High porosity Mo doped BiVO4 film by vanadium re-substitution for efficient photoelectrochemical water splitting

Journal Pre-proofs High porosity Mo doped BiVO4 film by vanadium re-substitution for efficient photoelectrochemical water splitting Xiang Yin, Weixin Qiu, Wenzhang Li, Chang Li, Keke Wang, Xuetao Yang, Libo Du, Yang Liu, Jie Li PII: DOI: Reference:

S1385-8947(20)30356-9 https://doi.org/10.1016/j.cej.2020.124365 CEJ 124365

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Chemical Engineering Journal

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13 December 2019 9 January 2020 5 February 2020

Please cite this article as: X. Yin, W. Qiu, W. Li, C. Li, K. Wang, X. Yang, L. Du, Y. Liu, J. Li, High porosity Mo doped BiVO4 film by vanadium re-substitution for efficient photoelectrochemical water splitting, Chemical Engineering Journal (2020), doi: https://doi.org/10.1016/j.cej.2020.124365

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High Porosity Mo doped BiVO4 Film by Vanadium Re-substitution for Efficient Photoelectrochemical Water Splitting

Xiang Yin a, Weixin Qiu a, Wenzhang Li a, b, Chang Li a, Keke Wang a, Xuetao Yang a, Libo Du a, Yang Liu a, *, Jie Li a, *

a

School of Chemistry and Chemical Engineering, Central South University, Changsha, 410083, China b

Hunan Provincial Key Laboratory of Efficient and Clean Utilization of Manganese Resources, Central South University, Changsha, 410083, China

Corresponding author: [email protected]; [email protected].

Abstract Bismuth

vanadate

(BiVO4)

is

one

of

the

most

studied

photoanode

whose

photoelectrochemical (PEC) performance is limited by the sluggish charge mobility and substantial recombination losses. Here, a high porosity Mo doped BiVO4 film was synthesized by excess Mo doping and following vanadium re-substitution. The as-prepared BiVO4 based photoanode has large contact area between the electrolyte and the film due to the dissolution of excess Mo, shown by the results of electrochemically active surface area tests. As a result, the photocurrent of VMo-BiVO4 is 6.12 mA/cm2 at 1.23 V vs. the reversible hydrogen electrode (RHE) in 0.2 M KPi + 0.2 M Na2SO3 with the charge separation efficiency of ~96%, and the photocurrent of VMo-BiVO4 (3.18 mA/cm2 at 1.23 V vs. RHE) is 6.91 times of pristine BiVO4 in 0.2 M KPi. This approach demonstrated that a reasonable doping structure design could guarantee PEC water splitting with high performance.

Keywords: Bismuth vanadate; Mo doping; Photoelectrochemical water splitting; Photoanode; Porosity structure.

1. Introduction Water splitting through photoelectrochemical (PEC) processes using semiconductor materials is a promising strategy to generate hydrogen as alternative energy. Because of the sluggish kinetics of the multi-step (4-hole) water oxidation reaction, a high efficient photoanode is indispensable for PEC water splitting. BiVO4 is one of the most widely investigated photoanode materials, owing to its visible light absorption and suitable valence band energy [1]. However, the slow charge mobility and unsatisfactory water oxidation ability limited the photocurrent to be lower than the theoretical value. To improve the PEC performance of BiVO4 photoanode, various strategies have been implemented over the last decades, such as designing nanostructure [2, 3], regulating crystal facet [4-8], constructing heterojunction [9-12], depositing electrocatalyst [13-16], and doping [17-19], etc. Among these strategies, the doping not only adjusts the electronic structure of BiVO4 but also introduces a local built-in electric field, which facilitates the charge mobility and separation of photo-generated electron-holes. Gerardo Colón et al. synthesized Er3+ doped BiVO4 with the exceptional photocatalytic ability for MB degradation and oxygen evolution. The Er3+ doped BiVO4 prefers to be tetragonal phase instead of monoclinic phase due to the formation of tetragonal-ErVO4 seeds previous to BiVO4 formation [20]. Zhou et al. used In3+ as dopant to substitute partial sites of Bi3+ in BiVO4, and the PEC performance of BiVO4 nanoflakes film is enhanced due to the modification of surface states [21]. Besides these, hexavalent metals (W, Mo, Cr) as dopants to substitute partial sites of V5+ were also reported [22-24]. The charge donor density (electrons) can be increased after doping, which can facilitate charge mobility due to the increased conductivity. Moreover, the element Mo was found to be the most promising transition metal because the Fermi level pinning can also be narrowed for BiVO4 [19].

In order to combine the advantages, some researchers doped BiVO4 with dual- or tri-metal elements. C. Buddie Mullins et al. incorporated both Mo and W into the BiVO4, and the doped BiVO4 with 6% Mo and 2% W showed the best performance in the experimental situation [25]. W. Schuhmann et al. deposited transition metals (Ta, W, Nb) with Mo to synthesize multi-metal doped Bi(V-Mo-Ta/W/Nb)O4 material, which presented good PEC properties (> 1 mA/cm2) when V:Bi atomic ratio is 70:30 with Nb concentration > 10 at.% [19]. Seldom of the above published papers about doped material consider the morphology and microstructure, which may be one of the reasons why the photocurrent is still well below the theoretical value. As we all know, the larger surface area connected with reactive medium is in favor of the higher photocatalytic and PEC performance. Xie et al. formed Mo doped BiVO4 inverse opals film, which exhibited twofold photocurrent (~1.4 mA/cm2 at 1.23 V vs. RHE) of the normal Mo doped BiVO4 film (~0.7 mA/cm2 at 1.23 V vs. RHE) [26]. Thus, constructing a hexavalent metal doped BiVO4 with high surface area for connecting with electrolyte is a possible strategy to gain high photocurrent density. In this paper, we synthesized Mo doped BiVO4 film at first, and high porosity Mo doped BiVO4 film was constructed after V re-substitution and alkali solution dissolution. The increased major carrier density due to the substitution of V by Mo, the larger interfacial area between the electrolyte and the film due to the dissolution of excess Mo, and the high PEC performance due to the rich V on the surface result in the high efficient photoanode. 2. Experimental section 2.1 Preparation of BiVO4 seed layer The BiVO4 seed layer was synthesized according to the previous literature [27]. In the typical process, 2.5 mL of nitric acid was diluted by 5 mL of deionized (DI) water, and 1.2127 g of Bi(NO3)3·5H2O (Sinopharm Chemical Reagent Co., Ltd, 98%) and 0.2925 g of NH4VO3

(Sinopharm Chemical Reagent Co., Ltd) were dissolved in it assisted by sonication. Then, 1.0507 g of citric acid, 0.2 g polyvinyl alcohol, 0.1 g PEG 1000 and 1.875 mL acetic acid were added into the solution under stirring. After stirring for 3 h, the solution was dropped and spin-coated on a clean FTO substrate at 1000 rpm for 10 s and 3000 rpm for 20 s, followed by heat treatment at 450 ℃ for 2 h in air. 2.2 Preparation of Mo doped BiVO4 The synthesis process was shown in Schematic 1. Firstly, active material was grown on the BiVO4 seed layer. The 0.0024 g Na2MoO4·2H2O was dissolved in 7.5 mL ethylene glycol (EG), and 0.1164 g Bi(NO3)3·5H2O was dissolved in 11.25 mL EG. After the above two solutions were mixed, 41.25 mL anhydrous ethanol was slowly added under stirring. The mixture was stirred for another 30 min and poured into a 100 mL Teflon-lined stainless steel autoclave, which contained the BiVO4 seed layer substrate against the wall with active material facing down. It was sealed and placed in an oven at 180 °C for 12 h. After cooling to room temperature naturally, the prepared film was taken out, washed with DI water and absolute alcohol, and dried at 60 °C for 0.5 h. Secondly, a 0.2 mL dimethyl sulfoxide solution contained 0.2 M vanadyl acetylacetonate was placed on the as-prepared film. The sample was annealed at 500 °C for 1 h under air at a heating rate of 2 °C/min. Finally, the annealed sample was immersed in 1 M NaOH for 30 min under stirring to remove the excess V2O5 and molybdenum oxide, and the high porosity Mo doped BiVO4 was synthesized (noted as VMo-BiVO4). As for comparison, some samples were also prepared noted as V-BiVO4 (using 0.0012 g NH4VO3 instead of 0.0024 g Na2MoO4 in the precursor), MoBiVO4 (without dropping vanadyl acetylacetonate and alkali solution dissolution) and BiVO4 (using 0.0012 g NH4VO3 instead of 0.0024 g Na2MoO4 in the precursor without the following dropping vanadyl acetylacetonate), respectively.

Schematic 1 Synthesis process of BiVO4, Mo-BiVO4, V-BiVO4 and VMo-BiVO4 films. 2.3. Characterizations

The crystal structures of the samples were examined by X-ray diffraction (XRD) using a Rigaku X-ray diffractometer (D/max2250) with Cu Kα (λ = 1.5406 Å) irradiation. The morphology and microstructure characterization were probed by a scanning electron microscope (SEM, Nova Nano SEM 230) with an energy-dispersive X-ray spectrometer (EDS). The optical properties were investigated by UV-vis spectrophotometer (TU-1901) equipped with a diffused integrated sphere. The chemical composition was analyzed by X-ray photoelectron spectroscopy (XPS, K-Alpha 1063) with monochromatic Al Kα irradiation. Raman spectra were carried out using a confocal Raman microscope (LabRam HR800) with a 532 nm wavelength laser source. 2.4. Photoelectrochemical (PEC) and electrochemical measurements Photoelectrochemical measurements were accomplished with an electrochemical workstation (Zennium or CHI 760E) by using a conventional three-electrode electrochemical cell. The asprepared film was employed as the photoanode with active area of 1 cm2, while an Ag/AgCl (saturated KCl) electrode was used as the reference electrode, and a Pt mesh (2 cm × 2 cm) was used as the counter electrode. Light illumination supplied by a 500 W xenon lamp fitted with an AM 1.5 G filter, and it was adjusted to an intensity of 100 mW/cm2. All the potentials measured in this work were converted to the reversible hydrogen electrode (RHE) using the Nernst equation: ERHE = EAg/AgCl+ 0.197 + 0.059 pH

(1)

where ERHE is the converted potential vs. RHE, EAg/AgCl is the experimentally measured potential vs. Ag/AgCl (saturated KCl). 3. Results and discussion 3.1. Characterization of photoanode The powder X-ray diffraction patterns of BiVO4 based films are given in Fig. 1. It is observed that each sample is in the monoclinic phase (JCPDS 75-1867) with featured diffraction peaks noted

in club. There are also some peaks around 26.40°, 33.60° and 51.38° (noted in round) can be assigned to the FTO substrate. In the partially enlarged view (Fig. S1), there is a peak at around 32° for Mo-BiVO4, which can be ascribed as (002) reflection of monoclinic Bi2MoO6 (JCPDS 822067). The lattice parameters of BiVO4, Mo-BiVO4, V-BiVO4 and VMo-BiVO4 are estimated as shown in Table S1. Interestingly, the VMo-BiVO4 film has more obvious changes in lattice parameters compared with the BiVO4 film and V-BiVO4 film, which may results from the substitution amount of V5+ (0.036 nm) by Mo6+ (0.041 nm) with larger ion radius [28, 29].

Figure 1. XRD patterns of BiVO4, Mo-BiVO4, V-BiVO4 and VMo-BiVO4 photoanodes. Fig. 2 shows the morphology of samples. The BiVO4 film consists of small particles, but most of them are interconnected with each other. The V-BiVO4 film shows wormlike morphology, leading to higher porosity for touching electrolyte than BiVO4 film. Moreover, the Mo-BiVO4 film presents thin flake morphology, and it also changes to wormlike morphology with high porosity after V re-substitution and alkali solution dissolution for VMo-BiVO4 film. To further confirm the

existence of element Mo in the VMo-BiVO4 film, EDX mapping on the top of VMo-BiVO4 film was detected as shown in Figs. S2a-d, which demonstrates that the Bi, V, O, Mo elements are uniformly distributed inside.

Figure 2. SEM images of (a) BiVO4, (b) V-BiVO4, (c) Mo-BiVO4 and (d) VMo-BiVO4 films. To investigate the crystallization, local structure and electronic properties of materials, Raman spectroscopy was recorded as shown in Fig. 3. The Raman spectrum of the BiVO4 film shows five peaks centered at 827.42, 713.51, 368.52, 326.99 and 212.54 cm−1, which are the typical vibrational bands of BiVO4 [30]. The peaks at 827.42 and 713.51 cm−1 are related to the vibrational mode of the V–O bond [31]. The peaks centered at 368.52 and 326.99 cm−1 represent the asymmetric and symmetric formations of the VO4 tetrahedron, and the band centered at 212.54 cm−1 indicates the structural

information of BiVO4 [32]. VMo-BiVO4 film exhibits the similar peaks centered at 828.93, 711.98, 368.52, 326.99 and 212.54 cm−1. The shifts of peaks from 827.42 and 713.51 cm−1 to 828.93 and 711.98 cm−1 mean the changes of V–O bond, which is due to the substitution of V by Mo [33].

Figure 3. Raman spectra of BiVO4 and V-Mo-BiVO4 films. XPS characterization was revealed to determine the surface compositions and chemical states of samples. In the survey spectra (Fig. 4), each sample exhibits typical Bi 4f, V 2p, and O 1s peaks. For the high resolution of Bi 4f (Fig. 5a) and V 2p (Fig. 5b) spectra, both Bi3+ (164.23 eV and 158.88 eV) and V5+ (524.16 eV and 516.38 eV) exist on the surface of each sample. Compared to BiVO4, Mo-BiVO4 presents two newly-emerged binding peaks at around 234.96 eV (Mo 3d5/2) and 231.85 eV (Mo 3d3/2) are related to Mo6+ species, indicating the existence of Mo [23]. After V re-substitution and alkali solution dissolution, the signal intensity of Mo decrease while the intensity of V increase (Fig. S3). Fig. S4 shows the UV-Vis spectra, which provide details regarding the absorbance of the BiVO4 thin films. The spectra of V-BiVO4 film and VMo-BiVO4 show band edge at around 505 nm, which is consistent with the reported values of monoclinic BiVO4 film electrodes [34, 35].

Figure 4. XPS survey plots of BiVO4, V-BiVO4, Mo-BiVO4 and V-Mo-BiVO4 films.

Figure 5. XPS plots of BiVO4, V-BiVO4, Mo-BiVO4 and V-Mo-BiVO4 films: (a) Bi 4f, (b) V 2p and (c) Mo 3d. 3.2. Photoelectrochemical (PEC) properties To evaluate the PEC performance, linear sweep voltammetry (LSV) curves were carried out in 0.2 M potassium phosphate buffer (KPi) at a scan rate of 20 mV/s (Fig. 6a). The photocurrent of BiVO4 increases with the potential when the applied potential is higher than 0.8 V vs. RHE, and

it is ~0.46 mA/cm2 at 1.23 V vs. RHE. The onset potential extracted by extrapolating the linear part of the Butler plot (J2−V) is about 0.88 V vs. RHE (Fig. 6b). The Mo-BiVO4 presents photocurrent density of 0.18 mA/cm2 at 1.23 V vs. RHE with the onset potential of 1.00 vs. RHE, and they are 0.68 mA/cm2 at 1.23 V vs. RHE and 0.80 V vs. RHE for V-BiVO4. For VMo-BiVO4 film, the current density is improved over the whole operating potential range (0.6-1.4 V vs. RHE) under irradiation. The onset potential is about 0.56 V vs. RHE, which presents negative shift compared to other samples. At 1.23 V vs. RHE, the photocurrent of VMo-BiVO4 film is 3.18 mA/cm2, which is about 6.9 times of the value for BiVO4 film, and it is also higher compared to some reported literature values on element doped BiVO4 (Table S2). From the Tafel slops (Fig. S5) calculated from LSV curves, the VMo-BiVO4 film shows a smaller value compared to other samples, indicating an improved fill factor [36]. When measuring the LSV in 0.2 M KPi+0.2 M Na2SO3, the photocurrent of V-BiVO4 is 3.58 mA/cm2 at 1.23 V vs. RHE (Fig. 6c). Moreover, the onset potential is about 0.32 V vs. RHE (Fig. 6d). For VMo-BiVO4 film, the photocurrent and onset potential are 6.12 mA/cm2 at 1.23 V vs. RHE and 0.30 V vs. RHE, respectively. Thus, the holes injection efficiency (ƞinj), charge separation efficiency (ƞsep) and applied bias photon-tocurrent efficiency (ABPE) are calculated according to the following formulas: ƞinj = 100% × (JKPi) / (JKPi+Na2SO3)

(2)

ƞsep =100% × (JKPi+Na2SO3) / (Jabs)

(3)

ABPE = 100% ×JKPi × (1.23−Vapp)/ Pin

(4)

where JKPi+Na2SO3 and JKPi are the measured photocurrents in KPi with and without Na2SO3 as a hole scavenger, respectively; The Vapp and Pin are applied potential on the photoanode and power density of incident light (100 mW/cm2). Meanwhile, Jabs is the calculated photocurrent (6.33 mA/cm2 for VMo-BiVO4 and 6.46 mA/cm2 for V-BiVO4) assuming 100% absorbed photon–to–

current conversion efficiency for photons (details in Supporting information). As shown in Fig. 7 and S6, the VMo-BiVO4 film exhibits higher separation efficiency (96%), holes injection efficiency (52%) and applied bias photon-to-current conversion efficiency (ABPE, 0.44%) than the V-BiVO4 film (separation efficiency = 55%, injection efficiency = 19% and ABPE = 0.046%). It means that the VMo-BiVO4 photoanode has better charge mobility in bulk.

Figure 6. (a) J-V curves and (b) Butler plots for BiVO4, Mo-BiVO4, V-BiVO4 and VMo-BiVO4 photoanodes measured in 0.2 M KPi (pH ≈ 7); (c) J-V curves and (d) Butler plots for V-BiVO4 and VMo-BiVO4 photoanodes measured in 0.2 M KPi+0.2 M Na2SO3 (pH≈7).

Figure 7. (a) Charge separation efficiency and (b) charge injection efficiency for V-BiVO4 and VMo-BiVO4 photoanodes. Fig. 8 shows the electrochemically active surface area (ECSA) plots of BiVO4, V-BiVO4, Mo-BiVO4 and VMo-BiVO4 film, which is obtained by cyclic voltammetry (Fig. S7) with the potential region of 0.24-0.34 V vs. RHE at different scan rates in the dark. According to the data recorded in Figs. S7a-d, the charging current difference (ΔJ) between the anodic and cathodic charging current at the potential of 0.29 V vs. RHE was plotted against the scan rate (Fig. 8), and the linear slope is twice of Cdl. The estimated Cdl values are 0.070, 0.096, 0.079 and 0.154 mF/cm2 for BiVO4, V-BiVO4, Mo-BiVO4 and VMo-BiVO4. Meanwhile, the ECSA can be calculated from Cdl according to the following equation [37]: ECSA = Cdl /Cs

(5)

where Cs is the capacitance of an atomically smooth planar surface of the material per unit area under identical electrolyte conditions [38]. Therefore, the ECSA value is in proportion to the slope and Cdl of the sample, and the ECSA value of VMo-BiVO4 film is about 1.61 folds of that of VBiVO4 film. It means that higher porosity can be obtained for VMo-BiVO4 film compared to MoBiVO4 film after V re-substitution and alkali solution dissolution. Additionally, the photocurrent

measured in KPi (Fig. 6a) is normalized according to the ECSA value ration among different samples [39]. As shown in Fig. S8, the photocurrent density of V-BiVO4 film is similar to BiVO4 film, and the photocurrent density of VMo-BiVO4 film is still much larger than other samples. It means that both high valence metal doping (substitution of V5+ by Mo6+) and well designed structure (high porosity) are necessary to achieve great PEC performance.

Figure 8. The charging current difference (ΔJ) between the anodic and cathodic charging current at the potential of 0.29 V vs. RHE for the BiVO4, V-BiVO4, Mo-BiVO4 and VMo-BiVO4. Fig. 9 shows the results of Mott-Schottky measurements for V-BiVO4 and VMo-BiVO4 at frequency of 1 kHz. The charge carrier density and flat band potential can be calculated using the following equation [40]: 1/C2 = (2/qε0εNdA2)(E-Efb-kT/q)

(6)

In this equation, q, ε0, ε, Nd and A are the electron charge, the permittivity of the vacuum, the dielectric constant of the semiconductor, the carrier density and electrode surface area, respectively. The flat band potential can be extracted by extrapolating the linear part, and the measured values for V-BiVO4 film and VMo-BiVO4 film are 0.12 and 0.10 V vs. RHE, respectively, which are in agreement with the trend of the onset potential in Figs. 6b and 6d. Meanwhile, the charge carrier

density is in an inverse ratio to the slope of the linear part. It can be seen that the slope of VMoBiVO4 film is about 1/4 of that of V-BiVO4 film under same experimental conditions, indicating an increase of charge concentration after Mo doping (substitution of V5+ by Mo6+). It also results in a shorter depletion width that can enhance the band bending for hole collection, which can be supported by the high separation efficiency in Fig. 7a.

Figure 9. Mott-Schottky plots for the V-BiVO4 and VMo-BiVO4. The electrochemical impedance spectroscopy (EIS) was also used to study the charge transfer ability of BiVO4, V-BiVO4, Mo-BiVO4 and VMo-BiVO4. It was performed with a sinusoidal perturbation amplitude of 10 mV at frequency from 10 kHz to 0.1 Hz at 1.23 V vs. RHE in 0.2 M KPi electrolyte under illumination (Fig. 10a and S9). Each sample presents a semicircle in the Nyquist plot (Figure 10a), and it can be fitted by the equivalent circuit (Fig. S9b), which consists of three elements: a series resistance (Rs), a charge transfer resistance (Rct), and a constant phase elements (CPE) [41]. The parameters were estimated by fitting experimental data with the equivalent circuit using Z-view software, and the results are listed in Table S3. The lower values of Rct and phase value in the bode plot (Fig. S9c) demonstrate that VMo-BiVO4 film has the better transfer and separation efficiency of the photo-generated electrons and holes compared to other films [42]. Intensity modulated photocurrent spectroscopy (IMPS) was also examined to

investigate the charge transfer ability of photoanode. In Fig. 10b, there are two semicircles in first and fourth quadrants corresponding to low-frequency (LFI) and high-frequency intersects (HFI) with the real axis (HFI and LFI), respectively. The HFI corresponds to jphotoCH/(CH+Csc), and the LFI represents jphotoktr/(ktr+krec) [43]. Among of them, the Helmholtz capacitance (assumed to be 20 μF/cm2) can be neglected for measured material with moderate charge carrier density [44]. Thus, the charge transfer efficiency (ktr/(ktr+krec)) can be calculated as LFI/HFI, which are ~0.42 and ~0.88 for V-BiVO4 and VMo-BiVO4, respectively.

Figure 10. (a) Nyquist plots for BiVO4, V-BiVO4, Mo-BiVO4 and VMo-BiVO4 photoanodes; (b) intensity modulated photocurrent spectroscopy (IMPS) plots for V-BiVO4 and VMo-BiVO4 photoanodes. The incident photon to current conversion efficiency (IPCE) was used to investigate the photocurrent responses as a function of the incident light wavelength (Fig. 11). The IPCE of VBiVO4 and VMo-BiVO4 films drop to zero at wavelengths longer than 505 nm, which is consistent with the results of UV-Vis spectra (Fig. S4a). At the wavelength of 420 nm, the IPCE values are 22.23% and 46.99% for V-BiVO4 and VMo-BiVO4 photoanodes, respectively. As we know, IPCE value is closely relevant to light harvesting capability, charge separation and injection efficiency.

According to the results of light absorption ability (Fig. S4), charge separation efficiency (Fig. 7a), hole injection efficiency (Fig. 7b) and charge transfer efficiency (Fig. 10), the enhancement of IPCE and absorbed photon-to-current efficiency (APCE, Fig. S10) should be ascribed to the doping with Mo as well as increasing the contact area between photoanode and electrolyte. This provides a great strategy to further improve the photoelectrochemical performance of BiVO4 based photoanode.

Figure 11. IPCE plots for V-BiVO4 and VMo-BiVO4 photoanodes. 4 Conclusion We reported a Mo doped BiVO4 photoanode with high porosity by V re-substitution and alkali solution dissolution. The facile method can produce a larger contact area between photoanode and electrolyte, which is in favor of the charge transfer at the interface of electrode/electrolyte. At the same time, the element Mo replaces V in the lattice of BiVO4, leading to a large cathodic onset potential shift for the BiVO4 based photoanode. Thus, a high photocurrent of 6.12 and 3.18 mA/cm2 at 1.23 V vs. RHE can be obtained in the oxidation of SO32- and water, respectively. All these results open new avenues of research for fabricating Mo-doped semiconductors with high porosity as well as excellent charge transfer properties.

Acknowledgements This study was supported by the National Nature Science Foundation of China (51904356), and the Hunan Provincial Science and Technology Plan Project of China (No. 2016TP1007).

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2

Current density (mA/cm )

10.1039/c7sc00363c

10 9 8 7 6 5 4 3 2 1 0

High Porosity Mo doped BiVO4

0.2

0.4

0.6 0.8 1.0 Potential (V vs. RHE)

1.2

A high porosity Mo doped BiVO4 film was synthesized by excess Mo doping and following vanadium re-substitution, which exhibited great photoelectrochemical performance.

Highlights High porosity structure was constructed via dissolution of excess Mo in BiVO4 film.

The contact area of photoanode and electrolyte is increased. The VMo-BiVO4 photoanode exhibits great photoelectrochemical performance. The separation efficiency for VMo-BiVO4 reaches to 92%. Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: