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A Multi-step Ion Exchange Approach for Fabrication of Porous BiVO4 Nanorod Arrays on Transparent Conductive Substrate Cong Liu, Jinzhan Su, Jinglan Zhou, and Liejin Guo ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 01 Aug 2016 Downloaded from http://pubs.acs.org on August 1, 2016

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ACS Sustainable Chemistry & Engineering

A Multi-step Ion Exchange Approach for Fabrication of Porous BiVO4 Nanorod Arrays on Transparent Conductive Substrate Cong Liu, Jinzhan Su*, Jinglan Zhou and Liejin Guo International Research Centre for Renewable Energy, State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Shaanxi 710049, P. R. China.

ABSTRACT

BiVO4 is recognized as a promising semiconductor for photo-electrochemical (PEC) application. However, lacking of synthesis methods to prepare high-quality nanostructural BiVO4 film is rarely reported in the PEC field. In this study, we report a novel synthesis approach on preparing one dimensional BiVO4 nanowire arrays using a multi-step ion exchange approach through solvothermal-hydrothermal-annealing process. The resulting BiVO4 electrodes showed a nanorod structure with high porosity. In particular, the aspect ratio surface of BiVO4 nanostructure was found favorable for its PEC application. The BiVO4 nanostructure with optimized synthesized condition showed an efficient PEC water oxidation with a photocurrent of 1.67 mA/cm2 at 1.83 V (vs. RHE).

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KEYWORDS: Multi-step ion exchange approach; Porous bismuth vanadate; Solvothermalhydrothermal-annealing process; Photo-electrochemical application.

Since Fujishima and Honda discovered water photolysis on the TiO2 photoelectrode1, metal oxide semiconductors have become one of the most studied classes of materials for photocatalytic and photo-electrochemical (PEC) water oxidation2-5. Bismuth vanadate (BiVO4) as an n-type semiconductor with a bandgap of 2.4-2.5 eV has been identified as a promising semiconductor for photocatalytic and PEC water oxidation6-8. BiVO4 has the advantages of earth abundance, low cost, low toxicity, and relatively high stability in aqueous environment (pH=59)9. The theoretical solar-to-hydrogen conversion efficiency (STH) of BiVO4 approaches 9.3% with the maximum photocurrent of 7.6 mA/cm2 under standard AM1.5G solar light illumination (100 mW/cm2)10. In addition, a relatively higher conduction band (CB) edge position of BiVO4 is preferable for water splitting compared to other narrow bandgap oxides11. However, the low charge carrier mobility and inefficient charge separation in BiVO4 appears to be the main drawback for its photo-energy conversion applications6, 11. For this reason, many attempts are devoted to improve the intrinsic carrier transport through the semiconductor and/or its charge transfer kinetics at the semiconductor/electrolyte interface. Fabricating nanostructures (nanopyramid12, nanoporous13-14 and nanotextured pillars15) improved the charge carrier mobility of BiVO4. Doing with W16-17 and/or Mo18 quickly enhanced the transport properties of BiVO4. Combining with WO319-22, ZnFe2O423, ZnO24 and TiO225 formed heterojunctions. A type II band alignment at the heterojunction interface helped to improve separation of photo-generated carriers. Coupling of BiVO4 with co-catalysts (CoPi26, CoOx27, FeOOH/NiOOH14) helped to improve the kinetics of oxygen evolution reaction (OER) significantly. Drop-casting Au


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nanoparticles onto the top surface of BiVO4 enhanced its PEC efficiency by Plasmon resonance28-29. Regardless of its importance, most of the recent work on BiVO4 has been addressed to the aforementioned modification methods and few studies have been for focused on the synthesis methods or processes for an eventual large scale implementation, especially for the nanostructure BiVO4 electrodes. Initially, our group reported the successful preparation of nanopyramid BiVO4 by seed-mediated growth in an aqueous BiVO4 suspension12. Later, Choi et al. focused on the preparation of nanoporous BiVO4 through changing of electrodeposited BiOI with vanadium compounds14, 30. Yoon et al. worked on the preparation of nanotextured pillars BiVO4 by an electrospray method15. In that case, their pure BiVO4 films were yet showed low photocurrent values. Recent studies show that the electron-hole separation in BiVO4 can be significantly improved by reducing the BiVO4 particles to sizes smaller than its hole diffusion length30-31. Therefore, it is of great importance to develop more preparation techniques and/or feasible manufacture procedures to produce nanostructural BiVO4 electrode for eventual large scale PEC applications. In this study, a facile multi-step ion exchange converting process was used to fabricate porous BiVO4 photoanodes with high surface-area and efficient PEC water oxidation. The specific process included three main steps as shown in Scheme 1. Firstly, SnSx nanosheets films were prepared by a solvothermal route using ethanol solution of tin tetrachloride (SnCl4) and thioacetamide (TAA). Then, the as-prepared SnSx films were converted to BiSm nanorod films through a hydrothermal route with aqueous solution of bismuth chloride (BiCl3) and hydrochloric acid (HCl). At last, BiVO4 films were fabricated via drop casting dimethyl sulfoxide (DMSO) solution containing vanadyl acetylacetonate (VO(acac)2) onto the as-prepared


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BiSm films and followed by annealing at 450 ℃ in air. Residual V2O5 in BiVO4 thin film after annealing was removed by NaOH solution.

Scheme 1. Flowchart summarizing the porous BiVO4 films derived by a solvothermalhydrothermal-annealing prepared process. The colors of as-prepared films of SnSx, BiSm and BiVO4 were russet, black and yellow respectively. The bandgaps of SnSx, BiSm and BiVO4 were determined to be 2.13 eV, 1.22 eV and 2.51 eV, respectively, according to the corresponding Tauc plots (Figure S1). Scanning electron microscopy (SEM) images of as-prepared films are shown in Figure 1. Ultrathin SnSx nanosheets (Figure 1a-b) of 20-35 nm in widths were grown perpendicular to substrate surface with a thickness of 2.69 µm. After the hydrothermal treatment, these thin SnSx nanosheets were converted to BiSm nanorods (Figure 1c-d) of 200-350 nm in diameter. The thickness of the film increased to 6.09 µm. Followed with drop casting of solution containing VO2+ and annealing, the BiSm nanorods were converted to porous BiVO4 nanorods (Figure 1e-f) with a thickness of 9.86 µm.


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Figure 1. SEM top view and cross sectional view images of (a, b) SnSx, (c, d) BiSm, (e, f) BiVO4 films. Insets show top view images at high magnification. X-ray diffraction (XRD) patterns of as-prepared films as well as F-doped SnO2 layer on FTO substrate are revealed in Figure 2a. The diffraction peaks with low peak intensities of sample


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SnSx could be indexed into that of cubic SnS and hexagonal SnS2 with relatively low degree of crystallinity. For BiSm, all peaks can be indexed to orthorhombic Bi2S3. The crystallinity of SnSx and BiSm could be improved by heat-treating in nitrogen atmosphere (Figure S2). XRD pattern of monoclinic BiVO4 (JPCDS NO. 00-014-0688) was obtained for the porous BiVO4 nanorods. More detail about structures of the as-prepared samples was obtained by transmission electron microscopy (TEM) analysis (Figure 2b-d). The TEM analysis reveals that the samples SnSx, BiSm and BiVO4 had morphologies of nanosheet, nanorod and porous nanorod, respectively. The high-resolution TEM (HRTEM) provides a clear inter-planar distance (d) of the as-prepared samples. The d=0.314 nm (Figure 2b) can be assigned to the (111) crystal plane of cubic SnS, which is consistent with the strongest XRD peak. Similarly, the d=0.311 nm (Figure 2c) and d=0.308 nm (Figure 2d) can be assigned to the (121) crystal plane of orthorhombic Bi2S3 and monoclinic BiVO4, respectively. Both the crystal planes are consistent with the corresponding strongest XRD peaks (Figure 2a). X-ray photoelectron spectroscopy (XPS) was used to track chemical structure changes during the three converting treatments (Figure 2e-g). The composition of SnSx sample was determined to be SnS and SnS2 (Figure S3). The peaks at 495.53 eV and 487.12 eV are corresponding to the binding energy of Sn 3d3/2 and Sn 3d5/2 of SnS, respectively. While the peaks with low intensities at 496.51 eV and 488.20 eV are corresponding to the binding energy of Sn 3d3/2 and Sn 3d5/2 of SnS2, respectively. The difference of binding energy between the Sn(IV) and the Sn(II) was in agreement with the reported value

0.6-1.0 eV32-33. The film surface compositions were

calculated by integrating the XPS peaks of Sn 3d and S 3p. The S/Sn atomic ratio was determined to be 1.35, indicating that the content of Sn2+ was larger than that of Sn4+. Thus, SnS was confirmed as the main composition of SnSx.


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The S 3p and Bi 4f spectra of Bi2S3 sample were also measured (Figure S4). The peaks at 162.73 eV and 161.45 eV correspond to S 3p1/2 and S 3p3/2, respectively. The Bi 4f5/2 and Bi 4f7/2 core level spectra were fitted with four peaks. The peaks at 164.04 eV and 158.75 eV are consistent with Bi 4f5/2 and Bi 4f7/2 of the Bi2S3, respectively. However, two small peaks at 165.03 eV and 159.82 eV could be assigned to Bi 4f5/2 and Bi 4f7/2 of the BiOCl34, respectively, the existence of BiOCl could be the excess of Bi3+ used in the hydrothermal precursor. The XPS analysis of BiVO4 (Figure S5) revealed that the oxidation states of Bi and V consistent with that of pure BiVO4. The peaks located at 164.19 eV and 158.89 eV (Figure S5a) correspond to Bi 4f5/2 and Bi 4f7/2, respectively. The peaks of 523.78 eV and 516.39 eV (Figure S5b) were identified as binding energy of V 2p1/2 and V 2p3/2, respectively. For the case of O (Figure S5c), O 1s peak is complicated by overlap of three different types of oxygen. The O 1s peak could be deconvoluted into three peaks at 531.90, 530.54 and 529.49 eV, which can be assigned to chemisorbed oxygen (OC), native oxygen vacancies (OV) and oxygen lattice (OL), respectively.


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Figure 2. (a) XRD patterns, (b, c, d) TEM images and (e, f, g) XPS spectra of as-prepared films. (b, e) SnSx; (c, f) BiSm and (d, g) BiVO4. Based on the characterization results, the mechanism for the specific converting process was discussed as follows. The Schematic illustration of converting process was shown in Figure 3. In


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the first step, the reduced tin ions reacted with S2- to form SnSx nanoflakes as the template. The oxidation-reduction potentials of the reaction between Sn2+ and ethanol can be expressed as follows: + 2 +2 ↔ + 2 (1) = −0.197 +2 ↔ (2) = +0.154 Equation 1 and 2 indicate that the reaction between Sn4+ and ethanol in a basic environment is possible33. It was also reported that Sn2S3 can be obtained by reducing Sn4+ by a mixed solvent (ethanol and methanol)35. In the second step, the HCl facilitated the spontaneous /converting of SnSx to Bi2S3 because the solubility product constant (pKsp) of Bi2S3 (115.1) is much larger than those of SnS2 (70.8) and SnS (33.6) 36. To understand the role of the HCl, hydrothermal route without HCl was prepared. When there was no HCl added into the precursor solution, Bi2S3 nanobelts (Figure S6a) of ca. 50 nm in widths were found on the top of the films while the lower part of the films was SnSx yet (Figure S6b). This implied the HCl not only contributed to the reaction rate, but also influenced the morphology of BiSm. When the HCl was added into the precursor solution, a typical characteristic of crystal growth was found in BiSm films as shown in Figure S6c, S6d. A bunch of BiSm nanorods grown from the same nucleation point implied the formation of BiSm was a typical dissolution/recrystallization process. The structure of the Bi2S3 nanorods evolved with the hydrothermal reaction time. When the hydrothermal reaction time was short, some scattered nanorods diameter of ca. 250 nm were observed on the top of the films (Figure S7a) and the nanosheets of SnSx still can be found beneath BiSm. The XRD peaks located at 2θ=25.8︒


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confirmed the presence of orthorhombic SnS (JPCDS NO. 01-073-1859) (Figure S8b). With the increase of hydrothermal reaction time, BiSm nanorods became perpendicular to substrate surface their diameter decreased and length increased (Figure S7). In addition, the content of the residual SnSx in BiSm films was also decreased (Figure S8) and the Uv-vis absorption capacity of BiSm was increased (Figure S9) with the reaction time increasing. As the hydrothermal reaction time increased to 48 h, the diameter of BiSm decreased to values smaller than 200 nm (Figure S7d).

Figure 3. Schematic illustration of formation mechanism and the morphological evolution in the multi-step ion exchange approach. The a b c arrows indicate the three different unit cell edges selected as axes. In the third step, the anions in Bi2S3 were replaced with VO2+ to form BiVO4 nanorods. The SEM top view images showed that the nanorod structure retained in BiVO4 from the original BiSm nanorods, but with a coarse surface (Figure S10). The photocurrent responses of these porous films were investigated under chopped illumination (Figure 4 and Figure S11) from back side (close to FTO). The better hole mobility of BiVO4


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compared to its electron mobility37-38 is beneficial to the back illumination to get a high photocurrent (Figure S12). After annealed in N2 atmosphere, the SnSx and BiSm showed a photocurrent density of 0.26 mA/cm2 and 4.28 mA/cm2 at 1.83 V (vs. RHE) (Figure S11), respectively. The photocurrent density of the final product BiVO4 annealed in air was 1.67 mA/cm2 at 1.83 V (vs. RHE). As the hydrothermal reaction time for SnSx to BiSm converting increasing, the photocurrent densities of BiVO4 films were increased (Figure 4). This implied that complete converting of SnSx to BiSm with longer hydrothermal reaction time is beneficial to obtain a high quality BiVO4. The residual SnSx would eventually be converted to SnO2 after annealing in air (Figure S13c). The presence of SnO2 in FTO/SnO2/BiVO4 could scatter much portion of visible light (wavelength of smaller than 400-440 nm, Figure S13a, b) but generated very little photocurrent (Figure S13d). There was then less visible light (wavelength of 400-440 nm to 500 nm) left for the underlayer BiVO4 when illuminated from the back side (close to FTO), resulted in poor photocurrent responses. It is noteworthy that the influences of SnO2 in this study is different from the reported10,

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. The main reason can be attributed to the thicker SnO2

thickness blocking the electron transport and scattering large portion of visible light away from underlayer BiVO4 in this study, while these influences can be ignored in the other reported studies as thicknesses of those SnO2 were much thinner. When the hydrothermal reaction time increased to 24 h, all SnSx will be converted to BiSm and a high photocurrent density of 1.67 mA/cm2 at 1.83 V (vs. RHE) was obtained for BiVO4-24 h film. Further increase of hydrothermal reaction time to 48 h decreased the photocurrent density slightly.


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Figure 4. Photocurrent response of BiVO4 films under chopped illumination. CONCLUSION In summary, we have successfully developed a facile solvothermal-hydrothermal-annealing process that can be used to prepare porous BiVO4 nanorod arrays. SnSx nanosheets were grown on the FTO substrate by a solvothermal reaction as the start step and then converted to BiSm nanorods with a hydrothermal reaction. Porous BiVO4 nanorods film was formed by the reaction of Bi2S3 and VO(acac)2 during annealing process. The obtained Porous BiVO4 nanorods showed a high photocurrent density of 1.67 mA/cm2 at 1.83 V (vs. RHE). This work provides a facile synthesis method for complex porous ternary metal oxide photo-electrodes through multi-step ion exchange approach. ASSOCIATED CONTENT Supporting Information. Details of experimental section and characterizations, Uv-vis absorption spectra, additional XRD patterns, XPS spectrum, SEM images and photocurrent responses of films. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION


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Corresponding Author *(Jinzhan Su) E-mail address: [email protected]. ACKNOWLEDGMENT We thank Dr. Penghui Guo for the XPS measurement and Dr. Yazhou Zhang for the TEM measurement. This work was supported by the National Natural Science Foundation of China (NO. 51202186 and 51236007). REFERENCES (1) Fujishima, A.; Honda, K., Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37-38. (2) Kang, D.; Kim, T. W.; Kubota, S. R.; Cardiel, A. C.; Cha, H. G.; Choi, K. S., Electrochemical Synthesis of Photoelectrodes and Catalysts for Use in Solar Water Splitting. Chem Rev 2015, 115, 12839-12887. (3) Liu, L.; Chen, X., Titanium Dioxide Nanomaterials: Self-Structural Modifications. Chem Rev 2014, 114, 9890-9918. (4) Huang, Z. F.; Pan, L.; Zou, J. J.; Zhang, X.; Wang, L., Nanostructured Bismuth VanadateBased Materials for Solar-Energy-Driven Water Oxidation: A Review on Recent Progress. Nanoscale 2014, 6, 14044-11063. (5) Gan, J.; Lu, X.; Tong, Y., Towards Highly Efficient Photoanodes: Boosting Sunlight-Driven Semiconductor Nanomaterials for Water Oxidation. Nanoscale 2014, 6, 7142-7164. (6) Park, Y.; McDonald, K. J.; Choi, K. S., Progress in Bismuth Vanadate Photoanodes for Use in Solar Water Oxidation. Chem. Soc. Rev. 2013, 42, 2321-2337.


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(31) Pala, R. A.; Leenheer, A. J.; Lichterman, M.; Atwater, H. A.; Lewis, N. S., Measurement of Minority-Carrier Diffusion Lengths Using Wedge-Shaped Semiconductor Photoelectrodes. Energy Environ. Sci. 2014, 7, 3424-3430. (32) Whittles, T. J.; Burton, L. A.; Skelton, J. M.; Walsh, A.; Veal, T. D.; Dhanak, V. R., Band Alignments, Valence Bands, and Core Levels in the Tin Sulfides SnS, SnS2, and Sn2S3: Experiment and Theory. Chem. Mater. 2016, 28, 3718-3726. (33) Chen, D.; Shen, G.; Tang, K.; Lei, S.; Zheng, H.; Qian, Y., Microwave-Assisted Polyol Synthesis of Nanoscale SnSx (X=1, 2) Flakes. J. Cryst. Growth 2004, 260, 469-474. (34) Malakooti, R.; Cademartiri, L.; Akçakir, Y.; Petrov, S.; Migliori, A.; Ozin, G. A., ShapeControlled Bi2S3 Nanocrystals and Their Plasma Polymerization into Flexible Films. Adv. Mater. 2006, 18, 2189-2194. (35) Su, H.; Xie, Y.; Xiong, Y.; Gao, P.; Qian, Y., Preparation and Morphology Control of Rod-Like Nanocrystalline Tin Sulfides Via a Simple Ethanol Thermal Route. J. Solid State Chem. 2001, 161, 190-196. (36) Licht S., Aqueous solubilities, solubility products and standard oxidation-reduction potentials of the metal sulphides. J. Electrochem. Soc. 1988, 135, 2971-2975. (37) Abdi, F. F.; Savenije, T. J.; May, M. M.; Dam, B.; van de Krol, R., The Origin of Slow Carrier Transport in BiVO4 Thin Film Photoanodes: A Time-Resolved Microwave Conductivity Study. J. Phys. Chem. Lett. 2013, 4, 2752-2757. (38) Liang, Y.; Tsubota, T.; Mooij, L. P. A.; van de Krol, R., Highly Improved Quantum Efficiencies for Thin Film BiVO4 Photoanodes. J. Phys. Chem. C 2011, 115, 17594-17598.


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(39) Saito, R.; Miseki, Y.; Sayama, K., Highly Efficient Photoelectrochemical Water Splitting Using a Thin Film Photoanode of BiVO4/SnO2/WO3 Multi-Composite in a Carbonate Electrolyte. Chem. Commun. 2012, 48, 3833-3835. TABLE OF CONTENTS

A Multi-step Ion Exchange Approach for Fabrication of Porous BiVO4 Nanorod Arrays on Transparent Conductive Substrate Cong Liu, Jinzhan Su*, Jinglan Zhou and Liejin Guo International Research Centre for Renewable Energy, State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Shaanxi 710049, P. R. China.


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A multi-step ion exchange approach through solvothermal-hydrothermal-annealing process was successfully used to prepare porous BiVO4 nanorod arrays on FTO substrate with a good PEC performance.


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A Multistep Ion Exchange Approach for Fabrication ... - ACS Publications

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