CN CATALYSIS

This journal is © The Royal Society of Chemistry 2023 Chem. Soc. Rev.
Cite this: DOI: 10.1039/d3cs00213f
Multifunctional carbon nitride nanoarchitectures
for catalysis
Prashant Kumar, *a
Gurwinder Singh,a
Xinwei Guan,a
Jangmee Lee,a
Rohan Bahadur, a
Kavitha Ramadass,a
Pawan Kumar,b
Md. Golam Kibria, b
Devthade Vidyasagar, c
Jiabao Yia
and Ajayan Vinu *a
Catalysis is at the heart of modern-day chemical and pharmaceutical industries, and there is an urgent
demand to develop metal-free, high surface area, and efficient catalysts in a scalable, reproducible and
economic manner. Amongst the ever-expanding two-dimensional materials family, carbon nitride (CN)
has emerged as the most researched material for catalytic applications due to its unique molecular
structure with tunable visible range band gap, surface defects, basic sites, and nitrogen functionalities.
These properties also endow it with anchoring capability with a large number of catalytically active sites
and provide opportunities for doping, hybridization, sensitization, etc. To make considerable progress in
the use of CN as a highly effective catalyst for various applications, it is critical to have an in-depth
understanding of its synthesis, structure and surface sites. The present review provides an overview of
the recent advances in synthetic approaches of CN, its physicochemical properties, and band gap
engineering, with a focus on its exclusive usage in a variety of catalytic reactions, including hydrogen
evolution reactions, overall water splitting, water oxidation, CO2 reduction, nitrogen reduction reactions,
pollutant degradation, and organocatalysis. While the structural design and band gap engineering of
catalysts are elaborated, the surface chemistry is dealt with in detail to demonstrate efficient catalytic
performances. Burning challenges in catalytic design and future outlook are elucidated.
a
Global Innovative Center for Advanced Nanomaterials, College of Engineering, Science and Environment (CESE), The University of Newcastle, University Drive, Callaghan,
2308, NSW, Australia. E-mail: Prashant.Kumar@newcastle.edu.au, Ajayan.Vinu@newcastle.edu.au
b
Department of Chemical and Petroleum Engineering, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada
c
School of Material Science and Engineering, Kyungpook National University, Daegu, 41566, Republic of Korea
Prashant Kumar
Prashant Kumar is presently a
senior lecturer at the University
of Newcastle, Australia. He got
his PhD in Physics in 2009 and
worked with top-ranking scien-
tists including Prof. C. N. R. Rao
(JNCASR/IISC Bangalore), Prof.
T. S. Fisher (UCLA) and Prof.
Gary J. Cheng (Purdue University).
He has explored the evolution of
crystallographic phases of quantum
materials under exotic thermo-
dynamic conditions. He has
exploited these advanced quantum
materials as well as their doped and hybrid versions in various
frontline applications in electronics, optoelectronics, spintronics, gas/
molecular/strain/light ultrafast sensing, electronic cooling, brain–com-
puter interface, energy generation/storage and catalysis.
Gurwinder Singh
Gurwinder Singh is working as a
Research Fellow at the Global
Innovative Centre for Advanced
Nanomaterials (GICAN), the Uni-
versity of Newcastle, Australia.
He received his PhD degree in
Materials Science (2018) under
the supervision of Prof. Ajayan
Vinu. After completing his PhD,
he joined Prof. Vinu’s research
group at GICAN as a research
associate and recently got pro-
moted to Research Fellow. His
current research interests include
the design and development of micro/mesoporous materials for
carbon capture/conversion, energy storage, and various environ-
mental applications.
Received 12th May 2023
DOI: 10.1039/d3cs00213f
rsc.li/chem-soc-rev
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Chem. Soc. Rev. This journal is © The Royal Society of Chemistry 2023
1. Introduction
Catalysis plays a significant role in the production of chemicals,
fuels and energy which are required to fulfil the day-to-day
needs of modern society.1–3
For example, catalysis is heavily
involved in 480% of manufactured products in chemical
industries and refinery-related catalysis has a global market
of 43.5 billion USD nowadays, which enormously contributes
to the global economy. Keeping in mind the exponential surge
in energy demand per capita, to provide next-generation citi-
zens with fossil fuel-free renewable green energy for the crea-
tion of a circular economic system, the role of catalysis is
crucial as it can provide a powerful platform for the production
of clean energy through sustainable pathways,4–6
Therefore, the
industry and research community are vigorously looking out for
highly efficient and economically suitable catalytic processes
that require highly active, stable, and low-cost catalysts. In most
of the existing catalytic processes, including oxygen/hydrogen
evolution reactions (for green energy),7
the oxygen reduction
reaction,8
water oxidation, nitrogen reduction, CO2 reduction
(for generating renewable fuels),9–11
and single atom catalysis,12
various catalytic materials (primarily metallic or metalorganic)
have been developed and implemented over the last few decades.
However, these materials are costly and suffer from various
Xinwei Guan
Xinwei Guan is currently a Post-
doctoral Fellow at the Global
Innovative Centre for Advanced
Nanomaterials (GICAN), the Uni-
versity of Newcastle. He received
his PhD degree in Materials
Science & Engineering from the
University of New South Wales
(UNSW, Australia) in 2021 and
M.S. degree from King Abdullah
University of Science and Tech-
nology (KAUST, Saudi Arabia) in
2017 under the supervision of
Prof. Tom Wu. His research
focuses on halide-perovskite-based optoelectronics, including non-
volatile memories, transistors, and photodetectors. His research
activities also include energy applications like solar cells,
photothermal applications, and catalysis.
Pawan Kumar
Pawan Kumar is presently work-
ing at the University of Calgary,
Canada. He has 5 years of re-
search experience in the fabri-
cation and development of new
semiconductors, 2D materials,
and hybrid catalysts using wet/
solid-state synthesis for electro-
catalysis, photocatalysis, organic
synthesis, optoelectronics and
photovoltaics. He has worked on
modulation of electronic band-
gap, defect engineering, surface
functionalization, molecular/
atomic level control on catalysts design and reactor design. His
research eﬀorts are reflected by ca. 3700 citations and an H-
index of 37.
Jiabao Yi
Jiabao Yi is an associate professor
in the Global Innovative Centre
for Advanced Nanomaterials,
School of Engineering, University
of Newcastle, Australia. His
research is focusing on oxides
and oxide-based magnetic semi-
conductors, 2D materials and
their magnetic properties, soft
and hard magnetic materials,
magnetic nanoparticles for bio-
applications as well as advanced
techniques, such as X-ray absorp-
tion spectroscopy, X-ray magnetic
circular dichroism (XMCD), neutron scattering and neutron
reflectometry as well as muon spin relaxation, demonstrated by
more than 200 original research papers in high impact factor
journals with ca. 12 400 citations and an H-index of 59.
Ajayan Vinu
Ajayan Vinu is the Global Inno-
vation Chair and Director at the
Global Innovative Centre for
Advanced Nanomaterials, the
University of Newcastle. In his
20 years of research, Prof. Vinu
has made tremendous contribu-
tions in the field of nanoporous
materials and their applications
in sensing, energy storage, fuel
cells, adsorption and separation,
and catalysis. His contribution to
the field of nanoporous materials
is also clearly reflected by 520
original research papers in high-impact factor journals with ca.
31 500 citations and an H-index of 90. He was selected as an
academician and fellow of the World Academy of Ceramics, World
Academy of Arts and Sciences, and Asia-Pacific Academy of
Materials, and is a Fellow of the Royal Society of Chemistry, the
Royal Australian Chemical Institute, and the Maharashtra
Academy of Science.
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drawbacks such as poor stability and not being eco-friendly.13,14
These concerns prompt the catalysis-specific scientific community
to think of minimizing the use of metals in catalysts and optimis-
ing synthesis methods to accomplish them in a scalable, repro-
ducible, and economic manner.
Ever since the discovery of graphene in 2004, various classes
of two-dimensional (2D) materials have been developed, such
as elemental atomic sheets (Xenes), hexagonal boron nitride
(hBN), transition metal dichalcogenides (TMDCs), 2D metal
oxides (2DMOs), metal carbides and nitrides (MXenes), etc.,
and the family of 2D materials is still evolving, creating a new
landscape and further promising limitless scientific and tech-
nological manoeuvres (Fig. 1).15–50
In contrast to other existing
2D materials, carbon nitride-based materials are metal-free
organic semiconductors with tunable band gaps. A variety of
molecular structures can be achieved with different x and y
values for the CxNy system. Depending on the nitrogen content,
each structure with a different electronic band structure and
hence energy band gap value can be realised. The tunability of
the band gap and the nature of the band gap are crucially
important as they determine how the material responds to
incident light and also participates in chemical reactions. Thus,
carbon nitride (g-CN) is well-suited for catalysis, thanks to its
unique properties such as tunable basicity and band gaps, good
thermal, chemical and mechanical stability51
and its unique
molecular structure with high nitrogen content that offers extra
electrons for catalysis. The number of active sites in CN can
also be tuned by introducing ordered porosity or increasing the
nitrogen content in the framework structure, which is benefi-
cial for many catalytic reactions.52,53
In addition, g-CN can offer
a unique platform for heteroatom doping or hybridization with
other 2D nanostructures, which can provide a great opportunity
to tune its band structure by various means.54–57
g-CNs also have electronic charge instability, generating
enormous orbital strain in the hexagonal planar phase, render-
ing them unstable and resulting in numerous crystallographic
phases (C1N1, C2N, C3N, C2N, C3N, C3N4, C2N3, C5N2, etc.)
(Fig. 2 left panel).58–60
These g-CNs can be prepared by a simple
thermal polymerization of nitrogen-containing organic pre-
cursors such as urea, thiourea, cyanamide, guanidine
hydrochloride, melamine, etc. However, these materials suffer
from low specific surface area, large band gap and fast recom-
bination of charge carriers, which limit their performance in
electro- and photocatalysis. Therefore, various synthetic strate-
gies have been developed to develop highly efficient g-CN-based
nano-catalysts. Fig. 3 summarizes different synthetic strategies
or protocols developed for altering the catalytically active sites
of g-CN-based 2D materials. For example, the textural proper-
ties of these g-CNs can be fine-tuned by varying the synthesis
strategies including templating assisted, salt-molding, self-
assembly process, etc. For example, charge instability and
polymorphism in g-CNs can suitably be exploited to attain
designer shapes/nanoarchitectures for various catalytic, sen-
sing, adsorption and separation and electrochemical applica-
tions. Such leverage of versatile material designs, in addition to
edge N-atoms with extra electrons, has inspired world leaders
in catalysis to implement g-CN systems for various catalytic
reactions relevant to the energy sector and pharmaceutical
industries. In addition, with proper defect engineering, mole-
cular modification, doping, and heterostructure coupling with
other 2D nanostructures, the surface chemistry of g-CNs includ-
ing surface area, charge-recombination rate, and band gaps,
which are key parameters that dictate the performance of the
designed g-CN materials for photo- and electro-catalytic appli-
cations, can be fine-tuned.61
g-CNs can also be molded into
various morphologies/nanoarchitectures with suitable physical/
chemical processing,62–64
as demonstrated in Fig. 2 right panel.
Various crystallographic structures of g-CN polymorphs with
high nitrogen contents and molecular structures exhibit differ-
ent electro- and photo-catalytically active sites owing to the
presence of distinct electrostatic environments and electronic
band structures.65
In these high N-containing g-CN materials,
defects and strain play a crucial role in structure determination
and hence their physical/chemical character.65–70
It should be
noted that the electrostatic charge localization at bond termi-
nation sites in these novel g-CNs acts as the primary source
responsible for catalytic activities. Therefore, the knowledge
and thorough understanding of rational design of these g-CN-
based nanostructures are highly critical as they could provide
efficient routes for designing and developing highly efficient
g-CN-based nanostructures for various catalytic applications.
There are several reviews covering the use of g-CNs in
electrochemical devices, photocatalytic hydrogen evolution
reactions (HERs), phase-specific catalytic applications, CO2
reduction to fuels, and so forth.71–75
However, none of these
reviews provides an outlook on the usage of g-CN in catalysis in
a broader sense. In this review, we aim to provide a compre-
hensive overview of the use of 2D g-CN-based nano or hetero-
structures in various classes of catalysis, providing a complete
picture of the recent developments in this ever-emerging dis-
cipline, as shown in Fig. 3. The emergence of 2D flatland vis-a-
vis the unique role of CN has been elaborated. In particular, we
discuss the evolution of g-CN in detail, including synthesis,
characterization, and physico-chemical properties to provide a
complete background of these unique material systems. In the
following section, we systematically describe and elaborate
Fig. 1 Schematic showing a calendar of band gaps (eV) of CNs vis-à-vis
other 2D materials.
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various manoeuvres, including defect engineering, band
gap engineering via heteroatom doping, hybridization
with various Xenes, TMDCs, MOFs, etc., functionalization,
metal nanoparticle decoration, and sensitization, aiming at
efficient excitation and prevention of recombination, so that
generated excitons can exclusively be exploited for catalytic
purposes.
Then, we provide a detailed summary of the use of g-CN for
various catalytic purposes in which nitrogen reduction reac-
tions, HER, water oxidation reactions, overall water splitting,
CO2 reduction, pollutant degradation, and organocatalysis are
just a few to count with. Then, we provide a detailed account of
the advantages of g-CN-based hybrids and their improvement
in various catalytic reactions. Finally, we provide a detailed
summary in the conclusion section together with some insight
into the recent developments of g-CN-based materials and
further present the existing challenges and future outlook.
2. Synthesis and characterization of
CNs
g-CNs with abundant porous structures have been successfully
synthesized in the past few decades.71,76–79
In general, nitrogen-
containing organic molecules are used as precursors for the
fabrication of g-CNs with different nitrogen contents. Various
top-down and bottom-up synthesis strategies, including solvo-
thermal, chemical vapor deposition (CVD), pulse vapor deposition
(PLD), sputtering, thermal polycondensation, chemical and physi-
cal exfoliation, microwave, sonochemical, self-assembly, salt mold-
ing, and templating, have been developed.80–84
Indeed, thermal
decomposition and recondensation of various nitrogen-containing
molecules, such as cynamide, dicynamide, melamine, thiourea,
and urea, guanidine hydrochloride, guanidine thiocyanate and
modified nitrogen precursors such as sulfuric acid treated mela-
mine, sulphur mixed melamine at 450–700 1C under air/nitrogen/
argon atmosphere end up with the formation of g-CN sheets.85,86
Most importantly, the nature of the precursors plays a significant
role in controlling the surface area, chemical structures, and band
gap of the g-CN materials. For example, the use of non-aromatic
precursors (urea or thiourea) results in more surface defects
than the materials prepared with cyclic CN precursors (amino-
triazine, melamine etc.) Interestingly, thermal treatment of the
urea molecules at 550 1C in a muffle furnace at a rate of 5 1C
min1
for 2 hours can offer g-CNs with a large band gap and
higher surface area whereas thiourea gives g-CNs with a
smaller band gap with lower specific surface area.87,88
Among
these techniques, microwave synthesis is rather nascent yet in
the synthesis of 2D materials, and the synthesis of g-CN
has been attempted by direct exposure of microwaves to
nitrogen-containing organic precursors.89,90
Nevertheless,
g-CNs prepared via these conventional pyrolysis or microwave
approaches suffer from poor crystallinity with a high density of
defect sites which significantly affect the electronic band
Fig. 2 Varieties of atomic structures and morphologies/nanoarchitectures of CNs62,63
(Copyright 2020 Elsevier, Copyright 2014, American Chemical
Society).
Fig. 3 A schematic of 2D CN-based nanostructures in a wide range of
catalysis fields.
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structure and band gap, which ultimately affects its activity,
mostly in electro- and photocatalysis.
Consequently, various advanced synthesis routes, including
CVD and PLD, have been given much attention to developing
g-CNs with high crystallinity. For instance, CVD is a well-known
technique for high-purity graphene deposition, and it has been
used widely for the growth of g-CNs on diﬀerent substrates.80
By varying the fabrication conditions, the defects in the formed
g-CN can be easily controlled. Sputtering is yet another com-
mon technique used for the fabrication of g-CN nanostructures.
Indeed, radio-frequency magnetron sputtering and ion beam
sputtering techniques have been successfully employed for
g-CN growth on flat substrates.81,82
The PLD of 2D materials
is still catching up and has been extensively used for the
preparation of g-CN-based thin films in a nitrogen atmosphere
(nitrogen pressure 7.5 103
Pa and Q-switched Nd: YAG laser
with the second (532 nm) or third (355 nm) harmonic).83
Nitrogen ion-assisted PLD is yet another way to accomplish
it.84
Various synthesis strategies for g-CNs and their various
morphologies are summarized in Fig. 4. Among these methods,
the vapour deposition methods (sputtering, PLD, CVD) involve
sophisticated instrumentation and are costly and moreover,
they result in substrate-supported g-CN layers/films, although
they are apt for device fabrication. As the crystallinity is the key
component for device fabrication, thermodynamic and field
(P + T + E) conditions in bottom-up growth need to be
optimised as they determine the crystalline order. For catalysis,
freestanding catalysts are needed; however these can be
achieved via various bottom-up and top-down methods. Var-
ious top-down physical and chemical approaches for the exfo-
liation of g-CN sheets have been developed, such as
sonochemical, solvothermal, thermal oxidation, chemical and
electrochemical methods. The mechanical exfoliation process
may be assisted using a chemical route in acidic/alkaline
media.91,92
It is seen that g-C3N4 may eliminate by-products
like NH3 at 390 1C during the formation of melem and further
may be unstable at temperatures above 600 1C which results in
the evolution of nitrogen and cyanogen fragments.93
Chemical
exfoliation involves an oxidative reaction with a solid mixture of
g-CN with KMnO4 and an acid solution of sulphuric and
phosphoric acid and reaction termination using H2O2. Exfolia-
tion is carried out by ultrasonication. After the exfoliation, the
formed oxygen functionalities are then suitably reduced by
employing reducing agents such as NaBH4. Reduction can also
be achieved by solvothermal or microwave treatment in redu-
cing solvents such as dimethylformamide (DMF). Chemical
exfoliation yields monolayers or few-layers of atomic sheets
with high specific surface areas, which are highly desirable for
catalysis. Bottom-up vacuum-based physical vapour deposition
results in a very high-purity carbon nitride material; however,
the method is very costly. Solution phase bottom-up methods
such as solvothermal synthesis from individual carbon and
nitrogen precursors or from precursors containing both C and
N are also economical, yet need an adequate level of optimiza-
tion of synthetic parameters to obtain high-quality samples.
Top-down synthesis involves a high surface energy solvent
which applies shear force on individual sheets or intercalates
amongst them giving rise to exfoliation of bulk crystals of
carbon nitride. This method is facile, economic, and single-
step. When carried out in a reducing solvent without any extra
chemical reagents, relatively pure samples with minimal sur-
face functionalities are obtained. With this method, a few
grams of mono/few layered samples can be obtained.
2.1. Mesoporous carbon nitride (MCN) by the template
approach
Synthesis of bulk g-CNs aﬀords poorly exposed surface area due
to prodigious stacking and inter-sheets cross-linking resulting
in few active sites that are available for the reaction. Opportu-
nities exist for material manipulation (structure, morphology
and porosity) by changing synthesis parameters such as tem-
perature, precursors, solvent, concentration, activation, and
template (top panel of Fig. 5). The surface properties of
g-CNs, including porosity and morphology, can be improved
by using hard/soft templates, two or more precursors with
gas evolving properties (NH4Cl), salt-templates, or the use of
hydrogen-bonded macromolecular conjugates, etc.94–96
Among
these, hard templates, such as SiO2 nanoparticles, SBA-15, SBA-
16, KIT-6, etc., are widely used to create nanoporous channels
and robust structures (Fig. 5a and b).97,98
The generic scheme
includes pore filling by CN precursors (individually for C, N,
or in the same precursor containing C and N) carbonization
(250–600 1C) within the pores of the hard template, and
template removal after the g-CN nanostructures are formed
within the porous channels.
Mostly, silica-based hard templates are removed by washing
with either dilute HF or hot NaOH solution. Through this
approach, micropores or mesopores can be introduced depend-
ing on the size of the pores and the walls of the templates. For
microporosity, zeolites or zeotype materials are generally used
as templates, whereas ordered porous silica templates with 2D
and 3D structures are employed for introducing well-ordered
mesoporosity in g-CN.9
However, these templates need to be
prepared in advance, and the templates are sacrificed at the
end of the process.
Vinu et al. first introduced the hard template strategy for the
fabrication of the first MCN materials with well-ordered meso-
porous structure through a simple polymerization reaction
between ethylenediamine using 2D mesoporous silica SBA-15
as a hard template. In this synthesis process, calcined SBA-15
was mixed with an ethylenediamine-carbon tetrachloride mix-
ture and refluxed at 90 1C for 6 hours before carbonising the
resultant mixture at 600 1C. After removing the silica template
by HF, MCN material was obtained. The prepared MCN mate-
rial exhibited high specific surface area, large pore volume
and well-ordered uniform mesopores.98
Since then, Vinu et al.
demonstrated the synthesis of various mesoporous/nanopor-
ous triazine/heptazine-based CN with enhanced N/C ratios (4/3
to 7/3) such as MCN-1, MCN-2, MCN-3, MCN-4, MCN-5, MCN-6,
MCN-7, and hetero atom doped MCN for various applications
including electrochemical reactions and transesterification of
b-keto esters of aryl, aliphatic, and cyclic primary alcohols,
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Friedel–Crafts acylation of benzene with hexanoyl chloride,
deacetalization-Knoevenagel.66,76,99–104
Recently, a micro-
porous g-CN with tetrazine units and C3N5.4 stoichiometry
was prepared using USY-zeolite as a template and aminogua-
nidine hydrochloride (AG). The mixture was heated in an oven
at 100 1C for 6 hours followed by another 6 hours at 160 1C.
The polymerisation was done for 5 hours at 400 1C with a
ramping rate of 3 1C per minute in a nitrogen environment. The
resultant carbon nitride material demonstrated improved CO2
adsorption (Fig. 5c).77
In another work, Chen et al. synthesized
a thiophene unit incorporated MCN (MCN–ATCNx) with an
extremely high surface area (200 m2
g1
) by copolymerization
Fig. 4 Summary of various synthetic strategies for g-CNs.
Fig. 5 Hard-templating methods using (a) SBA-15, (b) KIT-6, and (c) USY zeolites for the synthesis of MCNs77,97,98
(Copyright 2021 Wiley, Copyright 2014
American Chemical Society, Copyright 2017 Elsevier).
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of 3-aminothiophene-2-carbonitrile (ATCN) and dicyandiamide
using SiO2 nanoparticles as a hard template.105
The incorpora-
tion of thiophene units in the MCN–ATCNx network down-
shifted the conduction band, and the population of superoxide
radicals was significantly decreased. However, singlet 1
O2 was
the dominant intermediate species catalysing the oxidation of
alcohols to aldehydes.
Soft template methods have also been used to improve the
surface areas, which can be achieved by mixing CNx precursors
with organic surfactants, such as P123, CTAB, and TRITON-X
100, followed by annealing to remove organic surfactants. The
soft-templating approach for the preparation of porous g-CN
was realised by Antonietti et al., who used various organic
surfactants as soft templates and dicyandiamide as CN pre-
cursors and found that Triton X-100 was the best owing to its
high decomposition temperature.106
The low decomposition
temperature of the organic surfactants is the key issue in
obtaining an ordered porous structure in g-CN as most of the
syntheses of g-CN require a temperature higher than 500 1C.
Another issue with the soft-templating approach is the intro-
duction of carbon from the decomposed surfactant which
affects the band gap of the final product. Yan et al. reported
the synthesis of porous g-CN photocatalysts using a non-ionic
surfactant (Pluronic 123) as a soft template and melamine as a
CN precursor and demonstrated its enhanced performance
in photocatalytic hydrogen production.107
For avoiding the
decomposition of the CN matrix, sulfuric acid was added
together with the soft-template and the CN precursor.108
Fan
et al. used this idea and combined the Triton X-100 surfactant
with melamine and sulfuric acid to prepare g-CN with a porous
structure and enhanced surface parameters, which showed
improved photocatalytic activity for the degradation of RhB.
Ionic liquids are also used to avoid the early decomposition of
the surfactant for creating porous g-CN. Paraknowitsch et al.
demonstrated the preparation of porous g-CN using 1-ethyl-3-
methylimidazolium (EMIM) and 3-methy-1-butylpyridine (3-MPB)
as organic cations and dicyandiamide (DCDA) as the anion and
the carbonization was done at 1000 1C.109
The porous CN prepared
through this process showed high electronic conductivity similar
to graphite. The high-temperature carbonization, together with the
ionic liquid, is responsible for the improved electrical properties.
This strategy can also be used for the preparation of hetero-atom
doped g-CN using B, F and P containing ionic liquids. B/F doped
g-CN was prepared by mixing urea with different amounts of
BmimBF4 (1-butyl-3-methylimidazolium tetrafluoroborate), which
were mixed with water and stirred at 80 1C. The final samples were
prepared by calcining the solids in air at 550 1C for 2 h at a rate of
5 1C per minute.110
A combination of hard and soft template
approaches has been found to further improve the surface area of
the materials. For instance, Kumar et al. used a sol–gel mediated
thermal condensation approach for the synthesis of P- and F- co-
doped CN (PFCN) using dicyandiamide, ionic liquid and CTAB
(soft-template) and tetraethyl orthosilicate (TEOS – hard template)
with an exceptionally high Brunauer, Emmett and Teller (BET)
surface area of 260.9 m2
g1
compared to CN (12.0 m2
g1
).111
Due
to the synergistic effect of doping and improved surface properties,
the PFCN displayed excellent performance in the conversion of
monomer sugars into furanic compounds under aqueous condi-
tions. Indeed, soft and hard template-assisted synthesis improves
the surface properties; however, blockage of active sites by residual
silica, increased C:N ratio due to deposition of carbon in micro-
pores, altered active sites due to the use of etchants, and low yield
are the associated drawbacks. The best approach to improve the
surface properties including the degree of polymerization is the
synthesis of hydrogen-bonded macromolecular aggregates using
suitable solvents followed by annealing to get hollow/porous
structures.
2.2. Supramolecular preorganization of CN
Dreams of low-cost, facile synthesis of CN nanosheets (CNN)
can be realized by a low-temperature, aqueous-phase supramo-
lecular preorganization approach, which results in high-surface
area sheets and alleviates the necessity of mesoporous silica
templates. The as-obtained g-CN by this method is formed by
self-assembly and has a well-designed composition. Essentially,
hydrogen bonding helps in shaping of a self-assembled structure.
Several supramolecular assemblies by changing the precursors
(cyanuric acid + melamine + 2,4-diamino-6-phenyl-1,3,5-
triazine,112
barbituric acid + cyanuric acid,113
cyanuric acid +
urea + melamine),114
2,4-diamino-6-methyl-1,3,5-triazine,115
caffeine,116
solvents (water, acetonitrile), and hydrothermal
conditions have been reported to make spherical, nanotube,
nanorod, honeycomb and nanocube morphologies.117–119
Apart
from enhancing the surface area, the use of N-rich molecular
modules enables supramolecular assembly that can further con-
trol the band structures. Amino acids protected with aromatic
9-fluorenylmethoxycarbonyl (Fmoc) have been found to afford 1D
nanofiber morphology while increasing visible absorption.120
In a representative work, Shalom et al. reported the synth-
esis of the cyanuric acid–melamine (CM) complex in ethanol,
which yields a hollow structure under appropriate annealing
conditions, with a BET surface area of 45 m2
g1
.121
In such
synthesis approaches, the pre-organization of the supramolecular
structure is significantly influenced by solvents. For example,
Jun et al. found that CA complex aggregation in dimethyl sulfoxide
(DMSO) gives a spherical hollow structure which is mostly
destroyed at the higher annealing temperature due to the evolu-
tion of NH3.122
Likewise, Wu et al. demonstrated the synthesis of a
unique asymmetric supramolecular precursor using L-arginine
amino acid and melamine. The arginine amino acid and mela-
mine mixture was heated to 500 1C in a tube furnace in nitrogen at
a rate of 3 1C min1
for 90 min, and then washed with dilute
sodium hydroxide solution.123
Due to the presence of a basic
guanidine group on L-Arg, a fraction of melamine can hydrolyze to
cyanic acid. The resulting melamine and cyanuric acid bind with
L-Arg bonds via an amidation reaction to form a supramolecular
precursor (Fig. 6a). The thermal annealing of the asymmetric
supramolecular precursor actiniae-like CN (ACN) gives a specific
surface area of 171 m2
g1
(Fig. 6b and c).
The SEM and TEM images of ACN display hollow sea
anemone-type structures with several micrometre lengths that
are arranged orderly on the core stems (Fig. 6d and e). 13
C ssNMR
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spectra of the conjugate validated the presence of hydrogen
bonding between the precursors, while the NMR pattern was close
to the CN structure after annealing, suggesting that the basic CN
framework remained intact after synthesis. EELS spectra of ACN
display slight shifting in the core energy loss peak due to the
presence of aldehyde groups, while an increased p*/s* ratio
suggests the contribution of the N-rich precursors to the conju-
gated network (Fig. 6f). The optimized sample displayed increased
visible absorption extended up to the red region attributed to the N
2p contribution and introduction of additional energy levels from
CHO groups. The average lifetime for ACN calculated from the
time-resolved PL spectra was found to be 19.2 ns compared to bulk
g-CN (53.12 ns), demonstrating faster charge transfer from s* and
p* to mid-gap energy levels.
2.3. Highly crystalline g-CN (CCN)
Another deleterious component that reduces the performance
of CN-based materials is poor crystallinity due to hydrogen
bonding in the inner sheets between NH/NH2 of uncondensed
heptazine units. These hydrogen-bonded regions work as trap
centres, hindering the facile charge separation and transport.
Conventional thermal polycondensation synthesis of g-CN from
N-rich precursors (e.g., DCDA and urea) results in amorphous
CN. For improving the crystallinity of the g-CN materials,
molten salts or salt melts have been widely used as they play
a crucial dual role as a solvent and a template. One of the major
advantages of these molten salts is the avoidance of carbon
contamination and the easy removal of the salts with less
corrosive acids or water. This was first realised by Bojdys
et al. who demonstrated the preparation of high CCN using
the ionothermal method with temperature-induced condensa-
tion in the molten salt of LiCl/KCl.124
As the molten salts have a
wide operating temperature, various g-CNs can be prepared at
diﬀerent temperatures. The major advantage of using eutectic
salt for the synthesis of CCN is that the lower melting point of
the eutectic salt can completely melt the CN precursor owing to
Fig. 6 (a) Amidation reaction and assembly process between L-Arg, melamine, and cyanuric acid during hydrothermal processes. L-Arg, a Lewis base, its
guanidine group causes the aqueous solution to be alkaline, and so the melamine undergoes a hydrolysis reaction to produce cyanic acid. Meanwhile,
L-Arg could bond with melamine and cyanuric acid based on the amidation reaction to form a unique asymmetric supramolecular precursor. (b) The
ultrathin porous ACN assembly can be obtained by thermal condensation of a supramolecular precursor. (c) The molecular structure of the final product
(ACN) (d) SEM image of ACN14 (the inset shows natural anemones). (e) TEM image of ACN14. (f) EELS spectra near the carbon K-edge for BCN and
ACN14123
Copyrights 2020 Wiley-VCH.
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the higher condensation point of the heptazine unit. Mostly,
binary alkali metal chlorides are employed owing to their high
stability and low melting point. To this end, molten salt-
assisted ionothermal synthesis using LiCl + KCl, NaCl + LiCl
and various precursors can afford crystalline N- and NH-linked
polytriazine/polyheptazine.125–127
For example, Savateev et al.
demonstrated the synthesis of highly crystalline potassium
poly(heptazine imides) (K-PHI) by ionothermal synthesis, indi-
cating enhanced hydrogen evolution performance.128
Maintain-
ing crystallinity not only ensures enhanced photocatalytic
performance but also opens the door to fabricating robust
and high-performing electrodes for photo/electrocatalytic
applications.71
Bojdys et al. also demonstrated an improved
synthesis of crystalline triazine-based CN (TGCN) directly on
quartz using KBr + LiBr salts.129
Interestingly, the measured
out-of-plane conductivity of TGCN was found to be 1.01
104
S m1
that is almost twice than in-plane conductivity
(1.55 106
S m1
). Zhou et al. also synthesized crystalline
CN (CCN) by thermal annealing of melamine followed by the
molten salt treatment, demonstrating an enhanced perfor-
mance for oxidation of alcohols to aldehydes.130
A recent report
by Savateev et al. demonstrated that crystalline K-PHI could
generate singlet oxygen (1
O2) due to a triplet state transition
with a relatively long lifetime, which can drive a series of
cascade reactions.131
The singlet 1
O2 generated by KPHI sensi-
tization triggers the [3+2] cycloaddition of aldoximes to 1,2,4-
oxadiazoles under visible light irradiation. Furthermore, the
better charge transport probed by quenched photolumines-
cence spectra, increased transient photocurrent and O2-TPD
demonstrated that CCN can improve O2 adsorption and activa-
tion simultaneously. These findings suggest that CNx-based
materials will find multipurpose applications in the future
from photo-electrocatalysis to electrocatalysis owing to their
ability to minimize the efficiency loss due to improved charge
transport and the resulting charge delocalization.
2.4. 2D layered g-CN
Apart from improving crystallinity, the transformation of bulk
g-CN into 2D sheets can improve the processing and perfor-
mance of g-CN. Ultrasonication of bulk g-CNx in various
solvents, such as water, DMSO, isopropanol, ethanol, metha-
nol, and N-methyl-2-pyrrolidone, is the most adopted strategy
to transform the bulk g-CN into sheet structures.132,133
The
advantage of liquid phase exfoliation lies in the fact that the
crystallinity of sheets remains intact with low-cost and nontoxic
solvents. For instance, Lotsch et al. reported the synthesis of
highly crystalline polytriazine imides (PTIs) based CNN using
exfoliation in water, which displayed improved performance in
photocatalytic splitting of water.134
In another study, Niu et al.
reported the synthesis of a few-layered CNN by direct thermal
oxidation ‘‘etching’’ which gradually oxidized away the bulk
material leaving few-layered sheets.135
The resulting CNN
displayed an increased HER performance. Several other meth-
ods such as intercalation of ions (Li+
), freeze-drying and
annealing, use of a silica-graphene template, gas templates
such as NH4Cl, carbon dots bottom-up approach, etc. have also
been reported.136–143
Unfortunately, the transformation of bulk
CN into a sheet structure compromises the visible absorption
due to the confinement effect. Exfoliation of heteroatom-doped
sheets can compensate for the relative absorption loss.144,145
Wang et al. reported the synthesis of visible absorbing
(B578 nm) CNN (0.3 to 0.8 nm) via fluorination, followed by
thermal defluorination of CNN by thermal etching. The fluor-
ination step removes the stranded NH2, while the cyano groups
were introduced during the thermal defluorination step.146
2.5. N-rich C3Nx structures
Even though C3N4 (constituting triazine and heptazine motifs)
is thermodynamically more stable and constitutes a highly
crystalline structure, it performs poorly in several photo and
electrocatalytic reactions. In fact, enriching N content would
thermodynamically destabilize the CN network and simulta-
neously increase catalytically active sites with new molecular
structures.147,148
The precise molecular architectures of N-rich
C3Nx are determined by the nature of the CN precursors, the
nitrogen content of the CN precursor, synthesis techniques and
reaction conditions. By incorporating more nitrogen atoms into
the C3Nx lattice, it becomes N-rich. Various ways to connect and
arrange these C and N atoms can result in different N-rich C3Nx
structures, such as 2D layered structures and 3D porous frame-
works. In addition, defects and nitrogen functionalities also
affect resulting atomic structures. The nitrogen content in CN
can be increased by using either lowering carbonisation tem-
perature or high N-containing CN precursors. This was first
realised by Vinu and his co-workers,100
who used aminoguani-
dine as a CN precursor and prepared highly stable mesoporous
C3N6 with a tetrazine framework structure at the reaction
temperature of 400 1C using a hard templating approach.149
In this case, AG undergoes polymerization to form a highly
stable diamino-s-tetrazine moiety that is linked trigonally with
the nitrogen atoms. The stoichiometry of C3N6 is different from
C3N4 and it exhibited a distinct C and N bonding environment
compared to g-C3N4. The absence of graphitic C–N–C bonds in
the wall structure of C3N6 is notable and displays a lower energy
shift of the N-K edge in NEXAFS compared to g-C3N4. The C3N6
phase exhibits a band gap in the range of 2.25–2.49 eV and
displayed a much higher activity for Friedel–Crafts acylation of
benzene with hexanoyl chloride when compared with CN with
lower nitrogen contents, revealing the importance of high N
contents that increase the number of active sites. In 2017, Vinu
et al. also first demonstrated the synthesis of highly ordered
mesoporous C3N5 with a cubic structure and a low band gap
(2.2 eV) using 3-amino-1,2,4-triazole (ATZ) as a precursor and
KIT-6 as a template.147
Owing to its high N content and lower
band gap, superior performance in photocatalytic water split-
ting under visible light irradiation was achieved. Very recently,
Vinu’s team also used 5-amino-1H-tetrazole (5-ATTZ) for
the fabrication of mesoporous C3N5 with different molecular
structures containing 1 triazole and 2 triazine moieties at a
temperature of 400 1C for 4 h duration under nitrogen
environment.150
Controlled sintering of 5-ATTZ results in the
breaking of C–N and N–N bonds and thus gives cyanamide and
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hydrazoic acid. Under the same environment, the atoms of
these units reconstruct themselves into a new C3N5 lattice
network having single triazole and double triazine units. XRD
and near-edge X-ray absorption fine structure (NEXAFS) char-
acterization methods helped to establish the formation of the
triclinic crystal structure of C3N5. Their experimental results
were well supported by density functional theory (DFT). Simi-
larly, Yao et al. carried out pyrolysis of ATZ precursor at 500 1C
for 3 h and attained C3N5 rod-like structures with a band gap of
1.9 eV.151
In another interesting report, Kumar et al. used
pyrolysis of 2,5,8-trihydrazino-s-heptazine at 450 1C for 2 h to
attain C3N5 with a completely different molecular structure
having two s-heptazine units bridged together with an azo
linkage.152
The proposed structure was completely different
from the structure proposed by Vinu et al. for the mesoporous
C3N5 prepared from ATZ and 5-ATTZ.147,150
Vinu et al. also found that g-CN with different nitrogen
contents and molecular structures can be prepared by carbo-
nising the high N-containing precursors at different pyrolysis
temperatures. Pyrolysis of 5-ATTZ at 250 1C yields another new
phase C3N7, with even richer nitrogen content.153
A higher
band gap of 3.20 eV was registered with the compromised
optical response, which resulted in lower electrical conductivity
and lower ORR performances. Various N-rich crystallographic
phases of g-CN are shown in Fig. 7.
As has been experimentally explored in detail in the Vinu’s
group, it turns out that the selection of precursors and the
carbonization temperature primarily determine the resulting crys-
tal structures. While a high temperature of B550 1C yields C3N4, a
medium temperature of B450 1C yields C3N5, a low temperature
of B350 1C results in C3N6 and a far lower range of B250 1C yields
C3N7. Moreover, a restricted supply of vapour results in even non-
integral x in C3Nx. Thus, various structural features in CN with N-
rich active sites can be attained in a designer manner.
2.6. Carbon-rich carbon nitrides
Carbon nitrides are metal-free photocatalysts in pure form,
without metal doping/decoration. The past few years have
witnessed plenty of literature regarding N-rich C3Nx, which
has been demonstrated as better catalyst candidates, thanks
to its contribution to local electron enrichment at N-sites and in
providing feasibility of band gap engineering to efficiently
couple with the incident light and also due to enhancement
of catalytically active sites. However, the long-range crystalline
order and the overall conductivity (electronic transport) and
mobility are poor in N-rich C3Nx. The photocatalytic efficiency
of carbon nitrides could be better if enhanced crystallinity is
achieved and there is adequate light absorption through a
narrow band gap. Dong et al.154
demonstrated through DFT
calculations that introducing carbon atoms into g-C3N4 gener-
ates delocalized p bonds, which could effectively promote the
material’s electrical conductivity. This is critical for potential
applications where high electrical conductivity is desirable,
such as in electrochemical/photoelectrochemical catalysis.
Zhang et al.155
developed C-rich carbon nitride nanosheets
(from melamine and glucose precursors), which exhibited
enhanced photocatalytic performance and was attributed to
the modified electronic character of the carbon-rich C3Nx.
A new type of C-rich carbon nitride was synthesised using
melamine and 2 hydroxypropyl b-cyclodextrin using combina-
torial hydrothermal (180 1C, 24 h) and calcination (550 1C, 3 h)
strategies. The introduction of carbon atoms into the carbon
nitride lattice leads to the formation of delocalized p bonds
between the substituted carbons and the hexatomic rings,
which play a decisive role in facilitating electron transfer within
the material. The carbon-rich carbon nitride exhibited abun-
dant cross pore channels resulting in a larger surface area,
which provided more active sites for photocatalytic reactions
(Fig. 8).156
Fig. 7 Molecular structures of N-rich CNs: (a) C3N4
149
Copyright, 2017 Royal Society of Chemistry, (b) C3N4.8
150
Copyright 2018 Wiley, (c) C3N5
147
Copyright 2017 Wiley, (d) C3N5.4
77
Copyright 2021 Wiley, (e) C3N6
149
Copyright, 2017 Royal Society of Chemistry, and (f) C3N7
56,153
Copyright 2014 Wiley,
Copyright 2020 Wiley).
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Moreover, its unique structure helped in efficiently harvest-
ing light, enabling better utilization of solar energy for photo-
catalysis. The abundant carbon rings in the material facilitated
a faster separation of photoinduced charge carriers, reducing
the chances of recombination and increasing the overall effi-
ciency of the photocatalytic process. Visible light sensitivity was
observed to be enhanced due to band gap narrowing and hence
improved solar harvesting. The hydrogen generation rate of the
carbon-rich carbon nitride without the addition of Pt cocata-
lysts was approximately 117 times higher than that of bulk
carbon nitride obtained by calcination of melamine and
2.3 times higher than that of pure bulk carbon nitride. A new
Z-scheme photocatalyst based on C-rich carbon nitride/TiO2
(2CCN/TiO2) was developed using a simple self-assembly
technique157
and its photocatalytic performances were regis-
tered to be excellent compared to individual carbon-rich carbon
nitride (CCN) and TiO2 photocatalysts for degrading RhB
(rhodamine B), which was attributed to several factors such
as the Z scheme transfer path, increased light absorption and
enhanced surface area. Gashi et al.158
employed melamine (M)
and polycarboxylic acids (oxalic/tartaric/citric) (A) in different
ratios as precursors in the self-assembly synthesis of adducts
(MAy), which upon thermal conversion yielded carbon-rich
graphitic carbonitride materials (CNx) with an N/C ratios (x)
in 0.66–1.4 range and the result was compared with g-C3N4 (x =
1.33). This study highlighted that increasing C content in the
carbon-rich graphitic carbon nitride materials led to changes in
their structural, bonding, and optical properties (Fig. 9).
Incorporating carbon atoms into the polymeric carbon
nitride (PCN) matrix has recently been observed to result in
C-rich carbon nitride (CCN) nanosheets which exhibit the
extension of the aromatic p-conjugated electronic system and
Fig. 8 (a) Synthesis route to develop carbon-rich carbon nitrides with 2-hydroxypropyl-b-cyclodextrin (b) molecular structure of C-rich carbon nitrides
derived from the interaction of the precursors melamine and glucose156
Copyright, 2021 American Chemical Society.
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the generation of numerous oxygen-functionalities on the
material’s surface.159
The improved electronic as well as surface
properties of CCN nanosheets play a crucial role in enhancing
the adsorption and activation of H2S (hydrogen sulphide) and
O2 (oxygen) during the desulfurization process (Fig. 10).
Oxygen-functionalities facilitate the interaction with the
S-containing molecules, making it conveniently easier to
remove S-compounds from the gas stream. Strikingly though,
the nanosheet morphology of CCN with a large surface area and
enhanced adsorption sites seamlessly promotes the mass trans-
fer of H2S and O2 during the desulfurization process, and 99%
H2S conversion rate with B95% S selectivity is registered for
selective oxidation of H2S at 200 1C.
2.7. Characterization of g-CN
g-CN can be characterized by various diagnostic tools. X-ray
diffraction (XRD) is the most powerful technique to explore the
crystalline order of g-CN, in which its XRD pattern exhibits
peaks at 2y values of 11.31, 13.01, 14.51 and 17.31.68
However,
XRD is unsuitable for 2D materials due to the limited number
of periodic atomic planes: signal can be poor for a few-layered
2D materials; for the single monolayer, the signal is almost
insignificant. On the other hand, the vibration signals for
monolayered materials are much stronger compared to the
bulk counterparts; therefore, Raman spectroscopy is supposed
to be a more appropriate characterization for 2D materials.
Generally, g-CN atomic sheets exhibit characteristic vibrational
modes at 707 and 1232 cm1
.160
Transmission electron microscopy (TEM) is one of the key
imaging techniques that provide direct evidence of the surface
morphology of materials, including atomic-scale ripples/wrin-
kles/voids (very common in 2D materials). In addition, high-
resolution transmission electron microscopy (HRTEM) along
with selected area electron diffraction (SAED) patterns are
useful to provide visual details of atomic ordering and crystal
symmetries present in 2D g-CN in detail.68,161
C 1s and N 1s
peaks in X-ray photoelectron spectroscopy (XPS) can be decon-
voluted to provide exact information on the presence of various
types of chemical bonds (e.g., C–C, CQC, C–N, N–N, CQO) and
their relative strengths.68
UV-Vis absorption spectroscopy has
been extensively used for band gap determination whereas
photoluminescence spectroscopy can be used for understand-
ing the emission behaviour of g-CN.162
To validate the observed
experimental band gap, the DFT calculation has been carried
out to confirm the band structures.163
Fig. 9 C-rich C3N4 heterostructures with embedded six-carbon-ring
nanometer domains within a g-C3N4 lattice158
(Copyright, 2022 Wiley).
Fig. 10 Molecular structures and charge density difference of (a) oxygen and (b) hydrogen sulphide adsorption on PCN, and (c) oxygen and (d) hydrogen
sulphide adsorption on C-rich PCN. Electron depletion and accumulation are denoted by the violet and blue regions, respectively159
(Copyright, 20220
Elsevier).
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The layer-dependent behaviour is prominent in g-CN as
apparent from UV-Vis and PL spectroscopy and from the DFT
band structure of monolayer g-CN vis-à-vis its bulk. It should be
noted that the band structural features and band gap/nature of the
monolayer are completely different from those of the bulk CN. The
role of twist and translation in determining interlayer coupling
and strain arising, as a result, is going to be explored in the coming
times. Recently, strain-mediated crystallographic phases arising in
free-standing atomic sheets are being explored and the atomic
sheets release strain by the evolution of vacancies and ridge-lines/
protrusions. While band alignment upon excitation and its inter-
play in excitonics are responsible for the photophysical and
photochemical behaviour of g-CN, resultant electrostatic charge
transfer has a crucial role in catalysis.129,164–168
Apart from these basic characterization methods, many
advanced characterization tools have been developed recently for
characterizing molecular structure and elemental information. For
example, high-angle annular dark-field scanning transmission
electron microscopy (HAADF-STEM) provides atomic resolution
imaging whereas electron energy loss spectroscopy (EELS) and
secondary ion mass spectrometry (SIMS) are widely used to obtain
precise elemental information. Near-edge X-ray absorption fine
structure (NEXAFS) is also extensively used for the determina-
tion of the energy band structure of g-CN-based materials.
These advanced tools work as complimentary to existing tools
and provide an in-depth understanding of materials. Various
crystallographic structures of CN polymorphs exhibit distinct
electrostatic environments and the atoms constituting the struc-
tures interact differently giving rise to distinct electronic band
structures (Fig. 11).65
Defects and strain assume a crucial role
in structure determination and hence its physical/chemical
character.65–70
It should be noted that the electrostatic charge
localization at the bond termination sites in CN sheets acts as the
primary source responsible for the catalytic activities.
The unpaired electrons present in defective g-CN can be
probed via electronic paramagnetic resonance (EPR) to obtain
useful information on their local structure and spatial distribu-
tion. EPR spectroscopy investigations reveal that when exposed
to an electron donor, the cyanamide-functionalised CN system
forms a radical species that is long-lived.169
EPR is also a useful
tool to understand the charge recombination process in g-CN.
In dark mode, the unpaired electrons on the sp2
carbon
produce a singlet EPR signal, which is intensified under the
light irradiation mode due to electron excitation.170
EPR can be
further utilised to probe the effect of the morphology of g-CN
on holes when subjected to light excitation.171
Solitary Lorent-
zian lines with a g value of 2.0021 under the 3.44–3.53 kG field
range are observed in EPR.172
Solid-state magic angle spinning
nuclear magnetic resonance (MAS NMR) is an excellent tool to
reveal cyano group formation in CN. Peaks at 158.6 and 166.5 ppm
Fig. 11 Structure and DFT band structure of various 2D CNs65
Copyright, 2019 AIP publishing.
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arise due to C3N and C2N-NHx
respectively of heptazines.173
Fourier
Transform Infrared spectroscopy (FTIR) reveals and identifies the
presence of surface functional groups on CN sheets. CQC and
C–N stretches are of interest from the CN material point of view.
Specifically, FTIR peaks in the 1200–1600 cm1
range correspond
to aromatic CN heterocycles and have been employed frequently
for the characterization of carbon nitride. Electron-vibration inter-
action involving p electrons is held responsible for the FTIR
sensitivity of CN.174
3. Band gap engineering of g-CN
In order to figure out the bonding and properties of the
material of interest, it is imperative to investigate its band
structure consisting of a number of molecular orbital (MO)
levels as a result of the wave function interaction of each atom.
In the band structure, of particular interest for us is between a
highest-lying filled band and a lowest-lying empty band, which
are generally referred as the valence band (VB) and the con-
duction band (CB) if it is an insulator or a semiconductor. g-CN
is well-known for its intrinsic semiconducting feature consist-
ing of the VB and the CB which arise out of N 2p and C 2p
atomic orbitals, respectively. Benefiting from an adequate band
gap energy covering the visible range along with the pertinent
CB position for H2 generation, over the last decade, the g-CN
has enormously been investigated in the field of photo-
catalysis.161
In such fields, the enhancement of functionality,
e.g., solar to H2 conversion efficiency, revolves mainly around
the efficient band gap engineering of CN that can be modulated
in terms of electronic properties. As the efficiency is directly
proportional to the number of excited charge carriers, it is a
pivotal issue to suppress the recombination rate of excited
charge carriers by appropriate band gap engineering.
For energy fields other than photo-functional ones, band gap
engineering is important because it is an effective way to improve
electronic conductivity, which is a universal prerequisite to be
applicable in energy applications. In the upcoming sections, we
will introduce the powerful ways for band gap engineering of the g-
CN structures that are classified into intrinsic and extrinsic
approaches. The approaches to modify the intrinsic band gap of
g-CN primarily include a substitution strategy using light ele-
ments, integration of single metal atoms into the six-fold cavity,
and replacement of tertiary N sites with polymeric monomers. For
the extrinsic manipulation of the band structure, the strategy of
band gap alignment will be systematically addressed by introdu-
cing organo-metallic molecules, transition metal compounds and
Xene species as a hybridizing counterpart (Fig. 12). Chemical
processes of materials manipulation to obtain band gap engineer-
ing in carbon nitride change the atomistic crystalline structure as
well as the morphology and porosity nature. Such material mod-
ification certainly impacts the absorption properties as well.
3.1. Intrinsic modification of band gap
3.1.1. Defect engineering. Manipulating defects has gained
significant attention as a means to modify the crystal structure
of the material to alter the electronic band structure, optical
properties, and chemical coordination. The theoretical band
gap of 2.7 eV is commonly observed for CN, which can be
suitably manipulated through the change in chemical coordi-
nation, layer thickness, crystal structure, and heteroatom dop-
ing. The creation of defects in the form of vacancies, disorder,
dislocations, grain boundaries, and dopants is a unique strat-
egy to boost the catalytic properties of CNs. This leads to
structural distortion, which might cause a change in electron
density in the vicinity of the defect site. However, the creation
of defects may lead to a reduction in the crystallinity of the
samples. Defects create distortion in the structure; however,
this may be controlled to obtain exciting physicochemical
properties in the CN for optimum trap states which facilitate
charge carrier transfer and separation.175,176
This control is
essential to maintain crystallinity while creating point defects
to achieve ideal photocatalytic performance. Apart from the
nitrogen vacancy, the oxygen vacancy may also interact with the
defects to enhance light harvesting, catalytic activity, and
charge transfer.177
However, the quantification and qualitative
characterisation of the defects still remain a major challenge. It
is understood that the defects might create a lattice strain
which can act as active sites for desirable reactions.178
Strengthening the photogenerated carriers through the
incorporation of N vacancy resulted in a 28.7 times higher
photocatalytic H2 evolution rate when compared to g-C3N4.179
Heteroatom doping with the electron-deficient B and the
electron-rich N in a defective CN structure converts the Lewis
acid catalyst into a frustrated Lewis pair catalyst, promoting N2
reduction to ammonia.180
g-CN synthesized with carbon vacan-
cies showed much broader light absorption, high donor den-
sity, and longer lifetime of charge carriers which led to 20 times
increase in the photocatalytic hydrogen production rate
Fig. 12 Strategies for manipulating the band gap of carbon nitride.
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compared to bulk g-CN.181
The use of single Pt sites along with
CN defects and hydroxyl groups results in high activity and
selectivity for photocatalytic reduction of CO2 to CH4.182
In a
similar approach, a defect-rich CN was combined with FeS2
which showed a significant improvement by four times in the
photocatalytic nitrogen reduction to ammonia.183
Single-atom
catalysts are also gaining significant attention due to their
exceptional and unique properties. These can be loaded at
the defect sites on the support matrix such as CN to obtain
single atom loaded g-CN through a low-cost, facile and eco-
friendly top-down mechanochemical abrasion and solvent-free
approach, coupled with mild thermal treatment.184
In a rather
unique approach, laser irradiation was utilized to fabricate
atomic defects in a CN structure, which resulted in piezoelec-
tricity with strong structural distortion when the laser power
was increased to more than 348.4 MW cm2
.185
Through the
thermal polymerization route, using KOH, C3Nx compounds
were synthesized with a tunable band structure due to the N
vacancies which led to enhanced separation in the photo-
excited charge carriers.186
Another strategy to introduce defects
is through steam engineering for developing carbon vacancies
in a CN polymer matrix which leads to improvement in CO2
reduction efficiency.187
Considering the advanced characteriza-
tion tools and the availability of facile synthetic strategies,
defect engineering is becoming an increasingly popular strat-
egy to improve the efficiency of the catalyst at an atomic level.
This could lead to many exciting prospects in this field as the
atomic level manipulation can lead to major transformations in
the physicochemical property of the material.
3.1.2. Light element substitution in the g-CN lattice. Ben-
efiting from their similar radial size and electronegativity of C
and N atoms, it is quite straightforward to substitute the light
elements B, S and P atoms in the lattice of the CN framework as
nonmetal doping plays an important role in controlling the
electronic structure, reducing the band gap, enhancing the
visible light absorption, suppressing the recombination of
charge carriers and creating more active sites.188–190
Along with
an exploration of boron nitride, B-doped g-CN (BCN) has
received lots of attention in order to reduce and modulate the
band structure of the boron nitride that possesses a wide band
gap of 45.7 eV.191
The band structure could be finely regulated
depending on the composition of the trio of elements. In
addition, B doping is efficient to uplift the catalytic activity of
g-CN as boron-containing species act as strong Lewis acidic
sites.192
Theoretical simulations predicted that the B atom
substituted at the C site of the g-CN, rather than the N site,
has more inclination toward the catalytic oxidation of CO with
O2. Recently, the simultaneous generation of N defects and B
doping in the g-CN framework through its thermal treatment
with a single precursor, NaBH4, has been reported to alter its
band structure and enhanced catalytic activity for the photo-
catalytic OER.193
The modulation of CB and VB allowed the
materials to absorb the energy from the visible light for
enhanced water oxidation and an illustration of the charge
transfer process is shown in Fig. 13a and b.193
Another similar
study also employed NaBH4-based thermal treatment of CNN
and reported B-doped N-deficient g-CN (BDCNN) with a low-
ering of the intrinsic band gap from 2.72 to 2.37 eV.194
The introduction of transition metal/s along with boron
doping of g-CN is an effective strategy to adjust its band gap
to a suitable value for absorption of visible light. For example,
Cr and B doped together in g-CN reduce the band gap from
2.67 eV to 0.94 eV, which is made possible through the higher
p–d repulsions between the close energy orbitals of B 2p
(–0.2655 eV) and Cr 3d (–0.2148 eV).196
Such catalytic materials
also reduce the rate of the recombination of the holes and
electrons, which leads to higher productivity in the electro- and
photocatalytic reactions. For reactions such as nitrogen
reduction reactions (NRR), the hybridization state of boron is
critical in determining the catalytic activity of boron-doped
CN.197
Theoretical simulations showed that based on the
binding energy with N2, the sp2
hybridized boron is more
effective for NRR as compared to sp3
. A plethora of other
studies related to BCN for various applications exist.198–203
There are obvious challenges related to retaining boron during
the thermal treatment with the CN precursor, controlling the
exact composition of the trio elements, and eliminating any
remnants of the boron precursor used during synthesis.
Recently, much progress has been made in S-doped CN as
the S-doping can not only increase the active sites but also
narrow the band gap, and further facilitate the visible light
absorption. The S-substituted CN material was first prepared
via the polymerization process of trithiocyanuric acid as a
precursor.204
Upon the S atom substitution, the position of
the VB becomes more positive by 0.2 V, which can be helpful to
overcome the kinetic limitation to drive the water oxidation
reaction. Another study suggested negative shifts of both CB
and VB positions by the S substituent, as shown in Fig. 13c.195
The inconsistency in the band structure might originate from
the different local atomic positions that the S atom replaces.
Notwithstanding an obvious effect on the band positions, the
very low thermal stability of S atoms strictly restricts the
amount of S substituents in the CN framework to less than
B1 wt% (Fig. 13d and e). Recently, higher sulfur content of CN
(up to 3.5 wt%) has been achieved by facile self-assembly of
5-amino-1,3,4-thiadiazole-2-thiol with a combined thiadiazole,
triazole, and triazine framework.205
However, doping a large
amount of S into the CN framework might oppositely hinder
the active sites on the surface of g-CN.
Not only S, P substitution in CN can also break the hydrogen
bonds in the CN layer and create more active sites which are
favourable for catalysis. P doping in the CN framework was first
attempted by Zhang et al. through a simple mixing of precur-
sors containing P, C and N, and then the polymerization
process afterward.206
The P atoms replaced the graphitic C
atom in the CN framework by forming a P-N coordination
based on the nitrophilicity of the P atoms. Upon P substitution,
a high absorption was found in the lower energy range than the
band gap energy of CN and the absorption edge became vague,
inferring substantial change in the electronic structure of the
CN framework.207
Afterwards, the DFT calculations infer that
the P substitution introduces an inter-band state within the
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intrinsic band positions of the g-CN, allowing the absorption
range to be lower as shown in Fig. 13f and g.144
Like the
previous study, P atoms replace the graphitic C atoms that
coordinate heptazine aromatic rings by forming P-N bonding.
In addition, the positions of the CB and the VB seem to be
slightly shifted to result in the band gap narrowing upon the P
atom substitution, which could be ascribed to an orbital over-
lapping, or a structural distortion caused by ionic size mis-
match. The larger ionic size of P atoms compared to those of C
and N atoms elevates lattice strain and restricts their rich
substitution while maintaining the crystal structure of the
g-CN framework. By applying a harsh synthetic condition of
high pressure and high temperature, Wang et al. reported a new
form of P-substituted g-CN with the chemical formula C3N3P
which contains a higher amount of P substituents.208
In such a
case, the P substituents replace the graphitic N sites that bridge
s-triazine units and thus form P–C coordination, which is in
sharp contrast with the aforementioned research where P–N
coordination had formed. Also, the rich P substitution in the
g-CN lattice causes a lack of long-range crystalline order.
In spite of the active studies on the substitution of CN with
diverse elements with the purpose of band gap engineering,
there is still a lack of understanding of the sites to be sub-
stituted by the foreign elements from a crystallographic view-
point. Given the fact that the crystallographic arrangement
significantly aﬀects the overall band structure of the material,
it is crucial to clarify a variation of band structure depending on
diﬀerent doping sites such as pyridinic, pyrrolic and graphitic
N sites through a selective substitution strategy. The substitu-
tion strategy could be further extended to the CN structures
Fig. 13 (a) and (b) Scheme of the charge transfer process in N-deficient CN with B atom doping.193
(c) Scheme of the synthetic procedure of S-doped
g-CN195
Copyright, 2018 Elsevier, (d) and (e) atomic structure and 2D charge density profile of S-doped g-CN. Blue and red regions indicate lowest and
highest electron densities, respectively104
Copyright, 2019 American Chemical Society, (f) and (g) Top and side views of P-doped g-CN structures as
compared with the bare CN and its corresponding total density of states (PDOS) and partial density of states (PDOS)144
Copyright, 2015 Royal Society of
Chemistry.
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that comprise different types of repeating moieties, different
molecular structures and higher N contents.
3.1.3. Metal co-catalysts embedded CN. Although the inser-
tion of heteroatom dopants into the carbon nitride lattice can
significantly reduce the band gap to allow them to absorb a
larger fraction of visible light, the product yield is barely above
the micromole regime due to significant charge recombination.
Metal nanoparticles due to their low-lying Fermi level can act as
an electron sink and provide charge transfer channels and as
an anchor to efficiently adsorb reactants which concomitantly
reduces the recombination process. Furthermore, plasmonic
metals can enhance visible light absorption due to surface
plasmon resonance (SPR)209
and subsequent charge transfer to
the reactant molecule. Noble metals such as Au, Ag and some
transition metal/nonmetal nitrides such as TiN and HfN and
boron phosphide demonstrate surface plasmon resonance
phenomena.210
When surface plasmons interact strongly (coher-
ently) with incident light, the electrical vector of light oscillates in
resonance with surface plasmons leading to charge polarization.
The size and shape of plasmonic nanoparticles also have a
significant influence on the photocatalytic activity as plasmonic
excitation, charge injection and photothermal relaxation pro-
cesses are governed by the particle size and edges on the
nanostructure. Shaik et al. demonstrated Au nanoprisms (Au
NPs) encapsulated in dense and hollow carbon nitride spheres
(Au@g-C3N4) which exhibited enhanced photoelectrochemical
performance (Fig. 14a and b).211
The plasmon resonance,
charge separation and catalytic properties of metal nano-
particles can also be tuned by the formation of bimetallic
structures. The introduction of secondary metals such as Pt,
Pd, Rh, and Ru can not only tune the optical properties but also
provide active sites for the reaction.212,213
Various bimetallic
structures such as alloys, core–shell morphology and antenna-
reactor have been previously reported for photocatalytic appli-
cations. Coupling bimetallic structures with CN can further
improve charge separation and photocatalytic efficiency.214
Xue et al. reported the synthesis of Au–Pt co-decorated CN
by photothermal reduction of Au and Pt precursors in the
presence of IPA as a scavenger (Fig. 14c).215
The HR-TEM
images of Au/Pt/g-C3N4 nanocomposites demonstrate Au/Pt
NPs sized 7–15 nm with Pt decorated on the Au nanospheres.
The UV-Vis spectra clearly displayed plasmonic absorption
Fig. 14 (a) TEM micrographs of Au@mSiO2 templates at 20 nm scale bar used for the synthesis of Au@g-C3N4 core–shell nanohybrids. (b) EDS mapping
of a single Au@g-C3N4 core–shell nanohybrid211
Copyrights 2022 American Chemical Society, (c) HRTEM image of Au–Pt–C3N4 nanocomposites215
Copyrights 2015 American Chemical Society, (d) and (e) TEM images of C3N4–PtCu CNCs. (f) Most stable configurations of CO2 adsorbed on Pt(100),
PtCu(100), Pt(730) and PtCu(730) facets together with the adsorption energies (dark blue ball-Pt atom; brown, dark and red ones for Cu, C and O atoms,
respectively217
Copyrights 2017 Royal Society of Chemistry, (g) and (h) TEM images of FeCo@NGC, (i) TEM-EDS element mappings of FeCo@NGC (j) The
bulk model and H adsorption simulation of FeCo@GC, FeCo@GC-H, FeCo@NGC and FeCo@NGC-H (k) Calculated free energy diagram for H2 evolution
of FeCo@GC and FeCo@NGC relative to the standard H2 electrode at pH = 0; (l) The calculation of the density of states of FeCo@GC and FeCo@NGC218
Copyrights 2021 Elsevier.
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extended into the visible region which enhanced the kinetics
for the photodegradation of tetracycline.
Apart from plasmonic noble metal-based bimetallic nano-
structures, non-plasmonic dual structures have also been
reported to increase photocatalytic activity. Careful design of
the bimetallic assembly was also found to control product
selectivity, for example, bare Pt induces the formation of H2
during the CO2 reduction process; however, when alloyed with
Cu, hydrocarbons generation is favored.216
The exposed facet of
these alloyed nanostructures has a significant influence on
photocatalytic activities. Lang et al. demonstrated that (730)
facet-covered PtCu concave nanocubes embedded in a CN
scaffold activate CO2 to hydrocarbons at an elevated rate
compared to (100) facet-enclosed PtCu nanocubes.217
The concave PtCu nanocubes were grown in situ on CN using
Pt and Cu salts and PVP/HCl as shape-determining agents in a
hydrothermal synthesis. The HR-TEM images and XRD pat-
terns of the Pt-Cu/g-C3N4 composite displayed uniformly dis-
tributed Pt–Cu nanoparticles of 6 nm size with exposed (730)
facets on C3N4 nanosheets (Fig. 14d and e). UV-vis spectra of
NCs decorating CN demonstrated enhanced absorption while
PL verified the reduced recombination of carriers. During the
photocatalytic reduction reaction, CO and CH4 were found to
be the dominating products (0.046 and 0.112 mmol h1
; 90.6%
selectivity for CH4). The absence of hydrogen decipher well
design cocatalysts can suppress side reactions. DFT studies of
CO2 adsorption on Pt(100) and PtCu(100) models provide
adsorption energies of 0.08 and 0.03 eV which suggest that
PtCu have better performance due to the synergistic enhance-
ment (Fig. 15f). However, adsorption energies for the most
stable CO2 adsorption configuration on Pt(730) and PtCu(730)
facets were calculated to be 0.36 and 0.61 eV, respectively,
attributed to the low coordinate number (6) of the Pt atoms on
the concave (730) surface compared with the 8-fold coordinated
Pt atoms on the flat (100) surface.
The use of noble metals is not feasible for a real-world
application, therefore the search for alternative combinations
of non-noble metal-based multi-metallic structures is under-
way. In such an effort, Chen et al. fabricated a noble-metal-free
FeCo@NGC bimetallic alloy cocatalyst by thermally annealing a
Na2CoFe(CN)6 complex.218
When FeCo@NGC was integrated
with g-C3N4, the photocatalytic hydrogen evolution rate of
42.2 mmol h1
was achieved which was higher than 1%Pt/
g-C3N4 and 234 times higher than that of pristine g-C3N4. The
nanostructural analysis of 10% FeCo@NGC by HR-TEM
demonstrates spherical nanoparticles of FeCo wrapped in a
g-C3N4 shell with distinct lattice fringes of 0.202 and 0.32 nm
for FeCo and g-C3N4, respectively (Fig. 14g and h). Elemental
mapping verifies overlapped Fe and Co alloyed structure on C,
N of carbon nitride(Fig. 14i). DFT studies were performed to
understand the mechanism of improved photocatalytic activity
over FeCo@NGC (Fig. 14j and k). Two models FeCo@C and
FeCo@NGC were evaluated. These results showed that the
H2-adsorption energy of 0.23 and 0.05 eV on FeCo@C and
FeCo@NGC, respectively was observed, suggesting that an
N-rich structure has a favorable H2 adsorption state at pH = 0.
Furthermore, the density of state (DOS) for FeCo@C and
FeCo@NGC reveals C atoms around the N atom have the
highest Fermi surface, and the p orbitals of these C atoms
overlap with the s orbitals of H atoms (Fig. 14l). These findings
suggest that the C atom around N in the FeCo@NGC structure
has a strong interaction between C atoms and H atoms.
In addition to direct photocatalytic applications, the plas-
monic nanoparticle decorated CN can be used for various other
catalytic applications such as pollutant degradation and photo-
organic chemistry.219,220
A new dimension of using plasmonic
enhancement is in lithium–oxygen batteries. Li–O2 batteries
include oxygen evolution reaction (OER) and the oxygen reduc-
tion reaction (ORR) during the charging–discharging cycle.
Visible light-driven electrons can catalyze the O2 reduction
reaction during discharging to form Li2O2 while during char-
ging LiO2 accepts holes from the valence band of the semi-
conductor and releases O2 to form Li2+
. CN has already been
tested for Li–O2 batteries which demonstrated a reduced
voltage.221
However, the limited absorption of CN in the blue
region requires immediate action to improve the visible absorp-
tion profile up to the NIR region. Plasmonic nanoparticles
decorated with CN provide a facile solution to such a problem
due to the strong field enhancement and the high redox
potential of hot electrons and holes.
Although metal nanoparticle-loaded semiconductor cata-
lysts can enhance the photocatalytic performance, the require-
ment of high amounts of noble metals, leaching of the co-
catalysts, limited exposed surface area, poisoning of the cata-
lytic sites and poor absorption to achieve the desired quantum
efficiency are some obvious challenges. Additionally, the cata-
lytic selectivity is highly dependent on the chemical nature of
the exposed sites. The size reduction of the metal led to the
confinement of the charge in a limited space and the discreti-
zation of the energy levels which reduced the charge density on
the d-orbitals. These under-coordinated metal centers due to
increased surface energy become extremely reactive, and there-
fore, cluster catalysis has become a hot topic in the past few
years to drive many organic reactions. Due to their extremely
small size, metal clusters are prone to agglomerate and there-
fore, appropriate supports are usually needed for better metal
support interaction and recovery. Further reduction in size to
the atomic scale and coordination with supporting ligands can
entirely introduce new catalytic properties. The following sec-
tion discusses atomically dispersed catalysts that can reach
maximum atom economy, selectivity and increased stability in
detail.
3.1.4. Single atom-embedded g-CN. Recently, single-atom
catalysts (SACs) composed of isolated single metal atoms
coordinated on an active support have emerged as a new
frontier in the field of catalysis.222,223
Due to the presence of
a unique under coordinated environment and synergy with the
support (ensemble effect), SACs demonstrate exceptional activ-
ity and product selectivity (495%).224,225
SACs, due to the
availability of each catalytic site for the reaction and high
surface energy, can catalyse the reaction at an accelerated rate
with a dilute metal concentration like metal complexes, while
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their heterogeneous nature ensures facile recycling. Though the
concept of SACs is relatively new, increased activity and product
selectivity have been reported long before, such as in supported
catalysts prepared by the wet impregnation method on meso-
porous silica, ceria, etc.226
For making highly effective SACs on
the support, it is critical that the support has suitable sites
where the metal atoms coordinate effectively with the surface
groups or atoms of the support. The size reduction to the
atomic scale enormously increases the surface energy and
single atoms species have a natural tendency to agglomerate.
Therefore, strong electronic metal support interaction (EMSI) is
essential to fabricate SACs.227
Depending on the interaction of
the metal d-orbitals with the supporting ligands (s and p), they
can acquire different spin states (low, intermediate, high)
which have a significant influence on the adsorption and
activation of reactant molecules.228
The most common strategy
to stabilize the SA site on the support is via defect engineering,
which includes the creation of vacancies, decorating SA as
ad-atoms, and coordination with heteroatoms on the support.
Easily reducible oxide supports such as CeO2, TiO2, and
Fe2O3 are widely investigated for pinning SA sites due to the
facile creation of defects and strong redox interactions with
Fig. 15 (a) Schematic illustration for preparing a Co1/C3N4 single-atom catalyst by crystal-assisted confinement pyrolysis method. (b) Spherical
aberration-corrected HAADF-STEM images of theCo1/C3N4 catalyst. (c) Intensity profiles along the lines at positions 1 and 2 in the HAADF-STEM image.
Co K-edge XANES profiles (d) and EXAFS spectra (e) of samples; (f) wavelet transform analysis for the k2
-weighted EXAFS signals of Co foil (upper panel)
and Co1/C3N4 catalyst (bottom panel); (g) the Co K-edge R space EXAFS fitting results of the Co1/C3N4 catalyst; the inset shows the schematic illustration
of the CoN3 moiety structure273
Copyrights 2020 Elsevier. (h) Magnified AC HAADF-STEM images of Fe2/mpg-CN. Scale bar, 1 nm. (i) Corresponding fits
of the EXAFS spectrum of Fe2/mpg-CN at R space and k space, respectively. The inset of c is the schematic model of Fe2/mpg-CN (Fe cyan, O red, N blue,
and C gray). (j) Epoxidation of trans-stilbene. Catalytic epoxidation of trans-stilbene using different catalysts. (k) Consumption and regeneration of the
active one-coordinated oxygen species274
Copyrights 2018 Nature Publishing. (l) AC HAADF-STEM images of Pt2/mpg-CN. (m) The FT EXAFS fitting
spectrum of Pt2/mpg-CN at R- and k-space, respectively. (n) WT EXAFS of Pt foil, Pt1/mpg-CN, Pt2/mpg-CN, and PtO2. (o) The schematic model of
Pt2/mpg-CN (C: gray; N: blue; O: red; Pt: purple).275
Copyrights 2021 Nature Publishing.
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single atoms.229
Numerous inorganic supports for stabilizing
SACs have been developed in the past decade, and indeed in
2020, a chemical plant using Rh single atom-based catalysts for
the hydroformylation–hydrogenation of alkenes with 50 000 ton
per year propanol production rate has been operational in
Jiangsu China.230
Due to their uniquely coordinated and tuned
electronic environments, SACs have been found to catalyze a
variety of catalytic reactions with high selectivity, including
organic transformation, photocatalysis, CO2 reduction reaction
(CO2RR), N2 fixation and pollutant degradation. Among these,
photocatalytic and organocatalytic transformations are impor-
tant as they reduce the dependence on expensive catalysts and
toxic solvents and can produce fine chemicals which otherwise
require multiple steps. A detailed account of the design, tuning
and application of SACs in organic synthesis have recenty been
reviewed.231,232
Despite encouraging performances, SACs have
not reached their full potential due to several challenges.
For example, the introduction of defect states into the crystal
structure also compromises the electronic, chemical, and opti-
cal properties. Most of the reported catalysts have a metal
loading of 41%, so that the reaction rate remains low com-
pared to bulk nanoparticulate catalysts. Furthermore, due to
the unidentical coordination environment of defect states on
different crystal facets and edges of the support, the chemical
nature of SA sites was found to be altered, reducing the catalytic
activity and selectivity.
Distinctly, 2D materials have emerged as a choice of support
due to the tunable chemical composition, the high specific
surface area to accommodate a maximum number of SA sites,
and the ease of creating defect sites. Many 2D materials such
as MoS2, WS2, MXene, N-carbon, and graphene have been
explored for fabrication of SACs.233–235
Usually, SA is either
stabilized in cavities of inorganic 2D materials coordinating to
edge atoms or being anchored on the sheets by weak ionic/
covalent/van der Waals interactions.236
The variable cavity size
and chemical composition significantly influence the electro-
nic environment of SA sites which introduced heterogeneity.237
The unidentical SA sites catalyze different chemical reactions,
thus compromising the product selectivity. Precise control over
inorganic 2D material’s cavity size is highly challenging. On the
other hand, surface-decorated SA sites are prone to agglomera-
tion due to weak interaction. Interestingly, heteroatom (usually
N and sometimes P, S and O) doped carbon supports (M–Lx–C;
L–ligand) can stabilize SA sites due to effective coordination
between metal site and heteroatom due to a d–p overlap
followed by M - L charge transfer.238
M–Nx–C catalysts are
usually prepared by thermal annealing of nitrogen-rich carbo-
naceous precursors with a metal salt, and during thermal
annealing, metal centers are entrapped in a carbon framework.
Unfortunately, due to the high temperature of synthesis (700–
900 1C), metal, form aggregates and acid leaching is required to
remove non-single atom sites.239
Furthermore, the population
of isolated SA sites barely reaches above 1%. The high density
of SA sites is necessary to industrial-scale deployment and
replacing conventional catalytic systems. Recently, few general
syntheses including metal entrapped preorganized precursors
have been reported to fabricate high-density single-atom cata-
lysts with a metal loading as high as 40%. For example, Xia
et al. demonstrated the synthesis of Ni, Ir SACs with 40 wt%
metal loading via thermal annealing of graphene quantum
dots confined metal centers which prevent agglomeration.240
Compared to flexible N precursors, rigid nitrogen sources can
afford distinct porous structures with more access to active
centers. For instance, Kumar et al. synthesized high-density
cobalt SACs using melem (C6N7) and tetrameric cobalt phtha-
locyanine with a high surface area and 10.6 wt% Co content
delivering a high OER.241
In another approach, Hai et al.
reported a 23 wt% metal trapping in nitrogenous carbon sheets
using a two-step thermal annealing of ligand-bound metal
centers.242
Thermal annealing of metal–organic frameworks
(MOFs) also provides a uniform distribution of SA sites with
high density; however, the cost of MOFs ligands, the use of
expensive/harmful solvents for the synthesis of MOFs, and the
requirement of high temperatures for synthesis are some
evident challenges.243,244
Unfortunately, M–Nx–C catalysts due
to zero band gap cannot be employed in photocatalysis and
their application is limited to electrocatalysis and thermal
catalysis.
Distinct from other 2D semiconductors, CNs are advanta-
geous in the ability to decorate single atoms owing to the
presence of N-terminated heptazine units constituting vacan-
cies that can eﬀectively accommodate metal centers.245
The
abundance of nitrogen in g-CN can oﬀer lone pairs of electrons
to form a strong bond with the empty or partially empty orbitals
of metal atoms, which significantly stabilises the formed
SACs.231,246–254
Furthermore, their thermal/chemical stability,
moderate band gap, extended p conjugation to stabilize the
metal centre via charge transfer, and periodicity of active sites
make them ideal materials to fabricate SACs.255–257
These SACs
can be loaded at the defect sites on g-CN to obtain single-atom
loaded g-CN through a low-cost, facile and eco-friendly top-
down mechanochemical abrasion and solvent-free approach,
coupled with mild thermal treatment.184
The SACs on the CN supports are classified based on the
nature, the size and the amount of the SACs. Ideally, the SACs
are referred to the highly dispersed and individual single atoms
on the surface. When the SACs are dispersed in the form of
di- and tri-metallic clusters together with the individual single
atoms, they are called atomically dispersed metal catalysts
(ADMC). When two metal atoms are coordinated with the
surface functional groups of the support and act as single
atoms, they are referred to as dual metal SACs (DM-SACs). High
loading of SACs on the supports with high dispersion is termed
as ultra-high density SACs (UHD-SACs). Mostly, the SACs are
formed at the edges of CN where most of the active functional
groups are located. Sometimes, SACs are confined between the
layers of the CN which are called interlayer coordinated SACs.
The g-CN-based SACs can be synthesized through direct syn-
thesis approaches including template-free, molten salt, self-
assembly or freeze-drying. Post-synthesis approaches such as
electrostatic adsorption, deposition, thermal, chemical and
microwave reduction and atomic layer deposition (ALD) have
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also been extensively used for the preparation of SACs on
CN.258,259
During the post-synthetic approach, SACs are intro-
duced by mixing the metal salts onto the g-CN through
wet-impregnation or wet-deposition, followed by the reduction.
In the case of high-temperature treatment, it is possible that
the metal clusters may be formed on the surface, and therefore,
the stabilization of the metal centers on the precursor mole-
cules is essential to get better distribution and higher loading.
Unlike the substitution behaviour of light elements, SACs
generally prefer to locate on the periodic cavities of the g-CN as
a form of single atom rather than directly replacing C or N
atoms.260–262
When the transition metal atom occupies the
cavity sites, the N atoms with sp2
bonding character at the
edge of the cavities can play as effective coordination sites that
stabilize the single atoms.263
Gao et al. reported the integration
of Pt and Pd single atoms on the g-CN with alterations in
the electronic and optical properties.264
The six-fold cavity of
the g-CN is simulated to be the most stable site to integrate the
single atoms. Another study suggested that the Pd atoms could
be coordinated in the cavity of the g-CN framework with
locating slightly away from the center due to the local polariza-
tion in the vicinity of the heptazine units.265
Strong interaction
between a single atom and adjacent pyridinic N atoms could be
confirmed through the charge distribution.264
In terms of
electronic structure, integrations of Pt and Pd atoms give rise
to a significant reduction in the band gap of g-CN down to 0.64
and 0.72 eV, respectively. Also, the absorption spectrum of g-CN
is widely extended to a lower energy range, broadening and
enhancing visible light harvesting. In addition to these noble
metals, the thermodynamically stable Fe–N4 coordination
enables the synthesis of a Fe atom-embedded g-CN structure
by locating the Fe atom on the cavity surrounding with 3
heptazine units and the increase of Fe atom affects the band
gap energy to be shifted toward lower energy.266
As a host, the
g-CN template behaves quite differently from graphene. The
simulated d-band center for a Mn single atom embedded in the
g-CN framework appears to be closer to its Fermi level than that
in graphene, underscoring the Mn-embedded g-CN would be
more favorable in catalytic activity.267
In particular, there is a
growing consensus that the metal–N–C coordination with
appropriate ligation is pivotal in the activation of the inter-
mediates in the catalytic mechanism of the oxygen reduction
reaction (ORR).268
Numerous investigations have been done in this hot area.
For example, Kong et al. carried out DFT calculations to investigate
the optimized conditions for single atom-embedded CN structure
using 12 diﬀerent transition metals.269
Among these, it was
revealed that the V, Cr, Mn and Zr atoms would be favourably
anchored on the CN as a form of a single atom by exhibiting
higher adsorption energy than the cohesive energy. In the V atom-
embedded g-CN structure, electron transfer from the V atom to the
N atom is observed by Bader charge analysis, which suggests a
slight oxidation of the V atom. In nanoparticles, almost 60–80% of
the surface remains unexposed for the reaction, while the presence
of unidentical catalytic sites leads to a random product distribu-
tion. SACs on supporting materials not only ensure the availability
of each catalytic site but also influence charge redistribution at
the metal centre, creating a unique under-coordinated environ-
ment.270
The resulting coordination sites can catalyse the
reactions, which is not otherwise possible in individual atom/
nanoparticle systems. Further, effective coordination with sup-
port makes SACs less prone to poisoning.
Due to the structural similarity with macrocyclic metal
complexes and bio-enzymes, CN-based SACs promoted many
organic reactions with high selectivity. For example, nickel–CN
(C3N4–Ni) metallophotoredox SACs catalyzed the cross-coupling
of aryl bromide with alcohols with turnover numbers 4500 and
96% selectivity under visible irradiation.271
Silva et al. reported
the synthesis of crystalline Fe-poly(heptazine imide) SACs by
exchanging Na with Fe. In this case, the formation of the
hypervalent Fe(IV)
QO state like in methane monooxygenage
can promote the C–H bond activation (aliphatic) of aromatic
alkanes under a 50 W 410 nm LED.272
Many industrial fine
chemical syntheses rely on the oxidoreductase NADH regenera-
tion cycle. Currently, existing approaches of regeneration
are crippling due to slow regeneration rate, high cost and
lower selectivity. To overcome these challenges, Liu et al.
demonstrated the synthesis of Co SA decorated CN nanosheets
(Co1/C3N4) which can promote NADH regeneration in the
presence of [Cp*Rh(bpy)(H2O)]2+
combining photo and enzyme
catalysis (Fig. 15a–g).273
For the synthesis of Co1/C3N4 catalysts,
a crystal-assisted confinement pyrolysis method was adopted.
In the initial step, Co precursors and dicyandiamide were
deposited on a NaCl crystal by freezing an aqueous mixture
in liquid nitrogen, followed by annealing to form NaCl grown
on Co1/C3N4 (Fig. 15a). Dissolution of NaCl in water afforded a
Co1/C3N4 2D nanoflake structure. The presence of lonely Co
species was established by aberration-corrected (AC) HAADF-
STEM, demonstrating the presence of a sharp contrast for Co
metal centers, which were also clearly observable in line profile
(Fig. 15b and c).
Furthermore, X-ray absorption near-edge structure (XANES)
spectra of Co1/C3N4 and Co1/C3N4–Rh catalysts exhibited a
positive near-edge shift while the absorption edge lies in
between the Co foil and the Co3O4 reference validating the +2
oxidation state of the Co centers. Furthermore, the absence of
pre-edge peak at 7715 eV due to the 1s - 4pz shakedown
transition was absent, excluding the presence of Co in square
planar geometry; suggesting that Co might be located in non-
planar atomic configuration (Fig. 15d). Fourier transform k2
-
weighted of extended X-ray absorption fine structure (EXAFS) of
Co1/C3N4 displayed a single peak at 1.6 Å and additional peak for
Co–Co and Co–O was absent, suggesting that Co was present in the
isolated atomic state. EXAFS fitting demonstrates that Co atoms
were present in the 3 coordination state (CoN3) with an average Co–
N bond length of 2.07 Å validating that Co2+
was bonded with three
pyridinic N atoms of tri-s-triazine in the CN matrix (Fig. 15e and g).
The photocatalytic NADH regeneration experiment in the presence
of [Cp*Rh(bpy)(H2O)]2+
exhibits 99% yield of regenerated NADH
using 2 mg mL1
of the Co1/C3N4 catalyst. The prepared catalyst
also displayed excellent activity in the sequential reduction of
aldehydes to corresponding alcohols under visible irradiation.
Published
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13
October
2023.
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CN CATALYSIS

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