Hollow CoSx Polyhedrons Act as High-Efficiency Cocatalyst for

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Hollow CoSx polyhedrons act as high-efficiency cocatalyst for enhancing the photocatalytic hydrogen generation of g-C3N4 Junwei Fu, Chuanbiao Bie, Bei Cheng, Chuanjia Jiang, and Jiaguo Yu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04461 • Publication Date (Web): 10 Jan 2018 Downloaded from http://pubs.acs.org on January 12, 2018

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Hollow CoSx polyhedrons act as high-efficiency cocatalyst for enhancing the photocatalytic hydrogen generation of g-C3N4

Junwei Fu,† Chuanbiao Bie,† Bei Cheng,† Chuanjia Jiang† and Jiaguo Yu*,†,‡ †

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan

University of Technology, Wuhan 430070, P. R. China. E-mail: [email protected]. ‡

Department of Physics, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi

Arabia. E-mail: [email protected] E-mail: [email protected] E-mail: [email protected] E-mail: [email protected] E-mail: [email protected]

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ABSTRACT: Seeking for suitable cocatalyst to realize highly efficient photocatalytic hydrogen (H2) production is a great challenge in the field of solar energy conversion. Herein, hollow cobalt sulfide (CoSx) polyhedrons derived from ZIF-67 metal–organic frameworks were used as cocatalyst to enhance the photocatalytic H2 generation performance of graphitic carbon nitride (g-C3N4). The polyhedral morphology with more exposed edges provides more surface active sites for H2 generation, while the interface between CoSx and g-C3N4 with intimate contact leads to better separation efficiency of photogenerated charge carriers. Moreover, the hollow structure not only favors mass diffusion/transfer, but also induces multiple reflections of light within the hollow cavity, enhancing the utilization efficiency of solar energy. As a result, the obtained CoSx/g-C3N4 composites showed excellent photocatalytic H2 generation activity, achieving a H2 evolution rate of 629 µmol g–1 h–1 under visible light (λ ≥ 400 nm), which was about 52 times higher than pure g-C3N4. This work proves that hollow CoSx polyhedrons can be a potential substitute for noble metal cocatalysts for photocatalytic H2 generation. KEYWORDS: cobalt sulfide, hollow polyhedron, g-C3N4 nanosheet, photocatalysis, hydrogen generation

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INTRODUCTION Photocatalytic hydrogen (H2) generation from water splitting using the solar energy as energy source is considered as one of promising tactics for resolving energy crisis.1-2 Since the pioneering work reported in 2009,3 graphitic carbon nitride (g-C3N4) has attracted much scientific interest in the field of photocatalysis due to its unique layered structure and special properties, such as metal-free composition, chemical stability, easy preparation and low cost.4,5 However, the pure g-C3N4 photocatalysts exhibit extraordinarily low photocatalytic performance due to their low specific surface area, insufficient light harvesting, quick recombination rate of photogenerated charge carriers and low surface reaction rate,6-9 which seriously affect its practical application. In general, to promote the photocatalytic activity of g-C3N4, loading cocatalysts is proven to be an efficient strategy.10-12 Pure g-C3N4 cannot deliver high H2 generation rate due to its scarce surface active sites and high activation energy of H2 generation reactions.13-15 Moreover, the photogenerated electron–hole pairs easily recombine before migrating to the surface for reactions. Coupling g-C3N4 with suitable cocatalyst can improve the separation efficiency of photogenerated electron–hole pairs, and promote the sluggish H2 generation kinetics of g-C3N4.16-18 Noble metals, such as Pt,18 Au,19 Ag20 and Pd,21 are often used as efficient cocatalyst for enhancing the photocatalytic performance of g-C3N4. On the one hand, the noble metals can efficiently trap the photogenerated electrons of g-C3N4 and then greatly retard the recombination of electron–hole pairs, due to their large work function. On the other hand, the abundant H2 reduction sites on the surface of noble metals with the trapped photogenerated electrons can reduce protons

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(H+) to generate H2. However, due to their high prices and low reserves, seeking for substitutions of noble metals is still an ongoing endevor. Transition metal sulfides have been recently recognized as promising cocatalyst for H2 generation reaction due to their unique optical and electrical properties.22,23 Moreover, the adsorption free energies of H2 on the surface of transition metal sulfides are zeroapproaching.22 This is beneficial for the release of generated H2 from the surface of cocatalysts and re-exposure of the reduction active sites. Metal sulfides, such as NiS,24-29 MoS2,30,31 CuS,32-34 WS235 and CoS,36 have been reported to be efficient in enhancing the photocatalytic H2 generation activity of g-C3N4. Extensive studies indicate that the active sites mainly locate at the edges or surface of cocatalysts.37 Thus, polyhedral structure with more exposed edges should be an appropriate morphology feature of cocatalysts. Moreover, hollow structures can exhibit kinetically favorable open surface structure and short diffusion paths for mass and charge transport. More interestingly, the multiple reflections of light in the hollow structure can also enhance the utilization efficiency of solar energy. These merits are all beneficial for the photocatalytic reactions. Therefore, controlled design of transition metal sulfide with hollow polyhedral structure may yield highly efficient cocatalysts for enhancing the photocatalytic of g-C3N4. Unfortunately, the fabrication of well-defined hollow polyhedra is still a challenge. Templating approaches have proven to be the most efficient strategy of constructing hollow structures.38 However, the complicated synthesis steps, such as surface modification and template removal, greatly limit the applications of this technique in constructing hollow structures with suitable components. Recently, metal–organic frameworks (MOF) have been widely used in photocatalysis for their porous


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characteristic, rich metal species and numerous hollow structures.39-41 On the other hand, Co-based cocatalysts have demonstrated high efficiency to facilitate the separation of photogenerated electron–hole pairs, and they are also believed to play important roles in facilitating proton reduction and H2 evolution reactions on their surface.12,23 As shown in Figure 1, ZIF-67, a kind of Co-containing MOF, exhibits exposed metal sites and excellent stability. Moreover, the ZIF-67 polyhedrons can be used as both sacrificial templates and metal precursors to prepare hollow CoSx polyhedrons.42 The polyhedrons present anisotropic shape and expose more edges and facets, while the hollow structure endows more surface active sites and better light harvesting. These features indicate that hollow CoSx polyhedrons can be used as efficient cocatalyst for enhancing the photocatalytic H2 generation of g-C3N4.


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Figure 1. (a) The ball-and-stick model and (b) morphology schematic illustration of ZIF67. (c) The morphology schematic illustration of a hollow CoSx polyhedron derived from ZIF-67.

As is known, bulk g-C3N4 has a similar layered structure to graphite. Exfoliation of bulk g-C3N4 into graphene-like nanosheets can greatly increase specific surface area.43 In this work, we firstly prepared g-C3N4 nanosheets and ZIF-67, and then their mixtures were hydrothermally treated for in-situ transformation of ZIF-67 into hollow CoSx polyhedrons, as illustrated in Figure 2. The obtained CoSx polyhedrons are embedded in curled g-C3N4 nanosheets, preventing the aggregation of these hollow CoSx polyhedrons. The hollow polyhedron structure of CoSx can maximally utilize the active sites of CoSx, which can greatly reduce the overpotential of photocatalytic H2 reaction. The general process of photocatalytic H2 production over the CoSx cocatalyst is as follows. Firstly, the catalytic sites of CoSx adsorb protons from the aqueous solution, and the photo-excited electrons of g-C3N4 transfer to the surface of CoSx. Two protons obtain electrons and form a H2 molecule on the catalytic sites of CoSx. Finally, the generated H2 desorb from the catalytic sites of CoSx, and then the re-exposed catalytic sites can participate in the reaction again. As a result, the CoSx/g-C3N4 composites exhibit higher photocatalytic H2 generation activity than pure g-C3N4. This work provides new insight for designing highly efficient cocatalyst for g-C3N4-based photocatalysts.

EXPERIMENTAL SECTION Preparation of ZIF-67 and g-C3N4 nanosheets. ZIF-67 was synthesized according to the reported method.42 Typically, 1.048 g of cobaltous nitrate (Co(NO3)2·6H2O) and 0.791 g


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of 2-methylimidazole (C4H6N2) were dissolved into 40 mL of methanol to form uniform solutions, respectively. Then, the obtained wine-red Co(NO3)2 solution was added into the 2-methylimidazole solution dropwise, and the mixture solution was aged at room temperature for 12 h. After that, the purple precipitates were collected by centrifugation and washed with methanol 6 times. The obtained ZIF-67 was suspended in 10 mL of ethanol and set aside. The g-C3N4 nanosheets were prepared by thermal exfoliation of the bulk g-C3N4 synthesized from thermal condensation of urea.44 In detail, 10 g of urea was firstly filled into a crucible and heated at 550 °C for 2 h with a heating rate of 5 °C. Then the obtained pale yellow powder (bulk g-C3N4) was ground and placed in a crucible with lid. Then the thermal exfoliation process was performed at 550 °C for 2 h with a heating rate of 10 °C, yileding the g-C3N4 nanosheets. Preparation of CoSx/g-C3N4 composites. CoSx/g-C3N4 composites were prepared by a hydrothermal method. Firstly, 0.1 g of as-prepared g-C3N4 nanosheets was dispersed in 80 mL of ethanol and ultrasonically treated for 30 min. Then 0.2 mL of ZIF-67 suspension solution and 300 mg of thioacetamide were added into the milk-like g-C3N4 suspension. After vigorous stirring for 10 min, the mixture suspension was transferred to a Teflonlined stainless-steel autoclave and kept at 120 °C for 5 h. Finally, the resulting precipitates (CoSx/g-C3N4 composites) were washed with water and ethanol thoroughly, and dried in an oven at 80 °C for 12 h. The sample was designated as 2%CoSx/g-C3N4. By changing the added volume of ZIF-67 suspension, CoSx/g-C3N4 composites with different CoSx contents can be obtained. Specifically, the composites preapred by adding 0, 0.2, 0.5 and 1 mL of ZIF-67 suspension were referred to as 0%CoSx/g-C3N4, 2%CoSx/g-C3N4, 5%CoSx/g-C3N4 and 10%CoSx/g-C3N4, respectively.


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Figure 2. The schematic illustration for the fabrication of CoSx/g-C3N4 composites.

Material characterization. The FESEM and TEM images of samples were taken on a scanning electron microscope (JEOL 7500F, Japan) and a transmission electron microscope (Titan G2, FEI, USA), respectively. The zeta potentials of g-C3N4 and ZIF-67 suspensions were measured by a zetasizer (Malvern, Zetasizer Nano, ZS90, UK). Powder XRD patterns were collected on a D/Max-RB X-ray diffractometer (Rigaku, Japan). The FTIR spectra were recorded on Nicolet iS50 spectrometer (Thermo Scientific, USA). Raman spectra were collected on a micro-Raman spectrometer (Renishaw inVia, England). A 633 nm Ar+ laser was used as the excitation source. XPS measurements were performed on an electron spectrometer (VG ESCALAB 210, UK) with Mg Kα radiation. All the binding energies were calibrated by the adventitious carbon C 1s peak at 284.8 eV. The N2 adsorption–desorption measurements were performed on a N2 adsorption apparatus (Micromeritics ASAP 2020, USA). The specific surface area was determined by the multipoint BET method using the adsorption data in the relative


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pressure (P/P0) range 0.05–0.3, and pore size distributions were calculated by the Barrett– Joyner–Halenda (BJH) method using the adsorption data. The UV-vis diffuse reflectance spectra were recorded on a UV-2600 spectrophotometer (Shimadzu, Japan) with BaSO4 as reference. Photoluminescence emission spectra were acquired a on Hitachi F-7000 fluorescence spectrophotometer (Japan) with 350 nm excitation light. Time-resolved transient PL decay curves were obtained using a FLS920 fluorescence lifetime spectrophotometer (Edinburgh Instruments, UK) with 325 nm excitation wavelength and 450 nm emission wavelength. Photocatalytic H2 generation test. The photocatalytic H2 generation tests were performed in a 100 mL Pyrex flask. Firstly, 50 mg of sample was dispersed in 80 mL of 20 vol% triethanolamine aqueous solution. After ultrasoic dispersion for 5 min, the flask was sealed, and then bubbled with N2 to ensure an anaerobic condition in the reaction system. A 350-W Xenon arc lamp with a 400 nm filter was used as the light source. The irradiation distance between the lamp and the flask was set as 20 cm. The sealed flask was irradiated, and 0.4 mL gas was extracted from the flask with a syringe. The amount of generated H2 was analyzed by gas chromatography (GC-14C, TCD, Shimadzu, Japan). Photoelectrochemical measurement. Photoelectrochemical tests were performed on a three-electrode system using an electrochemical workstation (CHI660C Instruments, China), using a Pt wire as the counter electrode and Ag/AgCl electrode as the reference electrode. The working electrode was FTO glass with a sample film on the conductive surface. The specific preparation method of the working electrodes is described as follows. Firstly, 50 mg of sample and 10 µL of 5% nafion solution (E. I. Du Pont Company) were dispersed in 1 mL of ethanol, and the mixture was ground to a sticky paste and then


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applied on the conductive surface of FTO glass using a smooth glass rod. The obtained working electrode was heated at 70 °C for 2 h to evaporate the residual solvent and achieve a better contact between the sample film and FTO glass. The light source was a LED monochromatic point lamp (3 W, 420 nm). The light spot effective area on the working electrode was set as 1 cm2. 20 mL of 0.5 M Na2SO4 aqueous solution was acted as the electrolyte. The open-circuit voltages were set as the initial bias voltages in the transient photocurrent responses and EIS tests. The EIS spectra were recorded over a frequency range of 1–105 Hz with an amplitude of 5 mV. The Mott-Schottky plots were recorded by using the Impedance-Potential mode. In the LSV measurements, the potential was scanned from –0.61V to –1.31 V (vs. Ag/AgCl, pH = 7). DFT computational details. DFT calculations were carried out by using the VASP code. The exchange–correlation interaction was described by generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) functional. The energy cutoff and Monkhorst–Pack kpoint mesh were set as 500 eV and 5 × 5 × 1, respectively. During the geometry optimization, the convergence tolerance was set as 1.0 × 10–4 eV for energy. For the construction of surface models, a vacuum of 20 Å was used to eliminate interactions between periodic structures. The DFT-D2 method of Grimme was employed to treat the van der Waals interaction.

RESULTS AND DISCUSSION Morphology and microstructure The morphologies of the samples were investigated by filed-emission scanning electron microscopy (FESEM). As shown in Figure 3a, ZIF-67 possesses uniform polyhedral shape with a mean size of 425 nm (shown in the inset of Figure 3a), and this morphology


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was maintained in the CoSx derived from ZIF-67 after hydrothermal treatment (Figure 3b). Interestingly, some cracked polyhedrons can be observed in Figure 3b, suggesting the hollow structure of the CoSx polyhedrons. Figure 3c shows the crinkly laminar morphology of g-C3N4 nanosheets prepared by thermal exfoliation of bulk g-C3N4. The higher magnification FESEM image (Figure 3d) further proves the nanosheet structure of g-C3N4. Moreover, the g-C3N4 nanosheets exhibit abundant pores, which are beneficial to the mass diffusion/transfer during the photocatalytic reactions. As for the 2%CoSx/g-C3N4 composite, the CoSx polyhedrons are closely attached to the surface of the g-C3N4 nanosheets (Figure 3e), and the size and morphology of the CoSx polyhedrons in the CoSx/g-C3N4 composite (Figure 3f) are consistent with that in pure CoSx sample. Furthermore, in the EDS element mapping images of 2%CoSx/g-C3N4 (Figure 3g), the distributionss of C, N, Co, and S elements all match well with the outline of the FESEM image, indicating that CoSx polyhedrons are evenly dispersed on the surface of g-C3N4 nanosheets.


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Figure 3. FESEM images of (a) ZIF-67, (b) hollow CoSx polyhedrons, (c,d) g-C3N4 nanosheets and (e,f) 2%CoSx/g-C3N4. (g) FESEM and EDS mapping images of 2%CoSx/g-C3N4. Insets of parts a and b show the size distributions of the samples.


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Figure 4. (a) TEM images of hollow CoSx polyhedrons. (b) HRTEM image and selected area electron diffraction pattern (inset in b) of the circle part in (a). TEM images of (c) gC3N4 nanosheets and (d) the 2%CoSx/g-C3N4 composite.

For better understanding the microstructure, the samples were characterizied by transmission electron microscopy (TEM). Figure 4a clearly shows the hollow structure of the CoSx polyhedrons. Figure 4b exhibits the HRTEM image and selected area electron diffraction pattern (inset in b) of the circle part in Figure 4a. No obvious lattice fringes and electron diffraction rings can be observed, which indicate the amorphous feature of the CoSx polyhedrons. Figure 4c exhibits the typical TEM image of the porous g-C3N4


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nanosheets. As for the 2%CoSx/g-C3N4, Figure 4d clearly shows that the hollow CoSx polyhedrons are uniformly dispersed and closely embedded in g-C3N4 nanosheets. The intimate contact between g-C3N4 nanosheets and hollow CoSx polyhedrons can be ascribed to the different surface charge properties of g-C3N4 and the precursor of CoSx (ZIF-67). Figure 5a shows the zeta potentials of g-C3N4 nanosheets and ZIF-67 at the pH = 7. The g-C3N4 nanosheets possess negative zeta potential of –21.4 mV. The high zeta potential value indicates the good dispersity and stability of g-C3N4 nanosheets in the suspension.45 On the contrary, the ZIF-67 exhibits positive zeta potential of 3.8 mV. The electrostatic attraction between g-C3N4 nanosheets and ZIF-67 can result in intimate contact between g-C3N4 nanosheets and the ZIF-67 polyhedrons. After hydrothermal treatment, the ZIF-67 in-situ transforms into hollow CoSx polyhedrons. The special structure of hollow CoSx polyhedron embedded in g-C3N4 nanosheets can enhance the contact area between CoSx and g-C3N4, and thus promote the charge transfer and photocatalytic reaction.


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Figure 5. (a) Zeta potentials of ZIF-67 and g-C3N4 nanosheets (pH = 7). (b) XRD patterns of g-C3N4, CoSx and 2%CoSx/g-C3N4. (c) FTIR spectra and (d) Raman spectra of g-C3N4 and 2%CoSx/g-C3N4.

Phase structure and surface chemical state X-ray diffraction (XRD) was used to investigate the phase structure of the samples. Figure 5b shows the XRD patterns of the obtained 2%CoSx/g-C3N4 with the pure g-C3N4 and CoSx as reference samples. The pattern of pure g-C3N4 exhibits two typical peaks located at 13.0° and 27.5°, which can be ascribed to the {100} and {002} facets of g-C3N4, respectively.46 For the pure CoSx, only a broad peak centered at 22° can be observed,


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which indicates that the CoSx polyhedrons are amorphous. This result is consistant with that shown in TEM results. The XRD spectrum of 2%CoSx/g-C3N4 resembles that of gC3N4, with no peaks of CoSx observed, which can be ascribed to the amorphous feature and low content of CoSx in the composites. The chemical compositions of the samples were further revealed by Fourier transform infrared (FTIR) and Raman spectroscopies. In Figure 5c, the FTIR spectra of both g-C3N4 and 2%CoSx/g-C3N4 show characteristic peaks of CN heterocycles located from 1200 to 1600 cm–1 and the typical sharp peaks of triazine units at 810 cm–1,44,46,47 which suggests no microstructure change of g-C3N4 after being coupled with CoSx. No obvious new peak can be observed in the FTIR spectrum of 2%CoSx/g-C3N4. This can be attributed to the low content of CoSx. Moreover, the main FTIR characteristic signals of Co–S bonds are located in the far infrared region (30–400 cm–1), beyond the detection region (400–4000 cm–1) of FTIR measurement.48 Figure 5d compares the Raman spectra of g-C3N4 and 2%CoSx/g-C3N4. The sharp peak at 710 cm-1 can be assigned to the in-plane bending vibrations of triazine rings, and the peak at 985 cm-1 to the symmetric N–breathing mode of triazine rings. The broad band at 1200–1700 cm-1 with a sharp shoulder peak at 1240 cm-1 can be attributed to the C–N stretching vibrations.49,50 These typical Raman resonances of melon unit without obvious structure change can be observed in both samples, further proving the good retention of g-C3N4 structure in the 2%CoSx/g-C3N4 composite. The surface chemistry of the samples was investigated by X-ray photoelectron spectroscopy (XPS). All the binding energies were calibrated by the adventitious carbon C 1s peak at 284.8 eV. Figure 6a shows the high-resolution C 1s spectra of g-C3N4, CoSx


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and 2%CoSx/g-C3N4. The C 1s spectrum of CoSx can be fitted to three peaks at 284.8, 286.3 and 288.8 eV, which can be ascribed to adventitious C (C–C), C bonded with S (C– S) and C bonded with N (C–N) from thioacetamide, respectively.51 As for g-C3N4, the spectrum can be deconvoluted into two peaks located at 284.8 and 288.0 eV. The former corresponds to adventitious C, and the latter is assigned to the carbon in N–C=N2 structure of triazine in g-C3N4.46 After coupling with CoSx, the C 1s spectrum of 2%CoSx/g-C3N4 shows a similar shape to that of g-C3N4. However, it is worth noting that the binding energy of C in N–C=N2 positively shifted to 288.2 eV. Figure 6b shows the N 1s spectra of g-C3N4 and 2%CoSx/g-C3N4. For g-C3N4, the N 1s spectrum can be deconvoluted into four peaks located at 398.4, 399.7, 400.8 and 404.2 eV, which can be assigned to the C=N–C, (C)3–N, N–H and π-excitation, respectively.47 Interestingly, these four peaks of 2%CoSx/g-C3N4 all slightly shifted to more positive binding energies, at 398.7, 400.0, 401.1 and 404.4 eV, respectively. Figure 6c exhibits the Co 2p spectra of CoSx and 2%CoSx/g-C3N4. As for CoSx, the Co 2p spectrum can be fitted to six peaks. The first doublet peaks located at 778.8 and 794.0 eV can be ascribed to the Co 2p3/2 and Co 2p1/2 of Co3+, and the second doublet peaks located at 781.8 and 797.0 eV are assigned to Co 2p3/2 and Co 2p1/2 of Co2+. The other two peaks located at 785.9 and 801.1 eV are two shake-up satellites.52,53 These results proves that the cobalt sulfide hollow polyhedrons are present in the form of CoSx. Compared with the Co 2p XPS spectrum of pure CoSx, the 2%CoSx/g-C3N4 just shows weak doublet peaks belonging to Co2+ (781.8 and 797.0 eV) and two shake-up satellites peaks (785.9 and 801.1 eV), and peaks for Co3+ cannot be discerned. The smaller percentage of Co3+ in 2%CoSx/g-C3N4 means that CoSx can obtain electrons from g-C3N4 and then part of Co3+ converts into Co2+. Figure 6d


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shows the S 2p spectra of CoSx and 2%CoSx/g-C3N4. Clearly, the spectrum of CoSx can be deconvoluted into four peaks. The two peaks located at 163.1 and 164.3 eV can be attributed to S (–II) in sulfide (CoSx), while the two peaks located at 168.7 and 169.9 eV can be assigned to S (VI) in sulfates (SO42–), which is formed from the intermediate of thioacetamide during hydrothermal treatment.1,51 As for 2%CoSx/g-C3N4, all four peaks exhibit negative shifts compared with those in CoSx, which further proves that the electrons transfer from g-C3N4 to CoSx at the interface of g-C3N4/CoSx. As a result, the binding energies of elements in the electron donor g-C3N4 (C 1s and N 1s) shift positively, while those in the electron acceptor CoSx (Co 2p and S 2p) shift negatively.

Figure 6. (a) C 1s, (b) N 1s, (c) Co 2p and (d) S 2p high-resolution XPS spectra of gC3N4, CoSx and 2%CoSx/g-C3N4.


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Figure 7. Electrostatic potentials of (a) the monolayer g-C3N4 {001} surface and (b) the CoS {001} surface. (c) The three-dimensional charge density difference for CoS/g-C3N4 composite model. The isosurface value is 0.0004 e/Å3. The yellow and cyan regions represent charge accumulation and depletion, respectively.

The charge transfer between CoSx and g-C3N4 was further explored by density functional theory (DFT) computational calculation.54-57 As suggested from the XPS results, the main component of CoSx in 2%CoSx/g-C3N4 is CoS. Hence, the CoS phase (PDF # 65-3418) was selected to construct a CoS/g-C3N4 composite model. Figure 7a and 7b exhibit the electrostatic potentials of the monolayer g-C3N4 {001} and CoS {001} surfaces, respectively. The work functions of monolayer g-C3N4 {001} surface and CoS {001} surface were calculated to be 4.68 and 4.91 eV, respectively. The Fermi energy of CoS {001} surface is lower than that of monolayer g-C3N4 {001} surface. At the interface of CoSx and g-C3N4, the electrons will transfer from g-C3N4 to CoS until the Fermi


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energies reach the same level. Figure 7c shows the three-dimensional charge density difference at the interface of CoS and g-C3N4. Clearly, the electrons mainly accumulate around CoS, while the charge depletion occurs around g-C3N4.58,59 This result is consistent with the charge transfer form g-C3N4 to CoS as revealed by XPS results.

Textural properties The specific surface area and pore structure of the samples were investigated by the N2 adsorption–desorption measurement. As shown in Figure 8a, g-C3N4 exhibits typical IVtype isotherms with H3-type hysteresis loop, indicating the presence of slit-shaped mesopores, which are formed by aggregated g-C3N4 nanosheets.60 As for the pure CoSx, a typical IV-type isotherm with a H2-type hysteresis loop at high relative pressure can be observed. This result proves the presence of ink-bottle mesopores in the CoSx.60 As described in morphology observation, the CoSx sample is composed of hollow polyhedrons. These ink-bottle mesopores are derived from the hollow structure of CoSx polyhedrons. After being coupled with CoSx, the shape of isotherms and hysteresis loop of 2%CoSx/g-C3N4 show no obvious difference from g-C3N4. In order to investigate the effect of hydrothermal treatment on the specific surface area and pore structure of g-C3N4, the hydrothermally treated pure g-C3N4 without CoSx (0%CoSx/g-C3N4) was also studied. Compared with g-C3N4, the 0%CoSx/g-C3N4 also exhibits similar typical IV-type isotherms with H3-type hysteresis loop without obvious change. Interestingly, the pore size distribution of 0%CoSx/g-C3N4 changed obviously. The percentage of pores with size of 2–5 nm (small mesopores) in 0%CoSx/g-C3N4 become much less, while the percentage of pores with size of 50–200 nm (macropores) become more than that in g-C3N4, which


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can be ascribed to the re-creation of macropores during the hydrothermal treatment process of g-C3N4. It is worth noting that the content of the small mesopores in 2%CoSx/g-C3N4 maintain the same as that in g-C3N4, suggesting that the aggregation of g-C3N4 and CoSx polyhedrons can be suppressed in 2%CoSx/g-C3N4. Table 1 summarizes the Brunauer–Emmett–Teller (BET) specific surface area (SBET), pore volume (Vpore) and average pore diameter (dpore) of the samples. Clearly, the Vpore and dpore of 0%CoSx/gC3N4 are slightly bigger than those of g-C3N4 and 2%CoSx/g-C3N4, further proving the recreation of macropores during the hydrothermal treatment of g-C3N4. Since the SBET is mainly derived from the contribution of the micropores and small mesopores, the value of SBET of 0%CoSx/g-C3N4 (74 m2/g) is slightly lower than that of g-C3N4 (77 m2/g) (Table 1). The pure CoSx exhibits a low specific surface area (13 m2/g), which can be attributed to the high mass density of CoSx as well as to the fact that part of big interior cavity of the hollow polyhedrons (>300 nm) cannot be measured by the N2 adsorption method. The 2%CoSx/g-C3N4 composite exhibits higher SBET (83 m2/g) than the pure g-C3N4 and CoSx, which can be ascribed to the better dispersity of g-C3N4 and hollow CoSx polyhedrons. Moreover, the re-creation of macropores during the hydrothermal treatment process of gC3N4 is also beneficial for the mass diffusion/transfer during the photocatalytic reactions.


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Figure 8. (a) N2 adsorption–desorption isotherms and corresponding pore size distribution curves (inset in a) of g-C3N4, CoSx, 0%CoSx/g-C3N4 and 2%CoSx/g-C3N4. (b) UV–vis spectra of g-C3N4, CoSx and CoSx/g-C3N4 samples with different contents. (c) the corresponding (αhν)2 versus hν curves of the pure g-C3N4 and CoSx.

Table 1. The BET specific surface area, pore volume and average pore diameter of gC3N4, 0%CoSx/g-C3N4, 2%CoSx/g-C3N4 and CoSx. Samples

SBET (m2/g)

Vpore (cm3/g)

dpore (nm)

g-C3N4

77

0.24

12.3

0%CoSx/g-C3N4

74

0.28

15.3

2%CoSx/g-C3N4

83

0.27

13.0

13

0.03

8.7

CoSx


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Optical absorption The optical absorption characteristics of the samples were investigated by the UV-vis spectra (Figure 8b). All the CoSx/g-C3N4 composite samples with different CoSx contents exhibit similar absorption edge to pure g-C3N4 without obvious shift, which means that the bandgaps of the CoSx/g-C3N4 remain the same as that of g-C3N4. However, the light absorption of the CoSx/g-C3N4 composites in the visible-light range (500–800 nm) is greatly enhanced, which can be attributed to the wider spectrum absorption of the black CoSx. The band gaps of pure g-C3N4 and CoSx were obtained from the Kubelka–Munk plots.52,53 As shown in Figure 8c, the band gaps of g-C3N4 and CoSx were calculated to be 2.90 and 1.53 eV, respectively, which are consistent with the previous reports.61

Separation efficiency of photogenerated charge carriers The photoluminescence (PL) spectra were used to reveal the separation and recombination tendency of the photogenerated electron–hole pairs. Figure 9a exhibits the PL spectra of g-C3N4 and 2%CoSx/g-C3N4. A broad emission peak located between 400 and 550 nm can be ascribed to the emission of bandgap transition of g-C3N4.46,62 The shape and peak position of g-C3N4 and 2%CoSx/g-C3N4 exhibit no obvious change. However, the peak intensity of 2%CoSx/g-C3N4 becomes much lower than that of g-C3N4, indicating a lower recombination rate of photogenerated electron–hole pairs after coupling with CoSx.


23


Figure 9. (a) PL spectra and (b) time-resolved PL spectra of pure g-C3N4 and 2%CoSx/gC3N4. (c) EIS spectra and (d) transient photocurrent response curves of g-C3N4, CoSx and 2%CoSx/g-C3N4.

The PL decay dynamics can reveal more information about the photogenerated charge carriers.44,62,63 When electrons are excited to the excited state by an infinite narrow pulse of light, they will return to the ground state by radiation or nonradiative transition. The two attenuation transition rates are defined as γ and knγ, respectively. Fluorescence lifetime is defined as the reciprocal of the rate of decay: τ = (γ+ knγ)–1. The time of fluorescence intensity attenuated to the 1/e of original intensity is the fluorescence


24

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lifetime of the material. The time-resolved PL spectra of pure g-C3N4 and 2%CoSx/gC3N4 are shown in Figure 9b, and the average lifetimes and percent contributions determined from a three-exponential fitting44,63 are summarized in Table 2, with the instrument response function (IRF) curve (Figure 9b) employed as the basic function for fitting process. As for 2%CoSx/g-C3N4, all the emission lifetimes (τ1, τ2 and τ3) are shorter than those of pure g-C3N4, and the average decay lifetime for 2%CoSx/g-C3N4 (5.68 ns) was smaller that of pure g-C3N4 (6.74 ns). These results can be ascribed to the higher nonradiative attenuation transition rate in 2%CoSx/g-C3N4, which derived from the electron transfer process from g-C3N4 to CoSx.

Table 2. The average lifetimes and percent contributions of PL decay data determined from a three-exponential fitting.

Samples

τ1 (ns)

g-C3N4 2%CoSx/g-C3N4

A2 (%)

τ3 (ns)

Average A3 (%) lifetime (ns)

4.99

49.66

18.07

20.69

6.74

4.24

49.52

14.76

22.14

5.68

A1 (%)

τ2 (ns)

1.77

29.65

1.11

28.34

The photoinduced interfacial charge transfer rate was further investigated by electrochemical impedance spectroscopy (EIS). In the Nyquist plots (Figure 9c), pure gC3N4 exhibits a large arc radius, while the pure CoSx shows a much smaller one, which can be ascribed to the polymer nature of g-C3N4 and good conductivity of CoSx.46,64 The 2%CoSx/g-C3N4 shows an obviously decreased arc radius than that of g-C3N4. This means that the interfacial electron-transfer resistance in 2%CoSx/g-C3N4 is much smaller than that in g-C3N4.


25


Transient photocurrent measurements were used to study the excitation and transfer of photogenerated charge carriers. As shown in Figure 9d, all three samples exhibit relatively stable photocurrent response for several on–off cycles under visible light (400 nm) irradiation. These three curves all show anodic photocurrent spike immediately after the irradiation started, which is caused by the separation of electron–hole pairs at the interface of electrode/electrolyte.65 When the light was turned off, the photocurrent of gC3N4 rapidly decreased to zero. By contrast, the photocurrents of CoSx would first decrease to a low value and then gradually increase to stable levels even in the absence of light irradiation. Combined with the XPS results, this gradual increase of photocurrent without light irradiation can be induced by the released electrons from the reoxidation process of Co2+/Co3+ in CoSx.65 When the light was turned off, the photocurrent of 2%CoSx/g-C3N4 slowly decreased to zero. This phenomenon can be ascribed to the presence of CoSx in 2%CoSx/g-C3N4. Furthermore, the 2%CoSx/g-C3N4 has the highest transient photocurrent density, whereas CoSx and g-C3N4 show the medium and minimum response, respectively. These results indicate the higher separation efficiency of photogenerated electron–hole pairs in 2%CoSx/g-C3N4 than in g-C3N4, which is consistent with the smaller interfacial electron-transfer resistance in 2%CoSx/g-C3N4.

Photocatalytic hydrogen generation performance Photocatalytic H2 generation rates over the as-prepared samples are shown in Figure 10a. Over pure g-C3N4, only trace amount of H2 was generated, which can be ascribed to the high recombination rate of photogenerated charge carriers and low surface reaction rate. After coupling with CoSx, the photocatalytic H2 evolution rates greatly increased, and the


26

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content of CoSx in the CoSx/g-C3N4 composites significantly affected the photocatalytic H2 generation activity, with the 2%CoSx/g-C3N4 exhibiting the highest H2 generation rate of 629 µmol g–1 h–1, which is about 52 times higher than the pure g-C3N4. Figure 10b shows the photocatalytic stability of 2%CoSx/g-C3N4, and the H2 generation of this sample did not slow down over four cycles. This result indicates that hollow CoSx polyhedrons can act as a promising cocatalyst for enhancing the photocatalytic H2 generation of g-C3N4.

Figure 10. (a) Photocatalytic H2 generation rate of g-C3N4 and CoSx/g-C3N4 composites with different CoSx contents (1% to 10%) under visible light (λ ≥ 400 nm) irradiation. (b) Time courses of photocatalytic H2 generation rate of 2%CoSx/g-C3N4 under visible light (λ ≥ 400 nm). (c) Photocatalytic H2 generation rate of Sample A (2%CoSx/g-C3N4 after treatment by grinding and sonication), Sample B (g-C3N4 modified with solid cobalt sulfide particles) and 2%CoSx/g-C3N4.


27


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In order to better understand the advantages of the hollow polyhedron structure in the photocatalytic hyrdrogen generation process, the photocatalytic activities of two control samples and 2%CoSx/g-C3N4 were compared (Figure 10c). Sample A was obtained by grinding and sonication of 2%CoSx/g-C3N4 to destroy the morphological structure of hollow CoSx polyhedrons. Sample B was g-C3N4 nanosheets loaded with solid cobalt sulfide particles, prepared by simple hydrothermal treatment of g-C3N4 in cobaltous nitrate (Co(NO3)2·6H2O) and thioacetamide mixture solution. The added amounts of cobaltous nitrate (Co(NO3)2·6H2O) and thioacetamide were consistent with those in preparing the 2%CoSx/g-C3N4. Clearly, both Sample A and Sample B showed lower activity than 2%CoSx/g-C3N4, which highlights the vital role of the hollow polyhedron structure of CoSx in the photocatalytic H2 generation process.

Table 3. Comparison of the photocatalytic-H2 generation activity of the metal sulfide cocatalyst modified g-C3N4 composites. Enhancement H2 factor generation

Sacrifice Dosage

Photocatalyst Light source

compared

Year Ref.

rate (µmol

agent

with pure gg–1 h–1) C3N4 350 W

80 mL of 20

Xenon arc

vol%

This CoSx/g-C3N4

50 mg

629

52

2017 work

lamp with a triethanolamine


28

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400 nm filter

aqueous solution 100 mL of 10

300 W vol%

Ref.

Xenon arc

triethanolamine100 mg

NiS/g-C3N4

447.7

58

2014

lamp with a

24 aqueous

420 nm filter solution Four low

80 mL of 10

power LEDs

vol% lactic

(3 W, 420

acid aqueous

nm)

solution

300 W

10 mL of 15

Xenon arc

vol%

Ref.

NiS2/g-C3N4

50 mg

116.3

/

2017 25

Ref.

NiS2/g-C3N4

10 mg

406

/

2017

lamp with a triethanolamine 420 nm filter

aqueous

300 W

120 mL of 25

Xenon arc

vol% methanol

MoS2/g-C3N4

27

Ref. 100 mg

lamp with a

aqueous

400 nm filter

solution

300 W

200 mL of 20

Xenon arc

vol%

231

2013 30

Ref.

CuS/g-C3N4

50 mg

17.2

lamp with a triethanolamine 420 nm filter

11

aqueous


29

14

2017 32


Page 30 of 43

solution

100 mL of 10 300 W

vol%

Ref. CuS/g-C3N4

Xenon arc

triethanolamine100 mg

1265

6

2017 33

lamp

aqueous solution 100 mL of 10

300 W vol%

Ref.

Xenon arc CoS/g-C3N4

triethanolamine 50 mg

~ 400

lamp with a

/

2014 36

aqueous 420 nm filter solution

Table 3 shows the comparison of photocatalytic-H2 generation activity of the metal sulfide cocatalyst modified g-C3N4 composites. Our CoSx/g-C3N4 composite performs as one of the best H2-generation photocatalysts. To further investigate the role of CoSx in H2 generation process of CoSx/g-C3N4, the electrochemical linear sweep voltammetry (LSV) was performed in a typical three-electrode cell,36,65 with the working electrode prepared by coating the sample on a fourine-doped tin oxide (FTO) glass slide. The LSV curves of the g-C3N4/FTO, CoSx/FTO and 2%CoSx/g-C3N4/FTO electrodes are shown in Figure 11a. Compared with the g-C3N4/FTO electrode, the CoSx/FTO electrode exhibits an enhanced cathodic current, which is consistent with the good electrocatalytic H2 generation activity of CoSx.36 The 2%CoSx/g-C3N4/FTO electrodes also show enhanced cathodic current when compared with the g-C3N4/FTO electrode. This result indicates that CoSx can act as


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a cocatalyst, reducing the over-potential and thus facilitating the catalytic reduction of H+ to H2.36 Moreover, the afore-discussed results demonstrate that the CoSx can promote the separation of photogenerated charge carriers in CoSx/g-C3N4. The fast kinetics of H2 generation on CoSx surface and the promoted charge separation efficiency across the CoSx/g-C3N4 interface can explain the greatly enhanced photocatalytic H2 generation activity of the 2%CoSx/g-C3N4 composite.

Figure 11. (a) Linear sweep voltammograms of g-C3N4/FTO, CoSx/FTO and 2%CoSx/gC3N4/FTO electrodes. (b) The XPS valance band spectra and (c) Mott-Schottky plots of g-C3N4, CoSx and 2%CoSx/g-C3N4.

Band structure and photocatalytic mechanism


31


The band structures of g-C3N4, CoSx and 2%CoSx/g-C3N4 were investigated by the XPS valance band (VB) spectra. As shown in Figure 11b, the extrapolated dominant VB edges of g-C3N4 and 2%CoSx/g-C3N4 are close without obvious shift. By contrast, the extrapolated dominant VB edge of CoSx exhibits a considerable difference from that of gC3N4, with the former more negative than the latter. The Mott–Schottky plots were used to further identify the band alignment structure of the samples. As shown in Figure 11c, the Mott–Schottky plot of g-C3N4 exhibits a positive slope, indicating the typical n-type characteristic of g-C3N4. Clearly, the extrapolated dominant conduction band (CB) edge of g-C3N4 is more negative than that of CoSx, which means that the CB of g-C3N4 should be much higher than that of CoSx. This band structure is consistent with the charge transfer direction (i.e., from g-C3N4 to CoSx). As for 2%CoSx/g-C3N4, the Mott-Schottky plot exhibits similar shape with that of g-C3N4. Moreover, the extrapolated dominant CB edge of 2%CoSx/g-C3N4 is closer to that of gC3N4, which can be ascribed to the low loading amount of CoSx. The possible band structure alignment and photocatalytic mechanism over the CoSx/gC3N4 photocatalyst are illustrated in Figure 12a. Under light irradiation, the VB electrons of g-C3N4 will be excited to the CB, and the photogenerated holes will remain in the VB of g-C3N4. Due to the difference in Fermi level between g-C3N4 and CoSx, the photogenerated electrons in the CB of g-C3N4 will transfer to the surface of CoSx. As mentioned in the Introduction, the CoSx with zero-approaching adsorption free energies of H2 can act as H2 generation active site. The accumulated electrons on the surface of CoSx can reduce H+ to produce H2. Meanwhile, the holes in the VB of g-C3N4 can be consumed by the sacrificial agent (triethanolamine in this case).


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Apart from the promoted charge separation and surface reaction activation by CoSx, the hollow structure of the CoSx polyhedrons may benefit the photocatalytic process in the following ways. Firstly, the hollow structure can provide more exposure surface of CoSx, which can act as reduction reaction active sites. Secondly, the hollow structure allows multiple reflections of light in the cavity, which enhances the light harvesting. The possible light path in the hollow structure is illustrated in Figure 12b. Thirdly, the blackcolored CoSx can absorb most of the incident light, including the infrared region. The possible photothermal effect raises the surface temperature of the hollow CoSx polyhedron, which is benefical for the mass diffusion/transfer and H2 desorption.

Figure 12. (a) The possible mechanism for the photocatalytic H2 evolution over the CoSx/g-C3N4 composite photocatalyst. (b) The schemes of light path and photothermal effect in the hollow CoSx polyhedron.

CONCLUSIONS In summary, hollow CoSx polyhedrons derived from ZIF-67 has been used as highefficiency cocatalyst for enhancing the photocatalytic H2 generation of g-C3N4. The


33


CoSx/g-C3N4 was fabricated by firstly electrostatic self-assembly of the precursor ZIF-67 and g-C3N4, and subsequently in situ sulfidation of ZIF-67 into CoSx on the surface of gC3N4. The hollow polyhedrons of CoSx can greatly boost the surface active site density and light harvesting. The obtained 2%CoSx/g-C3N4 exhibited excellent photocatalytic H2 generation activity, achieving a H2 generation rate of 629 µmol g–1 h–1 under visible light, which is about 52 times higher than pure g-C3N4. The greatly enhanced activity can be ascribed to the better separation of photogenerated charge carriers, the faster H2 generation reaction kinetics and the more surface reaction active sites. This work proves that the hollow CoSx polyhedrons can be a potential cocatalysts for photocatalytic H2 generation.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes: The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by NSFC (21433007, 21573170, U1705251 and 51320105001), NSFHP (2015CFA001) and Innovative Research Funds of SKLWUT (2017-ZD-4).

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Table of Content

High-efficient hollow CoSx polyhedrons cocatalyst on g-C3N4 to generate sustainable hydrogen energy by photocatalytic water splitting.


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Hollow CoSx Polyhedrons Act as High-Efficiency Cocatalyst for

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