. (a) Illustration of the preparation of the GO−CN samples. (b) Rate of hydrogen evolution on CN loaded with different quantities of GO. (c) Photocatalytic activities of CN, CA−CN, GO−CN, 5CA−CN, and 5GO−CN samples. (d) Cycling test of photocatalytic H2 generation of the 5GO−CN hybrid. Reprinted with permission from ref 752. Copyright 2021 Elsevier. Figure 31. (a) Schematic illustration for the construction of 2D/2D graphitic carbon nitride/graphdiyne heterojunction on the 3D GDY nanosheet array. SEM images for (b) the 3D GDY nanosheet array and (c) g-C3N4/GDY. (d) HADDF image for g-C3N4/GDY. The corresponding elemental mapping images for (e) C, (f) N, (g) C, and N elements. (h) XPS valence spectra of GDY and g-C3N4. (i) Photoluminescence spectra of g-C3N4 and g-C3N4/GDY. (j) Mott−Schottky plots. (k) Band structures of g-C3N4 and GDY. Reprinted with permission from ref 762. Copyright 2018 Wiley-VCH. Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX AV
- 49. For example, when GO and melamine were calcined together at 550 °C for 2 h, a 2D/2D composite was formed, which can degrade phenol more rapidly than other catalyst compo- nents.749 Further, the hydrothermal treatment gives smaller graphenic fragments, which can improve the performance due to up-conversion. Utilizing the innate negative charge on the surface of GO, protonated carbon nitride prepared by the treatment of g-C3N4 with acids can form a 2D/2D heterostructure merely by simple mixing.750 The protonated g-C3N4 and GO (pCN/GO) with 5% of GO demonstrated excellent visible light photodegradation of RhB (4 times of g- C3N4) due to better charge separation on the surface of GO. The GO/g-C3N4 nanocomposite synthesized via sonochemical synthesis displayed antibacterial activity (E. coli) under visible light irradiation.751 Song et al. made a 2D CN@graphene@CN sandwich (5GO-CN) structure via in situ local thermal oxygen erosion strategy using melamine and GO precursor followed by two-step thermal annealing (Figure 30a).752 The afforded structure demonstrated a porous sandwiched structure. Because of the addition of GO, the visible light absorption of the catalyst was significantly improved in the visible to NIR region. When used as a photocatalyst for water splitting, an impressive and repeatable H2 evolution rate of 5.58 mmol g−1 h−1 was obtained, which was almost 14.3 times that of the pristine CN (0.39 mmol g−1 h−1 ) (Figure 30b−d). 10.2. Carbon Nitride−Graphdiyne. Graphdiyine (GDY) is a member of a broad class of compounds called “graphynes” which are the 2D allotrope of carbon constituted of sp- and sp2 -hybridized carbon atoms.753,754 Due to the unique 2D structures containing diacetylene linkages (−CC−CC−), the connected benzenic structure, GDY, shows some remarkable properties entirely different from sp2 carbon- based structures such as graphene, CNTs, etc.755,756 The theoretically calculated band gap for the GDY monolayer was found to be 0.44−1.47 eV.757 Additionally, GDY has high hole mobility and has been used as hole-transporting materials in various applications.758−760 The 2D nature, conjugated system, and high hole mobility make them a suitable candidate to integrate with 2D g-C3N4 in a 2D/2D fashion.761 Han and co-workers prepared a graphdiyne honeycomb structure on the copper substrate and then integrated it with carbon nitride sheets in a hydrothermal reaction to prepare g- C3N4/GDY 2D/2D heterostructure (Figure 31a).762 The afforded C3N4/GDY 2D/2D heterojunction showed good structure interfacial interaction (Figure 31b−g). The valence band positions of g-C3N4 and GDY calculated from XPS valence spectra were found to be 2.4 and 1.7 eV, suggesting thermodynamically favorable hole transfer from g-C3N4 to GDY (Figure 31h). While the CB position of GDY was less negative than that of g-C3N4, the afforded structure was in a type I configuration. The efficient charge separation was evident from a decreased PL intensity in the g-C3N4/GDY heterojunction and increased photocurrent density in PEC water-splitting experiments (Figure 31i−k). In another work, a 2D/2D heterojunction of GDY and g-C3N4 was prepared by high temperature (400 °C) annealing, which established a C− N bond between GDY and g-C3N4 and served as a charge carrier channel to accelerate the migration of photogenerated electrons from g-C3N4 to GDY.763 The prolonged charge carrier lifetime and decreased overpotential in g-C3N4/GDY enhanced the performance by a factor of 6.7 compared to g- C3N4. Furthermore, GDY interacted with a few-layered g-C3N4 exfoliated using liquid N2 also increased the H2 evolution performance by a multiplication factor of 3 over that of g- C3N4.764 10.3. Carbon Quantum Dot Implanted Carbon Nitride. Carbon quantum dots (CQDs) are quasi-spherical nanoparticles of graphitic or turbostratic carbon (sp2 carbon) comprised of either amorphous or crystalline form.765 Apart from amorphous carbon/graphitic carbon, a small fragment of graphene and graphene oxide also belongs to the CQD family.766 Since the accidental discovery of CQDs during the purification of carbon nanotubes, CQDs have emerged as future quantum materials for various applications such as LEDs, bioimaging, sensing, photocatalysis, and energy applications due to their unique optoelectronic and phys- icochemical properties.767−771 Recently, another new class of CQDs called carbon nitride quantum dots are replacing CQDs due to their high N content, bright luminescence, thermo- chemical stability, and resistance to photobleaching.42,772−774 Due to their spherical to subspherical morphologies, CQDs are put in the 0D family, and their properties and applications including their 0D/2D and 0D/3D structures are discussed elsewhere.722,775−777 Focusing on the scope of this review, we will discuss the role of carbon quantum dots to achieve the 2D/2D heterojunction, which essentially is not limited to van der Waals interaction. Physical interaction of CQDs with g-C3N4 affords 0D/2D heterojunction, which shows leaching to the solution and self- degradation due to less effective interfacial contact and is not desirable for long-term usage. On the other hand, implantation of carbon quantum dots in the carbon nitride heptazine (C6N7) network using a thermal approach has been found to transform the 0D system into 2D graphene-like domains giving 2D/2D in-plane heterostructures.778 This resulted because of graphitization of CQDs at elevated temperature and simultaneous accommodation in the polymerizing heptazine structure. Wang et al. demonstrated the implantation of CQDs in the g-C3N4 network by thermal annealing of freeze-dried urea and carbon quantum dot precursors.779 The implanted CQDs were visible in HR-TEM images. Due to localized grafting of the conductive sp2 carbon-rich domain in the g- C3N4 network, better intralayer charge separation was achieved, evident from the improved photocurrent response and H2 evolution rate. In another study, Han et al. used a strategy to synthesize carbon dot implanted carbon nitride (CCNS) using dicyandiamide and selenium precursors.780 Selenium not only prevents stacking of carbon nitride sheets during synthesis but also facilitates the release of nitrogen, ammonia and nitrile groups, which leads to the in situ formation of carbon quantum dots (CDs) without any added precursors. The presence of CDs in the carbon nitride scaffold was visible in HRTEM images, while the thickness of the sheets was 5.5 nm, which verifies the presence of CDs implanted in few-layered sheets. When used for the CO2 reduction under visible light, the CCNS photocatalyst afforded an excellent CO2 reduction, hydrogen evolution, and RhB degradation rate, which was attributed to better charge separation and electron transport on the few-layered sheets. The evidence of increased charge separation and transport came from PL and time-resolved PL (TRPL) measurement, which displayed a significant quenching and increased PL lifetime. In addition to conventional CQDs incorporated in the g- C3N4 structure, a reversed configuration where carbon nitride quantum dots (CNQDs) were embedded in carbon nano- Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX AW
- 50. sheets was synthesized using formamide as a single precursor and displayed remarkable apoptosis of cancer cells in the IR region.781 In very recent work, Li et al. fabricated CQDs containing a double-deck frame-like carbon nitride (CN) nanostructure by using a melamine-cyanuric acid super- molecule (CM) and CQDs precursor (CM-CQDs) for enhancing the photocatalytic activity (Figure 32a, Table 9).782 Due to the work function difference between CQDs and CN, two heterogeneous interfaces stitched in-plane and out-of-plane were obtained. CN-CQDs displayed towel gourd shape-like nanostructures where two channels were connected through a rod-like structure (Figure 32b−d). Such morphology arose because of the difference between the crystallization rate of the amino group-containing precursor, which crystallizes the edge faster, and gas that evolved from the central ladder-type eruption. Such morphology implanted with in-plane and out- of-plane CQDs affords better intra- and interplane charge separation increases TC and RhB degradation (Figure 32e,f). Due to the incorporation of CQDs in the CN framework, the CB and VB were downshifted while the band gap was reduced to 2.71 eV for CN-CQDs-100 (Figure 32g). In another work, an ultrathin tubular porous g-C3N4 implanted carbon dot (CN/C-Dots) lateral heterostructure was synthesized, which showed that electrostatic potential for the lateral structure was much less than the vertical heterostructure, which afforded a 113-fold increased H2 evolution rate compared with that of pristine CN.783 11. CARBON NITRIDE−2D POLYMER 2D/2D vdW STRUCTURES Polymeric semiconductors due to the possibility of facile band energy tuning by controlling the degree of polymerization, specific coordination, and chemical control over the nature of the constituting units are fast emerging as new candidates in photocatalysis. Poly(p-phenylene) was the first (1985) reported example of polymer photocatalysts demonstrating the hydrogen evolution under deep UV irradiation in the presence of sacrificial donors.784 Polyaniline (PANI), a conducting polymer, was also among the first few organic polymeric materials explored for the photolytic applications due to certain advantages such as p-type hole conducting behavior, easy solution processability, solubility, reversible redox behavior, and photostability.785,786 To better extract the holes and enhance the water oxidation kinetics, PANI has been integrated with several other organic/inorganic semiconduc- tors including carbon nitride.787,788 Later, a new combination including copolymers with electron donor−acceptor assembly such as phenyl and 2,1,3-benzothiadiazole units and metal chelated polymers (bipyridine ligand) has been reported for hydrogen evolution and other photocatalytic applications.789 Such linear conducting polymers leave little room for further modification, and the 3D morphology of the bulk agglom- erated state with unidirectional charge transport limits their application. However, 2D carbonized polydopamine was also prepared and integrated into carbon nitride for accelerated RhB degradation.790,791 Later, several new organic semi- conductor materials such as poly(azomethine) networks, conjugated microporous polymers (CMPs), covalent triazine- Figure 32. (a) Modulating the interfacial charge kinetic by simultaneously building two kinds of heterojunctions. The controllable CQD embedded CN nanoframes possess two kinds of heterogeneous interfaces within seamlessly stitched microarea two-dimensional in-plane and out-of-plane domains, which can effectively enhance its intrinsic driving force in different directions to accelerate the separation and transfer of charge. (b−d) CN-CQDs-40. (e) UV/vis absorption spectra and band gap energies (inset) of CN, CNs, CN-CQDs-20, CN-CQDs-40, and CN-CQDs-100. (f) Mott−Schottky plots for CN, CNs, CN-CQDs-20, CN-CQDs-40, and CN-CQDs-100 with 1000 Hz frequencies. (g) Band structure of alignments for CNs, CN-CQDs-20, CN-CQDs-40, and CN-CQDs-100. Reprinted with permission from ref 782. Copyright 2020 Royal Society of Chemistry. Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX AX
- 52. based frameworks (CTFs), covalent organic frameworks (COFs), and planarized-fluorene-based conjugated polymers have been investigated, some of which display photoactivity higher than molecular and carbon nitride photocatalysts.792,793 The pioneering work by the Cooper group on pyrene based conjugated microporous polymers (CMPs) synthesis using Suzuki Miyaura polycondensation demonstrates that the band gap of CMPs can be tuned in the whole spectrum just by varying the precursor stoichiometric ratio and polymerization degree.794 Contemporary development by the Wang group demonstrates the insertion of electron output tentacle dibenzothiophene-S,S-dioxide units in pyrene based donor− acceptor CPs reaching AQYs as high as 8.5% at 420 nm.795 Considering the 2D planarity and surface properties such as high surface area and tunable optical band gap, covalent organic frameworks (COFs) and covalent triazine frameworks (CTFs) are more successful candidates for photocataly- sis.796−798 Various synthetic approaches, chemical attributes, and photophysics of polymer photocatalysts are magnificently summarized in previous reports.799,800 Thomas et al. demonstrated TAPD-(Me)2 and TAPD-(OMe)2 COFs prepared by condensation of N,N,N′,N′-tetrakis(4-amino- phenyl)-1,4-phenylenediamine with 2,5-dimethylbenzene-1,4- dicarboxaldehyde/2,5-dimethoxybenzene-1,4-dicarboxalde- hyde ((OMe)2, which produces H2O2 from water at a rate of 22.6 μmol/16 h.801 Luo et al. demonstrated that when imine based COFs are grown on carbon nitride sheets (CNS) via an in situ approach, the CNS-COF heterostructure can reach HER as high as 9.1 mmol h−1 g−1 with an associated AQY 31.8% (425 nm).802 Such improved performance was attributed to surface passivation of CNS by utilization of the residual −NH2 group on CN in imination. As evident from the enhanced EPR signal, the CNS-COF assembly can achieve a better charge separation. CTFs with high nitrogen content and compositional similarity (CNxHy) with carbon nitride are more appealing for photocatalytic applications. CTFs are generally prepared by molten salt or low temperature coupling routes. Tang et al. demonstrated CTF-0 with relatively high N content synthesized by a microwave-assisted heating route in a stacked AB-fashion compared to the ionothermal approach (AA stacking) that positively influenced band edge alignments to benefit HER and OER.803 Similarly, the 2D/2D heterojunction Figure 33. Chemical structures of potential polymeric materials to form the vdW heterojunction with g-C3N4: (a) C3N3, (b) C3N, (c) C2N, (d) C4N3, (e) C3N2, (f) azo-linked C3N5, (g) triazole containing C3N5, (h) melem/PMDA carbon nitride polydiimide CN/PDI, and (i) 2D/2D heterostructure of CN:PDI/graphene. (a) Adapted with permission from ref 810. Copyright 2020 Wiley-VCH. (b) Adapted with permission from ref 811. Copyright 2016 National Academy of Sciences. (c) Adapted with permission from ref 812 by Mahmood et al. under the terms of the Creative Commons Attribution 4.0 International License (CC BY) (https://creativecommons.org/licenses/by/4.0/). Copyright 2015 Mahmood et al. (d) Adapted with permission from ref 813 by Zhou et al. under the terms of the Creative Commons Attribution 4.0 International License (CC BY) (https://creativecommons.org/licenses/by/4.0/). Copyright 2018 Zhou et al. (e) Adapted with permission from ref 814. Copyright 2021 Elsevier. (f) Adapted with permission from ref 73. Copyright 2019 American Chemical Society. (g) Adapted with permission from ref 815. Copyright 2018 Wiley-VCH. (h) Adapted with permission from ref 706 Copyright 2014 Wiley-VCH. (i) Reprinted with permission from ref 707. Copyright 2016 American Chemical Society. (j) DFT calculated 2D/2D heterojunction between C2N and g-C3N4. Reprinted and modified with permission from refs 77 and 817. Copyright 2018 Royal Society of Chemistry and Copyright 2016 Wiley-VCH, respectively. (k) Chemical structure of CTF-1. Adapted with permission from ref 819. Copyright 2021 American Chemical Society. (l) 2D/2D heterojunction of CTF-1/g- C3N4. Adapted with permission from ref 804. Copyright 2020 Elsevier. Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX AZ
- 55. between polymer semiconductors and carbon nitrides can be realized for mutually benefitted interaction including band edge modulation, enhance visible absorption, and charge separation (Figure 33). Cao et al. demonstrated the synthesis of an all organic 2D/2D heterojunction between amine- functionalized graphitic carbon nitride (GCN) nanosheets (CNNS) and carboxyl rich CTF nanosheets (CTFNS) via electrostatic interaction.804 Acid-assisted exfoliation of CTFs not only provides CTFNS but also overcomes the size requirement for efficient interaction, resulting in improved photocurrent generation and 95.8% removal efficiency for sulfamethazine. Isolated heptazine or triazine-based carbon nitride struggle with the issue of charge separation due to the absence of charge collection sites. Coupling triazine and heptazine units together in the same carbon nitride framework was found to solve this issue by the formation of a donor− acceptor network. Zhang et al. displayed that changing the LiCl/KCl with NaCl/KCl in the ionothermal molten salt method led to a diversion of the polymerization process due to the high melting point of the NaCl/KCl mixture and afforded triazine−heptazine based carbon nitride.805 The HER for CN- NaK was found to be 278 mmol h−1 with an AQE of 32% (at 420 nm). Molten salt assisted synthesis in the presence of LiCl/KCl usually produces crystalline polytriazine imides (PTI) in which two triazine units are connected with the −NH− group; however, 5-aminotetrazole precursor under identical conditions leads to formation of polyheptazine imides (PHI).806 CN is not considered as a good water oxidation catalyst due to less positive VB, limiting its application in high oxidation potential demanding reactions such as H2O2 formation. Polydiimides (PDI) synthesized by coupling of anhydrides and melem/melamine units have more positive valence bands than have been widely explored for H2O2 generation. Apart from the previously mentioned melem- PMDA based PDI, several other substituents such as naphthalene dianhydride, biphenyl tetracarboxylic dianhydride (BTCDA), perylene dianhydride, mellitic trianhydride with three coordination sites, melamine, etc. have been used for PDI synthesis.703,705,807 Taking advantage of the layered structure of the CN/PDI polymer, a 2D/2D hybrid with graphene was prepared which can photocatalyze water oxidation to H2O2 at a high rate, reaching a solar to hydrogen (STH) efficiency of 0.20%.707 When a high CB position containing carbon nitride with less positive VB is integrated with CN/PDI, usually a solid-state Z-scheme is the preferred mechanism of charge separation, leading to enhanced kinetics of overall water splitting. For example, Miao et al. demonstrated a perylenetetracarboxylic diimide (PDI) and carbon nitride Z-scheme heterojunction that can reach an H2 evolution rate of g-C3N4/PDI (1649.93 μmol g−1 h−1 ), which is 2.03 times higher than that of the g-C3N4 nanosheet (814.03 μmol g−1 h−1 ).808 In another work, benzo[ghi]- perylenetriimide/graphitic carbon nitride (BPTI/g-C3N4) synthesized by N-amidation reaction displayed enhanced RhB degradation in a direct scheme mechanism.809 With the advent of new 2D polymeric semiconductor materials such as C2N, C3N, C3N2, C3N3, C4N3, C3N5, C3N6, etc., the choices of fabrication of 2D/2D vdW heterojunctions are expanding (Figure 33, Table 10).73,810−816 For example, distinct from traditional six-member ring carbon nitride, a novel five- member ring (imidazole) containing carbon nitride with C3N2 stoichiometric composition and C−C bridging coordi- nation can demonstrate a band gap as low as 0.81 eV and is employed for PEC biosensing applications.814 Apart from photo/photoelectrochemical applications, the new band gap tuned carbon nitride containing 2D/2D heterojunctions will find applications in other fields including optoelectronic device, FET, LEDs/OLEDs, organic solar cells, etc. As CTF, PTI, and PHI iso-element conjugates (CxNy), there exists no distinct boundary. While generally referring to carbon nitrides, depending upon their coordination (bridging N in CN, fused benezenic ring), C/N content, and structural similarity, they can be categorized as graphene type (low band gap) and carbon nitride type structures (moderate band gap). For example, C2N, also called nitrogenated holey graphene, has a direct band gap of 1.96 eV.812 Theoretical studies using DFT suggests C2N stacking on carbon nitride sheets forms a direct scheme type-II heterojunction with suitable band edge positions for water splitting.817 Sadly, most of such semi- conductor heterojunctions with g-C3N4 are just reported based on the theoretical calculation, and more work is needed in this direction. 2D conjugated metal complex polymers such as a Schiff base polymer synthesized by reaction of tert-amino functionalized porphyrin and 2,5-dihydroxyterephthalaldehyde, which demonstrated almost 10 mA cm−2 current density, might also be explored for such applications.818 12. MISCELLANEOUS 2D/2D vdW STRUCTURES Apart from the above-mentioned materials, several other layered materials such as Ni(OH)2, Mo2N, montmorillonite, cobalt phosphide (CoP), UNiMOF, MgFe MMO, and In2Se3 MO2C have been reported, forming a 2D/2D vdW heterojunction with carbon nitrides to improve the photo- catalytic performance (Table 11).821−829 Among them, the large band gap hexagonal boron nitride (h-BN) is worth mentioning.830−832 The h-BN possesses a 2D graphene-like structure, an excellent chemical stability, a high thermal conductivity, and a melting point which makes it suitable for various applications including lubricants to the high surface area supporting materials. However, due to a significantly high band gap (5.5 eV), it is among the less explored 2D materials in photocatalytic application. Though from the point of visible light collection, h-BN does not fit in visible light mediated photocatalysis but essentially provides the large oxidation potential necessary for the oxidation of various recalcitrant pollutants and water oxidation.833,834 Further, like graphene, the charge distribution on the h-BN surface can be manipulated by the formation of a 2D/2D interface. Indeed, various 2D/2D interfacial catalysts amalgamated with h-BN and inorganic semiconductors have been reported. BN can form excellent lattice matched stacking with CN due to analogous structure followed by favorable charge redistribution in the close-packed CN-BN heterostructure. Besides band gap modulation, the high electronegativity of BN compared to CN facilitates efficient hole collection from the CN to accelerate rate-limiting oxidation kinetics.835 In the CN-BN 2D/2D host−guest structure, due to the electron-rich and deficient pattern, a donor−acceptor relationship can be established while close 2D/2D interfacial contact will minimize the recombination losses. In a study, Tu et al. reported the synthesis of h-BN and g-C3N4 heterojunction by a thermal recrystallization process using diluted aqueous HNO3 at 180 °C in an autoclave.836 Unexpectedly, the visible absorption of h-BN/g-C3N4 was found to be higher than that of the carbon nitride, suggesting an electronic charge redistribution. The band gap with 40% BN containing material was shifted to 2.44 Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX BC
- 56. eV compared to 2.70 eV for the pristine CN material that resulted in 99% degradation of acid red in 90 min under UV− vis irradiation. Further, the doping of BN with carbon, which provides extra electrons in the π-conjugated system, resulted in narrowing the band gap value.837,838 The band gap value can be tuned by controlling the amount of carbon doping in the BN lattice, and frequently it is referred to as boron carbon nitride. In recent work, metal-free 2D/2D carbon nitride/C- doped BN (CN/BCN) van der Waals (VdW) heterojunctions were prepared where the BN due to significant C doping has a smaller band gap than CN with less electronegative behavior.830 In such cases, a Z-scheme mechanism was preferred where electrons from the CB of CN were recombining with holes of BCN. The combination of CN/ BCN was able to afford an astonishing HER (3357.1 μmol h−1 g−1 ) with an associated AQE of 16.3% that was much higher than single CN (1298.8 μmol h−1 g−1 ) under the visible light. 13. CONCLUSIONS AND FUTURE PERSPECTIVES The development of a photocatalyst with sufficient visible absorption, better electron−hole separation, and a long lifetime to actuate a reaction before the annihilation of charges are a few key factors that will decide the future of the photocatalysis field. Almost 48 years have passed since the first photoelectrode promoting water splitting using energy from light was observed. Significant progress has been made to demonstrate the potential of photocatalysis to solve energy and environmental issues. Hitherto, no photocatalyst materials exist which can sustain the oxidation and reduction reaction at both ends of their band edges. Indeed, increasing visible absorption and galvanizing both reactions are paradoxical as one can be attained at the cost of sacrificing the other. Another ultimate challenge is to chain the electrons and protons derived from the oxidation reaction at the valence band with the reduction reaction at the conduction band. Heterojunction formation between two semiconductors envisaged solving these problems by harvesting more light without sacrificing the redox power of the catalysts. Particularly, the Z-scheme and S-scheme heterojunction constituted of two different reducing and oxidizing catalytic components has shown great promise. However, a significantly large number of traveling charge carriers trying to reach another semiconductor gets to recombine in the bulk and at the epitaxially mismatched interface. 2D materials, due to their large specific surface area available for maximum effective interaction, numbers of the active site, and excellent electronic mobility, found a specific place in the photocatalysis field. Fabrication of the 2D/2D heterojunction using two different semiconductors not only provides benefits of conventional heterojunction such as synergistic absorption and large band potential difference but also overcomes the issue of charge separation due to effective interaction between two interfaces and angstrom to nano regime travel distance between 2D sheets. With the advancement of materials genomics, numerous new 2D semiconductor/conductor materials have been developed that can be easily exfoliated in the monolayer to few-layered sheets. The past few years have witnessed the evolution of many resilient and effective 2D/2D heterointerface photocatalysts showing a 20- to 200- fold increment in the performance for many photocatalytic applications. Still, the efficiency is far from a realistic use due to the presence of the defect state, the requirement of specific plane matching from various permutations for effective charge transport, limited electronic mobility, etc. The carbon nitride-based 2D/2D heterojunction is giving hope as g-C3N4 possesses a suitable band structure and electronic mobility, and 2D electron-rich sheets can interact with almost any semiconductor to form a vdW heterostructure. New 2D materials such as phosphorene, antimonene, tellurene, transition metal oxides, dichalcogenides, LDHs, etc., due to their unique properties, are expanding the choice of the materials to fabricate an optimized vdW heterostructure. Unfortunately, nonchanneled bidirectional charge transport between two 2D sheets results in colossal carrier recombina- tion on the second semiconductors. Tangible advancements to channel the charge transport between 2D sheets was achieved by intercalation of alkali metals (K+ ) and noble metals (Ag), which provide an interlayer gallery and, in some cases, better light absorption too.697 The directionality of charge flow can be controlled by coupling “adjuster” atoms in the system.702 We observed that, in most of the cases, only a modest fraction of 2D sheets were present in the 2D/2D state, divergent from ideally represented schematics which might be another reason for lower performance than expected. Ideally, 2D/2D heterojunctions should be the more efficient catalyst; unexpectedly, lateral heterojunction triumphed on some occasions due to better charge separation in conductive domains present in high precision.713,714 Unfortunately, pristine carbon nitride synthesized at high temperature has some inherent drawbacks such as limited blue photon excitation, insolubility in most of the solvents, undisciplined polymerization, lack of long-range crystallinity, low electronic mobility, and uncondensed hydrogen-bonded fragments work- ing as trap centers. Most of the reported 2D/2D vdW heterojunction catalysts utilized conventional carbon nitride and concomitantly inherit the bottlenecks of regular carbon nitride, and the reported yield is still in the micromole regime. Considering the future development of the 2D/2D vdW heterostructure, switching to new carbon nitride-based materials is essential to fully cultivate the benefits of 2D/2D configuration. Molecular engineering by doping (nonmetal, alkali metal, and single atoms), alteration of the coordination pattern (bridging N, C atoms, or azo linkages), insertion of N- rich units such as triazole units, and replacing the basic triazine/heptazine units with new construction units are some fundamental strategies to advancing the intrinsic physicochem- ical properties of carbon nitrides.839,840 Several new variants of the CxNy family with an entirely different stoichiometric C:N ratio such as C2N, C3N, C3N2, C3N3, C4N3, C3N5, and C3N6 and novel photophysical behavior have been synthesized in recent years to conquer the drawbacks of conventional C3N4.810,813,814,841 Another grueling issue with carbon nitride-based 2D/2D heterojunctions is the indigent surface adsorption of the reactant and poor adsorption−desorption kinetics. Surface engineering of carbon nitride by introducing certain functional groups/units with a high affinity for reactants can solve this problem. C3N5 with two six-membered ring triazines and one five-membered ring triazole due to the presence of basic N on the triazole unit can virtually adsorb CO2, while the presence of a suitable band gap promotes on- site photoreduction.842 Two closely packed flat 2D sheets with a differential band gap is an ideal arrangement for efficient 2D/ 2D heterojunction to ensure flawless charge flow from one semiconductor to another. The faulty condensation due to cross-linking and intersheet hydrogen bonding in CN disrupts Chemistry of Materials pubs.acs.org/cm Review https://doi.org/10.1021/acs.chemmater.1c03166 Chem. Mater. XXXX, XXX, XXX−XXX BD
- 57. the periodicity/crystallinity, resulting in a poorly interacted heterojunction. Thus, maintaining crystallinity in carbon nitride will ensure uninterrupted charge migration on the CN surface. Molten salt (LiCl/NaCl/KCl) assisted ionother- mal synthesis has proven to be a promising approach for the synthesis of crystalline CN and PTI polymers but needs further improvement due to the associated disadvantage of pressurized reaction conditions, surface contamination, >NH bridging coordination, and undesirable doping.92,843 Based on current knowledge, we can predict the ideal 2D/2D photocatalyst design constituted of two semiconductors with sufficient oxidative and reductive band edges coupled with intercalated atoms and adjustors for directional charge flow. Additionally, the fabrication of the 2D/2D heterojunction between two lateral heterojunction sheets containing a conductive (gra- phenic or carbonaceous) zone in a manner that the conductive zone of one sheet is facing the semiconductive part of the other sheets will ensure the efficient charge capture and separation. We believe that the present report will encourage the photocatalytic community to gain a current understanding of the field and excel in the knowledge to develop resilient and sustainable photocatalysts for future applications. ■ AUTHOR INFORMATION Corresponding Authors Pawan Kumar − Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, Alberta T2N 1N4, Canada; orcid.org/0000-0003-2804-9298; Email: [email protected] Ajayan Vinu − School of Engineering, College of Engineering, Science and Environment, The University of Newcastle, Callaghan, New Sourth Wales 2308, Australia; orcid.org/ 0000-0002-7508-251X; Email: ajayan.vinu@ newcastle.edu.au Jinguang Hu − Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, Alberta T2N 1N4, Canada; orcid.org/0000-0001-8033-7102; Email: [email protected] Md. Golam Kibria − Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, Alberta T2N 1N4, Canada; orcid.org/0000-0003-3105-5576; Email: [email protected] Authors Devika Laishram − Department of Chemistry, Indian Institute of Technology Jodhpur, Jodhpur, Rajasthan, India 34201; Present Address: (D.L.) School of Chemical and Bioprocess Engineering, University College Dublin, Belfield, Dublin 4, Ireland; orcid.org/0000-0001-6953- 8309 Rakesh K. Sharma − Department of Chemistry, Indian Institute of Technology Jodhpur, Jodhpur, Rajasthan, India 34201; orcid.org/0000-0002-0984-8281 Complete contact information is available at: https://pubs.acs.org/10.1021/acs.chemmater.1c03166 Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS The authors would like to thank the Department of Chemical and Petroleum Engineering in the Schulich School of Engineering and the University of Calgary CFREF fund for financial assistance. ■ REFERENCES (1) Rao, N. D.; Pachauri, S. Energy Access and Living Standards: Some Observations on Recent Trends. Environ. Res. Lett. 2017, 12 (2), 025011. (2) Iyer, G. A Decent Life. Nat. Energy 2019, 4 (12), 1010−1011. (3) International Energy Agency, IEA. Key World Energy Statistics 2020; IEA: Paris, 2020; https://www.iea.org/reports/key-world- energy-statistics-2020 (accessed 2021-10-05). 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