Enhanced Visible-Light-Driven Hydrogen Production of Carbon Nitride

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Enhanced Visible-Light-Driven Hydrogen Production of Carbon Nitride by Band Structure Tuning Hongmei Wang, Wei Zhou, Peng Li, Xin Tan, Yanyu Liu, Wenping Hu, Jinhua Ye, and Tao Yu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04224 • Publication Date (Web): 10 Jul 2018 Downloaded from http://pubs.acs.org on July 11, 2018

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The Journal of Physical Chemistry

Enhanced Visible-Light-Driven Hydrogen Production of Carbon Nitride by Band Structure Tuning Hongmei Wang b,c, Wei Zhou*c, Peng Lid, Xin Tanb, Yanyu Liuc, Wenping Huc, Jinhua Yed,e,f and Tao Yu*a,f a

School of Chemical Engineering and Technology, Tianjin University, 92 Weijin

Road, Nankai District, Tianjin 300072, PR China b

School of Environmental Science and Engineering, 135 Elegant Road, Jinnan

District, Tianjin 300350, PR China c

School of Science, Tianjin University, 135 Elegant Road, Jinnan District, Tianjin

300350, PR China d

TU-NIMS Joint Research Center, School of Materials Science and Engineering,

Tianjin University, 92 Weijin Road , Nankai District , Tianjin 300072 , PR China e

Environmental Remediation Materials Unit, International Center for Materials

Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan f

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin),

Tianjin, 300072, PR China.

ABSTRACT

In this study, the band gap of carbon nitride has been decreased to 2.21 eV by merely optimizing the synthesis temperature to achieve a much higher utilization of visible 1 ACS Paragon Plus Environment

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light. To investigate the effect of synthesis temperature on the band gap structure and hydrogen production, a series of carbon nitrides were synthesized by directly heating melamine to between 500 and 650°C in Ar. The band gap structures of the samples prepared were studied based on characterization analysis in combination with the results of theoretical calculations, which revealed that their variation was caused by nitrogen vacancies. Higher synthesis temperatures cause the loss of -NH2 groups, and the nitrogen vacancies make the valence band level significantly more negative, thus allowing a remarkably enhanced H2 production rate under visible light (λ>480 nm). This work explains and demonstrates the reduction of the carbon nitride band gap by structural means and presents a simple method of developing high-performance visible light photocatalysts for hydrogen production.

INTRODUCTION The production of clean hydrogen energy through solar photocatalytic water splitting is of great importance for the sustainable development of human society.1-7 There are two important issues that must be addressed to realize the harvesting of sunlight by photocatalytic materials. The first is that the band gap structure of the catalyst should be suitable for the absorbance of visible light, i.e., the band gap should have a value between 1.6 and 3.2 eV. The second is that the H+/H2 and O2/H2O electrode potentials should lie between the edges of the conduction band (CB) and the valence band (VB).8-11 Thus, the search for suitable photocatalytic materials is a vital issue in current research. 2 ACS Paragon Plus Environment

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Metal-free graphitic carbon nitride is an efficient visible light-driven photocatalytic material with a band gap of ca. 2.7 eV.12-15 Furthermore, it exhibits suitable CB and VB levels for generating hydrogen under visible light irradiation. Consequently, there are many literature reports of carbon nitride showing excellent performance in H2 production.13, 16-22 Despite the considerable interest that these layered materials have attracted, there remains the need to tune the band gap of carbon nitride to allow a higher utilization of visible light. Scholars have undertaken extensive research on this issue.23 For instance, heteroatoms such as boron,17,

24-25

phosphorus,26 sulfur,27-28

iodine,29 iron30 and manganese31 have been introduced in the attempt to narrow the band gap, but most of these studies did not yield satisfactory results.32 However, there is an easy and effective way to accomplish this aim that involves changing the temperature used in carbon nitride synthesis. Wang et al.12, 33 found that an increase in the synthesis temperature resulted in a decrease in the band gap of their carbon nitride samples, and Dong et al.34 reported similar results. However, neither group provided a detailed explanation for this phenomenon. Thonhauser et al.35 analyzed the relationship between the structure and the band gap by theoretical calculations based on X-ray diffraction (XRD) results. Their work explored the relationship between the band gap and the structure of graphitic carbon nitride with reference to the separation between two adjacent layers and the stacking configuration.35 The structure of the inner layers may also play an important role here, e.g., vacancies may be important here in the aspect of introducing additional energy levels and / or as reaction sites. 3 ACS Paragon Plus Environment


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The influence of synthesis temperature on the band structure and hydrogen production performance of photocatalytic materials is of great importance for their design and application. But related reports are still lacking due to the characteristics of the sample itself and the limitation of characterization methods. The formation energies of the different crystal forms of carbon nitride as predicted by theoretical calculations are very similar. Thus, the competitive growth of different phases is thought to happen during crystal growth, and XRD spectra overlap is thought to frequently occur. Taking the poor crystallinity of carbon nitride synthesized in the laboratory into consideration, it is different to get accurate structural information from the identification and assignment of the characteristic peaks in XRD patterns. Detail information of the network of carbon nitride are very hard to obtain by analyzing suitable pattern fringes and diffraction rings using transmission electron microscopy because carbon nitride materials are very unstable under the test conditions. And what’s more, there are also discrepancies between the crystal structures of samples prepared under laboratory conditions and those of ideal crystals which also caused some difficulty for the comprehensive analysis and evaluation. Compared to other methods, X-ray photoelectron spectroscopy (XPS) is a more effective way here for the analysis of the structures, compositions, and bonding states of materials that contain nitrogen atoms and carbon atoms, all of which can be determined by fitting the C1s and N1s spectral lines. Accordingly, discussions and descriptions based on a combination of these characterization techniques augmented by theoretical calculations are more robust and informative. 4 ACS Paragon Plus Environment

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In this study, a series of carbon nitride materials with different band structures were synthesized by directly heating melamine to temperatures between 500 and 650°C in Ar. The influence of synthesis temperature on the crystal structure and morphology was explored through a series of characterization methods. The band gap tuning due to temperature variation was studied based on the aforementioned work in combination with theoretical calculations. Moreover, the relationship between the hydrogen production performance and the structure of the materials was also discussed based on the experiment and the theoretical calculation. EXPERIMENT SECTION Materials Synthesis Melamine (2 g, Wako) was placed into an alumina boat with a cover and heated to a certain temperature between 500 and 650°C in a muffle furnace for 4 h at a heating rate of 4°C·min−1 under Ar flowing at 50 mL·min−1. The resultant samples were denoted as CN(x), where x is the calcination temperature. Characterization XRD patterns of the samples were recorded on an X-ray diffractometer (Rint 2000, Altima III, Rigaku Co., Japan) using a Cu Kα source. UV-Vis diffuse reflectance spectra were recorded on a UV-2600 spectrophotometer (Shimadzu) with BaSO4 as the reflectance standard reference. The morphology and the C/N atomic ratios of the samples

were investigated

by

scanning

electron

microscopy

(SEM)

and

energy-dispersive X-ray spectroscopy (EDS, S4800, Hitachi Co., Japan). Nitrogen 5 ACS Paragon Plus Environment


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adsorption-desorption isotherms were recorded using nitrogen physisorption (Autosorb-iQ 2-MP; Quantachrome Corp., USA). The oxidation states of C and N in the samples were determined using XPS (PHI Quantera SXM, ULVAC-PHI Inc., Japan). Photoluminescence (PL) spectra were recorded on a JASCO FP-6500 spectrofluorometer. Photoelectrochemical measurement The sample anodes were prepared using a spin-coating method. Photoelectrochemical measurements were performed with a CHI electrochemical station (ALS/CH model 650A, Japan) using a three-electrode cell with a 0.1 M Na2SO4 solution as the electrolyte. The graphitic carbon nitride photoanodes, a Pt sheet, and a Ag/AgCl electrode (RE-1C; BAS Inc.) were used as the working electrode, counter electrode, and reference electrode, respectively. The illumination source was AM 1.5 solar simulation (WXS-80C-3 AM 1.5G, 100 mW cm−2). The Mott–Schottky (M-S) plots were derived from impedance potential testing at 962 Hz. The photocatalytic H2 evolution test was carried out in a glass reactor with a closed gas circulation system. The light source was a 300 W Xe lamp. A 0.5 wt% Pt-loaded graphitic carbon nitride sample (100 mg) was dispersed with a magnetic stirrer in an aqueous triethanolamine solution (30 mL of triethanolamine as an electron donor and 220 mL of H2O). The evolved gas including H2 was analyzed by an online gas chromatographer (GC-8A; Shimadzu) equipped with a thermal conductivity detector. The apparent quantum efficiency (AQE) was measured and calculated according to Eq. 1:13 6 ACS Paragon Plus Environment

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AQE% = × 100 =

×

× 100

(1)

Calculation details In this work, all calculations were performed using the projector augmented wave pseudopotentials, as implemented in the Vienna ab initio Simulation Package.36-38 The electron wave functions were expanded in plane waves up to a cutoff energy of 450 eV. Monkhorst-Pack k-point sets of 9 × 9 × 5 were used for the conventional cell, and k-points of 2 × 2 × 1 were set for the slab model with a vacuum layer of 14 Å. For the bulk phase, the Van der Waals correction of Grimme’s DFT-D2 was used in this work.39 The

atomic

positions

were

relaxed

with

the

Perdew-Burke-Ernzerhof

exchange-correlation function of the GGA approximation until the residual forces were below 0.01 eV Å−1. The electronic structure calculations were then performed by HSE06 functionals with a mixing parameter of 0.25. since the hybrid functional could reproduce the band gap of bulk g-C3N4 to 2.9 eV which is much close to the experimental values. RESULTS AND DISCUSSION

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Figure 1a shows the XRD patterns of the graphitic carbon nitride samples CN(500), CN(550), CN(600), and CN(650). The patterns of the samples exhibit peaks at ~27°, which correspond to the (002) crystal planes of graphitic carbon nitride. Highly graphite-like structures are observed, suggesting the formation of a well-developed carbon nitride layer structure.41 This peak moves from 27.12 to 27.74° as the temperature increases from 500 to 650°C, corresponding to a reduction of the separation between two adjacent layers from 3.28 to 3.22 Å.

Figure 1. (a) X-ray diffraction patterns of CN(500), CN(550), CN(600), and CN(650); (b) (Ahν)1/2 vs. photon energy for CN(500), CN(550), CN(600), and CN(650) (inset: images of the samples). The band gaps are given in the legend; (c) Mott–Schottky plots for CN(500), CN(550), CN(600), and CN(650); and (d) Electronic band structures of CN(500), CN(550), CN(600), and CN(650). 8 ACS Paragon Plus Environment

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The typical UV-Vis absorption edge of carbon nitrides is gradually blue-shifted from ca. 440 nm to more than 560 nm (Figure S1). CN(650) shows significantly enhanced absorption in the visible light region compared with the other samples. The band gap energy is estimated from the intercept of the tangents to the plots of (Ahν)1/2 vs. photon energy, as shown in Figure 1b. The corresponding band gap energy of the samples decreases from 2.83 to 2.21 eV. The inset of the figure clearly shows that the color of the samples changes from light yellow to pale brown. Figure 1c shows the M-S plots for the samples. When we consider that the products are n-type semiconductors, as shown by the positive slopes of the linear M-S plots, the values of the found flat-band potentials correspond, approximately, to the CB positions. The value of the CB positions shifts negatively as the synthesis temperature rises. The flat-band potentials of CN(500), CN(550), CN(600), and CN(650) are −0.69, −0.83, −0.90, and −0.92 eV, respectively. Their band gaps obtained from UV-Vis absorption curves are shown in Figure S1, and the details are shown clearly in Figure 1d. It is clear that the CB and VB potentials are dependent on the synthesis temperature. Both the CB and VB levels become more negative. The CB potentials are tuned from −0.69 to −0.92 eV, while the VB potentials have been tuned much more significantly from +2.14 to +1.29 eV. To confirm the contribution of interlayer space to band gap narrowing, the relationship between the two factors was calculated. The interlayer distances of the samples showed in XRD patterns decrease as the synthesis temperature increases. However, no further structural information can be obtained from the XRD spectra (the 9 ACS Paragon Plus Environment


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peak around 45°, which indicates the stacking configuration of the carbon nitride samples, was not detected in our work).

Figure 2. Layer stacking of carbon nitride used in this study. The layer stacking of carbon nitride exhibits several stable configurations located at local minima in the potential energy surface. The most stable stacking arrangement with taking into account the Van der Waals correction is shown in Fig.2. The calculation results show that the layer separation has only a minor influence on the band structure because the band gap changes by only 0.05 eV, from 2.9 eV to 2.85 eV, when the interlayer distance is compressed by 8%. This clearly indicates that the layer separation is not the main reason for the structural band gap tuning of the samples. Thus, the key factor may come from differences inside of the layers. Based on this assumption, XPS analysis of the carbon nitride samples prepared was performed to identify the specific chemical environments and oxidation states of the C and N components. 10 ACS Paragon Plus Environment

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Figure 3. (a) XPS corresponding to the core levels of the C1s region for CN(650); (b) XPS corresponding to the core levels of the N1s region of CN(650). Figure 3 shows the XPS spectrum of CN(650) as an example. The C1s XPS spectra in Figure 3a show one predominant C1 peak with a binding energy of ca. 284.5 eV for C–C. The C2 peak indicates N–C=N bonding at 287.9 eV.42 The N1s spectra shown in Figure 3b are also fitted to help elucidate the bonding structures. The contribution located at 398.2 eV is assigned to sp2-hybridized aromatic N atoms bound to carbon atoms (C–N=C), while the peaks centered at 400.1 and 400.9 eV are assigned to the tertiary nitrogen N–(C)3 groups that link the structural motif and the N of C–N–H bonds, respectively.34 The different N contents of CN(500), CN(550), CN(600), and CN(650) obtained from XPS (Figures S3–S5) are shown in Table 1. Table 1. Contents of N elements in CN(500), CN(550), CN(600), and CN(650) from XPS result C–N=C

N (C)3

C–N–H

500℃ ℃

61.96%

23.78%

14.25%

550℃ ℃

65.69%

18.31%

16.00%

600℃ ℃

57.98%

26.94%

15.07%

650℃ ℃

73.04%

18.15%

8.81%



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The ratio of N in C–N–H bonds is only 8.81% for CN(650).41 This indicates the loss of some nitrogen atoms with C–N–H bonding from the sample during the amorphization, which is also supported by the variation in the C/N atomic ratio determined by elemental analysis in Table 2. Table 2. Contents of C, N, and H elements in CN(500), CN(550), CN(600), and CN(650) from elemental analysis results

N(%)

C(%)

H(%)

C/N ratio

C/H ratio

500℃ ℃

55.795

31.480

1.788

0.564

17.606

550℃ ℃

56.465

32.495

1.521

0.575

21.364

600℃ ℃

55.760

32.340

1.477

0.580

21.904

650℃ ℃

55.475

32.435

1.455

0.585

22.299

To obtain more insight into the band gap narrowing in these carbon nitride samples,35 the influence of -NH2 group on the electronic structure of carbon nitride was simulated. A slab model of carbon nitride with -NH2 group was constructed here (Figure 4a), which also has been successfully used to simulate nitrogen vacancy in g-C3N4 in previous literatures 32.

The minimum distorted layer of the crystal lattice

is denoted by the dotted box, and the lost NH2 group is denoted by the red ellipse. The calculated density of states is shown in Figure 4b. Although the values of band gap are relatively larger than the bulk g-C3N4 due to the introduction of -NH2 group and quantum size effect, the intrinsic trend of band gap change still can be obtained. Removing one -NH2 leads to a 5.6% decrease in nitrogen content. Band gap narrowing of carbon nitride occurs after has lost one NH2, and the shift is in 12 ACS Paragon Plus Environment

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accordance with the trend shown in Fig.1d. From the calculation results (Figure S6), the band gap of carbon nitride is determined by the hybridization between N 2p and C 2p orbitals. Furthermore, the N 2p states dominate the VB maximum, while the CB minimum mainly consists of the C 2p states. Therefore, the substitution of–NH2 by–H in the samples will decrease the hybridization contributions and narrow the intrinsic band gap, which is beneficial for enhancing optical absorption.

Figure 4. (a) Schematic of the dimensional sheets of the carbon nitride melon. H, C, and N atoms are represented by the white, heavy gray, and light gray balls, respectively. (b) Partial density of states for pure carbon nitride and carbon nitride that has lost one -NH2.



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Figure 5. SEM images of (a)CN(500), (b)CN(550), (c)CN(600), and (d)CN(650).

The SEM images of the samples are shown in Figure 5. The surface of CN(500) is flat, while the other samples present layered porous structures, which appear more obviously with temperature increase. This is in accordance with the tendency of specific surface area change. The surface areas of CN(500), CN(550), CN(600), and CN(650) are 5.1, 6.3, 7.8, and 26.6 m2g−1, respectively. The surface area of CN(650) is much larger than that of the other samples. This is due to the layered structure damage caused by increasing calcination temperatures. And the loss of nitrogen atoms takes place simultaneously in this process. This result also supports our previous assertion.


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Figure 6. (a) Hydrogen evolution over CN(500), CN(550), CN(600), and CN(650) under visible light (3 wt% Pt as a co-catalyst and 10 vol% triethanolamine as an electron donor; λ> 400 nm); (b) Hydrogen evolution over CN(500), CN(550), CN(600), and CN(650) under visible light (3 wt% Pt as a co-catalyst and 10 vol% triethanolamine as an electron donor; λ> 480 nm); (c) Cyclic hydrogen evolution using CN(650) under visible light (λ> 400 nm); (d) Wavelength dependence of the apparent quantum efficiency (AQE) of CN(650).

The photocatalytic activities of the prepared carbon nitride samples for hydrogen production from water under visible light (λ> 400 nm) are shown in Figure 6a. The samples exhibit stable hydrogen generation. The hydrogen evolution rate obtained over CN(650) is 1.73 mmol h−1 g−1, which is 6.7, 5, and 3.2 times those over CN(500), CN(550), and CN(600), respectively. The photocatalytic activities of carbon nitride for hydrogen evolution from water under visible light (λ> 480 nm) were also 15 ACS Paragon Plus Environment


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examined (Figure 6b). CN(500) and CN(550) show almost no activity under such conditions. The hydrogen evolution rate over CN(650) is 203.6 µmol h−1 g−1, which is 28.2 times that over CN(600) (7.21 µmol h−1 g−1). This indicates that CN(650) exhibits outstanding advantages in activity under light of longer wavelengths. The fluorescence intensity of CN(650) is much weaker than that of the other samples (Figure S7). This shows that CN(650) has better photogenerated electron-hole separation, which contributes to improved photocatalytic activity. As shown in Figure 6c, CN(650) is quite stable. Except for the ~10% drop in activity during the second run, there is no apparent further deactivation in the third run. The AQE of CN(650) corresponds to its optical absorption spectrum, suggesting that the H2 production is primarily driven by photoinduced electrons in the carbon nitride polymer (Figure 6d) 43

.

Thus, the hydrogen production performance of a carbon nitride photocatalyst under visible light can be significantly improved by simply increasing its synthesis temperature. To explore whether the performance can be further promoted, we increased the synthesis temperature to 700°C. However, no sample residue remained. When the synthesis temperature is adjusted to 660°C, the exterior appearance, UV-Vis absorption, and hydrogen production performance of the sample prepared at 660°C are essentially the same as those of CN(650). The other samples show similar optical properties to those of CN(650) when the synthesis temperature is raised to 670 and 680°C, but the product decomposes significantly. Consequently, the optimum


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synthesis temperature for the photocatalytic hydrogen production performance of carbon nitride is ca. 650°C under the conditions of these experiments. CONCLUSIONS In this work, the influence of synthesis temperature on the band structure and hydrogen production performance of carbon nitride materials has been investigated in detail. With increasing synthesis temperature, the lamellar structure of the carbon nitride samples cracked, and their surface areas increased. Higher synthesis temperatures cause the loss of -NH2 groups from the samples, and the nitrogen vacancies decrease the hybridization contributions and narrow the intrinsic band gap to 2.21 eV. The optical absorption, utilization of visible light, and the hydrogen production performance of the samples was enhanced significantly. The photocatalytic water splitting rate over CN(650) is 28.2 times of CN(600) while CN(550) and CN(500) show almost no activity under visible light (λ> 480 nm). Increased specific surface area, suitable band structure, and higher separation efficiency of photogenerated carriers contribute to the improvement of the photocatalytic activity of the photocatalyst. This work demonstrates the possibility of reducing the band gap of carbon nitride by structural means and presents a simple method of developing high-performance visible light-active photocatalysts for hydrogen production. ASSOCIATED CONTENT Supporting Information 17 ACS Paragon Plus Environment


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Details such as UV-Vis absorption of CN(650), CN(600), CN(550), CN(500), XPS of CN(600), CN(550), CN(500), calculated partial density of states of carbon nitride with –NH2, steady –state Photoluminescence (PL) spectra of CN(650), CN(600), CN(550), CN(500) AUTHOR INFORMATION Corresponding Author *Email: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21406164, 21466035, 21706188), the National Key Basic Research and Development Program of China (973 program, No. 2014CB239301). and scholarship from The China Scholarship Council (CSC). REFERENCES (1). Lin, Y.; Shi, H.; Jiang, Z.; Wang, G.; Zhang, X.; Zhu, H.; Zhang, R.; Zhu, C., Enhanced Optical Absorption and Photocatalytic H 2 Production Activity of G-C 3 N 4 /Tio 2 Heterostructure by Interfacial Coupling: A Dft+ U Study. International Journal of Hydrogen Energy 2017. (2). Iwase, A.; Ng, Y. H.; Ishiguro, Y.; Kudo, A.; Amal, R., Reduced Graphene Oxide as a Solid-State Electron Mediator in Z-Scheme Photocatalytic Water Splitting under Visible Light. J Am Chem Soc 2011, 133, 11054-7. (3). Yan, M.; Hua, Y.; Zhu, F.; Sun, L.; Gu, W.; Shi, W., Constructing Nitrogen Doped Graphene Quantum Dots-Znnb 2 O 6 /G-C 3 N 4 Catalysts for Hydrogen Production under Visible Light. Applied Catalysis B: Environmental 2017, 206, 531-537. (4). Listorti, A.; Durrant, J.; Barber, J., Artificial Photosynthesis: Solar to Fuel. Nat Mater 2009, 8, 929-30. (5). Kim, D.; Sakimoto, K. K.; Hong, D.; Yang, P., Artificial Photosynthesis for Sustainable Fuel and


Page 19 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Chemical Production. Angew Chem Int Ed Engl 2015, 54, 3259-66. (6). Li, H.; Zhou, Y.; Tu, W.; Ye, J.; Zou, Z., State-of-the-Art Progress in Diverse Heterostructured Photocatalysts toward Promoting Photocatalytic Performance. Advanced Functional Materials 2015, 25, 998-1013. (7). Fujishima, A.; Honda, K., Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37. (8). Guo, Z.; Sa, B.; Pathak, B.; Zhou, J.; Ahuja, R.; Sun, Z., Band Gap Engineering in Huge-Gap Semiconductor Srzro3 for Visible-Light Photocatalysis. International Journal of Hydrogen Energy 2014, 39, 2042-2048. (9). Li, S.; Cao, W.; Wang, C.; Qiu, H., Band Gap Engineering of Ba5nb4o15 for Efficient Water Splitting under Visible Light. Journal of Alloys and Compounds 2015, 644, 757-762. (10). Bak, T.; Nowotny, J.; Rekas, M.; Sorrell, C. C., Photo-Electrochemical Hydrogen Generation from Water Using Solar Energy. Materials-Related Aspects. International Journal of Hydrogen Energy 2002, 27, 991-1022. (11). Wang, Y.; Wang, X.; Antonietti, M., Polymeric Graphitic Carbon Nitride as a Heterogeneous Organocatalyst: From Photochemistry to Multipurpose Catalysis to Sustainable Chemistry. Angewandte Chemie 2012, 51, 68. (12). Wang, X.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J. M.; Domen, K.; Antonietti, M., A Metal-Free Polymeric Photocatalyst for Hydrogen Production from Water under Visible Light. Nature materials 2009, 8, 76-80. (13). Xiang, Q.; Yu, J.; Jaroniec, M., Preparation and Enhanced Visible-Light Photocatalytic H2-Production Activity of Graphene/C3n4composites. The Journal of Physical Chemistry C 2011, 115, 7355-7363. (14). Liu, J.; Liu, Y.; Liu, N.; Han, Y.; Zhang, X.; Huang, H.; Lifshitz, Y.; Lee, S. T.; Zhong, J.; Kang, Z., Water Splitting. Metal-Free Efficient Photocatalyst for Stable Visible Water Splitting Via a Two-Electron Pathway. Science 2015, 347, 970. (15). Wang, X.; Blechert, S.; Antonietti, M., Polymeric Graphitic Carbon Nitride for Heterogeneous Photocatalysis. Acs Catalysis 2012, 2, 1596–1606. (16). Martin, D. J.; Qiu, K. P.; Shevlin, S. A.; Handoko, A. D.; Chen, X. W.; Guo, Z. X.; Tang, J. W., Highly Efficient Photocatalytic H-2 Evolution from Water Using Visible Light and Structure-Controlled Graphitic Carbon Nitride. Angew Chem Int Edit 2014, 53, 9240-9245. (17). Wang, Y.; Wang, X. C.; Antonietti, M., Polymeric Graphitic Carbon Nitride as a Heterogeneous Organocatalyst: From Photochemistry to Multipurpose Catalysis to Sustainable Chemistry. Angew Chem Int Edit 2012, 51, 68-89. (18). Cao, S.; Yu, J., G-C3n4-Based Photocatalysts for Hydrogen Generation. J Phys Chem Lett 2014, 5, 2101-7. (19). Jiang, W.; Luo, W.; Zong, R.; Yao, W.; Li, Z.; Zhu, Y., Polyaniline/Carbon Nitride Nanosheets Composite Hydrogel: A Separation-Free and High-Efficient Photocatalyst with 3d Hierarchical Structure. Small 2016, 12, 4370-8. (20). Liang, Q.; Li, Z.; Yu, X.; Huang, Z. H.; Kang, F.; Yang, Q. H., Macroscopic 3d Porous Graphitic Carbon Nitride Monolith for Enhanced Photocatalytic Hydrogen Evolution. Adv Mater 2015, 27, 4634-9. (21). Ye, J.; Li, Y.; Xu, H.; Ouyang, S.; Lu, D.; Wang, X.; Wang, D., In-Situ Surface Alkalinized G-C3n4 toward Enhancement of Photocatalytic H2 Evolution under Visible-Light Irradiation.



Page 20 of 28

J.mater.chem.a 2015, 4, 2943-2950. (22). Lu, X.; Xu, K.; Chen, P.; Jia, K.; Liu, S.; Wu, C., Facile One Step Method Realizing Scalable Production of G-C3n4 Nanosheets and Study of Their Photocatalytic H2 Evolution Activity. Journal of Materials Chemistry A 2014, 2, 18924-18928. (23). Xiong, T.; Cen, W.; Zhang, Y.; Dong, F., Bridging the G-C3n4interlayers for Enhanced Photocatalysis. ACS Catalysis 2016, 6, 2462-2472. (24). Wang, Y.; Zhang, J.; Wang, X.; Antonietti, M.; Li, H., Boron- and Fluorine-Containing Mesoporous Carbon Nitride Polymers: Metal-Free Catalysts for Cyclohexane Oxidation. Angewandte Chemie 2010, 49, 3356-9. (25). Zhang, J.; Chen, X.; Takanabe, K.; Maeda, K.; Domen, K.; Epping, J. D.; Fu, X.; Antonietti, M.; Wang, X., Synthesis of a Carbon Nitride Structure for Visible-Light Catalysis by Copolymerization. Angewandte Chemie 2010, 49, 441-4. (26). Zhang, Y.; Antonietti, M., Photocurrent Generation by Polymeric Carbon Nitride Solids: An Initial Step Towards a Novel Photovoltaic System. Chemistry, an Asian journal 2010, 5, 1307-11. (27). Zhang, J.; Sun, J.; Maeda, K.; Domen, K.; Liu, P.; Antonietti, M.; Fu, X.; Wang, X., Sulfur-Mediated Synthesis of Carbon Nitride: Band-Gap Engineering and Improved Functions for Photocatalysis. Energy Environ. Sci. 2011, 4, 675-678. (28). Liu, G.; Niu, P.; Sun, C.; Smith, S. C.; Chen, Z.; Lu, G. Q.; Cheng, H. M., Unique Electronic Structure Induced High Photoreactivity of Sulfur-Doped Graphitic C3n4. J Am Chem Soc 2010, 132, 11642-8. (29). Zhang, G.; Zhang, M.; Ye, X.; Qiu, X.; Lin, S.; Wang, X., Iodine Modified Carbon Nitride Semiconductors as Visible Light Photocatalysts for Hydrogen Evolution. Adv Mater 2014, 26, 805-9. (30). Chen, X.; Zhang, J.; Fu, X.; Antonietti, M.; Wang, X., Fe-G-C3n4-Catalyzed Oxidation of Benzene to Phenol Using Hydrogen Peroxide and Visible Light. J Am Chem Soc 2009, 131, 11658-9. (31). Zhang, W.; Zhang, Z.; Kwon, S.; Zhang, F.; Stephen, B.; Kim, K. K.; Jung, R.; Kwon, S.; Chung, K.-B.; Yang, W., Photocatalytic Improvement of Mn-Adsorbed G -C 3 N 4. Applied Catalysis B: Environmental 2017, 206, 271-281. (32). Niu, P.; Yin, L. C.; Yang, Y. Q.; Liu, G.; Cheng, H. M., Increasing the Visible Light Absorption of Graphitic Carbon Nitride (Melon) Photocatalysts by Homogeneous Self-Modification with Nitrogen Vacancies. Adv Mater 2014, 26, 8046-52. (33). Zhang, G.; Zhang, J.; Zhang, M.; Wang, X., Polycondensation of Thiourea into Carbon Nitride Semiconductors as Visible Light Photocatalysts. Journal of Materials Chemistry 2012, 22, 8083. (34). Dong, F.; Wu, L.; Sun, Y.; Fu, M.; Wu, Z.; Lee, S. C., Efficient Synthesis of Polymeric G-C3n4 Layered Materials as Novel Efficient Visible Light Driven Photocatalysts. Journal of Materials Chemistry 2011, 21, 15171. (35). Zuluaga, S.; Liu, L. H.; Shafiq, N.; Rupich, S. M.; Veyan, J. F.; Chabal, Y. J.; Thonhauser, T., Structural Band-Gap Tuning in G-C3n4. Phys Chem Chem Phys 2015, 17, 957-962. (36). Kresse, G.; Furthmüller, J., Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comp Mater Sci 1996, 6, 15-50. (37). Blöchl, P. E., Projector Augmented-Wave Method. Physical Review B Condensed Matter 1994, 50, 17953-17979. (38). Kresse, G.; Furthmüller, J., Efficient Iterative Schemes for Ab Initio Total-Energy Calculations


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Using a Plane-Wave Basis Set. Physical Review B Condensed Matter 1996, 54, 11169-11186. (39). Grimme S. Semiempirical GGA ‐ type density functional constructed with a long‐ range dispersion correction[J]. Journal of Computational Chemistry, 2006, 27(15):1787-99. (40). Zhang, G. G.; Zhang, J. S.; Zhang, M. W.; Wang, X. C., Polycondensation of Thiourea into Carbon Nitride Semiconductors as Visible Light Photocatalysts. J Mater Chem 2012, 22, 8083-8091. (41). Hu, S. Z.; Li, F. Y.; Fan, Z. P.; Wang, F.; Zhao, Y. F.; Lv, Z. B., Band Gap-Tunable Potassium Doped Graphitic Carbon Nitride with Enhanced Mineralization Ability. Dalton T 2015, 44, 1084-1092. (42). Kang, Y.; Yang, Y.; Yin, L. C.; Kang, X.; Liu, G.; Cheng, H. M., An Amorphous Carbon Nitride Photocatalyst with Greatly Extended Visible-Light-Responsive Range for Photocatalytic Hydrogen Generation. Adv Mater 2015. (43). Zhang, J.; Zhang, M.; Yang, C.; Wang, X., Nanospherical Carbon Nitride Frameworks with Sharp Edges Accelerating Charge Collection and Separation at a Soft Photocatalytic Interface. Adv Mater 2014, 26, 4121-6.



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Figure 1. (a) X-ray diffraction patterns of CN(500), CN(550), CN(600), and CN(650); (b) (Ahν)1/2 vs. photon energy for CN(500), CN(550), CN(600), and CN(650) (inset: images of the samples). The band gaps are given in the legend; (c) Mott–Schottky plots for CN(500), CN(550), CN(600), and CN(650); and (d) Electronic band structures of CN(500), CN(550), CN(600), and CN(650). 151x117mm (300 x 300 DPI)

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Figure 2. Layer stacking of carbon nitride used in this study. 153x74mm (150 x 150 DPI)


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Figure 3. (a) XPS corresponding to the core levels of the C1s region for CN(650); (b) XPS corresponding to the core levels of the N1s region of CN(650). 153x62mm (300 x 300 DPI)



Figure 4. (a) Schematic of the dimensional sheets of the carbon nitride melon. H, C, and N atoms are represented by the white, heavy gray, and light gray balls, respectively. (b) Partial density of states for pure carbon nitride and carbon nitride that has lost one -NH2. 155x56mm (300 x 300 DPI)


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Figure 5. SEM images of (a)CN(500), (b)CN(550), (c)CN(600), and (d)CN(650). 163x113mm (150 x 150 DPI)



Figure 6. (a) Hydrogen evolution over CN(500), CN(550), CN(600), and CN(650) under visible light (3 wt% Pt as a co-catalyst and 10 vol% triethanolamine as an electron donor; λ> 400 nm); (b) Hydrogen evolution over CN(500), CN(550), CN(600), and CN(650) under visible light (3 wt% Pt as a co-catalyst and 10 vol% triethanolamine as an electron donor; λ> 480 nm); (c) Cyclic hydrogen evolution using CN(650) under visible light (λ> 400 nm); (d) Wavelength dependence of the apparent quantum efficiency (AQE) of CN(650). 169x117mm (300 x 300 DPI)


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Enhanced Visible-Light-Driven Hydrogen Production of Carbon Nitride

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