Clarifying the Roles of Oxygen Vacancy in W ... - ACS Publications

Article Cite This: ACS Appl. Energy Mater. 2018, 1, 3410−3419

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Clarifying the Roles of Oxygen Vacancy in W‑Doped BiVO4 for Solar Water Splitting Xin Zhao,§,‡ Jun Hu,*,§,#,‡ Xin Yao,§ Shi Chen,∥ and Zhong Chen*,§ §

School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore School of Physical and Mathematical Sciences, Nanyang Technological University, 637371, Singapore # School of Chemical Engineering, Northwest University, Xi’an, 710069, P. R. China ∥

ACS Appl. Energy Mater. 2018.1:3410-3419. Downloaded from pubs.acs.org by OPEN UNIV OF HONG KONG on 01/12/19. For personal use only.

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ABSTRACT: Most oxide semiconductor photoanode materials for water splitting are synthesized in ambient environment. Oxygen vacancy exists in these samples making them intrinsically n-type at the assynthesized state. Oxygen vacancy has been widely reported for enhancing the performance of a photoanode by improving the electron conductivity. Besides the effect on the bulk materials properties, oxygen vacancy also plays an important role in the interfacial charge transfer to electrolyte, on which much less attention has been paid in the past. Herein, we found that although air-annealed W-doped BiVO4 has a higher electron density, lower surface charge transfer resistance, and a slightly better light absorption than the O2-annealed sample, the latter displays a higher photocurrent density. Experimentally we found that the enhanced performance comes from a better charge separation efficiency, despite that the presence of oxygen vacancy does lead to a better charge transfer efficiency. Theoretical calculation finds that there is a localized state formed inside the bandgap in W-doped BiVO4 with oxygen vacancy, which serves as recombination center to reduce its charge separation efficiency. Oxygen vacancy on the V site activates two different kinds of V into reactive sites for improved surface catalysis. Oxygen vacancy also facilitates the adsorption of the OHads, Oads, and OOHads involved in a water splitting process, which benefits the surface catalytic process. It is predicted from this study that better performance can be achieved by introducing oxygen vacancy on the surface of a doped BiVO4 and simultaneously avoiding oxygen vacancy in the bulk. The current study provides an important understanding of the roles played by oxygen vacancy in doped photoanode materials. KEYWORDS: BiVO4, oxygen vacancy, surface catalysis, DFT calculation, reactive sites, water splitting

1. INTRODUCTION Photoelectrochemical (PEC) water splitting is a promising route to generate hydrogen fuel to solve energy and environmental issues by converting and storing intermittent and inexhaustible solar energy into chemical bonds.1−3 Compared with photocathode semiconductor for hydrogen evolution, the low efficiency in oxygen evolving of photoanode faces a greater challenge to an overall water splitting.4 As a result, much efforts have been made to enhance the performance of photoanode. Most oxide photoanode materials, such as TiO2, WO3, Fe2O3, and BiVO4, are synthesized in ambient environment and contain native oxygen vacancy in the as-synthesized state. The presence of oxygen vacancy is usually seen as an advantage since they endow these oxides with the electron conductivity (n-type) needed for the photoanodes. However, the electron conductivity usually remains at a lower level than expected for water splitting applications. Many researchers have employed elemental doping such as W or Mo in BiVO4, Ti, Sn, Si, W, Nb in Fe2O3 to improve the electron conductivity by increasing the electron density.2,5−19 An © 2018 American Chemical Society

alternative to elemental doping for better electron conductivity is to introduce oxygen vacancy in an oxide semiconductor. It has been reported oxygen vacancies generated by hydrogen treatment can improve the performance of TiO2, WO3 and BiVO4 for PEC water splitting.20−22 Similarly, oxygen vacancies can be created by annealing in a controlled oxygen-deficient atmosphere. This has been demonstrated in Fe2O3 photoanode, which has tremendously improved the photocurrent density to around 3 mA/cm2.23 The roles of oxygen vacancies can be viewed from two perspectives, the bulk and surface. For the bulk material, free electrons are generated with the creation of oxygen vacancies, which can be seen as electron donors.24 Thus, proper amount of oxygen vacancy can efficiently enhance the conductivity and improve the PEC performance. On the other hand, oxygen vacancy in the bulk also become the recombination center of Received: April 6, 2018 Accepted: June 28, 2018 Published: June 28, 2018 3410

DOI: 10.1021/acsaem.8b00559 ACS Appl. Energy Mater. 2018, 1, 3410−3419

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ACS Applied Energy Materials

2. METHODS AND CHARACTERIZATION

the photogenerated electrons and holes; therefore, too much oxygen vacancy would harm the photocatalytic performance.25 Hence, a good performance is a result of proper balance of the electron carrier density and charge recombination at the oxygen vacancy site. For the surface, oxygen vacancy plays an important role in the interfacial charge transfer by modulating many surface properties, such as water adsorption sites and active reaction sites.26,27 Recently, our theoretical work found that oxygen vacancy has two roles in enhancing the surface catalysis on pristine BiVO4. First, oxygen vacancy increases the active sites by turning the V site into active site. Second, stronger adsorption of H2Oads, OHads, and Oads are observed in the presence of oxygen vacancy, which indicates an easier charge transfer to the electrolyte and improved surface catalysis.26 In the Fe2O3 system, it was found from experiment that oxygen vacancy plays different roles on the surface catalysis for pristine and Ti-doped Fe2O3: it contributes positively to the activity of pristine Fe2O3 but negatively affects the surface activity of Ti-doped Fe2O3.27 The reason lies with the fact that oxygen vacancy has introduced an active surface state in pristine Fe2O3, which has greatly enhanced the surface hole transfer. However, for Ti-doped sample, Ti doping generated two active surface states and oxygen vacancy had some negative effect on one of the states, which has resulted in a lower charge transfer efficiency of Ti-doped Fe2O3 with oxygen vacancy compared with that without oxygen vacancy.27 Hence, oxygen vacancy plays different roles in different materials. On the basis of previous studies, we believe it is of great interest and importance to fully understand the roles of oxygen vacancy in a doped BiVO4 photoanode materials. BiVO4 is one of the most promising photoanode materials for PEC watersplitting, which has a theoretical conversion efficiency of 9.1% with a bandgap of 2.4 eV.8,10,28 BiVO4 is an n-type semiconductor materials, oxygen vacancies exist when annealed in ambient condition. However, BiVO4 has a poor electron conductivity, thus n-type doping (W or Mo for most cases) is necessary to increase its PEC activity.5−8,10 In this study, we employed both experimental and theoretical approaches to investigate the roles of oxygen vacancy in a W-doped BiVO4. The W-doped BiVO4 thin films were prepared by a drop-casting method reported by our previous work.29 Heat treatment in ambient condition and pure oxygen environment was conducted to generate different state in oxygen vacancy and to study its effect. Oxygen vacancies can be produced under oxygen-deficient conditions or in a reductive environment such as hydrogen.30,31 Hydrogen treatment method has the potential influence of hydrogen impurities.32 And annealing in N2 or Ar gas produced black BiVO4 based on our past experience. Thus, in this study, we use air and pure oxygen environment to generate and compare oxygen vacancy effect, considering the difference of O2 content in air (about 21%) and pure oxygen (99.99% used in this study). The effects on bulk properties like electronic and optical properties and surface catalysis of oxygen vacancy in Wdoped BiVO4 were analyzed. It was found from experiment that oxygen vacancy decreased the charge separation and improved the surface charge transfer activity. The mechanisms were then understood through theoretical computation. These results enrich our understanding of the roles that oxygen vacancy plays in W-doped BiVO4 photoanode on PEC performance.

2.1. Preparation of W-Doped BiVO4 Thin Films. W-doped BiVO4 thin films were fabricated by a drop-casting method reported before.29 Bismuth trioxide, ammonium metavanadate, and ammonium tungstate hydrate were dissolved in DI water with proper amount of nitric acid to form 0.1 M Bi, V and W aqueous precursor solutions, respectively. The solutions were mixed well according to stoichiometric ratio of Bi:V:W = 100:97:3, which corresponded to a volume ratio of 500 μL: 485 μL: 15 μL) for 3% W-doped BiVO4. Citric acid (CA) was also added according to stoichiometric ratio of CA:M = 1.5:1 (M is the total amount of cation). After that, the mixture was added with 1 mL of water and 1 mL of ethylene glycol (EG). After mixing, 60 μL of the precursor solution were dropped cast onto the exposed half side of a 1 cm × 2 cm FTO substrates with the other half of the substrate covered by a thermal tape. The samples were then dried at 120 °C for 30 min to obtain gel films. After that, the tape was peeled off, and the sample was placed into a tube furnace for heat treatment at 500 °C for 2 h with a ramping rate of 10 °C/min. The annealing was carried out in air or pure O2 gas environment. Before the annealing, the tube furnace was first vacuumed, after which the required gas was flowed through at a rate of 50 cc/min. 2.2. Characterization. The morphologies and thickness of the Wdoped BiVO4 films treated in air and O2 gas were observed using field emission scanning electron microscopy (FESEM, JEOL JSM-7600F). The light absorption efficiencies of the obtained samples were performed by measuring the reflectance and transmittance spectra using UV−vis-NIR spectrophotometer with an integrating sphere (Lambda 950, PerkinElmer). Crystallinity of the W-doped BiVO4 films were identified by X-ray diffraction (XRD, Shimadzu 6000 X-ray diffractometer) with Cu Kα radiation (λ = 0.154 nm). The chemical state of O was investigated by X-ray photoelectron spectroscopy (XPS, Omicron EA125). The binding energy was calibrated by C 1s (284.6 eV). PEC measurements of the W-doped BiVO4 films were measured using a three-electrode configuration (PCI4/300 potentiostat with PHE200 software, Gamry Electronic Instruments, Inc.), with the W-doped BiVO4 film as the working electrode, Pt mesh as the counter electrode, and Ag/AgCl as the reference electrode. A solar simulator (HAL-320, Asahi Spectra Co., Ltd.) with power intensity of 100 mW·cm−2 calibrated by a solar reference cell was used as light source for PEC measurement. The photocurrents of water oxidation were measured in 0.5 M Na2SO4 aqueous solution with a scan rate of 30 mV·s−1. The photocurrents tested with hole scavenger were measured in 0.5 M Na2SO4 aqueous solution mixed with 0.1 M Na2SO3. Na2SO3 was selected as the hole scavenger rather than others because it has a relatively large electrochemical window. Electrochemical impedance spectroscopy (EIS) measurements were carried out under AM 1.5G solar simulator illumination in 0.5 M Na2SO4 electrolyte at the applied potential of 1.23 V vs RHE using an AUTOLAB potentiostat-galvanostat (AUTOLAB PGSTAT302 N). The Mott−Schottky plots were also carried out using the same AUTOLAB potentiostat-galvanostat at a fixed frequency of 1 kHz in 0.5 M Na2SO4 solution. 2.3. Calculation Methods. The CASTEP module of the Materials Studio software (Accelrys Inc.) was employed for the quantum chemistry calculations. During the calculations, a 2 × 1 × 2 BiVO4 (space group 15) supercell with crystallographic parameters of 14.602 Å × 11.704 Å × 10.290 Å was used for bulk property calculation. One V atom is replaced by W atom to represent the W doped ms-BiVO4. Two different kinds of oxygen vacancies were simulated. One is the one connects V and Bi (V−O−Bi), denoted as OV on V. The other one is the one connects W and Bi (W−O−Bi), denoted as OV on W. Two ×2 supercells of W doped BiVO4 with and without oxygen vacancy on the (010) surface with a vacuum region of 15 Å were selected to investigate the surface property. The (010) surface was chosen since it is the most stable surface under the realistic conditions.33−35 Self-consistent periodic density functional theory (DFT) calculations were used. The generalized gradient approximation (GGA), in the form of the Perdew−Burke−Ernzerhof (PBE) approximation, was used to calculate the exchange-correlation 3411


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Figure 1. SEM images of W-doped BiVO4 (a) heated in air and (b) heated in pure O2. Panels c and d are cross section images of W-doped BiVO4 heated in air (denoted as Air) and pure O2 (denoted as O2) respectively. (e) X-ray diffraction of W-doped BiVO4 heated in air and pure O2 respectively. (f) O 1s XPS spectroscopic spectra (intensity normalized for easy comparison) of the W-doped BiVO4 samples obtained by annealing in different atmosphere. energy. Broyden−Fletcher−Goldfarb−Shanno (BFGS) scheme was chosen as the minimization algorithm, and the DFT-D correction was used for dispersion corrections. Hubbard U-corrections to the d electrons of V (LDA+U, effective U(V) = 2.5 eV) and spin-polarized were performed during the calculations. The energy cutoff is 380 eV and the SCF tolerance is 1.0 × 10−6 eV/atom. The optimization finishes when the energy, maximum force, maximum stress and maximum displacement are smaller than 5.0 × 10−6 eV/atom, 0.01 eV/ Å, 0.02 GPa, and 5.0 × 10−4 Å, respectively. Two ×2 × 3 k-points samplings were used for bulk and 2 × 1 × 1 for surface calculations. The Fermi level is simply defined as the valence band maximum (VBM) for semiconductors and insulators in the CASTEP code and some other codes.36,37

peaks belonging to other phases are present except the ones from the substrate FTO (SnO2). This demonstrates different gas atmosphere heat treatment has no obvious changes on the morphology and crystallinity of W-doped BiVO4. The chemical state of O was investigated by XPS as shown in Figure 1f for the O 1s. The low binding energy component located at 526.6 eV is assigned as lattice oxygen (Figure S2), which are almost the same for both samples.38,39 For the air treated sample, there is a peak at the higher binding energy, which corresponds to oxygen vacancy.38,39 Thus, it confirms that there are some oxygen vacancies in the air treated W-doped BiVO4. Compared with the air treated sample, pure O2 gas treated sample has no such peak, which shows that W-doped BiVO4 heated in pure O2 gas are free of oxygen vacancy (or minimized at least). Figure 2 shows the photocurrents of samples treated in air and O2 after optimizing the annealing time in air and O2 considering that annealing time might have great impact on the

3. RESULTS 3.1. Experimental Results. Figure 1a and b illustrates the surface structure of W-doped BiVO4 with the heat treatment in air (denoted as sample Air) and pure O2 (denoted as sample O2). Both samples show similar porous structures. The size of pores in sample Air is slightly larger than that in sample O2, while the amount of pores in sample Air is slightly smaller than that in sample O2. Thus, it is difficult to directly tell which sample has a larger surface area. Considering this, electrochemically active surface areas were measured from the capacitive region of cyclic voltammograms (Figure S1). It is found that the electrochemically active surface areas for both samples are almost the same (Figure S1). The cross section images (Figure 1c and d) further prove that the films are composed of porous structure with the thickness of around 400 nm, and adhere strongly to the FTO substrates. No obvious difference was observed for the W-doped BiVO4 heated in different gas atmosphere. X-ray diffraction (Figure 1e) shows all peaks of obtained W-doped BiVO4 thin films agree well with the ones of BiVO4 (PDF no. 14-0688). No

Figure 2. Photocurrents of samples treated in air and O2. 3412


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Figure 3. (a) Mott−Schottky plots of sample Air and O2 measured at a fixed frequency of 1 kHz in 0.5 M Na2SO4 aqueous solution. (b) Electrochemical impedance spectra (EIS) of sample air and O2 measured at the applied potential of 1.23 V vs RHE in 0.5 M Na2SO4 electrolyte under simulated solar illumination. The inset in Figure 3b is the equivalent circuit to simulate the EIS. Rbulk represents total resistance from external circuit; Q is the constant phase element; Rct is charge transfer resistance of holes from film surface to solution.

However, in this study sample O2 with a lower carrier density has a better photocurrent than sample Air. Thus, surface properties have to be investigated to understand the charge transfer process next. Electrochemical impedance spectra (EIS) was then measured. To reflect the real situation of the surface reaction, EIS was performed under simulated solar illumination. Figure 3b shows the Nyquist plots, in which the radius of the semicircle for the sample O2 is larger than that of sample Air, indicating a larger surface resistance sample O2. According to previous work, an equivalent circuit is taken to analyze the surface charge transfer (inset in Figure 3b).42 In the equivalent circuit, Rs represents the resistances of the FTO film, the external electrical contacts and the liquid electrolyte, Rct (charge transfer resistance) and Q (constant phase element) represents the direct charge transfer at the semiconductor/electrolyte interface. Ideally, an equivalent circuit Rs (Cbulk Rct) is taken to simulate the EIS. However, nontypical semicircle is observed in this study and thus a constant phase element (Q), representing a nonintuitive circuit element Q, is utilized to simulate such type of spectra.43 The fitting results show that Rs values are independent of the film annealing environment, which are around 40 Ω. The difference lies with the surface charge transfer resistance Rct. Rct of sample Air is about 1410 Ω, while that of sample O2 is larger at around 2224 Ω. This result also indicates that sample O2 shows no advantage on the surface charge transfer over sample Air. Hence, both Mott−Schottky and EIS evidence indicate unfavorable trend for sample O2. In order to gain a clear understanding of this apparent paradox, we have to analyze in details the light absorption efficiency ηabs, charge separation efficiency in the bulk of the film ηsep, and interfacial charge transfer efficiency ηtran in a PEC process. The product of the three efficiencies will be able to explain the observed photocurrent densities. The water oxidation photocurrent JH2O is expressed by44

oxygen vacancy concentration (Figure S3). The sample treated in air has a lower photocurrent (around 1.3 mA/cm2 at 1.23 V vs RHE), while the sample treated in O2 has relatively better performance (around 1.6 mA/cm2 at 1.23 V vs RHE). This corresponds to a ∼ 23% increase. In order to confirm the observed enhancement, the experiment was repeated for 3 times with consistent trend observed (Figure S4). At 1.8 V vs RHE, a ∼ 33% increase is observed (current from 2 to 2.66 mA/cm2). The observed enhancement in photocurrent density is believed to be attributed to the oxygen vacancy, in view of early evidence of almost identify film morphology, thickness, and crystallinity. Based on the mass action law on the equilibrium Equation 1, oxygen vacancy concentration, nc, in W-doped BiVO4 is dependent on oxygen partial pressure, pO2, according to nc ∝ pO−1/6 .24 2

1 OO ⇔ O2 + 2e′ + VÖ 2

(1)

Under air treatment, oxygen vacancies will be created due to the lower oxygen partial pressure compared pure O2 gas. According to eq 1, free electrons are created with oxygen vacancy generation. Thus, oxygen vacancy is a shallow donor which provides free electrons as charge carriers.23,25,40,41 On the basis of this analysis, the difference between the samples treated in air and O2 should be reflected in the carrier density. Mott−Schottky plots of sample Air and O2 are obtained to investigate the carrier density (Figure 3a). The majority carrier (i.e., electron) density can be estimated by the following eq 2 Nd =

2 [d(1/C 2)/dV ]−1 A e0εε0 2

(2)

where A is electrode area (1 cm2 in this study), e0 the electron charge (1.60 × 10−19 C), ε the dielectric constant of BiVO4 (68), ε0 the permittivity of vacuum (8.85 × 10−12 F·m−1), Nd the donor density, and V the potential applied at the electrode. The slope of sample Air is lower than sample O2, which indicates a higher carrier density of the sample treated in air. The enhanced carrier density of sample Air is consistent with the results told from XPS, which proves that air treated sample has created oxygen vacancy as shallow donors and therefore increase the donor density. The calculated carrier density of sample Air and O2 are 6.1 × 1020 cm−3 and 4.8 × 1020 cm−3 respectively according to eq 2. Generally higher carrier density improves the conductivity and a better charge separation efficiency,27 which leads to a higher photocurrent performance.

JH O = J0 × ηabs × ηsep × ηtran = Jabs × ηsep × ηtran 2

(3)

where J0 is the theoretical solar photocurrent density which assumes that all the solar energy corresponding to the band edge can be fully converted to fuel energy (7.3 mA/cm2 for BiVO4).45 Water oxidation has a slow oxidation kinetics. When an effective hole scavenger Na2SO3 is utilized, the charge transfer efficiency can be approximated to be 100% (ηtran = 1) because of the fast oxidation kinetics of Na2SO3.44 The Na2SO3 oxidation photocurrent can be determined by44 3413


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Figure 4. (a) Light absorption efficiency, (b) charge separation efficiency, and (c) charge transfer efficiency of sample Air and O2.

Figure 5. (a) Crystal structure of W-doped BiVO4, W-doped BiVO4 with oxygen vacancy (Ov) on V atom, and W-doped BiVO4 with oxygen vacancy (Ov) on W atom. Bi, V, O, W atoms are represented by purple, gray, red, and blue colors. (b) Density of states (DOS) of W-doped BiVO4, W-doped BiVO4 with oxygen vacancy (Ov) on V atom, and W-doped BiVO4 with oxygen vacancy (Ov) on W atom. (c) Optical absorption spectra of W-doped BiVO4, W-doped BiVO4 with oxygen vacancy (Ov) on V atom, and W-doped BiVO4 with oxygen vacancy (Ov) on W atom on the (100) plane derived from frequency dependent imaginary part of dielectric function.

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Figure 6. (a) Side view, (b) top view, (c) HOMO, and (d) LUMO of W-doped BiVO4 (010) facet and W-doped BiVO4 (010) facet with oxygen vacancy.

JNa SO = J0 × ηabs = ηsep × Jabs × ηsep 2

3

compared with sample Air. Thus, the good charge separation is the main reason for the observed enhanced photocurrent for the sample annealed in O2. In addition, it is also found that the oxygen vacancy can facilitate the surface charge transfer, which is consistent with predicted results from first principle calculation in our previous work in pristine BiVO4. The theoretical calculation indicates that oxygen vacancy can enhance holes transfer from the pristine BiVO4 surface to the electrolyte by lowering the adsorption energies of some intermediates such as H2Oads, OHads, and Oads.26 Considering that the air atmosphere will not necessarily result a lower amount of oxygen vacancy, the O2 annealed sample was treated in the N2 gas at 300 °C for 2 h. The photocurrent dropped compared with that before N2 treatment (Figure S5). The charge transfer efficiency of N2 treated sample was also calculated as shown in Figure S6, which is similar to that of Air annealed sample. This confirms that oxygen vacancy has positive effect on the surface catalysis.

(4)

where JNa2SO3 is the oxidation photocurrent using Na2SO3. Combining eqs 3 and 4, we can obtain the charge separation efficiency ηsep = JNa2SO3/Jabs and the charge transfer efficiency ηtran = JH2O/JNa2SO3. Figure 4a shows the light absorption efficiency of sample Air and O2 gas by measuring the light transmittance and reflectance.45 The sample Air shows slightly enhanced light absorption compared with sample O2, which is probably due to the presence of oxygen vacancy. Jabs was calculated by integrating the light absorption efficiency over the solar spectrum (AM 1.5 G). The wavelength was integrated up to 510 nm, the light absorption edge of BiVO4. Jabs is estimated to be 4.56 and 4.24 mA/cm2 for sample Air and O2, respectively. The calculated charge separation efficiency, ηsep, of sample O2 shows about 37% increase at 1.23 V vs RHE compared with sample Air, while the charge transfer efficiency, ηtran, has a smaller decrease of about 12% at 1.23 V vs RHE for sample O2 3415


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Figure 7. Adsorption structures and energies of OHads, Oads, and OOHads involved in splitting processes on (a) W-doped BiVO4 (010) facet, (b) W-doped BiVO4 (010) facet with Ov on W site and (c) W-doped BiVO4 (010) facet with Ov on V site. The “-” and “@” signs stand for bond and adsorption state on the facet, respectively.

3.2. Theoretical Calculations. The 2 × 1 × 2 supercell model of W-doped BiVO4, W-doped BiVO4 with oxygen vacancy (Ov) on V atom (the missing oxygen atom is the one connects V and Bi, V−O−Bi), and W-doped BiVO4 with oxygen vacancy (Ov) on W atom (the missing oxygen atom is the one connects W and Bi, W−O−Bi are shown in Figure 5a. Monoclinic BiVO4 is composed of a layered structure along the crystallographic c axis (Figure 5a), in which two polyhedra units, VO4 tetrahedra and BiO8 dodecahedra link to each other by edge sharing. The doping element, W, substitutes V shown in Figure 5a which is represented in blue color. As seen from Figure 5a, the generation of Ov causes some deformation around the Ov site, around which the well overlapped O atoms (red spheres) in the W-doped BiVO4 now are separated. The affected area is circled in the two Ov contained cells. Figure 5b shows the DOS of W-doped BiVO4, W-doped BiVO4 with Ov on V atom, and W-doped BiVO4 with Ov on W atom. The valence band (VB) and conduction band (CB) are mainly composed of O 2p states and V 3d states. The Bi 6p and 6s states also contribute to the composition of VB and CB. Compared with W-doped BiVO4, one feature is that the electron density near the bottom of the conduction band is enhanced for the Ov containing W-doped BiVO4. This finding is consistent with the enhanced carrier density measured by

Mott−Schottky. The other obvious feature is the formation of localized, filled states inside the bandgap, which are composed of O 2p and V 3d states. Such kind of localized states usually serve as recombination centers. Figure 5c shows optical absorption spectra of W-doped BiVO4, W-doped BiVO4 with oxygen vacancy (Ov) on V atom, and W-doped BiVO4 with oxygen vacancy (Ov) on W atom. From Figure 5c, oxygen vacancy can enhance the light absorption due to more free electrons provided by oxygen vacancy, which is consistent with the experimental measurement of light absorption. Such kind of light absorption change was also reported in H2-treated SrTaO2N.46 Considering the anisotropic optical property of BiVO4, the light absorption caused by oxygen vacancy on the (010) plane was also calculated (Figure S7), the same trend was observed. The optimized W-doped BiVO4 (010) facet with and without oxygen vacancy are shown in Figure 6. The loss of oxygen between Bi and V (denoted as Ov on V) has negligible influence to the surrounding atoms. The loss of oxygen between Bi and W (denoted as Ov on W) causes the change of the position of O(2) atom, which becomes top site from bridge site, but the bond length has no obvious change. While other atom positions are not affected by the Ov on W. According to the frontier molecular orbital theory of chemical reactivity by 3416


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ACS Applied Energy Materials Fukui, most of the electron transition will happen at the frontier orbitals, named the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of reacting species.26,47 As shown in Figure 6, the HOMO region of W-doped BiVO4 (010) facet is mainly distributed on one V atom while the LUMO region of Wdoped BiVO4 (010) facet is also distributed on the V atom but disperses to other V atoms (i.e., not localized on one V atom). Similar observation can be seen when introducing Ov on W. However, for Ov on V, LUMO has little change, but for HOMO one more V atom on the top atom layer also becomes a distribution sit. As well-known, the energy of the HOMO orbital is directly related to the ionization potential and characterizes the susceptibility of the molecule to attacks by electrophiles.26 Thus, HOMO distribution site has electron acceptor ability from intermediates of H2O such as O, OH, OOH. From this view, W-doped BiVO4 (010) facet with Ov on V has more active sites for water oxidation. The PDOS of Wdoped BiVO4 (010) with and without Ov (Figure S8) also shows that filled states inside the bandgap are observed, which is contributed by V 3d and O 2p orbitals. As seen in Figure S8, two different states in the bandgap emerge for the Ov on V atom, while only one state forms for W-doped BiVO4 without vacancy and W-doped BiVO4 with Ov on W atom. These calculation results indicate that oxygen vacancy on V site will activate two different kinds of V atoms into reactive sites for surface catalysis. Figure 7 displays the adsorption of several key species involved in the water oxidation on W-doped BiVO4 (010) facet with and without Ov. The adsorption energies of OHads, Oads, and OOHads on Bi atoms of the (010) facet of oxygen vacancy containing W-doped BiVO4 are much smaller than those of Wdoped BiVO4 without oxygen vacancy, especially Oads, which indicates that presence of oxygen vacancy favors the adsorption of the surface species. For V atom sites, the adsorption energies of Oads and OOHads show a decrease in the presence of oxygen vacancy, which indicates a better adsorption of these species. For the OHads, the situation is complicated. The adsorption energy increases for the oxygen vacancy generated between W atom and Bi atom, which means a poorer adsorption under this condition. However, if the oxygen vacancy is generated between V and Bi, it facilitates the adsorption of OHads. Considering that the doping level is small (3%), most generated oxygen vacancies should be adjacent to V atoms. Thus, oxygen vacancy will favor the adsorption of all the surface species. For the W sites, the adsorption energies of OHads, Oads, and OOHads on W atoms also decrease when oxygen vacancy is introduced on W site, indicating a better adsorption. The comparison of adsorption of surface species involved in the water oxidation apparently shows that oxygen vacancy favors the surface catalytic process.

Figure 8. Schematic description of the PEC water splitting process on a W-doped BiVO4 with or without oxygen vacancy.

electron conductivity in BiVO4 sufficiently; additional oxygen vacancy mainly promotes the charge recombination. If only oxygen vacancy is generated in undoped BiVO4, the photocurrent density improves with higher charge separation and transfer efficiencies (Figure S9).48 Hence, the good PEC performance of oxygen vacancy containing sample is the balance of good conductivity and recombination through vacancies. Similar phenomenon exists in other metal oxides such as hematite. It was reported that the photocurrent of hematite first enhanced with increasing oxygen vacancy then showed decrease when the oxygen vacancy concentration is too much.25,41 For the surface catalysis, two kinds of oxygen vacancies are studied: the one between Bi and V, and the other between Bi and W. The calculated results show that Ov on V can activate the surface V atom into a reactive site, which increase the surface reaction sites. This is likely to be the dominant type of oxygen vacancies, because the doping level of W is very small. Thus, air treated W-doped BiVO4 will favors the surface catalysis of water splitting. In addition, oxygen vacancy influences the adsorption of several key species on W-doped BiVO4 (010) facet, which plays a key role in enhancing the hole transfer for water oxidation. The calculation well explains the observed lower hole transfer resistance and consequent enhanced charge transfer efficiency in experiment. Moreover, we also compared the pristine BiVO4, which also showed that oxygen vacancy containing pristine BiVO4 had a higher charge transfer efficiency (Table S1). Thus, for both doped and undoped BiVO4, oxygen vacancy is favorable in the surface catalysis. In summary, oxygen vacancy serves as recombination center in the bulk but facilitates the surface catalysis. Based on the above analysis, it can be predicted that in a doped BiVO4 photoanode better performance can be achieved by introducing oxygen vacancy on the surface and simultaneously avoiding oxygen vacancy in the bulk.

4. DISCUSSION On the basis of the above analysis, the mechanism of how oxygen vacancy affects the PEC water splitting in a W-doped BiVO4 photoanode is shown in Figure 8. For the bulk charge separation, the oxygen vacancy plays a negative role, as it serves as charge recombination center. The generation of oxygen vacancy usually improves the electron conductivity that favors charge separation.27 However, its role in hindering charge separation through charge recombination should not be neglected when selecting dopant element and concentration. In the current study, W doping has already improved the

5. CONCLUSION Porous W-doped BiVO4 thin films are prepared by drop casting, followed by heat treatment in air and in O2. The heat treatment does not change the morphology and crystallinity of the thin films, but the air-annealed sample contains oxygen vacancy in this intrinsically n-type semiconductor. Air-annealed sample shows better electron density, lower surface charge transfer resistance, and a slightly higher light absorption than 3417



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the O2-annealed sample. However, the O2-annealed sample displays better photocurrent density than the air-treated sample, as it has a much better charge separation efficiency. Theoretical calculation confirms that the better charge transfer in the air-annealed sample is attributed to the presence of oxygen vacancy for enhanced surface catalysis. Oxygen vacancy enhances the electron density near the bottom of the conduction band, and simultaneously improves light absorption. Oxygen vacancy also facilitates the adsorption of the OHads, Oads, and OOHads involved in a water splitting process, which benefits the surface catalysis process. However, as oxygen vacancy acts as a recombination center for the photogenerated electrons and holes, care should be exercised when choosing this strategy to improve the photoanode materials for a PEC performance, particularly in a highervalence doped sample where the electron density is high enough to ensure good charge separation efficiency. Current study predicts that in a doped BiVO4 photoanode better performance can be achieved by introducing oxygen vacancy on the surface and simultaneously avoiding oxygen vacancy in the bulk.

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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/acsaem.8b00559. investigation of relative electrochemically active area of sample air and sample O2, O 1s XPS spectroscopic spectra, photocurrents of samples treated in air and O2, photocurrents of 3 repeated samples for each treatment, photocurrent of O2 annealed sample, charge transfer efficiency of N2 treated sample, optical absorption spectra and partial density of states of W-doped BiVO4, W-doped BiVO4 with oxygen vacancy on V atom, and W-doped BiVO4 with oxygen vacancy on W atom, water oxidation and Na2SO3 oxidation photocurrents, calculated charge separation efficiency and water oxidation charge transfer efficiency, and theoretical calculation methods (PDF)

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Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Xin Zhao: 0000-0002-7493-1014 Jun Hu: 0000-0002-3075-9291 Zhong Chen: 0000-0001-7518-1414 Author Contributions ‡

X.Z. and J.H. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial supports from Ministry of Education (grant RG15/ 16), Nanyang Technological University in form of SUG, Singapore National Research Foundation through the Singapore-Berkeley Initiative for Sustainable Energy (SINBERISE) CREATE Programme, the National Natural Science Foundation of China (No. 21676216). 3418


Article

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Clarifying the Roles of Oxygen Vacancy in W ... - ACS Publications

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