Ultrafine Cobalt Catalysts on Covalent Carbon Nitride Frameworks

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Ultrafine Cobalt Catalysts on Covalent Carbon Nitride Frameworks for Oxygenic Photosynthesis Guigang Zhang, Shaohong Zang, Lihua Lin, Zhi-an Lan, Guosheng Li, and Xinchen Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11167 • Publication Date (Web): 05 Jan 2016 Downloaded from http://pubs.acs.org on January 5, 2016

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ACS Applied Materials & Interfaces

Ultrafine Cobalt Catalysts on Covalent Carbon Nitride Frameworks for Oxygenic Photosynthesis Guigang Zhang, Shaohong Zang, Lihua Lin, Zhi-An Lan, Guosheng Li, and Xinchen Wang* State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou, 350002, China Email: [email protected] Homepage: http://wanglab.fzu.edu.cn/

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ABSTRACT: The rational cooperation of sustainable catalysts with suitable light-harvesting semiconductors to fabricate photosynthetic device/machinery has been regarded as an ideal technique to alleviate the current worldwide energy and environmental issues. Cobalt based species (e.g., Co-Pi, Co3O4 and Co-cubene) have attracted particular attentions because they are earth-abundant, costacceptable, and more importantly, it shows comparable water oxidation activities to the noble metal based catalysts (e.g., RuO2, IrO2). In this contribution, we compared two general cocatalysts modification strategies, based on the surface depositing and bulk doping of ultrafine cobalt species into the sustainable graphitic carbon nitride (g-C3N4) polymer networks for oxygenic photosynthesis by splitting water into oxygen, electrons and protons. The chemical backbone of g-C3N4 does not alter after both engineering modifications, however, in comparison with the bulk doping, the optical and electronic properties of the surface depositing samples are efficiently promoted, and the photocatalytic water oxidation activities are increased owing to much more exposed active sites, reduced overpotential for oxygen evolution and the accelerated interface charge mobility. This paper underlines the advantage of surface engineering to establish efficient advanced polymeric composites for water oxidation, and it also opens new insights into the architectural design of binary hybrid photocatalysts with high reactivity and further utilizations in the fields of energy and environment.

KEYWORDS: Photocatalysis, oxygenic photosynthesis, cobalt species, graphitic carbon nitride, sustainable energy production.


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1. INTRODUCTION: Sunlight has long been served as a charge-free, clean and inexhaustible energy resource, and the conversion and utilization of diffused solar irradiation into condensed chemical energy have been viewed as sustainable solution to address globe energy and environmental issues.1,2 Large scale H2 production via photocatalytic water splitting provides an ideal and clean manner for efficient solar energy conversion and utilization in a chemical fashion, as motivated by the pioneer Honda-Fujishima Pt-TiO2 photoelectrochemical cell.3-5 However, the water oxidation half reaction is the key step and major bottle-neck to control overall water splitting process due to the sluggish transfer of the four-electron and two-proton and high activation energy barrier (~700 mV) for O-O bond formation.6-8 Nowadays, the state-of-art water oxidation catalysts (WOCs) are still these Ru and Ir-based catalysts, which are restrained by the high cost, rare reserves and high toxicity for wide applications.9-11 Thus, the development of new semiconductors and kinetic promoters which are low cost, stable, efficient, eco-friendly, and sustainable are actively pursued in the research communities. In nature, the water oxidation active sites (cubical CaMn4O5) in photosystem II (PSII) was bonded with the protein polymer under neutral conditions with low overpotential (~ 160 mV) and high turnover frequency (TOF) number (~1).12,13 This inspires the materials designers and structural chemists to synthesis and develop a well designed support that provides large surface area and chemical binding sites to adhere the WOCs and capture more incident photons to magnify the water oxidation activity. Typically, inorganic materials, such as Mn3O414 and silica15 have been established as the hard scaffold to disperse and stabilize WOCs for water oxidation, which is benefited from the large surface area and strong synergistic attraction between the host


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and guest. The activity of the hybrid was not only depended on the structural, morphology, and textural properties of the substrate, but also relied on the size, dispersion, and deposition sites of WOCs and even their interactions. The substrate with typical structure, such as 1D nanowires,16 2D nanosheet,17 and 3D porous architecture,18 aids the high dispersion of the visiting particles on the substrate materials, contributing to establish strong surface adhesion for rapid charge transfer to promotes the sluggish water oxidation reaction. Besides, when the semiconductor scaffolds are excited under band-gap irradiation, the photo-induced electrons and holes are immediately transferred to the interface and then to the surface of active sites and subsequently participated in the following redox reaction.19,20 Unfortunately, most of the current inorganic semiconductors contain metal or rare metal contents, and some of them can only function under UV light irradiation. Thus, more future attentions should be spared into the architectural design of new nanomaterials that composed with sustainable elements and could function with visible light for the utilizations in the fields of energy and environment. Recently, melon-based carbon nitride (denoted as g-C3N4) semiconductors have been widely investigated especially in the fields of photocatalysis due to their cheap, stable, light-weight, and metal-free features.21 This polymer shows robust stability under light and solution corrosion even in crude acid solution due to the extension of the π-conjugated system in the tri-s-trazine units, enabling its emerged utilization in heterogeneous catalysis, especially in photocatalysis.22-25 Contrary to most traditional 1D polymers, this 2D polymer has a medium band gap (2.7 eV) to absorb the incident visible photons (λ < 460 nm). During the past few years, various strategies such as texture/morphological engineering, doping, molecular design, super-molecular chemistry, solvothermal synthesis, and sol & thin-film fabrication have been adopted to modify the bulk and surface properties,26-32 to improve the materials performance for photochemistry and catalysis


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applications. Compared with the high activities for H2 evolution (the high quantum yield of ~26 % has been established),33 the water oxidation process (with a quantum yield of only ~1%)21 is still poor due to the relative weak oxidation ability to resist the huge energy barrier and sluggish electrons transfer for O2 evolution. To enhance the water oxidation activity, either optimizing the bulk electronic structure or constructing surface hetero-junctions on the polymer to accelerate the kinetics rate is recommended. For instance, implanting metal (Zn, Fe, Cu) species into the carbon nitride frameworks as the active sites could evidently enhance the photocatalytic activities and extend the utilization for metal catalysis.34,35 Interestingly, no particle accumulation on the surface was observed due to the intense interaction between the metal ions and polymeric host. The rich lone pair electrons of the nitrogen atoms enable carbon nitride as “ligand” for binding transition metal ions which have empty or partial empty orbital. However, this bulk doping strategy cannot fully boost the activity because some metal species were submerged into the bulk structure and acted as the recombination centers to impede the activity rather than exposed as highly-dispersed active sites to promote the catalytic behavior.35 Combining a second material with the host with well matched band alignment of the two different materials is an efficient strategy to facilitate the charge carrier transfer and to promote the subsequent electrons and holes migration to the interface to participate in the photo-redox reactions. For instance, Co3O4 nanoparticles or layered Co(OH)2 have been deposited on the surface of carbon nitride or hexagonal BCN to generate fierce adhesion.36-40 These binary hybrids show sharp reduced over-potential for the water oxidation and accelerated charge carrier transfer, which are no matter contributed to improve the activity towards photocatalytic water oxidation. This modification also merges photocatalysis with metal catalysis, be valuable for creating tandem photosynthetic systems. Furthermore, same nanomaterials with different


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crystalline or phases (e.g. CN/CNS, α/β-Ga2O3, red/black P hybrid)41-43 could generate heterophase junctions at the interface for charge carrier separation. This inspires more attentions to design and construct new binary water oxidation photocatalysts with sustainable elements to boost the photocatalytic water oxidation efficiency. In this study, we developed two different chemistry protocols based on the surface depositing and bulk doping of polymeric carbon nitride with ultrafine cobalt species for improving the activity of photocatalytic water oxidation. Various characterizations have been carried out to investigate the structure, texture, morphology, and optical and electronic properties. The photocatalytic water oxidation activities of the as-prepared samples were evaluated in an assay of O2 evolution in the presence of electron acceptors with light irradiation. This paper aims to develop an efficient binary hybrid photocatalyst for sustainable water oxidation, which may also open new insights in the design and development of efficient photocatalysts for CO2 reduction, water splitting and photovoltaic cells. 2. EXPERIMENTAL SECTION Materials: All chemicals are of analytical grade and were used as received without further purification. Synthesis of g-C3N4: The typical g-C3N4 photocatalyst was prepared by a traditional thermal polymerization strategy as reported previously. In a typical procedure, 5 g urea was placed in a crucible with a cover, and then it was annealed at 550 oC for 2 h in the air. The final yellow powder was then collected. It was denoted as g-C3N4. Synthesis of cobalt bulk doped g-C3N4: Typically, 10 g of urea and different amounts (10, 20, 30, 50, 100, 200 mg) of Co(NO3)2 were mixed together to form a homogeneous solution. Then, it was evaporated with water vapor to generate a solid mixture. After grinded into powders, they


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were placed in a crucible with a cover to polymerize the precursors in a muffle furnace at 450, 500, 550, 600 oC for 2 h. The final products were obtained after naturally cooled down to the room temperature. They were denoted as B-Co-g-C3N4 for simplify. Synthesis of cobalt surface deposited g-C3N4: In the first place, g-C3N4 was obtained according to the above thermal polymerization methods. Then, 0.2 g of the prepared g-C3N4 solids were immersed in an aqueous solution containing different amounts of Co(NO3)2 (10, 20, 30, 50, 100, 200 mg). After evaporation with water vapor, the as-prepared mixture was calcined at 250, 300, 350, and 400 oC for 1h. The final obtained solids were denoted as S-Co-g-C3N4 for simplify. Characterization: XRD measurements were performed on a Bruker D8 Advance diffractometer with CuKα1 radiation. Fourier transformed infrared (FTIR) spectra were recorded using a Nicolet Magna 670 FTIR spectrometer. The UV/Vis spectra were recorded on a Varian Cary 500 Scan UV/Vis system. TEM was performed on a FEI Tencai 20 microscope. X-ray photoelectron spectroscopy (XPS) data were obtained on Thermo ESCALAB250 instrument with a monochromatized Al Kα line source (200 W). Photoluminescence spectra were recorded on an Edinburgh FI/FSTCSPC 920 spectrophotometer. Electron paramagnetic resonance (EPR) measurements were recorded using a Bruker model A300 spectrometer. Electrochemical measurements were conducted with a BAS Epsilon Electrochemical System in a conventional three electrode cell, using a Pt plate as the counter electrode and an Ag/AgCl electrode (3 M KCl) as the reference electrode. The working electrode was prepared on F-doped tin oxide (FTO) glass that was cleaned by sonication in ethanol for 30 min and dried at 353 K. The boundary of FTO glass was protected using Scotch tape. The 5 mg sample was dispersed in 1 mL of DMF by sonication to get a slurry mixture. The slurry was spread onto pretreated FTO glass. After airdrying, the working electrode was further dried at 393 K for 2 h to improve adhesion. Then, the


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Scotch tape was unstuck, and the uncoated part of the electrode was isolated with epoxy resin. The electrolyte was 0.1 M KOH aqueous solution without additive (pH =13). The scan rate is 10 mV/s. Photocatalytic test for water oxidation: Photocatalytic O2 production was carried out in a Pyrex top-irradiation reaction vessel connected to a glass closed gas circulation system. For each reaction, 50 mg catalyst powder was well dispersed in an aqueous solution (100 mL) containing AgNO3 (0.01M) as an electron acceptor and La2O3 (0.2g) as a pH buffer agent. The reaction solution was evacuated several times to remove air completely prior to irradiation with a 300 W Xeon lamp with a working current of 15 A (Shenzhen ShengKang Technology Co., Ltd, China, LX300F). The wavelength of the incident light was controlled by applying some appropriate long-pass cut-off filters. The temperature of the reaction solution was maintained at room temperature by a flow of cooling water during the reaction. The evolved gases were analyzed insitu by gas chromatography equipped with a thermal conductive detector (TCD) and a 5Å molecular sieve column, using Argon as the carrier gas. 3. RESULTS AND DISCUSSION: The preparation details of the photoatalysts, the characterization conditions and the photocatalytic activities evaluation informations were described in the experimental section. Typically, for the surface modification, we first obtain the g-C3N4 polymers via a thermalinduced polycondensation of urea into melon based carbon nitride solids. Then, the powders were loaded with different amounts of cobalt via a immersion strategy. The final powders were obtained after subsequently thermal treated at 250-400 oC in the air to construct the firece interface adhesion. They were denoted as S-Co-g-C3N4 for simplicity, where S is corresponded to surface deposition. To create the cobalt doped g-C3N4 hybrid, urea and cobalt were first mixed


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together to form a homogenous mixture. The final solids were collected after calcined in the air at 500-600 oC. Similarly, the as-prepared samples are marked as B-Co-g-C3N4, also B is corresponded to bulk doping modification. The two different synthesis procedures of the g-C3N4 binary photocatalysts can be seen clearly in Scheme 1. In the first place, the chemical structure of S-Co-g-C3N4 and B-Co-g-C3N4 have been characterized by powder XRD, FT-IR, and high-resolution XPS analysis. In Figure 1, both S-Cog-C3N4 and B-Co-g-C3N4 were checked by powder XRD. It is clearly to observe that two typical diffraction peaks located at 2 θ = 13.0o and 27.4o, which are corresponded to the in-plane repeating units of tri-s-triazine and the graphite stacking of the polymer, respectively.43-46 None new peaks were emerged for both of the two series of samples, just with a slightly decrease in the peak intensity. Besides, nearly no peak shift for S-Co-g-C3N4 samples can be viewed in the picture, while about 0.32o peak shift to the higher angle was achieved for the B-Co-g-C3N4 polymers, indicating the depositing modification just interrelated with van der waals attraction, while the doping decoration linked in the form of metal and polymer coordination. However, no evident change was checked for both two embellished protocols, confirming the robust stability of the polymer for heterogeneous photocatalysis. In addition, same changes were also revealed by the FT-IR variations of the S-Co-g-C3N4 and B-Co-g-C3N4 samples. As shown in Figure S1, all the characteristic variations assigned to typical g-C3N4 are clearly seen for all the samples, indicating the successful evolution of the g-C3N4 structure. The sharp peaks at 805 cm-1 is attributed to the breathing mode of the heptazine heterocyclic ring (C6N7) units, while the strong bands at 1200-1600 cm-1 are related to the stretching variations of the repeating units.47,48 No evident change can be found for both S-Co-g-C3N4 and B-Co-g-C3N4 samples, once again certifying that the polymers remain unchanged after the surface or bulk modification.


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The local chemical state of both S-Co-g-C3N4 and B-Co-g-C3N4 polymers was tested by the high resolution XPS analysis. As shown in Figure 2a, two weak peaks with binding energy of 781.7 and 797.1 eV are ascribed to Co 2p3/2 and Co 2p1/2 of CoOx species.36 In Figure 2b, the binding energy of O 1s is located at 532.0 eV. Combining both binding energy of the Co 2p and O 1s, it is easy to certify the successful formation of CoOx incorporated with g-C3N4. The binding energy of C 1s and N 1s in Figure 2 c and d for S-Co-g-C3N4 and B-Co-g-C3N4 samples are centered at 288.1 and 398.7 eV, respectively, which are the same as those in the pure gC3N4,49,50 suggesting that no noticeable change in the major building block of the polymer was obtained after the inclusion of the cobalt species into the framework. This once again implies that these modifications do not alter the main skeleton construction of the polymer, which is the perquisite for the utilization in the fields of catalysis or photocatalysis. Since we have successfully implanted the cobalt species into the triazine polyunits of the polymer, more characterizations are demanded to reveal the local morphology, optical and electric properties after hybridizations. The morphology and texture structure of both S-Co-g-C3N4 and B-Co-g-C3N4 polymers were investigated by transmission electron microscopy (TEM). In Figure 3a, no obvious particle accumulation on the surface was observed for S-Co-g-C3N4 sample, revealing the homogenous dispersion of the cocatalysts. To further illustrate the distribution of the catalysts, scanning transmission electron microscopy (STEM) was carried out. In Figure 3b, it shows the TEM picture of S-Co-g-C3N4 in dark field. In Figure 3 c-f, the elemental mapping of Co, O, C, and N was presented for S-Co-g-C3N4, once again displaying the well distribution of the cobalt species on the interface of g-C3N4 that intends to promote the photocatalyic performance.34,35 The morphology of both pure g-C3N4 and B-Co-g-C3N4 polymers was also checked by TEM images.


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As shown in Figure S2, both of the two samples show similar silk-like two-dimensional (2D) nanosheet, indicating that no evident change has been obtained after surface and bulk engineering with CoOx cocatalysts. Besides, it is easy to find that there are some pores on the interface of B-Co-g-C3N4 polymers, which is mainly due to the partial decomposition of the polymer in the presence of transition metal under high thermal treatment temperature (550 oC).35 This no doubt induces the enlargement in the surface area, which will be explained in the following section. There are also no particle accumulation for the B-Co-g-C3N4 polymers, which is accordant with the previous reports.34,35 The optical absorption of pure g-C3N4, S-Co-g-C3N4 and B-Co-g-C3N4 polymers was characterized by UV-Vis diffused reflectance spectra (DRS). In Figure S 3a and b, very few difference in the optical absorption have been observed for both S-Co-g-C3N4 and B-Co-g-C3N4 polymers, implying that few amounts of metal contents do not alter the basic band structures seriously. This can be clearly certified in the picture of Figure 4a. The surface area of the S-Cog-C3N4 and

B-Co-g-C3N4

polymers

were

measured

by

the

low

temperature

N2

adsorption/desorption isotherms. All the three samples show the typical IV type adsorption/desorption isotherms, suggesting the presence of micropores. Besides, nearly no difference in the adsorption/desorption isotherms between pure g-C3N4 and S-Co-g-C3N4 can be found in the picture, while an noticeable enlargement in the surface area for B-Co-g-C3N4 polymers can be obtained. The BET surface area for pure g-C3N4, S-Co-g-C3N4 and B-Co-gC3N4 polymers was determined as 62, 54, and 108 m2 g-1, which is accordant with the TEM characterizations. The slightly decreased BET surface area for S-Co-g-C3N4 is mainly ascribed to the blockage of the pores. The charge carrier behaviors of the samples were evaluated by the room temperature photoluminescence (PL) with excitation wavelength of 400 nm.51,52 In Figure


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S4, both S-Co-g-C3N4 and B-Co-g-C3N4 polymers show broad emission peaks at 450-650 nm, confirming the visible light response. Besides, evidently restrained PL emission intensity were recorded for both S-Co-g-C3N4 and B-Co-g-C3N4 materials in comparison with the pristine one, well elaborating the restrained charge carrier recombination rate of the photoinduced electrons and holes, which is expected to accelerates the fast separation and migration rate of the charge carriers. In Figure 4c, it is notably to see that the S-Co-g-C3N4 exhibits much decreased PL emission intensity in comparison with the B-Co-g-C3N4 materials, suggesting that the surface decoration with cobalt species is superior in promoting the charge carrier transfer than that of the bulk construction. More electronic properties of the pure g-C3N4, S-Co-g-C3N4 and B-Co-g-C3N4 polymers can be further studied by room temperature electron spin resonance (ESR). In Figure 4d, all samples show same g-factor at 2.0034, confirming the complete derivation of graphitic stacking configuration.53 Besides, when the polymeric solids were irradiated under band-gap illumination, largely enhanced ESR intensities in comparison with pristine g-C3N4 were emerged for S-Co-gC3N4 and B-Co-g-C3N4 counterparts, indicting more unpaired electrons were generated when cobalt was imparted in the framework to establish the efficient surface heterojunction for photoredox reaction. In fact, the S-Co-g-C3N4 shows higher increased EPR intensity than B-Cog-C3N4, it is well to illustrate the advantage of the surface depositing in comparison with the bulk doping modification. In order to gather more electronic properties of S-Co-g-C3N4 and B-Co-g-C3N4 samples, a series of electrochemical experiments (eg: I-t, LSV, EIS) were carried out in a typical threeelectrode cell. In Figure 5a, it shows the n-type transient photocurrents of the engineered electrodes with +400 mV bias voltage in 0.1 M KOH. Evidently, much enhanced transient


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photocurrent was produced when the electrodes were illuminated under visible light (λ > 420 nm), indicating the generation and separation of photoinduced e--h+ pairs at the electrode and water interface. Only about 1 µA cm-2 of photocurrent density was emerged for pure g-C3N4, while 10 and 15 µA cm-2 of photocurrent density were created for the B-Co-g-C3N4 and S-Co-gC3N4 electrodes, implying the well electrical conductivity and prolonged life time of charge carriers for the B-Co-g-C3N4 and S-Co-g-C3N4 electrodes than the pristine one. Besides, another advantage of the cobalt modification is the reduced overpotential for oxygen evolution. In Figure 5b, it shows the liner cyclic voltammetry curves (LSV) of the electrode under dark in 0.1 M KOH. Notably increased current was created at about 0.7 V (vs Ag/AgCl, pH=13) for both BCo-g-C3N4 and S-Co-g-C3N4 electrodes, revealing the efficiently reduced overpotential for water oxidation and a better photocatalytic performance can be expected. Figure 5c and d depict the electrochemical impedance spectroscopy (EIS) with a bias voltage of 0.4 V in the dark and under visible light irradiation, respectively. It is clearly to observe that both B-Co-g-C3N4 and S-Co-gC3N4 electrodes exhibit decreased impedance in comparison with pure g-C3N4. The radius of the semicircle was further reduced when they were irradiated under visible light, elucidating the fast generation and separation of the photo-induced electron and hole pairs. The electrochemical impedance of the three electrodes are calculated to be 1047, 503, and 182 KΩ for pure g-C3N4, B-Co-g-C3N4 and S-Co-g-C3N4 electrodes. The photocatalytic performance of both B-Co-g-C3N4 and S-Co-g-C3N4 polymers were evaluated in an assay of oxygen evolution in the presence of electrons accepter (0.1 M AgNO3). To achieve the optimum activities, in the first place, we investigated the effects arise from the thermal treatment temperature. In Figure S5, when cobalt species were deposited on g-C3N4 at 250-400 oC, all S-Co-g-C3N4 polymers show increased water oxidation activities than the pure g-


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C3N4, while the optimum was located at 300 oC. Similarly, when B-Co-g-C3N4 was treated at 500-600 oC, the activities were increased as the temperature varied from 500 to 500 oC (Figure S6). Few improvements were achieved when further raised the temperature to 600 oC, which indicated that the optimum polymerization value is 550 oC. Besides, the loading and doping contents were also investigated. As depicted in Figure 6 a and b, both surface deposition and bulk doping modifications show similar water oxidation activities variations in the cobalt contents. The photocatalytic water oxidation for O2 evolution rate was first increased and then decreased when varied the cobalt densities. The optimal oxygen evolution rate (OER) of S-Co-gC3N4 (0.3 wt. % CoOx) was 75.4 µmol h-1, while that for B-Co-g-C3N4 (0.2 wt. % CoOx) was 46 µmol h-1, which is nearly 10 and 6-fold faster than the pure g-C3N4 (8 µmol h-1) under UV-Vis irradiation (λ > 300 nm). Evidently, the surface deposition shows superior advantages in promoting the photocatalytic water oxidation performance than that of the bulk doping construction. It is not difficult to understand that the surface exposed cobalt active sites are prone to participate in the interface redox reaction, while most of the active sites were immerged in the bulk framework that cannot utilized efficiently for the heterogeneous photocatalysis. Except for the enhanced water oxidation activity, the oxygen evolution selectivity for both optimized gC3N4 samples was greatly improved. The oxygen evolution selectivity for pure g-C3N4 is 61 %, while the value was increased to 95 and 92 % for S-Co-g-C3N4 and B-Co-g-C3N4 polymers, respectively. To certify cobalt species are superior in promoting the water oxidation activities of g-C3N4, some other traditional transition metal oxides, including NiOx, MnOx, FeOx, and CuOx have been deposited on pure g-C3N4 with the same surface depositing strategy for photocatalytic water oxidation. Evidently, all of the above metal oxides show negligible enhanced activities in


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comparison with the pristine g-C3N4, well elucidating that cobalt shows better activities than the other transition metals for improving the photocatalytic water oxidation performance, which is in accordance with the previous studies.36-38 Moreover, the long time course water oxidation activities were also characterized for pure, gC3N4, S-Co-g-C3N4 and B-Co-g-C3N4 polymers under UV-Vis and visible light irradiation. In Figure 6c, no gas evolution was emerged for all the samples in dark, indicating the light is perquisite to drive the water oxidation reaction. When the samples were illuminated under UVVis (λ > 300 nm), obvious gases evolution was recorded for all the samples. The amounts of the collected O2 gases for B-Co-g-C3N4 and S-Co-g-C3N4 were increased to 137 and 262 µmol, respectively, as prolonged the reaction time to 11 hours. It should be noted that the water oxidation rate was decreased as the reaction continued, which is mainly ascribed from the coating of excess amounts of Ag nanoparticles on the surface of the polymer to reduced the optical absorption of photons for g-C3N4 excitation. This can be well proclaimed by the TEM pictures and XPS analysis of the samples collected after the photocatalytic reactions (Figure S8 and S9), which are accordance with the previous studies.36 4. CONCLUSION: In this study, two different optimized and general synthetic strategies have been developed, based on surface and bulk imparting of ultrafine cobalt species into the covalent framework of gC3N4 to establish efficient surface heterojunctions that benefits the interface charge carrier transfer for promoting the photocatalytic water oxidation activities. Interestingly, these two different modifications show quite different efficiency in the optical, electronic properties and photocatalytic behaviors for water oxidation. Surface depositing of ultrafine cobalt species are more efficient in promoting the photocatalytic behavior than the bulk doping construction. This


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paper underlines the advantage of surface engineering technology to establish efficient surface junctions for water oxidation, and it also opens new insights into the architectural design of binary hybrid photocatalysts with high photoactivity and further utilizations in the fields of energy and environment.54,55 ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. Additional data, including FT-IR, TEM, HR-TEM, DRS, and PL spectra of g-C3N4, B-Co-g-C3N4 and S-Co-gC3N4 polymers. Water oxidation rates of S-Co-g-C3N4 samples prepared at different temperatures. Water oxidation rates of different metal oxides deposited g-C3N4 samples. TEM and high resolution XPS analysis of samples after reaction. AUTHOR INFORMATION Corresponding Author Email: [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. ACKNOWLEDGEMENT This work is financially supported by the National Basic Research Program of China (973 Program) (2013CB632405), the National Natural Science Foundation of China (21425309 and 21173043).


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Scheme 1. Illustration for the synthesis process of surface and bulk imparting of ultrafine cobalt species into the covalent framework of g-C3N4.


24

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a)

I / a.u.

I / a.u.

S-Co-g-C3N4

g-C3N4 10

20

30

2θ/

40

50

60

24

o

26

28

2θ/

30

o

o

0.32

b)

I / a.u.

B-Co-g-C3N4 I / a.u.

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


g-C3N4 10

20

30

40

2 θ/

50

60

0

24

26

2θ/

28

30

0

Figure 1. Powder XRD patterns of pure g-C3N4, (a) S-Co-g-C3N4 and (b) B-Co-g-C3N4 polymers.


25


b)

a) Co 2p

O 1s

S-Co-g-C3N4

I / a.u.

I / a.u.

S-Co-g-C3N4

B-Co-g-C3N4

B-Co-g-C3N4 810

805

800

795

790

785

780

775

540

538

536

534

532

530

528

526

BE / eV

BE / eV

c)

d) C 1s

N 1s

S-Co-g-C3N4

B-Co-g-C3N4

S-Co-g-C3N4

I / a.u.

I / a.u.

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

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B-Co-g-C3N4 294

292

290

288

286

284

282

410

408 406

BE / eV

404 402

400

398 396

BE / eV

Figure 2. High-resolution XPS analysis of (a) Co 2p, (b) O 1s, (c) C 1s, and (d) N 1s of S-Co-gC3N4 and B-Co-g-C3N4 polymers.


26

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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


Figure 3. TEM imagine (a) and elemental mapping in dark (b-f) of S-Co-g-C3N4. The scale bar of c-f pictures are 200 nm.


27


b)

a) g-C3N4

400 350

g-C3N4

300

S-Co-g-C3N4

250

B-Co-g-C3N4

3

Vads / cm ⋅ g

F(R)

B-Co-g-C3N4

-1

S-Co-g-C3N4

200 150 100 50

400

500

0 0.0

600

0.2

0.4

λ / nm

0.6

P/P

c) g-C3N4

0.8

1.0

1.98

1.96

o

d)

light dark

S-Co-g-C3N4

B-Co-g-C3N4

450

500

550

600

I / a.u.

S-Co-g-C3N4

I / a.u.

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

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650

B-Co-g-C3N4 g-C3N4

2.04

2.02

2.00

geff

λ / nm

Figure 4. (a) UV-Vis diffuse reflectance spectra (DRS), (b) low temperature N2 adsorption/desorption isotherms, (c) Room temperature photoluminescence spectra (PL), and (d) room temperature electron spin resonance (ESR) of S-Co-g-C3N4 and B-Co-g-C3N4 polymers.


28

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b) 10

-2

Current density / µA cm

B-Co-g-C3N4

light on

S-Co-g-C3N4

15

g-C3N4

-2

g-C3N4

20

10

5

Current density / mA cm

a)

8

B-Co-g-C3N4 S-Co-g-C3N4

6

4

2

0

0 100

150

0.4

200

c)

3000

g-C3N4

d)

dark

B-Co-g-C3N4

2500

0.6

0.8

1.0

Potential / vs Ag/AgCl pH=13

t/s

g-C3N4 600

light

B-Co-g-C3N4

S-Co-g-C3N4

S-Co-g-C3N4

2000

400

Z'' / KΩ

Z'' / KΩ

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


1500 1000

200 500 0

0 0

500

1000

1500

2000

Z' / ΚΩ

2500

3000

0

200

400

600

Z' / ΚΩ

Figure 5. (a) Transient photocurrents with 400 mV bias voltage of the engineered electrodes, (b) Polarization curves in the dark, Nyquist plots of electrochemical impedance spectroscopy (EIS) in the dark (c) and under visible light irradiation (d) of S-Co-g-C3N4 and B-Co-g-C3N4 polymers. Conditions: 0.1 M KOH, traditional 3 electrode cell.


29


a)

90 80

S-Co-g-C3N4

O2

0.3 wt.

b) 50

B-Co-g-C3N4

O2

0.2 wt.

N2

N2

40

-1

60

0.5 wt.

0.2 wt.

50

1 wt.

40 30

0.1 wt.

OER / µmol h

OER / µ mol h

-1

70

20 10

0.3 wt. 0.1 wt.

30

0.5 wt. 1.0 wt.

20

10

0 wt

0 wt.

0

0

c)

d) λ > 300 nm

250

S-Co-g-C3N4

200

B-Co-g-C3N4

150

100

light on 50

g-C3N4

0 0

2

4

6

8

10

12

Amounts of evolved gases / µmol

Amounts of evolved gases / µmol

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

Page 30 of 31

50

λ > 420 nm

S-Co-g-C3N4

40

30

B-Co-g-C3N4 20

light on 10

g-C3N4

0 0

2

t/h

4

6

8

10

12

t/h

Figure 6. Oxygen evolution rates of different amounts of S-Co-g-C3N4 (a) and B-Co-g-C3N4 polymers (b) under UV (λ> 300 nm) irradiation; time course water oxidation activities of g-C3N4, S-Co-g-C3N4 and B-Co-g-C3N4 polymers under (c) UV (λ> 300 nm) and (d) visible light irradiation (λ> 420 nm).


30

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TOC Graphics


31

Ultrafine Cobalt Catalysts on Covalent Carbon Nitride Frameworks

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