Effect of Surface Self-Heterojunction Existed in BixY1–xVO4 on

Research Article pubs.acs.org/journal/ascecg

Effect of Surface Self-Heterojunction Existed in BixY1−xVO4 on Photocatalytic Overall Water Splitting Wenjian Fang, Junying Liu, Dong Yang, Zhidong Wei, Zhi Jiang, and Wenfeng Shangguan* Research Center for Combustion and Environment Technology, Shanghai Jiao Tong University, 800 Dong Chuan Road, Shanghai 200240, China S Supporting Information *

ABSTRACT: BixY1−xVO4 solid solution, with absorption edge about 410 nm, is a new visible light photocatalysts based on V with d0 electron configuration for overall water splitting. However, BixY1−xVO4 prepared by solid state reaction always shows low photocatalytic activity and bad repeatability. In this paper, diluted acid was introduced to modify the BixY1−xVO4 prepared by solid state reaction. The photocatalytic activity of BixY1−xVO4 can be increased nearly four times after diluted acid treatment. The apparent quantum efficiency for overall water splitting at 380 nm is 3.4%. The enhanced photocatalytic water splitting activity is mainly attributable to the disappearance of BiOy clusters on the surface of BixY1−xVO4. The adverse effects for water splitting induced by BiOy clusters is explained by a novel surface selfheterojunction built between BiOy clusters and BixY1−xVO4. Without diluted acid treatment, BiOy clusters on the surface could capture photogenerated electrons by this surface self-heterojunction, which is bad for water splitting due to its lower conduction band. KEYWORDS: BiVO4, Solid solution, Heterojunction, Solar energy conversion, Hydrogen production

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INTRODUCTION

So far, the best photocatalysts reported for overall water splitting are La-NaTaO3 loaded with Ni/NiOx cocatalyst under UV light irradiation8 and (Ga1−xZnx)(N1−xOx) loaded with Rh2−yCryO3 cocatalyst under visible light irradiation.9 Recently, it is reported that a solar-to-hydrogen energy conversion efficiency of particulate photocatalyst sheets exceeds 1%.10 However, it is still far from the practical application requirement of at least 10% solar energy conversion efficiency.7 Thus, discovering new materials and improving solar energy conversion efficiency are still two major themes in research of heterogeneous photocatalytic water splitting.11−17 BiVO4, an excellent photocatalyst for producing O2 from AgNO3 aqueous solution, failed to reduce H2O to H2 due to the redox potential of its conduction band slightly lower than that of H2O/H2.18−25 So properly modified BiVO4 may be a new material for overall water splitting. In our previous study, we first discovered that the BiVO4:YVO4 solid solutions are stable and efficient photocatalysts for photocatalytic water splitting into H2 and O2.13 Moreover, a series of mixed oxide photocatalysts BixM1−xVO4 (M = Y, Dy, Sm, etc.) prepared by solid state reaction were also proved to be effective photocatalysts based on V with d0 electron configuration for overall water splitting.13,26−31 Replacement of Bi by M cations not only

Hydrogen, as a potential alternative energy, is one of the most attractive measures for solving energy crisis and carbon emissions reduction. There are many ways to generate hydrogen, such as electrolysis of water, coal gasification, crude oil, and natural gas steam catalytic conversion, etc. However, these methods need excess energy consumption which is usually greater than the energy produced. So it is necessary to find a sustainable energy source to obtain hydrogen energy. Photocatalytic water splitting, known as the photodriven conversion of liquid water to gaseous hydrogen and oxygen, has attracted more and more attention since the discovery of the Honda-Fujishima effect in the 1970’s.1 In general, there are two main ways for photocatalytic hydrogen production: heterogeneous photocatalytic hydrogen production (HPC) and the photoelectric catalyzed hydrogen production (PEC).2,3 In terms of HPC, photocatalytic water splitting can be divided into two main reaction systems, including photocatalytic hydrogen production with sacrificial agent and overall water splitting. Though photocatalytic hydrogen production with sacrificial agent can always easily absorb visible light and achieve high quantum efficiency, the requirement of sacrificial reagents is a limit for providing sustainable hydrogen energy.4−6 In this case, overall water splitting is more significance which is hailed as the “Holy Grail” in chemistry.7 © 2017 American Chemical Society

Received: March 16, 2017 Revised: May 26, 2017 Published: June 6, 2017 6578

DOI: 10.1021/acssuschemeng.7b00808 ACS Sustainable Chem. Eng. 2017, 5, 6578−6584

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̂ Figure 1. (a) UV−vis diffuse reflectance spectra (DRS) of Y-BYV and W-BYV and (b) the plot of (αhν)(1/2) vs hν of Y-BYV and W-BYV.

Figure 2. (a) Powder X-ray diffraction patterns of Y-BYV and W-BYV; (b) a narrow range. Preparation of Photocatalyst. The BixY1−xVO4 solid solution was prepared by the solid state reaction method, referring to our previous work.30 Bi2O3, Y2O3, and NH4VO3 were used as starting materials without further purification. A typical procedure is as follows: stoichiometric amounts of Bi2O3 (1.165 g), Y2O3 (0.565 g), and NH4VO3 (1.17 g) were weighed and mixed with full grinding in a mortar. Then, the mixture above was calcined in air at 800 °C for 6 h and recalcined at 850 °C for 6 h with an intermediate full regrinding process to obtain yellow Bi0.5Y0.5VO4. The obtained Bi0.5Y0.5VO4 was then soaked in HNO3 (2 M) until the color of the precipitate changed to white. Finally, the precipitate was filtered and thoroughly washed with distilled water to pH = 7 to obtain white Bi0.5Y0.5VO4 (denoted as W-BYV). For comparison, the obtained Bi0.5Y0.5VO4 was soaked in pure water with the same process above to obtain yellow Bi0.5Y0.5VO4 (denoted as Y-BYV). Situ Photodeposition Pt. The cocatalysts were loaded on BYV by situ photodeposition method.21 0.4 g BixY1−xVO4 as prepared was suspended in 80 mL deionized water with ultrasound for 20 min in a Pyrex reaction cell. Then H2PtCl6 solution containing 4 mg Pt was added to the mixed solution above. The suspension was vigorous stirring in dark with a continuous flow of Ar for 30 min when the suspension was well-mixed and the air was expelled; it was then thoroughly degassed and irradiated by Xe lamp for 3 h. After that, the dark green sediment Pt-BYV was filtered and dried at 353 K in air for overnight. Photocatalytic Reaction for Water Splitting. The photocatalytic reactions were carried out in a Pyrex reaction cell connected to a closed gas circulation and evacuation system. Typically, 0.1 g PtBYV was suspended in 80 mL deionized water. Then the suspension was thoroughly degassed and irradiated by a Xe lamp (300 W). The amount of H2 and O2 were analyzed every hour using an online gas chromatography. Characterizations. XRD patterns were tested on a D8 DA VINCE (Bruker) X-ray diffractometer using a Cu Kα ray radiation source. The scanning speed was 6°/min, tube voltage was 40 kV, and tube current

gives indirect effect on band structure but also raises the position of conduction band to satisfy the H2O/H2 potential. Actually, some puzzling phenomena exist in BixM1−xVO4 solid solutions which may affect the photocatalytic water splitting activity. For example, samples prepared by solid state reaction are different every time for the photocatalytic activity as well as the color of the samples. Maybe there is a relationship between the color and the activity. In addition, there is a nonignorable trailing absorption to about 500 nm from UV−vis diffuse reflectance spectra of BixM1−xVO4 solid solutions which may have the potential to split water under visible light irradiation. However, it is disappointing that H2 is not detected under the irradiation with wavelength larger than 420 nm.13 This trailing absorption was also observed in the other composite oxides of Bi such as Bi2Ga4O9.32 In all, these incomprehensible problems are interesting and important for the improving of photocatalytic activity. So in this paper, we will give a possible explanation of this trailing absorption which causes the changing of the samples’ color. This trailing absorption is mainly caused by Bi−O or V−O existing on the surface of BixY1−xVO4 (denoted as BiOy and VOz clusters) that will disappear by diluted acid treatment. Additionally, this trailing absorption is bad for photocatalytic water splitting. Photocatalytic activity can be increased nearly four times after diluted acid treatment.

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MATERIALS AND METHODS

Chemicals. All reagents were purchased from Sinopharm Chemical Reagent Co., Ltd., China. Doubly distilled water was used throughout this work. All other reagents were of analytical reagent grade, without further purification. 6579

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ACS Sustainable Chemistry & Engineering was 40 mA. The UV−vis diffuse reflection spectra (DRS) were determined by a UV−vis spectrophotometer UV-2450 (Shimadzu, Japan) and were converted to absorbance by the Kulbelka−Munk method. The morphology of the products was investigated by scanning electron microscopy (FEI Nova NanoSEM). The XPS patterns were measured on an AXIS UltraDLD electronic energy spectrum (Kratos group) at 300 W using Mg Kα X-rays as the excitation source. The binding energies (BE) of the elements were calibrated relative to the carbon impurity with a C 1s at 284.8 eV. Raman spectra were recorded on a Raman spectrometer (Jasco NRS2100), in which a laser of 532 nm was used as an excitation source.

treatment. So the changing of photoabsorption property, such as absorption intensity and absorption edge shifting and trailing, can only be induced by some things with smaller band gaps existing in Y-BYV. These things can be washed by diluted acid treatment, which will be inexistence in W-BYV. In addition, to imitate this trailing absorption, we mix the WBYV and BiVO4 (2 wt %) mechanically. The trailing absorption has three stages, which is obviously different with Y-BYV (see Figure S1). Moreover, we found another interesting phenomenon. The disappearing trailing absorption of W-BYV appears again by tablet compressing (see Figure S1). Though the reason is still unclear, this phenomenon can exclude the impurity phase causing trailing absorption. Thus, the exotic impurity phase can be also ruled out. The crystal structures are studied by XRD to exclude that some things with smaller band gaps existed in Y-BYV are not exotic impurities. BiVO4 and YVO4 are the two end members of the solid solution BYV. They belong to tetragonal zircon type (I41/amd space group). From Figure 2a, Y-BYV and WBYV are of single phase with the zircon type structure without any impure peaks. The peaks of BYV fall in between BiVO4 and YVO4, which suggests the fully solid solution. Additionally, the diffraction peaks of W-BYV shifts about 0.02° to large-angle by acid treatment. The scanning resolution of XRD is 0.02°. So, the shifting is too small to be discussed. Moreover, BiOy clusters on the surface is the constituent part of BYV. When Bi−O on the surface is broken by diluted acid treatment, the x value of BixY1−xVO4 will decrease slightly. Thus, the diffraction peaks of W-BYV should also shift slightly to large-angle. Thus, there is no difference and no exotic impurities in the crystal structure of Y-BYV and W-BYV. As we know, the bonding properties of the metal−oxygen bridge accurately determined the physical and chemical properties of metallic oxide. So the bonding properties of Y(Bi)O8 dodecahedron and VO4 tetrahedra in the BYV are further confirmed using Raman spectroscopy shown in Figure 3. Four obvious vibration bands at 254.5, 371, 798, and 876 cm−1 were observed. The band at 254.5 cm−1 is assigned to the Y(Bi)-O symmetric stretching in a Y(Bi)O8 dodecahedron.13,34 The other three bands belonged to internal modes of the VO4 tetrahedron within zircon type. From Figure 3, peaks shifting of vibration bands is still not observed. Therefore, the structures of Y(Bi)O8 dodecahedron and VO4 tetrahedra in Y-BYV and W-BYV are identical.

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RESULTS AND DISCUSSION Structure and Morphology. The obtained BixY1−xVO4 (denoted as Y-BYV) was soaked in HNO3 (2 M) until the color

Figure 3. Raman spectra of Y-BYV and W-BYV.

of the precipitate changed to white (denoted as W-BYV). The color of Y-BYV is dark yellow. By diluted acid treatment, the color of Y-BYV changes to creamy white shown in Figure 1a. The absorption edge of Y-BYV from UV−vis DRS is about 420 nm. Specifically, an obvious trailing absorption to about 500 nm of Y-BYV solid solutions is observed. This trailing absorption was also observed in the other composite oxides of Bi.32 In our previous studies, it was thought that indirect-allowed transition of BYV caused this trailing absorption. However, it failed here to explain the disappeared trailing absorption of W-BYV. The ̂ band structure determined from the (αhν)(1/2) versus photonenergy plots (Figure 1b) reveals a band gap of Y-BYV and WBYV nearly the same of about 3.03 eV.33 It is concluded that the band structure of BYV is not changed by diluted acid

Figure 4. Scanning electron microscopy image of (a) Y-BYV and (b) W-BYV. 6580

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Figure 5. XPS survey spectra of Y-BYV and W-BYV.

On the basis of the morphology analysis, it is concluded the surface properties can be changed by diluted acid treatment. To gain insight into the chemical bonding between the component elements on the surface of Y-BYV and W-BYV, the highresolution of C 1s, O 1s, Bi 4f, V 2p, and Y 3p investigated by the XPS measurements are shown in Figure 5. The binding energies (B.E.) of the elements were calibrated relative to the carbon impurity with a C 1s at 284.8 eV. From Figure 5f, peaks of Y-BYV and W-BYV are essentially consistent with B.E. changing from 0 to 1000 eV. It suggests that elements except Bi, Y, V, and O are not brought into the surface of BYV after diluted acid treatment. In their C 1s spectra, the only peak located at 284.8 eV is typically ascribed to graphitic CC. Among the high resolution spectra of O 1s, Bi 4f, V 2p, and Y 3p, one common characteristic is that peaks shift toward the high binding energy. From the XRD in Figure 2, cell constants of BiVO4 and YVO4 with tetragonal zircon type are different. The unit cell volume of YVO4 is smaller than

Table 1. Surface Elemental Analysis by XPS % At Y-BYV W-BYV peak shifting (eV)

C

O

Bi

V

Y

Y/Bi

V/Bi

39.47 39.5 0

40.39 38.12 0.09

6.31 5.62 0.19

8.07 7.71 0.18

5.77 9.05 0.4

0.91 1.61

1.28 1.37

Excluding exotic impurities and metal−oxygen bridge differences which may induce the changing of photoabsorption property of BYV, the only remaining possibility is the surface differences. Morphology analysis of BYV solid solutions is shown in Figure 4. Irregular-shaped particles with nonuniform size are observed for both Y-BYV and W-BYV. In particular, signs of slight corrosion by diluted acid treatment can be found on the surface of W-BYV, though surfaces for both samples are very smooth. In addition, specific surface areas are very small, which make no different effect on the activities. 6581

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yellow inducing the trailing absorption. Through the diluted acid treatment, W-BYV prefers to show creamy white. Photocatalytic Activity. Photocatalytic water splitting over Y-BYV and W-BYV with 1 wt % Pt as cocatalyst were performed in a Pyrex cell (λ > 300 nm). For comparison, the test condition is completely the same. Time courses of H2 and O2 production by using the W-BYV with 1 wt % Pt as cocatalyst was conducted and shown in Figure 6. H2 and O2 were steadily produced at the rates of around 139 and 69 μmol/h, respectively. The apparent quantum efficiency at 380 nm is about 3.4%. In addition, the ratio of H2 to O2 is 2, the stoichiometric value, with a small experimental error. Nevertheless, Y-BYV shows lower photocatalytic activity with H2 36 and O2 39 μmol/h. Proposed Mechanism. By the analysis above, it is concluded that the surface properties of Y-BYV and W-BYV, especially surface chemical composition, play an important role in photocatalytic activity. Scheme diagram for enhanced photocatalytic water splitting activity of BixY1−xVO4 solid solution by diluted acid treatment is proposed in Figure 7. BiOy, YOt, and VOz are distributed randomly on the surface of BixY1−xVO4 without diluted acid treatment. The color of BixY1−xVO4 changes with the amount of BiOy on the surface. It explains that the color of samples prepared by solid state reaction are different every time. Moreover, BiOy has a narrower bandgap than the BixY1−xVO4. So the absorption tailing is connected with BiOy. The more amount of BiOy distributed on the surface, the more serious absorption tailing observed. Then a novel surface self-heterojunction built between BiOy clusters and BixY1−xVO4 is speculated. As we know, the bottom level of the conduction band of BiOy is about 0.29 eV, which is more positive than the redox potential of H+/ H2.36 As a result, the photogenerated electrons prefer to migrate to the lower energy contributed by BiOy, which have no ability to reduce water to H2. By diluted acid treatment, BiOy will be washed and the surface only formed by Y−O−V as shown in Figure 7. The electron photogenerated will keep in the conduction band formed by BYV which is able to split water.

Figure 6. Photocatalytic H2 and O2 evolution rates of Y-BYV and WBYV with 1 wt % Pt as cocatalyst. Photocatalytic reaction conditions: 0.1 g catalyst, 300 W xenon light (λ >300 nm), 100 mL pure water.

the BiVO4. So the binding energy between metal oxygen keys of YVO4 is stronger than that of BiVO4. Therefore, we speculate that the peak shifting to high binding energy is caused by the extinction of Bi−O on the surface of W-BYV. Additionally, XPS results have been fitted in Figure S2. It is found that each peak of C 1s, Bi 4f, and Y 3p can be deconvoluted into only one peak. It implies that the chemical bonding on the surface is simplicity. The peaks of O 1s and V 2p can be deconvoluted into two peaks, which is ascribed to V− O.35 Surface elemental analysis by XPS gives direct evidence shown in Table 1. The ratio of Y/Bi between Y-BYV and WBYV changes tremendously from 0.91 to 1.61. Bi−O on the surface will be broken by diluted acid treatment, while Y−O−V is stable enough to stop the further corrosion. The ICP result of the acid solution after treatment is also shown in Table S1. The concentration of Bi, Y, and V are 811, 273, and 314 mg/L, respectively. Due to a small quantity of Bi0.5Y0.5VO4 nanoparticles in the acid solution, the amount of Bi0.5Y0.5VO4 has been taken away from the ICP result. It is found that the content of Bi is very high in the acid solution after treatment. In addition, the color of BiOy with a narrow band gap is yellow and YOt with a relatively wide band gap is white. So Y-BYV with high amounts of Bi on the surface prefers to show dark

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CONCLUSIONS The BixY1−xVO4 solid solution was prepared by the solid state reaction method. It is found that the color of BixY1−xVO4 solid solution changes from dark yellow to creamy white by diluted acid treatment. Exotic impurities and metal−oxygen bridge

Figure 7. Scheme diagram for enhanced photocatalytic water splitting activity of BixY1−xVO4 solid solution by diluted acid treatment. 6582

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(9) Maeda, K.; Takata, T.; Hara, M.; Saito, N.; Inoue, Y.; Kobayashi, H.; Domen, K. GaN:ZnO Solid Solution as a Photocatalyst for Visiblelight-driven Overall Water Splitting. J. Am. Chem. Soc. 2005, 127, 8286−8287. (10) Wang, Q.; Hisatomi, T.; Jia, Q.; Tokudome, H.; Zhong, M.; Wang, C.; Pan, Z.; Takata, T.; Nakabayashi, M.; Shibata, N.; Li, Y.; Sharp, I. D.; Kudo, A.; Yamada, T.; Domen, K. Scalable Water Splitting on Particulate Photocatalyst Sheets with a Solar-to-hydrogen Energy Conversion Efficiency Exceeding 1%. Nat. Mater. 2016, 15, 611−615. (11) Wang, X.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J. M.; Domen, K.; Antonietti, M. A metal-free Polymeric Photocatalyst for Hydrogen Production from Water under Visible Light. Nat. Mater. 2009, 8, 76−80. (12) Liu, J.; Liu, Y.; Liu, N.; Han, Y.; Zhang, X.; Huang, H.; Lifshitz, Y.; Lee, S. T.; Zhong, J.; Kang, Z. Metal-Free Efficient Photocatalyst for Stable Visible Water Splitting via a Two-Electron Pathway. Science 2015, 347, 970−974. (13) Liu, H.; Yuan, J.; Jiang, Z.; Shangguan, W.; Einaga, H.; Teraoka, Y. Novel Photocatalyst of V-based Solid Solutions for Overall Water Splitting. J. Mater. Chem. 2011, 21, 16535−16543. (14) Yang, H. G.; Sun, C. H.; Qiao, S. Z.; Zou, J.; Liu, G.; Smith, S. C.; Cheng, H. M.; Lu, G. Q. Anatase TiO2 Single Crystals with a Large Percentage of Reactive Facets. Nature 2008, 453, 638−641. (15) Meng, M.; Wu, X.; Zhu, X.; Zhu, X.; Chu, P. K. Facet Cutting and Hydrogenation of In2O3 Nanowires for Enhanced Photoelectrochemical Water Splitting. ACS Appl. Mater. Interfaces 2014, 6, 4081−4088. (16) Karimi-Nazarabad, M.; Goharshadi, E. K. Highly Efficient Photocatalytic and Photoelectrocatalytic Activity of Solar Light Driven WO3/g-C3N4 nanocomposite. Sol. Energy Mater. Sol. Cells 2017, 160, 484−493. (17) Wang, Y.; Li, H.; Chen, G.; Wang, Z.; Sang, Y.; Liu, H. PdO/ TiO2 Nanobelt Heterostructures with High Photocatalytic Activities Based on an Exposed Highly Active Facet on Ultrathin TiO2 Nanobelts. Sol. Energy Mater. Sol. Cells 2017, 161, 297−304. (18) Obregón, S.; Caballero, A.; Colón, G. Hydrothermal Synthesis of BiVO4: Structural and Morphological Influence on the Photocatalytic Activity. Appl. Catal., B 2012, 117−118, 59−66. (19) Li, R.; Zhang, F.; Wang, D.; Yang, J.; Li, M.; Zhu, J.; Zhou, X.; Han, H.; Li, C. Spatial Separation of Photogenerated Electrons and Holes among {010} and {110} Crystal Facets of BiVO4. Nat. Commun. 2013, 4, 1432−1439. (20) Kim, C. W.; Son, Y. S.; Kang, M. J.; Kim, D. Y.; Kang, Y. S. (040)-Crystal Facet Engineering of BiVO4 Plate Photoanodes for Solar Fuel Production. Adv. Energy Mater. 2016, 6, 1501754. (21) Zhang, G.; Lan, Z.-A.; Lin, L.; Lin, S.; Wang, X. Overall Water Splitting by Pt/g-C3N4 Photocatalysts Without Using Sacrificial Agents. Chem. Sci. 2016, 7, 3062−3066. (22) Zhao, S.; Zhang, Y.; Zhou, Y.; Zhang, C.; Sheng, X.; Fang, J.; Zhang, M. Reactable Polyelectrolyte-Assisted Synthesis of BiOCl with Enhanced Photocatalytic Activity. ACS Sustainable Chem. Eng. 2017, 5, 1416−1424. (23) Zou, L.; Wang, H.; Wang, X. High Efficient Photodegradation and Photocatalytic Hydrogen Production of CdS/BiVO4 Heterostructure through Z-Scheme Process. ACS Sustainable Chem. Eng. 2017, 5, 303−309. (24) Kudo, A.; Omori, K.; Kato, H. A Novel Aqueous Process for Preparation of Crystal Form-Controlled and Highly Crystalline BiVO4 Powder from Layered Vanadates at Room Temperature and Its Photocatalytic and Photophysical Properties. J. Am. Chem. Soc. 1999, 121, 11459−11467. (25) Kudo, A.; Ueda, K.; Kato, H.; Mikami, I. Photocatalytic O2 Evolution Under Visible Light Irradiation on BiVO4 in Aqueous AgNO3 Solution. Catal. Lett. 1998, 53, 229−230. (26) Wang, Q.; Liu, H.; Jiang, L.; Yuan, J.; Shangguan, W. Visiblelight-responding Bi0.5Dy0.5VO4 Solid Solution for Photocatalytic Water Splitting. Catal. Lett. 2009, 131, 160−163. (27) Liu, H.; Yuan, J.; Jiang, Z.; Shangguan, W.; Einaga, H.; Teraoka, Y. Roles of Bi, M and VO4 tetrahedron in photocatalytic properties of

differences are excluded for the changing of photoabsorption property. The only difference brought by diluted acid treatment is chemical composition on the surface of BixY1−xVO4. The ratio of Y/Bi between Y-BYV and W-BYV changes tremendously from 0.91 to 1.61. Bi−O on the surface will be broken by diluted acid treatment. The absorption tailing will not be observed along with the disappearance of BiOy clusters on the surface. Moreover, BiOy clusters on the surface can capture photogenerated electrons, which is bad for water splitting due to its lower conduction band. So photocatalytic activity of BixY1−xVO4 can be increased nearly four times after diluted acid treatment. Additionally, this novel surface self-heterojunction built between BiOy clusters and BixY1−xVO4 is ubiquitous in bismuth-based composite oxide and may be significance in the field of photocatalysis.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00808. UV-vis diffuse reflectance spectra (DRS), XPS survey spectra, ICP result of acid solution after treatment, and AQY calculation (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Wenfeng Shangguan: 0000-0001-9229-2845 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (Grants 20973110 and 21577088) and the National Key Basic Research and Development Program (Grant 2009CN220000) for the financial support.

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