Cross-Linked Graphitic Carbon Nitride with Photonic Crystal Structure

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Cross-linked Graphitic Carbon Nitride with Photonic Crystals Structure for Efficient Visible-Light-Driven Photocatalysis Lu Sun, Wei Hong, Jing Liu, Meijia Yang, Wensheng Lin, Guojian Chen, Dingshan Yu, and Xudong Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14359 • Publication Date (Web): 04 Dec 2017 Downloaded from http://pubs.acs.org on December 4, 2017

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ACS Applied Materials & Interfaces

Cross-linked Graphitic Carbon Nitride with Photonic Crystals Structure for Efficient Visible-Light-Driven Photocatalysis Lu Sun, Wei Hong*, Jing Liu, Meijia Yang, Wensheng Lin, Guojian Chen, Dingshan Yu and Xudong Chen* Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, Guangdong Engineering Technology Research Center for High-performance Organic and Polymer Photoelectric Functional Films, School of Chemistry, Sun Yat-sen University, Guangzhou 510275, China KEYWORDS: graphitic carbon nitride, hydrogen evolution, photocatalysis, water splitting, conjugated polymers, photonic crystals

ABSTRACT: Highly cross-linked graphitic carbon nitride has been prepared by a thermal copolymerization of dicyanodiamide with tetramethyl-ammonium salts. The cross-linking can be evidenced by i) increased C/N ratio without new cabon species, ii) decreased specific surface area and iii) Tyndall effect after dissolution in concentrated sulfuric acid. The cross-linked graphitic carbon nitride with photonic crystal structure has highly efficient photocatalytic activity for water splitting under visible light due to the synergistic enhancement by the greatly

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suppressed photoluminescence, red-shifted absorption edges, strong inner reflections and effective PCs stop band overlaps. It exhibits an enhanced photodegradation kinetic of methyl orange and a high visible-light-driven hydrogen-evolution rate of 166.9 µmol h-1 (25 times higher than that of the pristine graphitic carbon nitride counterpart). This work presents a facile method for designing and developing high-performance graphitic carbon nitride photocatalysts, providing a broad range of application prospects in the fields of electronics and energy conversion.

INTRODUCTION The global energy crisis and severe environmental pollution have propelled the widespread research on phototcatalysts for water splitting and purification using sustainable solar energy.1 The inorganic metal-based phototcatalysts have been extensively developed while the organic semiconductor phototcatalysts were much less explored due to the unfavorable stabilities and low activities.2 Recently, a promising phototcatalysts of graphite-carbon nitride (g-C3N4) with a suitable band gap of ca. 2.7 eV has attracted considerable attention because of its attractive features of chemical stability, nontoxicity, metal free composition and visible-light activity.3,4 Nevertheless, the catalytic activity of pristine g-C3N4 is limited by its inherent deficiency such as low specific surface area, limited visible-light absorption range and significant recombination of charge carriers.5,6 Thus, various approaches have been proposed to improve the photocatalytic performance including chemical modification,7-22 nanostructuring,23-28 and heterojunction coupling.29-38


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The chemical modification of g-C3N4 focused on breaking the perfection in the hierarchical chemical structure of g-C3N4, from molecular structure,7-14 to hydrogen bonds,15,16 to the polymeric structure,17,18 to the layer stacking structure.19-22 The materials of g-C3N4 were considered as polydispersed polymeric structures based on the linear poly-melem (known as melon, see Scheme 1a) with cross-linked segments as evidenced by the liquid-state NMR.39 In 2009, the conceived cross-linked framework, poly (heptazine imide) or PHI, was proposed for the g-C3N4 (Scheme 1b),40 and the stoichiometric ratio of C3N4 appeared to be the ideal condensed form of the melem repetitive unit (Scheme 1c).3,18 In fact, most of the reported gC3N4 materials had the C/N atomic ratios between 0.667 and 0.750, with sizable amount of hydrogen (~2 wt%).3,7-27 Therefore, the cross-linking/condensation degree could be an important part in the hierarchical chemical structure of g-C3N4, dominating the geometry and electron transition of the carbon nitride framework. The organic conjugated phototcatalysts, including poly(p-phenylene)s,41 planarized fluorene-type conjugated polymers,42 conjugated triazine frameworks,43 and covalent organic frameworks (COFs),44 have the advantages of easily tuned electronic and structural properties via the synthetic modularity, including monomers and crosslinkers.2 Assuming that g-C3N4 behaves like a partially π-conjugated heptazine polymer,29,45 the aromatic system depends on not only the polymerization degree but also the cross-linked degree of the polymeric structures. In the pioneer work by Wang and co-workers, the densityfunctional-theory calculation results showed the band gap of 3.5 eV for melem molecule, 2.6 eV for polymeric melon and 2.1 eV for the fully condensed g-C3N4.3 Thus, increasing the condensation degree in g-C3N4 may lower the band gap through π-conjugation extension, enhancing the absorption range for visible light.


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Scheme 1. Structures, repetitive units and C/N atomic ratios of a) melon, b) cross-linked framework of poly(heptazine imide) and c) fully condensed g-C3N4. The nanostructuring of g-C3N4 mainly refered to the porous modification, which could benefit from the increased surface area by mesopores and effective inner reflections by macropores.23-27 For the last decade, nanostructuring based on the photonic crystals (PCs) structure has been extensively investigated in photocatalytic application and optoelectronic devices,46-53 owing to its special properties including slow light,54 inhibition of spontaneous emission and amplified photon absorption/emission which ascribed to its particular photonic stop-band.55 In addition, the PCs structures could provide increased surface area, shorten charge diffusion distance and effective light harvesting by inner reflections.46-53 However, most of the photocatalyst PCs had limited absorption range within the visible light, restricting the effects from inner reflections.47,50,53 Moreover, as the photonic stop-band of the three dimensional PCs


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has angular dependence,47,53,56,57 the irregular distribution of the photocatalyst powder in the solution could lead to significantly reduced interaction between the stop-band and the limited light-absorption range.47,52,53 Consequently, the simutaneous optical absorption extension of gC3N4 and PCs structure engineering could maximize the use of light in the photocatalysis. Herein we demonstrate the synthesis of cross-linked (highly condensed) graphitic carbon nitride by employing a co-monomer of tetramethyl-ammonium (TMA) salt for the modification of g-C3N4 conjugated polymeric networks as a cross-linking agent. The introduction of TMA red-shifts the absorption edge of g-C3N4 with an increased C/N atomic ratio. The cross-linked graphitic carbon nitride can inherit the specific surface area of the PC template more effectively and the expanded light-absorption range can lead to stronger inner reflections in the PCs structure, further improving the photocatalytic activity of the material. The resulting cross-linked graphitic carbon nitride PCs exhibit superior photodegrading activity and photocatalytic hydrogen revolution rate to the pristine g-C3N4 under visible light. This approach may provide a bottom-up way to alter the cross-linking structures and optical properties of the g-C3N4-based materials. EXPERIMENT SECTION Material Preparation. Preparation of silica photonic crystals. Stöber method was used to synthesize monodispersed silica microspheres with diameters ranging from 190 to 238 nm.58 Brifely, 25 mL tetraethyl orthosilicate (TEOS) was mixed with 360 mL ethanol in a flask equipped by a magnetic beater at room temperature. After that the mixture of ammonia (20 mL) and deionized water (18 mL) were slowly injecting into the flask within 15 minutes using a syringe pump. The mixture was kept stirring overnight. Silica microspheres with different diameters could be obtained by varying the amount of TEOS. Centrifugation and re-dispersion


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was carried out to remove residues and purify the monodispersed silica microspheres. Afterwards, monodispersed silica spheres in water with a concentration of 5 wt% were transferred into 10 mL vials. The silica photonic crystals could be obtained on the wall of the vials within a few days in a constant temperature and humidity incubator at 60 ℃ with a relative humidity of 65%. Synthesis of pristine g-C3N4. Pristine g-C3N4 was prepared by direct annealing dicyandiamide (DCDA) at 550 ℃ for 4 h with a ramp of 4 ℃ min-1 under N2 flow. The yield based on the mass of DCDA was about 50 %. Synthesis of bulk cross-linked graphitic carbon nitride (TCNx). The doping agents for bulk TCNx were prepared by mixing a certain amount of 25 wt% tetramethylammonium hydroxide aqueous solution (13, 25, 51, 76, 101, 127 µL for x=1.6, 3.2, 6.4, 9.6, 12.8, 16, respectively) and the corresponding amount of acetic acid (2.5, 5, 10, 15, 20, 25 µL, respectively) into 1 mL anhydrous ethanol to form TMA acetate. After the evaporation of ethanol and water at 50 ℃ overnight, sticky liquids were obtained due to the strong hygroscopicity of the quaternary ammonium salt. The dried doping agents was mixed with 0.2 g DCDA and directly heated in N2 with the same treatments of pristine g-C3N4. The yield based on the total mass of DCDA and TMA acetate was about 56%. Synthesis of g-C3N4 PCs. 0.2 g DCDA was mixed with 0.33 g silica photonic crystals, and calcined at 550 ℃ for 4 h with a ramp of 4 ℃ min-1 under N2 flow. The as-obtained product was etched using 4 M NH4HF2 solvent for 36 h under continuous oscillation to completely remove the silica template, afterwards the product was rinsed with pure water for several times and dried. The yield based on the mass of DCDA was about 50 %.


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Synthesis of cross-linked graphitic carbon nitride (TCNx) PCs. Firstly, the TMA precursors were prepared with same way for bulk TCNx, and then corresponding amounts (367, 383, 400, 417 mg for x=6.4, 9.6, 12.8, 16, respectively) of silica photonic crystals were immersed into the precursor solutions. The mixture was dried at 50 ℃ overnight. After 0.2 g DCDA was mixed with the TMA-loaded photonic crystals, the mixture was calcined and post-treated with the same process for g-C3N4 PCs. The yield based on the total mass of DCDA and TMA acetate was about 56%. Characterization. Powder X-ray diffraction (XRD) measurements were performed on Rigaku Co SmartLab. X-ray photoelectron spectroscopy (XPS) data were obtained on Thermo ESCALAB 250 instrument with a monochromatized AlKα line source (200 W). C and N elemental analyses were carried out in an elemental analyzer (Elementar Vario EL). NMR experiments were performed under ambient condition on a Bruker AvanceIII-600 HD spectrometer, and liquid NMR was measured with sample concentrations of 25 mg/mL in concentrated sulfuric acid-d2. Electrochemical measurements were carried out on a CHI 660 Electrochemical System. Reflectance spectra were collected on an Ocean Optics USB-2000+ fiber spectrophotometer coupled with a fiber optic reflection probe connecting a tungsten halogen light source. A Shimadzu UV-3600 spectrophotometry was used to record the UV-Vis diffuse reflectance spectra (DRS) for the catalyst samples and UV-Vis absorption spectra of the methyl orange (MO) solutions. The nitrogen adsorption-desorption isotherms were measured on a Micromeritics ASAP 2020 M nitrogen adsorption analyzer at 77 K. The scanning electron microscopy (SEM) images were collected on a Hitachi S-4800 field-emission microscope. Photoluminescence (PL) spectra and PL lifetimes were measured on an Edinburgh FLS 980 spectrophotometer.


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Photodegradation activities test. The photodegradation of methyl orange (MO) under visible light irradiation (under a 300 W xenon lamp, Microsolar 300, PerfectLight, Beijing, with a 420 nm cut-off filter) was used to evaluate the photodegradation activities of the samples. Before the photodegradation, 35 mg of powdered photocatalyst was dispersed in MO aqueous solution (35 mL, 20 mg L-1) via magnetically stirring for 2 hours in dark to reach adsorption equilibrium. 3 mL aliquots were sampled at certain time intervals and centrifuged to remove the catalysts. The concentration of MO was determined by the UV-Vis absorption maxima of the solution. To study the major reactive oxygen species, three kinds of scavengers were added during the photo-degradation experiment to determine the major reactive oxygen species, ammonium oxalate (5 mM) for h+, methanol (20 vol% in the solution) for HO﹒ and benzoquinone (1 mM) for O2-﹒, respectively. Photocatalytic H2 evolution test. Photocatalytic water splitting was carried out in a gas circulation system (Labsolar-6A, PerfectLight, Beijing) with a Pyrex top-irradiation reaction vessel. For the H2 evolution test, 20 mg of powdered photocatalyst was dispersed in 100 mL aqueous solution containing 10 vol% triethanolamine as a hole scavenger. In addition, H2PtCl6 was added into the reactant solution for loading 3 wt% Pt (~1.9 wt% determined by inductively coupled plasma analysis, ICP) onto the catalysts by photodeposition. The reaction solution was evacuated several times before the irradiation under a 300 W solar simulator (Microsolar300, PerfectLight, Beijing) with a 420 nm cut-off filter. Circulating water was used during the reaction to maintain the reaction solution at room temperature. The evolved gases were sampled and analyzed on a gas chromatography (GC-7900T, TCD, molecular sieve 5 Å, argon carrier, Techcomp Ltd, China). Wavelength specific hydrogen evolutions were measured with 25 nm FWHM light filters (420, 450, 475 and 500 nm, respectively) under the same incident light.


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RESULTS AND DISCUSSION A series of cross-linked graphitic carbon nitride was synthesized by changing the amount of TMA acetate added to the dicyanodiamide (DCDA) for copolymerization. The resulting samples were denoted as TCNx, where x presented the mass ratio of TMA to the dicyanodiamide precursor, ranging from 1.6 to 16. X-ray diffraction (XRD) patterns of the presine g-C3N4 and the TCNx samples were shown in Figure 1a. The XRD pattern for pristine g-C3N4 exhibited two characteristic peaks at 13.1° and 27.4°, attributed to the (100) and (002) crystal planes of g-C3N4, representing in-plane packing of heptazine framework and interfacial stacking of g-C3N4 planes, respectively.3,59 Both peaks progressively broadened with increasing TMA usage, indicating the formation of disordered in-plane packing and turbostratic stacking in the g-C3N4. In addition, the peaks of 27.4° shifted to lower 2θ angles with TMA usage, suggesting a larger distance between the planes. Fourier transform infrared (FTIR) spectra of TCNx (Figure S1, Supporting Information) and solid-state 13C NMR (Figure S2, Supporting Information) showed no much change in the feature bands when compared with the pristine g-C3N4. FTIR spectra of all samples showed the breathing mode of the triazine units at 810 cm-1, the stretch mode of N-C=N heterorings between 900 and 1800 cm-1, and N-H stretching vibrations around 3400 cm1 7,9,10,11,13

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Solid-state 13C NMR measurements showed highly related peaks at 149.5 and 157.2

ppm corresponding to the chemical shifts of inner C3N and outer C2N-NHx in the heptazine units, respectively,60 confirming that modifications with TMA maintained the basic atomic structures of the g-C3N4. However, the TCNx powders became progressively darker yellow and then finally black as shown in Figure 1c. Figure 1b showed UV–VIS diffuse reflectance spectra (DRS) for the pristine g-C3N4 and TCNx samples. The absorption spectrum of the pristine g-C3N4 showed a


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strong absorption band with a steep edge at about 460 nm corresponding to the intrinsic bandgap followed by a weak band tail ended at around 600 nm originating from the incomplete longrange ordering of atomic structures.15 Remarkable red shifts of the optical absorption were observed for the TCNx samples prepared with increasing amounts of TMA. For the TCN1.6 sample, the band tail dominated the absorption band, indicating the intralayer long-range disordering of the g-C3N4 atomic structures with the usage of TMA. As the x increased in TCNx, the samples could harvest light at λ > 800 nm more intensely. The X-ray photoelectron spectroscopy (XPS) valence band spectra (Figure S3, Supporting Information) showed the TCNx had similar binding energy with the pristine g-C3N4, indicating the similar valence bands (VB). On the other hand, the Mott–Schottky plots determined by electrochemical measurements exhibited unchanged conduction bands (CB) of the samples (Figure S4a and b, Supporting Information), supporting that the broadened absorption range of TCNx should be mainly attributed to the enhanced band tail rather than the intrinsic bandgap. Consequently, the bandgaps calculated from the Tauc plots of the TCNx samples (Figure S5, Supporting Information) narrowed from 2.62 to 1.25 eV corresponded to the dominating band tails. To explain the broadened absorption range of the TCNx samples, organic elemental analysis were carried out. The results revealed an increase in the C/N atomic ratio with the TMA usage (see Table 1). The prestine g-C3N4 exhibited a C/N atomic ratio of 0.680, close to the C/N ratio of melon (0.667) rather than that of the fully condensed g-C3N4 (0.750), indicating the major form of strands could be the linear structure. Since increasing of the condensation degree could lead to the rise of C/N ratio, the rising C/N ratio in TCNx might be related to the increasing cross-linking degree. On the other hand, the oxygen element for all TCNx samples remained lower than the detection limit (< 0.5 wt%), indicating that the broadened light-absorptions should


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be related to the C/N atomic ratios. In addition, as we changed the TMA acetate into Oktakis (tetramethylammonium)-T8-silisesquioxane whose anion was out of carbon, TCNx with similar C/N atomic ratio could be obtained. Thus, the changes in the C/N atomic ratio of TCNx samples were based on the copolymerzation of the TMA cation (not the acetate anion) with DCDA. Thermogravimetric analysis in Figure S6 indicated that the TMA might take part in the condensation during the rearrangement of melamine.61-63 A proposed synthesis mechanism for TCNx was described in Scheme S2. Considering the increasing C/N atomic ratio might be caused by carbon-doping as the higher carbon content of TMA than DCDA,8,9,14 XPS and liquid-state 13

C NMR were used to further investigate the possibly additional carbon in TCNx.

Figure 1. a) XRD patterns, b) UV-vis DRS and c) a digital photograph of pristine g-C3N4 and TCNx samples. For b and c, from left to right, the samples were pristine g-C3N4, TCN1.6, TCN3.2, TCN9.6 and TCN16, respectively.


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Table 1. C/N atomic ratios for pristine g-C3N4 and TCNx samples.

Sample C/N atomic ratios

g-C3N4 TCN1.6 TCN3.2 TCN6.4 TCN9.6 TCN16 0.680

0.686

0.691

0.703

0.709

0.734

As shown in Figure 2a, XPS spectra demonstrated that the major carbon and nitrogen species in the TCNx samples remained similar to those in the pristine g-C3N4. The C 1s peak of 288.2 eV corresponded to the C(N)3 structure and the peak of 284.8 eV corresponded to adventitious hydrocabons.7,8,10,12 The 284.8 eV signals were intensified with the increasing TMA usage, indicating excessive usage of TMA could lead to the formation of adventitious hydrocabons. In addition, the 288.2 eV signals slightly red-shifted with the increasing TMA usage probably corresponding to the C-N structures of the N-doped adventitious hydrocabons, which had the lower binding energy than C(N)3.64,65 The deconvolution of the overlapped N 1s signals included three peaks centered at 398.6 eV for N(C)2 structure, 399.9 eV for N(C)3 structure and 401.1 eV for amino groups, respectively.7,8,10,12 The 398.6 eV signals of N(C)2 exhibited a small shift to lower binding energy with the increasing TMA usage, which could be attributed to the pyridine structure of the N-doped adventitious hydrocabons.64,66 The 401.1 eV signals of amino groups were weakened with the increasing TMA usage, which could be taken as additional evidence for the crosslinking. The weak O 1s peaks for all the samples should be attributed to the surface adsorbed oxygen (Figure S7, Supporting Information).7,10 Liquid-state 13

C NMR spectra (Figure 2b) for the pristine g-C3N4, TCN9.6 and TCN16 exhibited the resonance

signals of the corner carbons (C2N-NHx, δ=154-161 ppm) and bay carbons (C3N, δ=143-147 ppm),


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indicating similar incompletely condensed heptazine units of the dissolved part.27,59 However, TCN9.6 and TCN16 showed additional peaks between 155-150 ppm compared with pristine gC3N4, which was believed as a result of the plane distortion caused by the highly crosslinking (see Figure S8, Supporting Information). To analyse the effect of the crosslinking on the plane distortion, density-functional-theory calculations were carried out (see Figure S9, Supporting Information).67 The calculation results indicated that the geometry would change from symmetric to distorted as the edge nitrogen atoms turned to SP3 hybridization when the heptazine units had highly crosslinking structure, facilitating the n→π* electron transition involving lone pairs of the edge nitrogen atoms of the heptazine units which led to the absorption bands tails.68-70 In addition, since the major interconnection of the strands in prestine g-C3N4 was hydrogen bonds,15,18,39 the cross-linking of the g-C3N4 framework could induce intralayer lattice misfit because of the difference in length between the covalent bonds and the hydrogen bonds, leading to the increased band tails in the light-absorption due to the incomplete long-range ordering of the intralayer structure.15 Therefore, the cross-linking was believed to be the major factor of the increased light-absorption. Furthermore, the resonance signals of the major carbon species showed reduced intensity with the TMA usage when compared with the background, which was caused by the reduced solubility in concentrated sulfuric acid as evidenced by Tyndall effect in the solution (see Figure 2c). Pristine g-C3N4 could form true thermodynamic solutions in concentrated sulfuric acid through heating at 100 ℃.27 Tyndall effect was normally not observed for gC3N4/H2SO4 solution. In the contrast, TCNx showed significant Tyndall effect after the solvation, indicating insoluble parts in the TCNx as a result of the cross-linking in the g-C3N4 framework.


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Figure 2. a) C1s and N1s XPS spectra of pristine g-C3N4 and TCNx samples. b) Liquid-state 13C NMR spectra and c) digital photographs of 7.5 mg/mL solutions of g-C3N4/H2SO4 (left), TCN9.6/H2SO4 (center) and TCN16/H2SO4 (right) and evaluation of Tyndall effect.


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N2 physisorption measurements at 77 K were performed in order to further investigate the specific surface area and pore structure of TCNx. The Brunauer-Emmett-Teller (BET) specific surface area of the TCN9.6 (7.82 m2 g-1) was lower than the prestine g-C3N4 (12.77 m2 g-1) supporting the conclusion of chemical cross-linking, while most of the reported doped g-C3N4 had increased specific surface areas.8,9,14 Considering the reduced surface area would lead to limited photocatalytic activity, PC structures were introduced for improving surface area and further enhacing light-absorption based on the slow-light effect and inner reflections. As shown in Figure 3, scanning electron microscope (SEM) image indicated spherical pores with threedimension periodic order were introduced into g-C3N4 samples through PC templating (Figure S10, Supporting Information).53 In addition, the PCs structures were further evidenced by the structural colors (Figure 4a) and increased specific surface area (Table S1, Supporting Information), while the chemical structure remained similar to that of the bulk counterparts as evidenced by the organic elemental analysis (Table S2, Supporting Information), XRD patterns (Figure S11, Supporting Information), electrochemical Mott-Schottky plots (Figure S4c and 4d, Supporting Information) and XPS spectra (Figure S12, Supporting Information). Interesingly, the TCN9.6 with PC structure (denoted as TCN9.6 PC, 44.21 m2 g-1) exhibited higher specific surface area than prestine g-C3N4 with PC structure (denoted as g-C3N4 PC, 38.30 m2 g-1), probably because the cross-linking prevented the collapse of the micropores during the template inheritance.18,19 Further analysis about the N2 physisorption measurements of the TCNx PC (Table S3, Supporting Information) indicated that the crosslinking had dual effects on the BET specific area: First, the crosslinking could enhance the mechanical strength of the graphitic carbon nitride to stabilize the pore structures, increasing the BET surface area of the TCNx PC samples for x < 6.4; Second, the crosslinking could reduce the BET surface area of bulk graphitic


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carbon nitride, thus when most of the pore structures were stabilized, the BET surface area of the TCNx PC samples decreased with the increasing usage of TMA for x > 6.4. UV-vis DRS of TCN9.6 PC showed significantly enhanced light-absorption when compared with the bulk TCN9.6 and prestine g-C3N4 PC, indicating the broadened light-absorption could lead to stronger inner reflection (Figure 4b and Figure S13, Supporting Information).19,53 As shown in Figure S13, the TCNx PC had significant enhanced absorbance at the wavelength ≤ 410 nm, supporting the photonic stopband of TCNx PC because the photonic stop of TCNx PC with sphere diameter of 195 nm in air was at the wavelength ≤ 410 nm (410 nm for normal incident light, and < 410 nm for higher incident angles).54,55,59

Figure 3. SEM images of the a) pristine g-C3N4, b) TCN9.6, c) g-C3N4 PC (spherical pore diameters of 195 nm), d) TCN9.6 PC (spherical pore diameters of 195 nm). Scale bar: 1 µm. Insets: the enlarged views of the corresponding images with scale bar of 200 nm.


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Figure 4. a) The reflectance spectra and images showing structural colors of the TCN9.6 PCs in water along the photonic crystal direction. Spherical pore diameters: 1) 195 nm, 2) 207 nm, 3) 215 nm and 4) 238 nm. b) UV-Vis diffuse spectra of pristine g-C3N4, g-C3N4 PC, TCN9.6 and TCN9.6 PC powder, respectively. c) Photoluminescence (PL) spectra under 420 nm excitation and d) the PL lifetime monitored at 470 nm under 406.2 nm excitation at room temperature for pristine g-C3N4, g-C3N4 PC, TCN9.6 and TCN9.6 PC powder suspension in water, respectively. For b) to d), the spherical pore diameter of the PCs was 195 nm. In addition to the band-gap structure and specific surface area, TCNx were expected to alter the recombination of charge carriers, which could be investigated by steady-state/time-resolved photoluminescence (PL) spectra. As shown in Figure 4c, an intense PL signal was observed for prestine g-C3N4 as a result of the radiative recombination of charge carriers. TCN9.6 exhibited a weak PL intensity at 470 nm decreased by two orders of magnitude from that of pristine g-C3N4.


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However, the time-resolved PL spectra indicated that the radiative recombination of TCN9.6 became faster than the pristine g-C3N4 (Figure 4d). Hence the significant PL suppression of TCN9.6 should be attributed to the PL emmision reabsorption as the TCN9.6 had significantly broadened absorption range. The rapid radiative recombination of the TCN9.6 was considered as a result of the increased density of the band tails which could trapped charge carriers.15 The gC3N4 PC and TCN9.6 PC showed weaker PL emmisions than their bulk counterparts, due to the enlarged surface area as charge traps and PL suppression by the overlapping between the PC stop bands and the PL emissions.53,55 In addition, the PL lifetimes of g-C3N4 PC and TCN9.6 PC were longer than that of the bulk counterparts, because of the decrease in photonic density of states within the PC stop band.55-57 The photocatalytic performances of the cross-linked graphitic carbon nitride and the PCs were evaluated by the photodegradation of methyl orange (MO) and H2 evolution under visiblelight irradiation (> 420 nm) at room temperature. As shown in Figure 5b, despite the expanded light-absorption range, the cross-linked graphitic carbon nitride showed reduced photodegradation activity at pH=5.6 when compared with pristine g-C3N4, due to the lowered specific surface area which could go against the MO adsorption. In the contrast, the TCNx PC samples exhibited significantly accelerated photodegradation of MO when compared with the bulk or PC counterpart. When the pH of the solution was adjusted to 3.0, the MO degradation speed of all samples exhibited enhanced activity (Figure S14a, Supporting Information), which was believed as a result of improved MO adsorption caused by the ion-exchange process on the positive surface charge of the g-C3N4 in acidic solutions. MO photodegradation were measured for all of the TCNx PC samples with the highest MO degradation rate observed for TCN9.6 PC (Figure S14b, Supporting Information), suggesting that there might be an optimum level of


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cross-linking in g-C3N4 for achieving high photocatalytic activity. The activity of the TCNx PC samples with higher cross-linking degree showed decreased activity when compared with TCN9.6 PC, because the increased light absorption > 550 nm of TCN12.8 PC/TCN16 PC had insufficient driving force for the photocatalysis and the band-tail with low energy level could trap the photogenerated electron/hole pair leading to decreased photocatalytic activity. In addition, the comparison between TCN9.6 with various PC structures in Figure S14c indicated the optimized PC structure with sphere diameter of 195 nm for visible light (> 420 nm) driven photocatalytic activity. To study the major reactive oxygen species during the MO photodegradation, three kinds of scavengers were added during the photo-degradation experiment, ammonium oxalate for h+,71 methanol for HO﹒,72 and benzoquinone (1 mM) for O2-﹒,73 respectively. As shown in Figure S15, the ammonium oxalate had no significant impact on the MO photodegradation and methanol partially reduced the photocatalytic activity, while benzoquinone evidently reacted due to the generation of O2-﹒ in the solution, indicating that the O2-﹒ could be the major species during the MO photodegradation by TCN9.6 PC and HO﹒ could be the minor species.


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Figure 5. a) The illustration of TCNx PC with the metal-g-C3N4 heterojunctions in photocatalysis. b) The concentration of MO versus the irradiation time for different photocatalyst samples under visible light irradiation (λ > 420 nm). pH=5.6, C0= 20 mg L-1. c) Hydrogen evolution of pristine g-C3N4, TCN9.6, TCN9.6 PC and g-C3N4 PC powders under visible light irradiation (λ > 420 nm) with 3 wt% Pt (~1.9 wt% determined by ICP), and triethanolamine as a sacrificial electron donor. The inset showed the average HER over ten hours. d) Wavelength dependence of the initial HER on TCN9.6 PC. For b) to d), the spherical pore diameter of the PCs was 195 nm. Light-induced hydrogen revolution rate (HER) was measured with Pt nanoparticles to form metal-semiconductor heterjunction. Figure S16 demonstrated that the Pt nanoparticles in diameters about 5 nm were dispersed on the sample. The amount of Pt added as H2PtCl6 was controlled to be 3 wt% of the samples but Pt loaded on the samples determined by IPC were 1.97


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wt% for TCN9.6 PC, 1.90 wt% for g-C3N4 PC, 1.85 wt% for bulk TCN9.6 and 1.92 wt% for pristine g-C3N4, respectively. The Pt loaded on the samples were lower than expected, but the amount of Pt on all samples were close enough for the HER comparison. As shown in Figure 5c, the HER of TCN9.6 was 7.20 µmol h-1, with an enhancement factor of 1.07 times compared with pristine g-C3N4 (6.76 µmol h-1). Despite the broadened absorption range, the TCN9.6 showed limited photocatalytic activity due to the deficiency of exposed surface and primary amines active sites, as the accepted postulate of “surface terminations and defects seem to be the real active sites”.18,74,75 The initial HER of TCN9.6 PC (with sphere diameter of 195 nm) reached 166.9 µmol h-1 (with an average permformance of 120.1 µmol h-1 over ten hours), with an enhancement factor of 24.7 times compared with pristine g-C3N4, and an enhancement factor of 1.8 times compared with g-C3N4 PC (initial HER of 88.35 µmol h-1 with an average permformance of 66.3 µmol h-1 over ten hours), indicating that even though the TMA modification itself could not effectively improve the photocatalysis activity, the improved absorption tail by TMA modification could boost the photocatalysis activity with the PC structure due to the increased surface area and inner reflections. In addition, the specific surface area of TCN9.6 PC was only 3.46 times larger than the pristine g-C3N4, demonstrating that the enhanced HER was mainly attributed to the synergistic enhancement of broadened absorption range by chemical cross-linking, as well as the consequently stronger inner reflection and slowlight effects by effective overlapping with the PCs stop bands.47-53 The TCN9.6 PC was one of the most effective g-C3N4 materials (Table S4, Supporting Information), and the HER of the Pt loeaded TCN9.6 PC remained stable over 10 h, confirming its chemical and mechanical stability. Figure 5d showed the wavelength dependence of the HER for TCN9.6 PC. The HER of TCNx at 420, 450, 470 and 500 nm were measured. The apparent quantum efficiency (AQE) of TCN9.6


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PC at 420 nm was calculated to be 7.2%. The HER at 470 nm remained 60% of that at 420 nm, supporting the enhancing light-absorption by broadened absorption range and the PC structure. CONCLUSIONS In summary, the thermal copolymerization of DCDA with TMA salts could modify the gC3N4 with cross-linking feature characterized by i) increased C/N ratio without new cabon species, ii) decreased surface area and iii) Tyndall effect in concentrated sulfuric acid solutions. The cross-linked graphitic carbon nitride favored the light harvesting via the red-shifted absorption edge and greatly suppressed the photoluminescence. In addition, the cross-linking reduced the structure collapse during template removal, resulting in the higher surface area with the templating method. Furthermore, the cross-linked graphitic carbon nitride with PCs structures induced stronger inner reflection and effective PCs stop bands overlapping, giving a significantly enhanced photocatalytic HER under visible light. This work provides an insight for the condensed structure of g-C3N4 and open up a new window for turning the band structures of g-C3N4 semiconductors for effective photocatalytic, optoelectronic and photoelectrocatalytic applications. ASSOCIATED CONTENT Supporting Information. Supplementary data associated with this article can be found in the online version. The following files are available free of charge. Measurement of conduction band and external quantum yield; calculation of the diffraction maxima of photonic crystals; FTIR, VB-XPS and O 1s XPS of pristine g-C3N4 and TCNx


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samples; solid-state 13C NMR of pristine g-C3N4 and TCN9.6; UV-Vis diffuse spectra of pristine g-C3N4, TCNx and TCNx PC; thermogravimetric analysis during the calcination of the samples; proposed mechanism for the condensation of TCNx; computational optimized geometries of poly(heptazine imide) with different crosslinking degree; BET, C/N ration, XRD, C 1s and N 1s XPS and MO photodegradation of g-C3N4 samples; SEM and reflectance spectra of PC (PDF) AUTHOR INFORMATION Corresponding Author E-mail: [email protected] (Dr. Wei Hong) [email protected] (Prof. Xudong Chen) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant No. 51233008 and 51503229) and the Natural Science Foundations of Guangdong Province of China (Grant No. 2014A030311035 and 2015A030310212). REFERENCES [1] Ong, W. J.; Tan, L. L.; Ng, Y. H.; Yong, S. T.; Chai, S. P. Graphitic Carbon Nitride (g‑C3N4)‑Based Photocatalysts for Artificial Photosynthesis and Environmental Remediation: Are We a Step Closer To Achieving Sustainability, Chem. Rev. 2016, 116, 7159-7329.


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Cross-Linked Graphitic Carbon Nitride with Photonic Crystal Structure

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