H2 and O2 Evolution from Water Half-Splitting Reactions by Graphitic

Article pubs.acs.org/JPCC

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.

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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 7178

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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.

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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.

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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 7180

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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 7181

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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% 7182

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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.

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CONCLUSIONS

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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: [email protected] (A.B.J.); [email protected] (P.F.M.). Notes

The authors declare no competing financial interest. 7183

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ACKNOWLEDGMENTS The authors acknowledge EPSRC (UK) as well as the UCL Enterprise fund for financial support.

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