Boosted Water Oxidation Activity and Kinetics on BiVO4 Photoanodes

Dec 3, 2018 - Ecomaterials and Renewable Energy Research Center, School of Physics, Nanjing University, Nanjing 211102 , China. Inorg. Chem. , Article...
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Boosted Water Oxidation Activity and Kinetics on BiVO4 Photoanodes with Multihigh-Index Crystal Facets Minji Yang,† Huichao He,*,† Aizhen Liao,‡ Ji Huang,† Yi Tang,† Jun Wang,† Gaili Ke,† Faqin Dong,† Long Yang,† Liang Bian,† and Yong Zhou*,‡ †

Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 12/08/18. For personal use only.

State Key Laboratory of Environmental-Friendly Energy Materials, Key Laboratory of Solid Waste Treatment and Resource Recycle of Ministry of Education, School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, China ‡ Ecomaterials and Renewable Energy Research Center, School of Physics, Nanjing University, Nanjing 211102, China S Supporting Information *

ABSTRACT: The crystal facet of the BiVO4 photoanode has potential influence on its charge-transfer and separation properties as well as water oxidation kinetics. In the present work, a BiVO4 polyhedral film with exposed {121}, {132}, {211}, and {251} high-index facets was synthesized by a facile Bi2O3 template-induced method and investigated as a photoanode for water oxidation. In comparison with the normal BiVO4 film with a {121} monohigh-index facet, the BiVO4 film with multihigh-index crystal facets shows higher activity and faster kinetics for photoelectrochemical water oxidation. Specifically, a higher photocurrent density of 1.21 mA/cm2 was achieved on the multihigh-index facet BiVO4 photoanode at 1.23 V versus reversible hydrogen electrode (RHE) in 0.1 M Na2SO4, which is about 200% improved over the normal BiVO4 photoanode (0.61 mA/cm2 at 1.23 V vs RHE). In addition, a negative shift of 300 mV onset potential for water oxidation was observed on the as-prepared BiVO4 photoanode (0.22 V vs RHE) relative to the normal BiVO4 photoanode (0.52 V vs RHE) in 0.1 M Na2SO4. Although the UV−vis absorbance property and water oxidation pathway not be changed, the charge-transfer and separation properties as well as the overall water oxidation kinetics on the multihigh-index facet BiVO4 film were boosted obviously. Theory calculations reveal that the adsorption of H2O molecules on BiVO4{121} and {132} high-index facets is energetically favorable for subsequent dissociation and oxidation relative to that on {010} and {110} low-index facets. Furthermore, the water oxidation limiting step on {121} and {132} high-index facets of BiVO4 is changed to the step of two protons reacting with •O to form •OOH species (•O + H2O(l) + 2H+ + 2e− → •OOH + 3H+ + 3e−), which is different from the limiting step on {010} and {110} low-index facets that corresponds to the dissociation of H2O to •OH (2H2O(l) + • → •OH + H2O(l) + H+ + e−). In addition, the overpotential of water oxidation limiting step on BiVO4{121} and {132} high-index facets is lower than that on {010} and {110} low-index facets.

1. INTRODUCTION Photoelectrochemical (PEC) water splitting has appealed intensity interest as a clean route to produce H2 fuel using solar energy. As the photoelectrode is the heart of the PEC water-splitting cell, the study of efficient photoelectrodes is the forefront of the field. Among the semiconductors, BiVO4 has currently been regarded as a promising photoanode material for PEC water oxidation.1,2 In thermodynamics, BiVO4 is of a moderate band gap of 2.4 eV and an appropriate valence band position of ∼2.40 V versus reversible hydrogen electrode (RHE),3 which provides adequate overpotential to drive water oxidation using partial visible light. A high solar-to-hydrogen efficiency theoretical value (ηSTH) of 9.3% with a maximum photocurrent theoretical value of 7.6 mA/cm2 can be achieved on the BiVO4 photoanode under standard solar light illumination.4,5 Nonetheless, the best performance of the BiVO4 photoanode developed to date has fallen short on its promising values because of its poor charge-transfer and © XXXX American Chemical Society

separation properties as well as sluggish water oxidation kinetics.6,7 Because of those limitations, a variety of modified strategies have been recently developed to improve the PEC water oxidation performance of the BiVO4 photoanode.8,9 Typically, tuning the composition (such as W and Mo codoping and the introduction of oxygen vacancies) and morphology (such as nanostructuring and structural functionalization) of BiVO4 photoanodes is purposefully conducted for addressing their charge-transfer issue.10,11 In addition, designed couple of BiVO4 with photocatalysts (such as WO3, TiO2, MoO3, and C3N4) has been demonstrated to be an effective approach to improve its charge separation efficiency.12−15 Furthermore, loading of proper oxygen evolution catalysts (such as FeOOH and NiOOH) on BiVO4 photoanodes is Received: September 10, 2018

A

DOI: 10.1021/acs.inorgchem.8b02570 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Bi2O3 film and heated in air to form BiVO4 film with excess V2O5. Finally, the excess V2O5 was removed by alkali dissolution. The preparation details are as follows: 1.45 g of Bi(NO3)3 (Aladdin, 99.0%) was dissolved into 59.5 mL of deionized water and 1.5 mL of triethanolamine (Aladdin, AR) by adding 0.5 mL of HNO3 (Aladdin, 68%) and magnetic stirring to form Bi precursor solution. During the stirring, the pH value of the Bi precursor solution was adjusted to 13.5 using 1 M KOH. After that, the cleaned FTO glass was vertically immersed into the Bi precursor solution and maintained at 60 °C for 20 min to grow Bi(OH)3 film. Then, the as-grown white Bi(OH)3 film was annealed at 350 °C in air for 1 h to form nanoporous Bi2O3 film on the FTO glass substrate. Next, NH4VO3 (Sigma-Aldrich, 97%) was dissolved in deionized water and ethylene glycol (Aladdin) to make up a 0.1 M V precursor solution. Twenty microliters of V precursor solution was evenly dropped on the nanoporous Bi2O3 film and subsequently be annealed in air at 500 °C for 2 h to obtain BiVO4 film with excess V2O5. Finally, the as-prepared film was soaked into 1 M NaOH solution for 12 h to remove the excess V2O5. The asprepared BiVO4 film with multihigh-index facets is marked as BiVO4A in subsequent work. For comparison, the normal BiVO4 film with the {121} monohigh-index facet (marked as BiVO4-B) was synthesized by a drop-casting method similar to our previous work.14 Briefly, the above V and Bi precursor solutions were mixed with a Bi/V atomic ratio of 1:1. Then, 30 μL of mixed solution was dropped on FTO glass and annealed at 500 °C in air for 2 h to form BiVO4 film with the {121} monohigh-index facet. 2.2. Material Characterization. The morphology of Bi2O3 and BiVO4 films was characterized by Zeiss SUPRA 55 VP scanning electron microscopy (SEM). The microstructure and lattice of BiVO4A film were examined by a JEM-2010 transmission electron microscope (TEM). A PANalytical X’pert X-ray diffractometer (XRD) was used to collect the crystal structure information of Bi2O3 and BiVO4 films. An X-ray photoelectron spectroscopy (XPS) equipment with monochromated Al Kα radiation was employed to investigate the chemical state of the BiVO4 film. The UV−vis absorption spectrum of two kinds of BiVO4 films was collected on a Shimadzu UV-2600/2700 spectrophotometer. To detect the composition, the BiVO4-A film was dissolved into 15 wt % nitric acid and analyzed by an inductively coupled plasma (ICP) technique (Thermo ICAP6300 Duo). After PEC stability testing, the content of dissolved V and Bi in the 0.1 M Na2SO4 electrolyte was also analyzed by ICP. A SPEX 500 M spectrometer (325 nm laser as an excitation source) was used to record the photoluminescence (PL) emission spectrum of two kinds of BiVO4 films. 2.3. Electrochemical and PEC Measurements. The electrochemical (EC) and PEC measurements were performed on a CH Instruments 660E workstation. The measurements were carried out in a three-electrode quartz cell (5.0 × 5.0 × 5.0 cm3). A 300 W xenon lamp with an AM 1.5G filter was serviced as the light source (Beijing China Education Au-light Co., Ltd). The working electrode was the BiVO4 film, which has an exposed geometric surface area of 1 cm2. Pt wire (99.99%, CHI Instrument) was serviced as a counter electrode, and a saturated calomel electrode (SCE, Shanghai INESA Scientific Instrument) was the reference electrode. The conversion of measured potential (vs SCE) into the RHE (vs RHE) scale used the following Nernst equation.

confirmed to be capable of accelerating its water oxidation kinetics.16,17 Notably, both experimental and calculation works have revealed that the water oxidation activity and kinetics on photoanodes are determined by their charge-transfer and separation properties.18,19 Meanwhile, the charge-transfer and separation properties of the photocatalyst are closely related to its crystal facets.20−22 In the work of Li et al., the photogenerated electrons and holes were found to be separated directionally between BiVO4{010} and {110} facets because of their different energy levels.21 In our very recent work, 30-faceted BiVO4 polyhedron with {132}, {321}, and {121} high-index crystal facets shows more rapid oxygen generation rate than their low-index counterparts.23 The higher water-splitting activity on 30-faceted BiVO4 powder is originated from the dissociation of H2O more energetically favorable on the surface of high-index crystal facets with lower overpotential for oxygen evolution reaction. Inspired by these information, the BiVO4 film photoanode with multihigh-index crystal facets is expected greatly favorable for PEC water oxidation. However, it is worth noting that the frequently used methods for the synthesis of BiVO4 film normally produce BiVO4 with a {121} monohigh-index facet,24,25 and thus few works concern with the influence of multihigh-index crystal facets on the PEC water oxidation activity and kinetics of BiVO4. Herein, the BiVO4 polyhedral film with exposed {121}, {132}, {211}, and {251} high-index facets was prepared with a facile Bi2O3 template-induced method and investigated as a photoanode for PEC water oxidation. Our experimental results indicate that the as-prepared BiVO4 film is of remarkably improved charge-transfer and separation properties as well as boosted kinetics for PEC water oxidation relative to the normal BiVO4 film with the {121} monohigh-index facet. Further theory calculations reveal that the adsorption and dissociation of H2O molecule on BiVO4{132} and {121} high-index facets are more energetically favorable than that on {010} and {110} low-index facets, which could be the origin of enhanced water oxidation activity and kinetics on the multihigh-index facet BiVO4 film photoanode.

2. EXPERIMENTAL SECTION 2.1. Preparation of the BiVO4 Film. The BiVO4 film with multihigh-index facets was prepared on fluorine-doped tin oxide (FTO) glass substrates (1.0 cm × 1.5 cm, sheet resistance < 50 Ω) using a Bi2O3 template-induced method (shown in Figure 1). First, the nanoporous Bi2O3 film as the inducing template was synthesized on the FTO glass by chemical bath deposition and postannealing.26 Then, the NH4VO3 precursor solution was placed on the nanoporous

Θ E RHE = ESCE + ESCE + 0.059pH

(1)

where EΘSCE = 0.241 V versus RHE at 25 °C, pH is the pH value of the used electrolyte. The actual electrochemically active surface area (ECSA) for BiVO4A and BiVO4-B films was evaluated based on their double-layer capacitance (Cdl). The corresponding cyclic voltammograms for BiVO4-A and BiVO4-B films are shown in Figure S3a,b, respectively. The difference (ΔI) between the anodic (Ia) and cathodic charging currents (Ic) at 1.2 V versus RHE was plotted against the scan rate (v), and the value of Cdl for the BiVO4 film is the half of the linear slope value of ΔI versus v curve (Figure S3c). The ECSA is calculated from Cdl via the following equation.

Figure 1. Schematic illustration of the preparation procedure of BiVO4 film with multihigh-index facets. B

DOI: 10.1021/acs.inorgchem.8b02570 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry ECSA = Cdl /CS

Perdew−Burke−Ernzerhof parameterization of the generalized gradient approximation was employed for the exchange correlation.31 The bulk lattice parameters of BiVO4 were relaxed and optimized using 400 eV as the cutoff energy for basis function and 6 × 6 × 3 Monkhorst−Pack-type k-point. The experimental value of angle β of monoclinic scheelite BiVO4 is 90.383°, which was set to 90° for simplification. The calculated lattice parameters for a conventional cell are a = 5.1773 Å, b = 11.7609 Å, c = 5.1450, and β = 908, which are in reasonable agreement with the experimental data in reference. The vacuum layer of 12 Å was used to avoid the interaction between periodic slabs for all of the surface calculations. The Monkhorst−Pack set of 3 × 3 × 1 k-points was applied during the optimization of the surfaces covered with adsorbates. The upper half of the slab and the adsorbates were allowed to relax, whereas the bottom half of the slab was held fixed at its optimized bulk position. The force on each atom is converged to 0.01 eV/A during the structural relaxation.

(2)

where CS is the specific capacitance of the BiVO4 film sample. However, it is not practical for most photoelectrodes to measure CS and to estimate ECSA. Therefore, the CS value of the FTO substrate in the same electrolyte is usually used to replace the CS value of the BiVO4 film for the calculation of its ECSA.27 In the present work, the photocurrent density of film (mA/cm2) was given based on its geometric area unless specified to its ECSA. EC impedance spectroscopy (EIS) of BiVO4 film photoanodes was performed at 1.23 V versus RHE in 0.1 M Na2SO4 under AM 1.5G irradiation with an ac amplitude of 5 mV. The EIS measurement frequency ranges from 0.01 to 100 000 Hz. The measured EIS data were further fitted by ZView software using an equivalent circuit model. Incident-photon-to-current conversion efficiency (IPCE) of the BiVO4 film was carried out in 0.1 M Na2SO4 at 1.23 V (vs RHE) under monochromatic light. Figure S15 shows the power density of monochromatic light used for IPCE measurements. The IPCE was calculated by the following equation. IPCE = (1240 × j)/(λ × Plight)

3. RESULTS AND DISCUSSION Figures 2a and S1a show the SEM image of the as-prepared Bi2O3 film grown on an FTO substrate. Clearly, the Bi2O3 film

(3)

Here, λ is the incident light wavelength (nm), j is the photocurrent density of the film under specific incident light (mA/cm2), and Plight is the used power density of monochromatic light (mW/cm2). Mott−Schottky plots of two kinds of BiVO4 films were conducted on a CHI 660E workstation. The flat band potential of BiVO4 film was determined using Mott−Schottky eq 4 1/C 2 = (2/eεε0A2 Nd)(Va − Vfb − kT /e)

(4)

The amount of O2 and H2 evolution on the BiVO4-A film was measured in a gas-tight PEC cell that connected with an Agilent 7890B gas chromatograph. Before the measurement, the electrolyte (0.1 M Na2SO4) was treated by Ar bubbling for 30 min. The faradaic efficiency [η(gas)] was calculated according to eq 5.

η(gas) = (amount of generated gas) × 100/(theoretical amount of gas)

(5)

The intensity modulated photospectroscopy (IMPS) of BiVO4-A and BiVO4-B films was conducted on a Zennium electrochemical workstation at different applied potentials (0.9 to 1.23 V vs RHE). The measurement frequency ranges from 100 K to 0.1 Hz. An incident light wavelength of 365 nm (100 mW/cm2) was used as an irradiation source. The charge-transfer time (τ) was investigated in the following equation. τ = 1/2πf Here, f is the frequency corresponding to the minimum imaginary components. The water oxidation pathway on BiVO4-A and BiVO4-B film photoanodes was checked by a rotating ring-disk electrode (RRDE) system. The RRDE system was consisted of a CH Instruments 760E Bipotentiostat and a RRDE instrument (HP-1). A glassy carbon disk electrode (r1 = 5 mm) and a Pt ring electrode (r2 = 6 mm) were connected on the RRDE instrument. For RRDE measurements, the BiVO4 film was prepared on the disk electrode by the following procedures. BiVO4 powder (5 mg) was first scraped off from BiVO4-A or BiVO4-B film. Next, the BiVO4 powder was dispersed in 625 μL of deionized water and 625 μL of ethanol by sonication of 30 min. Then, 25 μL of 5 wt % Nafion solution was added into the BiVO4 powdercontaining solution. After 30 min sonicating, 5 μL of the dispersion solution was placed onto the glassy carbon disk electrode and dried at 25 °C in air for 60 min. The RRDE measurements was performed in 0.1 M Na2SO4, and 25 mW/cm2 of simulated solar light was used as the irradiation source. 2.4. Theoretical Calculations. Theoretical calculations were performed using the method similar to a reported work.28 The unit cell of monoclinic scheelite BiVO4 was optimized using the VASP code with the projector augmented wave pseudopotentials.29,30 The

Figure 2. Typical SEM image of the as-prepared Bi2O3 film (a), BiVO4-A film (b,c,e,f), and BiVO4-B (d) film. The particle size distribution of BiVO4-A (g) and BiVO4-B (h) films.

consists of randomly oriented nanoparticles with a mean diameter about 54 nm (Figure S2). As the Bi2O3 nanoparticles are not dense enough, some space among nanoparticles was observed. The nanoporous characteristic of the Bi2O3 film plays an essential role in the synthesis of BiVO4, which provides suitable path for the diffusion of V5+ ions into the C

DOI: 10.1021/acs.inorgchem.8b02570 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Bi2O3 film to induce the formation of BiVO4 during postannealing. The morphology of BiVO4-A film prepared by the Bi2O3 template-induced method is shown in Figure 2b,c. In comparison with the Bi2O3 film, larger and denser BiVO4 nanoparticles were observed. The high-magnification SEM image reveals that the BiVO4 nanoparticles are of polyhedral structure with exposed multifacets (presented in Figure 2e,f). In the previous work related to BiVO4 powder photocatalysts, the polyhedral structure with exposed multifacets was demonstrated to be propitious to separate electrons and holes through the energy-level difference between different facets.21 Therefore, it is anticipated that the BiVO4 film with a polyhedral structure may have higher activity for PEC water oxidation. The statistics for the diameter of BiVO4 polyhedral indicates that its size distribution is not uniform (ranges from 100 to 400 nm, shown in Figure 2g), which may be attributed to some small nanoparticles joining together to form large nanoparticles due to their high surface energy.23 By contrast, the morphology of the normal BiVO4 film prepared by the drop-casting method is presented in Figure 2d and S1b. The BiVO4-B film is mainly composed of dense nanoparticles with relatively uniform shape and size distribution (see Figure 2h). In general, the side-view SEM images reveal that two kinds of BiVO4 films are of similar thickness of around 400 nm. The difference on morphology suggests that the crystal structure or phase for BiVO4-A and BiVO4-B films is different. To investigate the crystal structure and phase of the asprepared films, the XRD patterns of these films were collected and are shown in Figure 3a. The characteristic diffraction peaks indexed to monoclinic BiVO4 (JCPDS. 14-0688) were

observed on both BiVO4-A and BiVO4-B films. However, four diffraction peaks at 34.50°, 39.78°, 46.46°, and 56.07° were peculiarly presented in the XRD pattern of the BiVO4-A film, which correspond to the {002}, {211}, {132}, and {251} facets, respectively. These high-index facets are absent in the XRD pattern of the BiVO4-B film, indicating that the BiVO4-A film exposes more high-index facets. The texture coefficient (P) for both films was calculated by using the XRD data to estimate the degree of their preferential orientation. The calculation equation is P(hkl) =

I(hkl) ∑ I0(hkl) I0(hkl) ∑ I(hkl)

, where I(hkl) denotes

the measured peak intensity of the (hkl) facet and I0(hkl) is the standard intensity from JCPDS. 46-1088 card. The P = 1 represents randomly orienting crystallite, and a special facet with higher P value indicates the abundance of the crystal oriented in that facet direction.32 According to the calculation, it can be found that the {121}, {132}, and {251} facets are the most three preferential crystallite orientations for the BiVO4-A film (see Table S1). For the BiVO4-B film, the {121} facet is the only preferential high-index facet. Comparatively, the BiVO4-A film is predominant by more high-index facets relative to the BiVO4-B film. The predominance facet of {121} in the BiVO4-A film was further verified by the high-resolution TEM (HRTEM) observation. As shown in Figure 3d, the {121} facet is of typical lattice fringes of 0.31 nm. Meanwhile, BiVO4 particles with irregular shapes were observed in the TEM image (Figure 3c) because of their polyhedral structure, which is consistent with their morphology shown in the SEM images (Figure 2c,e,f). The transformation of Bi2O3 template and the removal of excess V2O5 were checked by XRD, XPS, and ICP analyses. Figure 3b shows the contrastive XRD pattern of Bi2O3 film (the template) and BiVO4-A film (the product). The diffraction peaks of monoclinic α-Bi2O3 crystals (JCPDS. 761730) entirely disappeared in the XRD pattern of the BiVO4-A film, showing complete transformation of Bi2O3 to BiVO4. Figure 3e,f displays the HR XPS spectrum of V 2p and Bi 4f collected from the BiVO4-A film. The V 2p3/2 and V 2p1/2 peaks were observed at 517.96 and 524.58 eV, respectively, and the Bi 4f5/2 and Bi 4f7/2 peaks were shown at 159.12 and 164.71 eV, respectively, closely matching with the standard Bi3+ and V5+ in BiVO4.25,33 According to the XPS spectrum, the atomic ratio of V5+/Bi3+ was calculated to 16.02:16.14. Furthermore, the ICP testing confirmed that the atomic ratio of V5+/Bi3+ in the BiVO4-A film is 0.99:1.01 (Table S2), which is close to the expected atomic ratio of 1:1 in standard BiVO4. The PEC water oxidation activity on BiVO4-A and BiVO4-B films was investigated contrastively and is shown in Figure 4a. For the BiVO4-A film, a higher photocurrent density of 1.21 mA/cm2 was achieved at 1.23 V versus RHE in 0.1 M Na2SO4, which is almost twofold for the BiVO4-B film at same condition (0.61 mA/cm2). According to the data shown in Figures S3 and S4, the actual ECSA for BiVO4-A and BiVO4-B films in 0.1 M Na2SO4 was detected to be 5.68 and 4.96 cm2, respectively. Figure S5 shows the ECSA-normalized photoresponse curves for two kinds of films, which indicates that the BiVO4-A film (the multihigh-index crystal facet film, 0.21 mA/ cm2 vs ECSA) still has almost twofold ECSA-photocurrent for the BiVO4-B film (the normal film, 0.12 mA/cm2 vs ECSA). The Faraday efficiency of BiVO4-A film photoanode and Pt wire cathode for O2 and H2 evolution was measured about 95 ± 5%, indicating that the photocurrent on BiVO4-A film is

Figure 3. (a,b) XRD pattern of BiVO4-B, BiVO4-A, and Bi2O3 film. (c) Low-resolution TEM and (d) HRTEM image of BiVO4-A film. The HR XPS spectrum for V 2p (e) and Bi 4f (f) from the BiVO4-A film. D

DOI: 10.1021/acs.inorgchem.8b02570 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 4a,b and the UV−vis absorbance spectrum of films (Figure S7a,b), the charge-separation and charge-injection efficiencies of BiVO4-A and BiVO4-B films were quantitatively calculated and are shown in Figure 4c,d. At 1.23 V versus RHE, the charge-separation and charge-injection efficiency on BiVO4-B film were determined to be 33.8 and 29.4%, respectively, whereas the charge-separation and chargeinjection efficiency on BiVO4-A film were 38.9 and 52.0%, respectively. In general, the BiVO4-A film is of higher chargeseparation and charge-injection efficiency than the BiVO4-B film. Notably, the charge-injection efficiency of BiVO 4 photoanode is related to the hole-to-oxygen transfer kinetics at the interface of BiVO4/H2O.35 The charge-injection efficiency of BiVO4-A film (52.0%) is particularly higher than that BiVO4-B film (29.4%) relative to their comparison of charge-separation efficiency (BiVO 4 -A: 38.9%; BiVO 4 B:33.8%). This outcome indicates that the multihigh-index facets exhibit more obvious influence on the hole-to-oxygen transfer kinetics of BiVO4 photoanode than that on the charge separation. Figure 4e shows the PEC water oxidation stability for BiVO4-A and BiVO4-B films at 1.0 V versus RHE in 0.1 Na2SO4 under AM 1.5G irradiation of 7200 s. After the rapid attenuation during the initial irradiation of 15 min, the photocurrent was stabilized gradually on two kinds of BiVO4 films in the following irradiation. In general, higher and more stable photocurrent was achieved on the BiVO4-A film. About 60% of initial activity was retained on the BiVO4-A film after 7200 s of PEC water oxidation reaction, whereas the BiVO4-B film only has 37% of its initial activity after the same reaction time. In the previous work, the dissolution of V5+ from the BiVO4 lattice during PEC water reaction was demonstrated to the direct cause of unsatisfactory stability of the BiVO4 photoanode.36 Essentially, the sluggish kinetics of BiVO4 photoanode is the primary cause of BiVO4 dissolution, which results in the holes cannot be consumed quickly enough by water reaction, and thus accumulates on the BiVO4 surface to induce the dissolution of V5+ from BiVO4.18 For demonstration of better PEC stability on the BiVO4-A film, the concentration of dissolved V and Bi in 0.1 M Na2SO4 solution that used as an electrolyte for PEC stability testing was detected with the ICP technique. V (1.814 μM) and Bi (0.335 μM) were detected in the Na2SO4 solution that used for BiVO4-B film stability testing, confirming the occurrence of BiVO4 photocorrosion during PEC water oxidation (see Figure 4f). For the BiVO4-A film used Na2SO4 solution, lower V concentration of 0.743 μM and Bi concentration of 0.218 μM were detected, suggesting that BiVO4 dissolution was obviously suppressed because of the presence of multihigh-index facets. Further comparative analyses of XRD pattern and SEM image for two kinds of BiVO4 film before and after stability testing indicate that the dissolution on the BiVO4-A film is much weaker than that on the BiVO4-B film (shown in Figures S8 and S9). Furthermore, higher IPCE was achieved on the BiVO4-A film (Figure S10), which is consistent with the findings of above LSV and j−t results (see the integrated photocurrent from the IPCE spectrum in Table S4). To understand the origin of enhanced PEC water oxidation kinetics and activity on the BiVO4 film with multihigh-index facets, the potential reasons in thermodynamics and kinetics were investigated. First, a comparative EIS measurement was performed for BiVO4-A and BiVO4-B films at 1.23 V versus RHE under AM 1.5G irradiation. In the Nyquist plots (Figure

Figure 4. LSV scans for BiVO4-A and BiVO4-B films in 0.1 M Na2SO4 without (a) and with (b) 1 M Na2SO3 under AM 1.5G irradiation. Calculated charge-separation (c) and charge-injection (d) efficiency of BiVO4-A and BiVO4-B films. (e) J−t curve of BiVO4-A and BiVO4B films in 0.1 M Na2SO4 at 1.0 V vs RHE under AM 1.5G irradiation. (f) After 2 h stability testing, the concentration of dissolved V and Bi in the used 0.1 M Na2SO4.

mainly originated from water oxidation reaction (shown in Figure S6). Empirically, the water oxidation kinetics on BiVO4 film photoanode mainly depends on the adsorption and dissociation energy of H2O molecules on its crystal facets. Therefore, the surface area of BiVO4 film photoanode has significant no effect on its water oxidation kinetics. Significantly, a more negative onset potential for photocurrent (0.22 V vs RHE) was observed on the BiVO4-A film relative to BiVO4-B film (0.52 V vs RHE), implying that the multihighindex crystal facet BiVO4 film is of lower overpotential for water oxidation as well as faster kinetics. This deduction was also supported by the comparison of PEC water oxidation activity on our multihigh-index crystal facet BiVO4 film to the reported BiVO4 film with the {121} monohigh-index facet (see Table S3). To further confirm the difference of PEC activity on two kinds of BiVO4 films, the charge-separation and chargeinjection efficiencies of BiVO4-A and BiVO4-B films were evaluated in 0.1 M Na2SO4-1 M Na2SO3 mixed solution. As an efficient hole scavenger, sulfite can be oxidized on the photoanode with favorable kinetics and thus the interfacial hole-transfer kinetics on photoanode is normally negligible.34 As shown in Figure 4b, the photocurrent and onset potential on BiVO4-A and BiVO4-B in 0.1 M Na2SO4-1 M Na2SO3 were significantly promoted relative to they measured in 0.1 M Na2SO4. Because of unsatisfactory charge-separation efficiency on the normal BiVO4 film (BiVO4-B film), more obvious promotion on photocurrent was observed for the BiVO4-B film than that on the BiVO4-A film. On the basis of the results in E

DOI: 10.1021/acs.inorgchem.8b02570 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry 5a), smaller semicircle arc was observed on the BiVO4-A film photoanode, demonstrating its faster PEC water oxidation

RHE, whereas a shorter charge-transfer time of 0.78 ms for the BiVO4-A film. Additionally, similar charge-transfer time difference was found at 0.8 and 1.0 V (vs RHE) between BiVO4-A and BiVO4-B films, indicating that the BiVO4 film with multihigh-index facets has higher electron−hole separation efficiency and faster charge-transfer kinetics during PEC water oxidation. This observation is further confirmed by our Mott−Schottky measurements for BiVO4-A and BiVO4-B films (Figure 5d). As shown in the Mott−Schottky plots, the flat band potential of the BiVO4-A film was measured around 0.05 V versus RHE, a negative shift of 150 mV relative to that of the BiVO4-B film (around 0.20 V vs RHE). The negative shift of flat band potential is associated with the accelerated interfacial charge transfer of photoanode,41,42 suggesting that the multihigh-index facet BiVO4 film photoanode is of faster charge transfer for water oxidation. Meanwhile, lower slope of the Mott−Schottky plot was observed on the BiVO4-A film photoanode. According to the Mott−Schottky equation (shown in eq 4), the carrier density of film is inversely proportional to the slope of its Mott−Schottky plot. It can be inferred that the BiVO4-A film has higher carrier density than the BiVO4-B film. A comparative analysis on the PL emission spectrum of BiVO4-A and BiVO4-B films demonstrates that the radiative recombination rate of carrier in the BiVO4-A film is indeed lower than that in the BiVO4-B film (Figure S14). However, slight difference of UV−vis absorption property was observed on the BiVO4-A and BiVO4-B films (Figure S5a,b). These above experimental results indicate that the BiVO4 film with multihigh-index facets is of favorable charge-transfer and separation as well as hole-to-oxygen property for PEC water oxidation. To get insight into the PEC water oxidation difference on BiVO4-A and BiVO4-B films, the adsorption energy of H2O molecules and the free energy of H2O oxidation on different index facets of BiVO4 were calculated in theory. As well known, water oxidation is typical liquid−solid interface reaction involving several steps, whereas the adsorption of H2O molecules on the photoanode surface is the initial step. Accordingly, the adsorption energy of H2O molecules on typical low- and high-index-facets of BiVO4 was first calculated to investigate their interactions. In our calculations, the H2O molecule adsorption energy on the surface of the BiVO4 film is defined as Eads = Etotal −Esurf − EH2O, where Etotal represents the total energy of the H2O-adsorbed surface after geometry relaxation, Esurf is the energy of surface slab without H2O molecule adsorption, and EH2O is the energy of a free H2O molecule. A more negative Eads value represents more exothermic and stronger adsorption. In addition, stronger adsorption suggests that the adsorption of H2O molecule state is more metastable to the dissociated waterstate.23 The adsorption energy of H2O molecules on (010) and (110) surfaces (typical low-index facets of BiVO4) is calculated to be −0.64 and −0.60 eV, respectively. As a contrast, lower adsorption energy of H2O molecules was found on the highindex facets of BiVO4. For example, an adsorption energy of −0.83 eV was found on the (121) surface and −0.82 eV on the (132) surface (Table 2). Therefore, the calculation indicates that the adsorption of H2O molecules on high-index surfaces of BiVO4 is energetically favorable for subsequent dissociation and oxidation relative to that on low-index surfaces of BiVO4.

Figure 5. (a) Nyquist impedance plots of BiVO4-A and BiVO4-B films in 0.1 M Na2SO4 solution at 1.23 V vs RHE under AM 1.5G irradiation. (b) Chopped j−t curve of BiVO4-A and BiVO4-B films in 0.1 M Na2SO4 solution measured on the RRDE system with a rotation rate of 2000 rpm. The disk and ring currents were measured at 1.23 and 1.68 V vs RHE, respectively. (c) IMPS plots of BiVO4-A and BiVO4-B films in 0.1 M Na2SO4 solution at 1.23 V vs RHE under an incident light wavelength of 365 nm (100 mW/cm2). (d) Mott− Schottky plots for BiVO4-A and BiVO4-B films in 0.1 M Na2SO4 under AM 1.5G irradiation.

kinetics. The Nyquist plots were further fitted with an equivalent circuit model (inset of Figure 5a), where Rct represents the hole-to-oxygen transfer resistance during water oxidation reaction.37,38 For the BiVO4-A film photoanode, the value of Rct is determined to be 1559 Ω (shown in Table 1), Table 1. Value of Elements in Equivalent Circuit Fitted in the Nyquist Plots of Figure 5a sample

Rs/Ω (error/%)

Rct/Ω (error/%)

CPE/F (error/%)

BiVO4-A BiVO4-B

84.59 (1.14) 91.4 (1.31)

1558 (1.51) 3952 (1.39)

2.71 × 10−4 (4.90) 1.09 × 10−5 (4.19)

which is much smaller than the Rct of BiVO4-B film photoanode (3952 Ω). However, further RRDE measurements reveal that the water oxidation pathway on the BiVO4-A film photoanode in kinetics is similar to that on the BiVO4-B film photoanode. As shown in Figures 5b and S11, the BiVO4-A film electrode displays higher disk current than BiVO4-B film electrode, but the ring current on both film electrodes is pretty close. The RRDE results indicate that the water molecules were mainly oxidized on both BiVO4-A and BiVO4-B film photoanodes through a four-electron pathway, and the formed H2O2 intermediate on two kinds of BiVO4 photoanodes during water oxidation via a two-electron pathway is negligible.39 On the other side, the charge-transfer time of inorganic photoanode is closely associated with its water oxidation kinetics.40 Accordingly, IMPS measurements were employed to detect the charge-transfer constant for BiVO4-A and BiVO4-B films. On the basis of the measured IMPS plots on two kinds of BiVO4 photoanodes (Figures 5c and S12), their charge-transfer time was calculated (Figure S13). For the BiVO4-B film, a charge-transfer time of 0.99 ms was achieved at 1.23 V versus F

DOI: 10.1021/acs.inorgchem.8b02570 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

onset potential of water oxidation on the BiVO4 photoanode with multihigh-index facets has a significant shift to lower potential relative to the normal BiVO4 photoanode. Combined with the experimental and calculated results, it can be concluded that the enhanced water oxidation activity and kinetics on BiVO4-A are caused by its exposed high-index facets.

Table 2. Calculation Adsorption Energies on Different Index Facets of BiVO4 (hkl) facets Adsorption energy (eV)

(010) −0.64

(101) −0.60

(121) −0.83

(132) −0.82

With respect to the water oxidation for oxygen evolution, which involves a four-electron pathway on the photoanode as follows.43 2H 2O(l) + • → •OH + H 2O(l) + H+ + e−

4. CONCLUSIONS In summary, the BiVO4 polyhedral film with exposed {121}, {132}, {211}, and {251} high-index facets was synthesized by a facile Bi2O3 template-induced method and investigated as a photoanode for water oxidation. Compared to the normal BiVO4 film with a {121} monohigh-index facet, the BiVO4 film with multihigh-index facets is of higher activity and faster kinetics for water oxidation. A higher photocurrent density of 1.21 mA/cm2 was obtained the as-prepared BiVO4 photoanode at 1.23 V versus RHE in 0.1 M Na2SO4, which is about 200% improvement over the normal BiVO4 photoanode (0.61 mA/ cm2 at 1.23 V vs RHE). Additionally, the BiVO4 film photoanode with multihigh-index facets shows a negative shift of 300 mV onset potential for water oxidation relative to the monohigh-index facet BiVO4 film photoanode (0.52 V vs RHE) in 0.1 M Na2SO4. Although the UV−vis absorbance property and water oxidation pathway did not change, the charge-transfer and separation properties as well as the overall water oxidation kinetics on the multihigh-index facet BiVO4 film are boosted obviously. Theory calculations reveal that the adsorption of H2O molecules on BiVO4{121} and {132} highindex facets is energetically favorable for subsequent dissociation and oxidation relative to that on {010} and {110} low-index facets. Furthermore, the water oxidation limiting step on BiVO4 {121} and {132} high-index facets is the reaction of two protons with •O to form •OOH species (•O + H2O(l) + 2H+ + 2e− → •OOH + 3H+ + 3e−), which is different from the limiting step on {010} and {110} low-index facets that corresponds to the dissociation of H2O to •OH (2H2O(l) + • → •OH + H2O(l) + H+ + e−). Meanwhile, the overpotential of water oxidation limiting step on {121} and {132} high-index facets of BiVO4 is lower than that on {010} and {110} low-index ones.

(6)

•OH+H 2O(l) + H+ + e− → •O + H 2O(l) + 2H+ + 2e− (7) +

−

+

•O + H 2O(l) + 2H + 2e → •OOH + 3H + 3e

−

(8) +

−

+

−

•OOH + 3H + 3e → O2(g) + 4H + 64e + •

(9)

For these reactions, the fundamental driving force is provided by holes at the valence band edge of BiVO4 under irradiation. On the basis of a calculation scheme developed by Nørskov and co-workers,44,45 the thermodynamics of water oxidation on typical low- and high-index facets of BiVO4 was studied. Figure 6 shows the calculated free-energy diagram for

Figure 6. Free-energy diagrams for the four steps of the PEC water oxidation on the (a) (010), (121) and (b) (110), (132) facets of BiVO4 at U = 0, pH = 6.8, and T = 298 K. The ΔG (vertical sold line with arrows) value of the rate-determining step is shown for each surface.

water oxidation steps on BiVO4 (121), (132), (010), and (101) surfaces at U = 0 V, pH = 6.8, and T = 298 K. The initial step of water oxidation that corresponds to the dissociation of H2O to •OH (eq 6) is generally considered as the rate-limiting step for water oxidation owing to its thermodynamic difficulty. Therefore, a computed overpotential was applied on the BiVO4 surface to trigger the reaction. As shown in Figure 6, the calculated overpotential for the (010) surface is 1.42 V (=2.65−1.23 V) and 1.14 V (=2.37−1.23 V) for the (110) surface. Compared with the low-index surfaces, lower overpotential was calculated on the high-index surfaces for the H2O dissociation step. The H2O dissociation overpotential for the (121) surface is 0.47 V (=1.70−1.23 V) and 0.18 V (=1.41− 1.23 V) for the (132) surface. Clearly, the H2O dissociation on (121) and (132) high-index surfaces is more energetically favorable than that on (010) and (110) low-index surfaces. Additionally, the computed results indicate that the ratelimiting step of water oxidation on (121) and (132) high-index surfaces shifts to the third step, in which two protons react with •O to form •OOH species (eq 8). The overpotential for such step on the (121) surface is 0.58 V (=4.90 − 3.09 − 1.23 V) and on the (132) surface is 0.65 V. The calculated results are consistent with our experimental observations, in which the

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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/acs.inorgchem.8b02570. Low-magnification SEM images; size distribution of corresponding Bi2O3 film; CV curves; ECSA normalized photocurrent density; time course; UV-Vis absorbance spectra and absorbed photon flux; XRD pattern; IPCE spectrum of BiVO4-A film and BiVO4-B film; LSV curves of bare glassy carbon electrode; IMPS plots of BiVO4-A and BiVO4-B films; charge-transfer time on the surface of BiVO4-A and BiVO4-B film photoanodes; PL spectra; typical incident light power density spectrum; comparison of standard and observed values; atomic ratio of V:Bi; comparison of PEC water oxidation activity on our BiVO4 film to the reported BiVO4 films; and integral photocurrent values (PDF) G

DOI: 10.1021/acs.inorgchem.8b02570 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +86 0816 2419201. Fax: +86 0816 2419201 (H.H.). *E-mail: [email protected] (Y.Z.). ORCID

Huichao He: 0000-0003-1193-1129 Liang Bian: 0000-0002-2769-7018 Yong Zhou: 0000-0002-9480-2586 Funding

National Natural Science Foundation of China (41702037, 21773114, 41872039, 41831285, and 21473091), Sichuan Science and Technology Program (2017JY0146 and 2018JY0462). Notes

The authors declare no competing financial interest.

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DOI: 10.1021/acs.inorgchem.8b02570 Inorg. Chem. XXXX, XXX, XXX−XXX