Electrophoretic Deposition of Carbon Nitride Layers for

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Electrophoretic Deposition of Carbon Nitride Layers for Photoelectrochemical Applications Jingsan Xu, and Menny Shalom ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02853 • Publication Date (Web): 05 May 2016 Downloaded from http://pubs.acs.org on May 6, 2016

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Electrophoretic Deposition of Carbon Nitride Layers for Photoelectrochemical Applications Jingsan Xu and Menny Shalom* Email: [email protected] Department of Colloid Chemistry, Max Planck Institute of Colloids and Interfaces, Potsdam 14476, Germany KEYWORDS: Carbon nitride electrodes, electrophoretic deposition, photoelectrochemistry, heterostructures, water oxidation

ABSTRACT: Electrophoretic deposition (EPD) is used for the growth of carbon nitride (C3N4) layers on conductive substrates. EPD is fast, environmentally-friendly and allows the deposition of negatively-charged C3N4 with different compositions and chemical properties. In this method, C3N4 can be deposited on various conductive substrates, ranging from conductive glass, carbon paper, and nickel foam possessing complex 3D geometries. The high flexibility of this approach enables us to readily tune the photophysical and photoelectronic properties of the C3N4 electrodes. The advantage of this method was further illustrated by the tailored construction of a heterostructure between two complementary C3N4, with marked photoelectrochemical activity.

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INTRODUCTION In the last decade, graphitic carbon nitride (C3N4) has attracted widespread attention due to its outstanding catalytic and photocatalytic activity.1,2 C3N4-based materials have been found useful for many applications such as photodegradation of pollutants,3 water splitting,1,4 CO2 reduction5 and many more.6-8 In the last years several modifications of the C3N4 synthesis, such as hard/soft templating,4,9,10 copolymerization of monomers,11,12 heteroatom doping,13-17, supramolecular preorganization of monomers18,19 and heterojunction construction20-22 were introduced to improve its electronic and catalytic properties. Despite this, most of these modifications are exclusively applicable to powdered C3N4 and improving their photocatalytic performance.23 An alternative approach to convert light into chemical or electric energy is through the exploitation of photoelectrochemical (PEC) and photovoltaic cells. One notable advantage of these systems includes more efficient charge separation upon illumination when compared to traditional photocatalytic systems. To harness this feature for practical photoelectrochemical cells the formation of intimate contacts between the photoactive materials and the conductive substrate, in addition to separation of photoinduced excitons. Several methods were developed to allow the growth of carbon nitride on film. For instance, Zhang et al. used the doctor-blade technique to fabricate C3N4 electrodes and showed photoactivity of the C3N4 material.24 Wang’s group employed a sol process to deposit C3N4 layers on fluorine-doped tin oxide (FTO) coated glass.25 Bian et al. and our group used vapor-transport deposition to grow C3N4 layers on FTO glass and TiO2 film. In every instance, the successful deposition enhanced the photoactivity of C3N4 photoanodes in PEC cells.26,27 In addition, our group developed a liquid-mediated growth approach for uniform, continuous C3N4 films, which can be used as active layers in organic solar cells28 and light-emitting diodes.29 Unfortunately these growth methods often involve a high-

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temperature step (≥500 oC) which prohibits the use of chemically or thermally unstable substrates. Low-temperature deposition alternatives (i.e. sol process) require the use of concentrated acid, which may be incompatible with C3N4. For these reasons the development of a mild, environmentally friendly deposition method is in a great demand to fully exploit C3N4 in photoelectrochemical devices. RESULTS AND DISCUSSION Electrophoretic deposition (EPD) was employed to deposit a wide range of particles, such as carbon nanotubes,30,31 quantum dots,32,33 metal nanoparticles34 and metal-organic framework35 onto conductive substrates. The deposited particles must be slightly charged in order to acquire a successful deposition. In a typical EPD process, the solvent can produce a surface charge (that can be altered in different solvents) due to its interaction with the given particle. The solvent should also have a large electrochemical window to avoid any decomposition under the applied voltage. Generally the EPD route is significantly simpler than other deposition methods and requires no expensive or specialized equipment. We demonstrate the facile, safe and fast deposition of C3N4 layers on several conductive substrates using EPD. The electrodes were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), UV-vis absorption and steady-state fluorescence spectroscopy. The photoactivity of the electrodes was tested by measuring C3N4 in photoelectrochemical cells. The EPD process is schematically illustrated in Figure 1a. A suspension of C3N4 particles was prepared by sonication of C3N4 powder in toluene followed by centrifugation to remove the residual aggregates and was used as a reactant. We note that toluene was used as a solvent due to its large electrochemical window. A direct current (DC) voltage of 200 V was applied between the target substrate and the counter electrode. The target substrate

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was held at positive potential and the counter electrode (blank FTO) held at negative potential. The C3N4 was deposited solely on the positively charged electrode and confirms that the C3N4 particles are negatively charged, as illustrated in previous publications.36,37 The photograph of the substrates after the EPD process clearly indicates the successful deposition of C3N4 (Figure 1b).

Figure 1. (a) A scheme illustrating the process of carbon nitride EPD film growth, (b) photograph of the deposited C3N4 on FTO glass, carbon paper and nickel foam. In former deposition techniques, all the chemical, electronic and photophysical modification of the C3N4 were limited to its powder form and could not be transferred to C3N4 films. The mild condition in our EPD process allows us to design the C3N4 electrode with finely tuned properties. This is a general approach as any known C3N4 powder with specific can be successfully deposited onto a conductive substrate as long as it is partially charged. To demonstrate this, we

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used EPD to deposit C3N4 materials with different chemical, photophysical and catalytic properties. This includes C3N4 obtained by heating the cyanuric acid-melamine (1:1 molar ratio) complex (denoted as CM) and the C3N4 with carbon substitution made from cyanuric acidmelamine-barbituric acid complex, where the amount of barbituric acid ranged from 5 to 50 mol% with respect to melamine38 (labeled here as CMBx, where x indicates the mol% of barbituric acid in the starting material). We found that using this method C3N4 can be deposited on various conductive substrates, ranging from bare FTO glass to carbon paper and nickel foam that have complex 3D geometries. The flexibility of this method allowed us to tune the composition, structure and photoelectric properties of C3N4 on surfaces, which is highly beneficial for the optimization of the C3N4-based PEC cells.

Figure 2. SEM images of (a) blank carbon paper, (b-c) CMB5 deposited on carbon paper; (d) blank nickel foam, (e-f) CMB5 deposited on nickel foam. The morphology of the C3N4 particles on carbon paper was observed by SEM. As shown in Figure 2a, the blank carbon paper is composed of smooth carbon fibers, whereas after the EPD

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process the fibers were coated with C3N4 particles (Figure 2b and c) with a sheet-like morphology, originating from the raw powder (Figure S1). Similar results can be observed for nickel foam (Figure 2d-f) and FTO glass, indicating the broad applicability of the EPD approach. The SEM image of C3N4 on FTO glass is shown in Figure S2. In the FTIR spectrum of the C3N4 coated carbon paper (Figure 3a), the typical stretching modes of C-N heterocycles located at 1200-1600 cm-1 were observed, along with the vibration peak at 812 cm-1, which is the characteristic absorption peak of the (tri-s-)triazine unit. The presence of residual toluene adsorbed on C3N4 was excluded as no characteristic vibration peaks were detected. We could not identify the C3N4 on carbon paper using XRD measurements due to the overlap of the strong diffraction peak (26.4o) from the graphitic carbon substrate with that of the C3N4 coating (Figure S3a). For the FTO/C3N4 system, due to the high crystallinity of FTO, C3N4 could not be detected (Figure S3b). Nevertheless, for the nickel foam, the strong interplanar stacking peak of aromatic systems around 27.2° (indexed as (002)), was observed. The amount of the C3N4 particles deposited on the substrate can be increased by carrying out multiple cycles of EPD. As shown in Figure S4, more particles were attached to the carbon paper and Ni foam with increasing EPD cycles from 1, 3 to 5 cycles, wherein each cycle lasted for 1 h.

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Figure 3. (a) FTIR spectra of blank carbon paper, C3N4 on carbon paper and the raw C3N4 powder; (b) UV-vis spectra and (c) PL spectra of CMB5, CMB50 and CMB50/CMB5 heterostructure on FTO. Afterward, we measured and recorded the optical absorption spectra of the deposited C3N4 layers on FTO (Figure 3b). The layers that were deposited from CMB5 showed an absorption edge at 515 nm while the absorption edge of the CMB50 sample was shifted to the expected value of 540 nm.39 One main advantage of EPD is that it allows the deposition of multi-layer C3N4based materials with altered absorption and energy levels position. Tailored design of these deposited layers can lead to the enhanced light harvesting and improved charge separation. We successfully built a C3N4 heterostructure by depositing CMB5 on top of CMB50 and expectedly, the absorption spectrum of the CMB50/CMB5 heterostructure consists of the contribution of both materials. The photoluminescence (PL) spectra (Figure 3c) of the CMB5 layer showed a peak centered around 500 nm while the PL of the CMB50 layer was broader and weaker (from 450 to 600 nm), indicating the presence of other recombination paths probably due to the additional carbon as previously described.38 We noticed that the emission was further quenched for the CMB50/CMB5 bilayer structure, suggesting that the materials are connected. The latter can be explained by the formation of a new charge separation path between the two C3N4, confirming the formation of a heterostructure (Figure 4d).40 The PEC activity of various C3N4 layers deposited on carbon paper was measured using a three-electrode setup in 1 M KOH. In order to improve the C3N4 adhesion to the substrate, all the electrodes were annealed in nitrogen at 300 oC for 1 h, dramatically improving stability upon contact with electrolyte and during PEC measurements. A platinum wire was used as the counter electrode and Ag/AgCl was used as the reference electrode. A white LED (λ > 410 nm) was used

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as the light source. As opposed to FTO glass, carbon paper has a three-dimensional structure and high surface area, which allow higher loading of C3N4 along with better contact between the electrolyte and the C3N4 layers. We found that the C3N4 layers on FTO showed a transient photocurrent of only around 1 µA/cm2 under zero bias, while on carbon paper the photocurrent improved significantly to 12 µA/cm2. It should be pointed out that the bare FTO and carbon paper showed no photocurrent in the present conditions (Figure S7). We further optimized the photoresponse of the electrodes by adjusting the carbon content in the C3N4 materials. Among all the single-layer CMBx electrodes, CMB5 was found to show the best performance with a photocurrent of 43 µA/cm2 (Figure 4a), and electrodes undergoing three deposition cycles demonstrated the highest photocurrent (Figure S6). Further carbon insertion (CMB10 and CMB50) resulted in activity decline (down to 20 µA/cm2) despite the extension of the light absorption, probably due to the high amount of defect states.39 We consider that the optimal PEC activity for CMB5 stems from good light harvesting properties, sufficient life time of photoinduced electrons and holes and low amount of defect states.38 The relatively long time for the electrodes to reach the maximum photocurrent (especially in the first run) indicates nonoptimal separation of charges upon excitation under illumination. The latter implies that the system is not yet optimized, leading to low energy conversion efficiency.

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Figure 4. (a) Transient photocurrent of the CMBx electrodes on carbon paper, (b) LSV curves of CMB50/CMB5 layers on carbon paper and blank carbon paper in dark and under illumination, scanning rate 20 mV/s in 1M KOH, (c) Mott-Schottky plots and the derived band structure (inset) of CMB5 and CMB50, (d) a schematic drawing of the designed heterojunction structure built by the deposition of CMB5 on CMB50 layer. To further enhance the PEC activities of the C3N4 photoanodes, we built a heterojunction structure by depositing CMB50 and CMB5 in sequence on carbon paper as noted above (Figure S5). According to the Mott-Schottky measurements (Figure 4c), the positive slopes of the linear plots indicated the n-type behavior of the obtained C3N4, agreeing well with previous reports.31 In addition, this measurement indicates that the flat-band potential of CMB5 and CMB50 is −1.46

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and −1.30 V vs. Ag/AgCl, respectively. The relative locations (Figure 4c inset) of the conduction band (CB) and valence band (VB) of CMB5 and CMB50 allow for the construction of a heterojunction with a suitable energy level between them, thus improving the charge separation process (Figure 4d). The addition of a second absorber with a lower bandgap resulted in the enhancement of the light harvesting properties of the PEC (see Figure 3b). Under zero bias, the transient photocurrent of the CMB50/CMB5 electrode reached up to 65 µA/cm2 (Figure 4a). The linear sweep voltammetry (LSV) measurements (Figure 4b) further demonstrated that the heterostructure has a strong impact on the photoresponse, especially at potentials higher than 0.6 V. The current peaked to 6.8 mA/cm2 at 0.8 V (vs. Ag/AgCl) under illumination resulting from the oxygen evolution reaction, while in the dark the value was only 3.5 mA/cm2. As a comparison, blank carbon paper showed very low polarization current both in the dark and under light, even under high overpotential (Figure 4b and Figure S7a). It is important to note that the high dark current is indicative of the high activity of C3N4 as electrocatalyst for the oxygen evolution reaction,42 further demonstrating the possibilities of this method not only for lightbased devices but also for electrochemical uses. CONCLUSIONS In summary, we demonstrated the facile growth of C3N4 on different conductive substrates, such as FTO glass, nickel foam and carbon paper by the use of electrophoretic deposition method. This C3N4 deposition is environmentally-friendly, fast and allows the tuning of the chemical, electronic, photophysical and photoelectronic properties of the C3N4 electrodes. The advantage of this EPD was illustrated by the tailored construction of a heterojunction between two complementary carbon nitrides, with enhanced photoelectrochemical activity. We believe

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that the use of EPD opens up even more possibilities for the fabrication of C3N4-based optical and electronic devices, such as solar cells, light-emitting diodes and transistors. Experimental Materials synthesis and EPD growth: All chemicals were purchased from Sigma-Aldrich and used without further purification. The C3N4 powders were synthesized by a thermal condensation method. Typically, the cyanuric acidmelamine-barbituric acid supramolecular complex was synthesized by mixing them with a 1:1:X molar ratio in water, shaken overnight, separated by centrifugation and dried at 60 oC in vacuum. Then the precursor was heated to 550 oC at a rate of 2.3 oC/min, and kept at this temperature for 4 hours in a nitrogen atmosphere. A C3N4 suspension was prepared by sonication of 10 mg of C3N4 powder in 100 ml of toluene for 5 hours, followed by centrifugation at 1000 rpm for 10 min to remove the residual big particles. The C3N4 layer was deposited onto different electrodes by EPD. A 200 V DC was used to drive the particles toward the target substrate, including carbon paper, FTO glass and nickel foam that was held at positive potential. A piece of blank FTO glass (1 × 2 cm) was used as the counter electrode held at a negative potential. The distance between the target substrate and the blank FTO is 1.4 cm. The duration of the deposition was 1 h for each cycle. Multiple cycles could be carried out to increase the amount of the C3N4 deposition. Before the PEC measurements, the electrodes were annealed in nitrogen at 300 oC for 1 h. Electrochemical and photoelectrochemical measurements: For the PEC measurements the C3N4 electrodes were prepared by 3 EPD cycles. The C3N4 substrates were used as the working

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electrode in the typical three-electrode photoelectrochemical cell with a Gamry Reference 3000 potentiostat (Gamry Instruments, US), using Pt wire as counter electrode and a saturated Ag/AgCl as reference electrode. Photocurrent response under visible light irradiation was recorded at zero bias and 1 M KOH aqueous solution as the electrolyte. Linear sweep voltammetry (LSV) was performed at a scan rate of 20 mV/s. A 50 W white LED (λ > 410 nm, HLN-60H-24A, Mean Well) was used as the light source. The light was chopped manually. Mott-Schottky measurements were carried out in the same cell configuration using 0.2M Na2SO4 aqueous solution as the electrolyte. Characterization: X-ray diffraction-patterns (XRD) were measured on a Bruker D8 Advance instrument using Cu-Kα radiation. Energy disperse X-ray analysis and morphology observation by scanning electron microscope (SEM) were performed on JSM-7500F (JEOL) equipped with an Oxford Instruments X-MAX80 mm2 detector. FT-IR spectra were recorded on a Nicolet iS5 FT-IR spectrometer (Thermal Scientific). Solid UV-vis absorbance spectra were measured using a UV-2600 spectrophotometer (Shimadzu, Japan) equipped with an integrating sphere. The emission spectra were recorded on LS-50B, Perkin Elmer instrument with the excitation wavelength of 350nm. ASSOCIATED CONTENT Supporting Information. XRD patterns, SEM images and LSV curves of bare carbon paper. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author Dr. Menny Shalom [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors thank Mr. Wenyao Zhang for his help in the revised manuscript. We thank Dr. Laurent Chabanne and Dr. Ryan Guterman for helpful suggestions and the Max Planck Society for financial support.

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(10) Yang, Z.; Zhang, Y.; Schnepp, Z. Soft and Hard Templating of Graphitic Carbon Nitride. J. Mater. Chem. A 2015, 3, 14081-14092. (11) Zhang, J. S.; Chen, X. F.; Takanabe, K.; Maeda, K.; Domen, K.; Epping, J. D.; Fu, X. Z.; Antonietti, M.; Wang, X. C. Synthesis of a Carbon Nitride Structure for Visible-Light Catalysis by Copolymerization. Angew. Chem. Int. Ed. 2010, 49, 441-444. (12) Zhang, J.; Zhang, M.; Lin, S.; Fu, X.; Wang, X., Molecular Doping of Carbon Bitride Photocatalysts with Tunable Bandgap and Enhanced Activity. J. Catalysis 2014, 310, 24-30. (13) 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, 11642-11648. (14) Lin, Z.; Wang, X. Nanostructure Engineering and Doping of Conjugated Carbon Nitride Semiconductors for Hydrogen Photosynthesis. Angew. Chem. Int. Ed. 2013, 52, 1735-1738. (15) Ma, T. Y.; Ran, J.; Dai, S.; Jaroniec, M.; Qiao, S. Z. Phosphorus‐Doped Graphitic Carbon Nitrides Grown In Situ on Carbon‐Fiber Paper: Flexible and Reversible Oxygen Electrodes. Angew. Chem. Int. Ed. 2015, 54, 4646-4650. (16) Ran, J.; Ma, T. Y.; Gao, G.; Du, X.-W.; Qiao, S. Z. Porous P-Doped Graphitic Carbon Nitride Nanosheets for Synergistically Enhanced Visible-Light Photocatalytic H2 Production. Energy Environ. Sci. 2015, 8, 3708-3717. (17) Lin, Z.; Wang, X., Nanostructure Engineering and Doping of Conjugated Carbon Nitride Semiconductors for Hydrogen Photosynthesis. Angew. Chem. Int. Ed. 2013, 52, 1735-1738. (18) Jun, Y. S.; Lee, E. Z.; Wang, X.; Hong, W. H.; Stucky, G. D.; Thomas, A. From Melamine‐ Cyanuric Acid Supramolecular Aggregates to Carbon Nitride Hollow Spheres. Adv. Funct. Mater. 2013, 23, 3661-3667.

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(27) Xu, J.; Herraiz-Cardona, I.; Yang, X.; Gimenez, S.; Antonietti, M.; Shalom, M. The Complex Role of Carbon Nitride as a Sensitizer in Photoelectrochemical Cells. Adv. Opt. Mater. 2015, 3, 1052-1058. (28) Xu, J.; Brenner, T. J. K.; Chabanne, L.; Neher, D.; Antonietti, M.; Shalom, M. Liquid-Based Growth of Polymeric Carbon Nitride Layers and Their Use in a Mesostructured Polymer Solar Cell with Voc Exceeding 1 V. J. Am. Chem. Soc. 2014, 136, 13486-13489. (29) Xu, J.; Shalom, M.; Piersimoni, F.; Antonietti, M.; Neher, D.; Brenner, T. J. K. ColorTunable Photoluminescence and NIR Electroluminescence in Carbon Nitride Thin Films and Light-Emitting. Adv. Opt. Mater. 2015, 3, 913-917. (30) Boccaccini, A. R.; Cho, J.; Roether, J. A.; Thomas, B. J. C.; Jane Minay, E.; Shaffer, M. S. P. Electrophoretic Deposition of Carbon Nanotubes. Carbon 2006, 44, 3149-3160. (31) Chunsheng, D.; Ning, P. High Power Density Supercapacitor Electrodes of Carbon Nanotube Films by Electrophoretic Deposition. Nanotechnology 2006, 17, 5314-5318. (32) Brown, P.; Kamat, P. V. Quantum Dot Solar Cells. Electrophoretic Deposition of CdSe− C60 Composite Films and Capture of Photogenerated Electrons with nC60 Cluster Shell. J. Am. Chem. Soc. 2008, 130, 8890-8891. (33) Farrow, B.; Kamat, P. V. CdSe Quantum Dot Sensitized Solar Cells. Shuttling Electrons Through Stacked Carbon Nanocups. J. Am. Chem. Soc. 2009, 131, 11124-11131. (34) Teranishi, T.; Hosoe, M.; Tanaka, T.; Miyake, M. Size Control of Monodispersed Pt Nanoparticles and Their 2D Organization by Electrophoretic Deposition J. Phys. Chem.B 1999, 103, 3818-3827.

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(35) Hod, I.; Bury, W.; Karlin, D. M.; Deria, P.; Kung, C. W.; Katz, M. J.; So, M.; Klahr, B.; Jin, D.; Chung, Y. W. Directed Growth of Electroactive Metal‐Organic Framework Thin Films Using Electrophoretic Deposition. Adv. Mater. 2014, 26, 6295-6300. (36) Yang, X.; Tang, H.; Xu, J.; Antonietti, M.; Shalom, M. Silver Phosphate/Graphitic Carbon Nitride as an Efficient Photocatalytic Tandem System for Oxygen Evolution. ChemSusChem 2015, 8, 1350-1358. (37) Zhu, B.; Xia, P.; Ho, W.; Yu, J. Isoelectric Point and Adsorption Activity of Porous g-C3N4. Appl. Surf. Sci. 2015, 344, 188-195. (38) Shalom, M.; Guttentag, M.; Fettkenhauer, C.; Inal, S.; Neher, D.; Llobet, A.; Antonietti, M. In Situ Formation of Heterojunctions in Modified Graphitic Carbon Nitride: Synthesis and Noble Metal Free Photocatalysis. Chem. Mater. 2014, 26, 5812-5818. (39) Zhang, J.; Zhang, G.; Chen, X.; Lin, S.; Möhlmann, L.; Dołęga, G.; Lipner, G.; Antonietti, M.; Blechert, S.; Wang, X. Co-Monomer Control of Carbon Nitride Semiconductors to Optimize Hydrogen Evolution with Visible Light. Angew. Chem. Int. Ed. 2012, 51, 3183-3187. (40) Li, X.-H.; Antonietti, M. Metal Nanoparticles at Mesoporous N-Doped Carbons and Carbon Nitrides: Functional Mott-Schottky Heterojunctions for Catalysis. Chem. Soc. Rev. 2013, 42, 6593-6604. (41) Ma, T. Y.; Cao, J. L.; Jaroniec, M.; Qiao, S. Z. Interacting Carbon Nitride and Titanium Carbide Nanosheets for High‐Performance Oxygen Evolution. Angew. Chem. Int. Ed. 2015, 55, 1138-1142.

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Table of Contents Graphic and Synopsis

Electrophoretic deposition is used for the growth of carbon nitride (C3N4) layer on conductive substrates. The electrophoretic deposition is fast, environmentally-friendly, and allows the tuning of the chemical, electronic, photophysical and photoelectronic properties of the C3N4 electrodes. The advantage of this method was illustrated by the tailored construction of a heterostructure between two complementary C3N4, with marked photoelectrochemical activity.

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