H2 and O2 Evolution from Water Half

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Article
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H2 and O2 Evolution from Water Half-Splitting Reactions by Graphitic
Carbon Nitride Materials
A. Belen Jorge,*,† David James Martin,‡ Mandeep T. S. Dhanoa,† Aisha S. Rahman,† Neel Makwana,†
Junwang Tang,‡ Andrea Sella,† Furio Corà,† Steven Firth,† Jawwad A. Darr,† and Paul F. McMillan*,†
†
Department of Chemistry, Christopher Ingold Laboratories, University College London, 20 Gordon Street, London WC1H 0AJ,
United Kingdom
‡
Department of Chemical Engineering, University College London, Torrington Place, London WC1E 7JE, United Kingdom
ABSTRACT: Graphitic carbon nitride compounds were
prepared by thermal treatment of C−N−H precursor mixtures
(melamine C3N6H9, dicyandiamide C2N4H4). This led to solids
based on polymerized heptazine or triazine ring units linked by
−N or −NH− groups. The H content decreased, and the C/
N ratio varied between 0.59 and 0.70 with preparation
temperatures between 550 and 650 °C due to increased layer
condensation. The UV−vis spectra exhibited a strong π−π*
transition near 400 nm with a semiconductor-like band edge
extending into the visible range. Samples synthesized at 600−650 °C showed an additional absorption near 500 nm that is
assigned to n−π* electronic transitions involving the N lone pairs. These are forbidden for planar symmetric s-triazine or
heptazine structures but become allowed as increased condensation causes distortion of the polymeric units. Photocatalysis
studies showed there was no correlation between the increased visible absorption due to this feature and H2 evolution from
methanol used for the anodic reaction. In the absence of any cocatalyst the sample synthesized at 550 °C showed 1.5 μmol h−1
H2 evolution with UV−vis irradiation, but this dropped to ∼0.23 μmol h−1 when the UV spectrum was blocked. Use of a Pt
cocatalyst was required to observe H2 evolution from the other samples. Using a more powerful (300 W) lamp led to higher H2
production rates (31.5 μmol h−1) with visible illumination. We suggest the distorted N sites caused by increased polymerization
result in electron/hole traps that counter the photocatalytic efficiency. Issues concerning sample porosity are also present.
Photocatalytic O2 evolution was determined for RuO2-coated samples using the 300 W lamp with aqueous AgNO3 solution as
the sacrificial agent. The materials all showed production rates ∼9 μmol h−1. A highly crystalline compound containing
polytriazine structural units ((C3N3)2(NH)3·LiCl) prepared in this study did not show measurable photocatalytic activity.
■
INTRODUCTION
A polymeric carbon nitride material initially reported by
Berzelius following ignition of mercury thiocyanate was
determined to have general formula (C2N3H)n and termed
“melon” by Liebig.1 Other related compounds prepared by
various decomposition and reactions of nitrogen-rich precursors were later named “melem” (C6N7(NH2)3) and “melam”
(C6N11H9).2 X-ray studies indicated that these had polymeric
or layered structures related to graphite resulting from
polymerization between heterocyclic aromatic units such as striazine or heptazine (tri-s-triazine) linked by −N or −NH−
units.3 There is now expanding interest in understanding the
structures and developing applications of these graphitic carbon
nitride materials (gCNMs) with functional properties.4−9
Application of modern characterization techniques combined
with ab initio theory is leading to a detailed understanding of
their structures.4−12 The polymeric solids within melem- or
melon-based series are assembled from fused heptazine units to
form chains of NH-bridged melem monomers: these strands
adopt a zigzag-type geometry and are linked by hydrogen bonds
to give a 2D array (Figure 1).11 Further condensation occurring
with continued elimination of NH3 component results in partly
© 2013 American Chemical Society
or fully polymerized graphitic layered structures with general
composition CxNyHz.4−6,8,9 Recent discussions of gCNM
structures and their properties have been based on such
polymerized heptazine models.4,5
However, other gCNM structures based on condensation of
s-triazine rings are also possible. These include nanocrystalline
g-C3N4 first produced by chemical vapor deposition (CVD)
techniques and then studied theoretically as well as highly
crystalline bulk samples produced from melamine and cyanuric
chloride (C3N3Cl3) under high pressure−high temperature
conditions13−15 and polytriazine imide (PTI) structures
obtained by crystallization from molten salt (LiCl−KCl)
mixtures.16,17 These structures contain planar s-triazine rings
linked by −NH− groups to form large (C6N6) voids within the
layers. Cl− and Li+ ions derived from the synthesis can be
contained within these voids or intercalated between the
layers.14,17
Received: January 28, 2013
Revised: March 1, 2013
Published: March 6, 2013
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Figure 1. (a) Structure of Liebig’s melon ([C6N7(NH2)(NH)]n). Zigzag chains of heptazine (tri-s-triazine) units are linked by bridging −NH−
groups and decorated on their edges by N−H groups.11 (b) A single layer of g-C6N9H3·HCl prepared by reaction between melamine and cyanuric
chloride under HPHT conditions.14 This is a polytriazine imide (PTI) structure containing large C6N6 voids within the graphitic layers. Cl− ions
occupy the centers of these voids that are decorated by N−H species. (c) Polymeric melon/melem type structure initially proposed for crystalline
graphitic CxNyHz material obtained by a molten salt (LiCl/KCl) route.16 (d) The actual structure of this material is based on polytriazine units
linked by −NH− groups, and the composition was determined to be [(C3N3)2(NH)3·LiCl] (PTI/Li+Cl−).17 Here the Cl− ions occur between the
layers and Li+ ions occur both between the layers and inside the C6N6 voids along with N−H species.
Ab initio calculations have indicated that polyheptazine
motifs are more thermodynamically stable than structures based
on triazine units;4−6 however, either of these motifs can be
produced under different synthesis and precursor conditions
and heptazine−triazine units might even be combined within a
single gCNM structure. It is clear from theoretical studies that
the planarity of the layers and the optoelectronic properties can
be affected by the degree of polymer condensation and the
presence of intercalated species. The result leads to a family of
graphitic carbon nitride materials (gCNMs) with tunable
optoelectronic properties depending on the synthesis and
processing conditions.
The compounds are readily produced by reactions between
and thermal treatment of nitrogen-rich compounds including
d i c y a n d i a m i d e (D C D A : C 2 N 4 H 4 ) a nd m el am i n e
(C3N6H9).4,5,14,16 We tested various precursor formulations
and found that a 1:1 DCDA/melamine mixture yielded best
yield for the photocatalysis experiments. The product X-ray
diffraction patterns display a broad feature corresponding to a d
spacing near 0.325 nm close to the 002 reflection of graphite
while weaker reflections near 0.720 nm correspond to the inplane dimension of the heptazine units or spacing among the
triazine units (Figure 1).
It has been >30 years since the initial report of photoassisted
electrochemical water-splitting by TiO2 appeared,18 and this
now constitutes a key area of current energy and environmental
materials research. Since that pioneering work, considerable
progress has been made toward developing new materials that
are able to photocatalytically evolve both O2 and H2 from water
or organic feedstock under solar illumination. Several classes of
compounds have been reported or predicted to show such
activity under visible light irradiation (λ > 400 nm). However,
no single material has yet been able to deliver the required
efficiency over a sustained period at an acceptable cost.19−22
New attention is being paid to g-CNMs that have been shown
to act as semiconductors with an intrinsic bandgap near 2.7 eV
and optical absorption extending into the visible range.23 These
materials exhibit catalytic activity associated with their
intercalation, ion exchange, and redox properties,23−29 and
they have shown signs of photocatalytic activity extending into
the visible range.29 Here we investigated the properties of wellcharacterized gCNMs prepared with different degrees of layer
condensation to study the photocatalysis effects in relation to
the UV−vis absorption properties.
■
EXPERIMENTAL METHODS
Synthesis. Polymeric/graphitic carbon nitrides were prepared by thermolysis and condensation reactions of 1:1 molar
ratio mixtures of dicyandiamide (C2N4H4) and melamine
(C3N6H9) at 550−650 °C. Finely ground samples were loaded
in an alumina boat into a quartz tube in a tube furnace under a
gentle nitrogen flow. The temperature was raised at 5 °C/min
and held for 15 h. The furnace was allowed to cool to room
temperature before samples were removed. Crystalline PTI/
Li+Cl− was also synthesized from DCDA in molten eutectic
LiCl/KCl (45:55 wt %) mixtures heated at 400 °C under N2(g)
for 6 h then sealed under vacuum and heated to 600 °C for 12
h.16 Some studies were also carried out for crystalline
C6N9H3·HCl compounds prepared previously by high-P,T
synthesis,14,15 but these showed no photocatalytic activity and
were not investigated further.
Characterization. C, N, and H analyses were performed
using a Carlo-Erba EA1108 system. Cl content was determined
by EDAX analysis. SEM was performed with a JEOL JSM7179
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6301F field emission imaging system at 5 kV acceleration
voltage. Powder X-ray diffraction data were obtained using a
Bruker-AXS D4 system with Cu Kα radiation. DSC/TGA
analyses were performed on a Netszch DSC/TGA instrument,
and BET measurements were carried out using a Micrometrics
ASAP 2420 surface area/porosity analyzer. UV−vis spectra
were recorded at room temperature using an Ocean Optics Inc.
diffuse reflectance spectrometer using a BaSO4 standard and
the Kubelka−Munk algorithm to determine absorbance from
the reflectance data. FTIR spectra were obtained using a
PerkinElmer Spectrum 100 system. Raman and photoluminescence spectra were measured using a Renishaw microRaman instrument with excitation wavelengths between 325
and 784 nm.
Photocatalysis Measurements. Photocatalysis experiments were conducted in a 50 cm3 quartz vessel according to
standard procedure.30 For H2 and O2 evolution measurements
0.25 cm3 of gas was removed from the headspace with a syringe
at regular intervals and analyzed by gas chromatography using a
Varian CP-3800 GC equipped with a 5 Å molecular sieve
column. The quantum efficiency was estimated using Φ (%) =
(2 × H/I) × 100 or Φ (%) = (4 × O/I) × 100, where H and O
represent the number of evolved H2 and O2 molecules,
respectively, and I represents the incident photon flux measured
by a calibrated Si photodiode. It was assumed that all incident
photons were absorbed by the photocatalyst.
■
RESULTS AND DISCUSSION
Synthesis and Chemical Characterization. Polymeric
and graphitic carbon nitrides were prepared by thermolysis of
DCDA and melamine mixtures treated at 550−650 °C under a
N2 atmosphere leading to materials with different C:N:H
stoichiometry as a function of the reaction temperature (Figure
2a). Different mixtures of C−N−H precursors (DCDA,
melamine, cyanamide, cyanuric chloride) were tried. The
mixture gave highest yield (∼26% for the 550 °C sample and
12% for the 650 °C sample) of solid samples for the
photocatalysis measurements and characterization experiments.
Beyond this maximum synthesis temperature volatility of the
precursors and thermal decomposition reactions limited
product formation. Following synthesis and recovery we
examined the thermal stability of the materials using DSC/
TGA analysis (Figure 2b). The gCNM synthesized at highest
temperature exhibited the greatest thermal stability, with no
weight loss observed until above 520 °C. By 700 °C 50% of the
sample had decomposed. HCN, N2, and NH3 were detected in
the evolved gases by mass spectrometry. Weight loss for the
550 °C sample begins above ∼440 °C, and decomposition was
complete by 670 °C. Initially mainly NH3 is evolved, and this is
joined by HCN and N2 at higher temperature. Both
endothermic and exothermic events are observed in the 600−
700 °C range. The polymers are likely to be based on heptazine
units linked by −N or −NH− species as in melon but may
also contain triazine rings derived from the melamine
precursor. The compounds prepared at lower temperature
have a higher internal surface area corresponding to a lower
degree of layer condensation. Upon initial heating the loss of
NH3 component indicates additional condensation to form a
more compact structure. This is evident from both the BET
measurements and SEM examination. During our photocatalysis experiments to study O2 evolution we could also
detect a very small amount of N2 that varied between samples.
This is likely to be due to N2 gas incorporated within the
Figure 2. (a) Dependence of the C/N and H/C ratios determined as a
function of reaction temperature between DCDA/melamine mixtures
during syntheses of polymeric gCNMs. (b) TGA/DSC data for
gCNMs in air.
porous material from the synthesis atmosphere rather than a
decomposition or condensation reaction.
X-ray Diffraction and SEM Examination. X-ray diffraction patterns of gCNMs prepared in this study are displayed
in Figure 3. The strong peak at around 27.5° 2θ corresponds to
Figure 3. X-ray diffraction patterns of graphitic CxNyHz solids
prepared from polycondensation of DCDA/melamine (1/1) mixtures
at different temperatures.
a repeat distance ∼0.325 nm that correlates approximately with
the 002 reflection of graphitic layered materials. This becomes
narrower and shifts slightly to smaller distance (0.322 nm for
the 650 °C sample), indicating a higher degree of crystalline
order and a reduction in the stacking distance with increasing
synthesis temperature. The broad feature at around 12.5° 2θ
corresponds to an in-plane repeat distance of 0.706 nm. This
agrees well with the dimension of a single tri-s-triazine unit
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(0.713 nm) and could be associated with formation of
polymerized structures within the layers. The relative intensity
of the two features changes as a function of preparation
temperature. Applying the Scherrer relation to the interplanar
reflection, we estimate that the average correlation length of the
layered graphitic structure increases from approximately 2.5 nm
(550 °C) to 6.8 nm (650 °C). However our SEM examination
indicates that the materials prepared at higher temperature are
hetereogenous (Figure 4). The 550 °C exhibits a latticework of
Figure 5. UV−vis diffuse reflectance spectra of gCNMs.
The intense band with maximum around 350−380 nm is
assigned to π−π* transitions that are commonly observed in
conjugated ring systems including heterocylic aromatics. The
absorption edge and maximum typically shift to longer
wavelength with increasing polymerization as we observed
here. The feature that appears near 500 nm with increasing
reaction temperature is interpreted as due to n−π* transitions
involving lone pairs on the edge N atoms of the triazine/
heptazine rings. Such transitions are forbidden for perfectly
symmetric and planar s-triazine or heptazine units, but they
become allowed as the structures develop distortions with
increasing layer condensation, including effects from both layer
buckling and deviation of the ring units from trigonal
symmetry.31
Room temperature photoluminescence (PL) spectra of the
samples excited using a UV laser (325 nm) are shown in Figure
6. The feature near 450 nm corresponds to the PL signal
Figure 4. SEM images of gCNMs prepared by polycondensation of
DCDA/melamine mixtures at (a) 550, (b) 600, (c) 625, and (d) 650
°C.
interlocking planar microstructures with individual layer
thicknesses on the order of 2−3 nm that give rise to porous
aggregates with pore sizes on the order of a few nanometers.
The aggregates are fused together to give rise to much larger
pores (1−2 μm) in the resulting solid. BET measurements
revealed that the porosity decreased when increasing temperature, being 28 and 27 m2/g for the 550 and 600 °C samples
and dropping to 13 and 11 m2/g for the 625 and 650 °C
materials, respectively. As the synthesis temperature was
increased to 600 °C, the individual graphitic sheet structures
appeared to maintain approximately the same thickness but the
overall texture became denser. The 625 °C sample contained
large blocky units with a detectable hexagonal or trigonal habit,
and by 650 °C the porous microstructure had largely
disappeared (Figure 4).
UV−vis Absorption and Photoluminescence Spectra.
The UV−vis spectra showed a maximum at 350−380 nm with a
steep rise in absorption in the blue/violet-UV region of the
spectrum that resembles a semiconductor bandgap onset
(Figure 5). Applying a Tauc plot to the 550 °C data indicated
an intrinsic bandgap near 2.7 eV, consistent with the pale
yellow color. Increasing the reaction temperature to 650 °C
caused the absorption edge to shift to longer wavelength as well
as emergence of an additional feature near 500 nm so that the
samples became yellow-brown in color along with an increased
absorption coefficient in the visible range (Figure 5). It was
thought that this light absorption behavior could be important
for determining the photocatalytic properties.
Figure 6. Room temperature photoluminescence (PL) spectra of
gCNMs prepared in this study following 325 nm laser excitation.
following excitation of the π−π* transitions, whereas that
around 500 nm is due to emission associated with the n−π*
manifold. This feature shifts to longer wavelength and grows
rapidly relative to the π−π* band with increasing synthesis
temperature, indicating that transitions involving the N lone
pair electrons become more allowed as layer condensation
proceeds.
FTIR and UV Raman Spectroscopy. The FTIR spectra
obtained for the gCNMs are shown in Figure 7. The sharp peak
found at about 800 cm−1 is assigned to out-of-plane bending
vibrations of six-membered rings common to either triazine or
heptazine units within the structures.12 The linkage of these
ring systems by −NH groups is shown by absorption bands in
the 1200−1400 cm−1 region that are characteristic of the C−
NH−C units in melam and melon.32 The multiple bands found
in the 1600−1200 cm−1 region are typical of C−N stretching
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methanol solution as the electron donor. The reaction
atmosphere was purged with Ar(g) for at least 30 min prior
to each experiment. In a first series of experiments we studied
our gCNM materials alone (i.e., with no added cocatalyst)
using a 75 W Xe lamp (UVA irradiance 17.5 mW cm−2) that
provided radiation throughout a wide range of UV−vis
wavelengths (300−800 nm). The sample prepared at 550 °C
showed a H2 production of 1.5 μmol h−1 corresponding to a
quantum efficiency (QE) of 1.4%. However, samples prepared
at higher T showed a reduction in H2 evolution rate under
these conditions (Figure 9), in agreement with previous
Figure 7. FTIR spectra of gCNMs.
and bending vibrations of nitrogen heterocycles. This region of
the IR spectrum does not change significantly with the reaction
temperature. No bands were observed in the 2200 cm−1 region,
indicating that there are no triple-bonded −CN groups or
double bonds −CNCN− present in the gCNM samples.
Absorption bands at 3310 and 3200 cm−1 are due to N−H
stretching vibrations.33,34 A substantial decrease in the intensity
of these bands occurred with increasing reaction temperature,
but complete elimination of the −NH species to form purely
CxNy materials could not be achieved in our study.
Raman spectra were obtained using UV excitation (325 nm)
to avoid intense fluorescence that obscures gCNM spectra
obtained using visible laser excitation (Figure 8). However, the
Figure 9. (a) H2 evolution for as-prepared gCNMs synthetized at
different temperatures under UV + visible irradiation using a 75 W Xe
lamp. (b) H2 evolution of as-prepared gCNM (prepared at 600 °C)
under UV + visible irradiation compared with a Pt-coated sample using
visible radiation using a 75 W Xe lamp. (c) H2 production for Ptcoated gCNMs prepared at different temperatures under visible
irradiation using a 300 W Xe lamp. (d) O2 evolution for RuO2-loaded
gCNMs prepared at different temperatures under UV + visible
irradiation using a 300 W Xe lamp.
results.29 We then placed a UV filter to limit the incident
radiation to >400 nm. The photocatalytic performance was
tested using a reference disk of Pt-coated P25 TiO2 prepared
using the same pelletization procedure. None of our gCNM
samples exhibited photocatalytic activity under visible light
illumination, and so we introduced Pt nanoparticles mixed with
the gCNM powders prior to compression into pellets to act as a
cocatalyst and promoter. We could now detect H2 evolution for
the nano-Pt-containing samples, but this was substantially
reduced compared with the results for UV−vis illumination
(Figure 9). The result for the gCNM sample prepared at 600
°C showed an initial rise between 0 and 200 min but then
reached a plateau, and the averaged data over a 0−400 min
period indicated H2 production of only 0.23 μmol h−1.
Further experiments were carried out using a more powerful
lamp (300 W) source that resulted in significantly higher H2
production for the Pt-coated samples under visible illumination,
especially for the gCNM material produced at lowest
temperature. Here the H2 evolution achieved 31.5 μmol h−1,
although the photocatalytic efficiency dropped rapidly with
increasing temperature of preparation (Figure 9). Using the
300 W excitation source, we also investigated O2 production for
samples containing incorporated RuO2 nanoparticles within the
powders. Here all the samples showed similar O2 evolution
Figure 8. UV Raman spectra (325 nm excitation) of gCNMs.
UV spectra may exhibit resonance Raman effects that could
probe specific functional groups or regions of the sample.14 The
spectra were dominated by an intense, broad, asymmetric
feature in the 1200−1700 cm−1 region of the spectrum. This is
attributed to C−N stretching vibrations and resembles the “G”
and “D” band profiles observed for structurally disordered
graphitic carbons and other (C, N) layered materials.35−38 The
broad bands observed in the 3000 cm−1 region are mainly due
to second-order Raman scattering associated with fundamental
C−N stretching vibrations in the 1200−1700 cm−1 range.
Sharp peaks were observed at 690 and 980 cm−1. The 980 cm−1
peak can be assigned to the symmetric N-breathing mode of
heptazine and/or triazine units.39−41 The second peak at 690
cm−1 is a doubly degenerate mode associated with in-plane
bending vibrations of the CNC linked triazine/heptazine
linkages.39−41 These features were observed for all samples
but became better defined for materials prepared at high
temperature (Figure 8).
Photocatalytic H2 and O2 Evolution. The activity of the
H2-evolving half-reaction photocatalyzed by the graphitic
carbon nitride materials was evaluated using an aqueous 10%
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Figure 10. (a) XRD pattern and (b) UV−vis absorption spectrum (inset SEM image) of PTI/Li+Cl− material.
rates (∼9 μmol h−1). During our gas chromatography
experiments to determine evolved gas quantities, we also
detected small amounts of N2. This could be produced by
oxidation of nitride anions at the gCNM surface, but Wang et
al.29 did not detect any N2 evolution in their study of similar
materials. We suggest that our synthesis approach carried out in
a nitrogen atmosphere could have resulted in minor amounts of
N2 incorporated within the solid materials and released during
the photocatalysis studies.
The gCNMs prepared at higher temperature exhibit
increased optical absorption in the visible range, but they
exhibit a reduction in photocatalytic activity for H2 evolution.
We can associate this loss in activity with the appearance of
charge trapping sites at the N centers that become distorted
away from planar sp2 geometry as the condensation process
takes place. More research will be needed in order to
understand why this effect is more pronounced in the H2
photoreduction than in the water oxidation.
Electronic band structures have been reported from DFT
calculations for a polymeric melon structure based on heptazine
structural units similar to those likely to be present within the
gCNM compounds synthesized here.29 The 1.23 V separation
between H+/H2O and O2/H2O potentials occurs within the
bandgap between the top of the valence band derived from N
lone pair orbitals in the xy-plane and the bottom of the
conduction band derived from C pz orbitals.29 The minimum
bandgap from DFT studies occurs at 2.6 eV and corresponds to
a n−π* transition that is observed for the buckled layers at high
degree of condensation (650 °C), but not for the planar
unconstrained materials produced at 550 °C. However, the
appearance of the n−π* absorption does not seem to be
coupled to the photocatalytic behavior.
Studies of a Crystalline Structure Based on Polytriazine Units. To investigate the effects of polyheptazine vs
polytriazine ring units within the gCNM structure, we explored
the UV−vis absorption and photocatalytic properties of a
highly crystalline phase produced from DCDA precursor by
molten salt LiCl/KCl mixtures.16 In contrast to the samples
prepared by thermal condensation, the PTI/Li+Cl− compound
exhibited a sharp series of peaks in its X-ray diffraction pattern
consistent with a P63cm unit cell.16,17 Initial studies suggested
the material was heptazine-based, but new work has indicated a
triazine-based (C3N3)2(NH)3·LiCl (PTI:Li+Cl−) structure with
Li+ and Cl− ions intercalated both within and between the
graphitic layers related to that found for graphitic C6N9H3·HCl
prepared by high-pressure techniques.17 The UV−vis spectrum
of PTI:Li+Cl− exhibits a main absorption profile between 300
and 400 nm with a sharp absorption edge, but an n−π* feature
is present at ∼450 nm. The features result in a yellow-brown
color indicating visible absorption. However, the materials did
not exhibit any photocatalytic activity for H2 or O2 evolution,
even when the compounds were coated with Pt. A similar
negative result was found for graphitic C6N9H3·HCl.
■
CONCLUSIONS
■
AUTHOR INFORMATION
The main result of our study is that gCNMs prepared under
different conditions are indeed photocatalytically active under
UV and visible light illumination. The photocatalysis can be
achieved even without addition of cocatalysts such as Pt or
RuO2 nanoparticles, but the efficiency depends dramatically on
the degree of gCNM polymer condensation that itself depends
on the precursors and the synthesis/processing conditions
used.
We can now begin to relate the photocatalytic properties of
gCNM compounds to their structures, optoelectronic properties, and synthesis/processing conditions. Syntheses carried out
by thermal polycondensation of DCDA/melamine mixtures
have resulted in graphitic carbon nitride materials with varying
degrees of polymerization and C:N:H ratios. The C/N ratio
increases and H content decreases with increasing reaction
temperature due to the condensation of the C−N−H precursor
by removing NH3 molecules. Increasing condensation
especially above 600 °C creates interchain −NH− bonds that
result in distortion of the triazine/heptazine units and
consequent loss of planarity and distortion in the triazine/
heptazine units. The optical absorption data indicate a
semiconductor-like rise with onset near 400 nm. Increasing
the reaction temperature causes the absorption edge to shift to
longer wavelengths as well as emergence of an additional
feature near 500 nm resulting in increased visible absorption.
However, this is not coupled with improved photocatalytic
efficiency. The 500 nm absorption is due to n−π* transitions
involving N lone pairs that become allowed for distorted
gCNM structures that also result in electron/hole traps, thus
impeding the photocatalytic process. gCNMs provide a new
class of materials to develop for photocatalysis applications, but
we must understand better the relationships between the
nanoscale structures and optoelectronic properties.
Corresponding Author
*E-mail: a.sobrido@ucl.ac.uk (A.B.J.); p.f.mcmillan@ucl.ac.uk
(P.F.M.).
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
The authors acknowledge EPSRC (UK) as well as the UCL
Enterprise fund for financial support.
■
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