Spray-processed nanoporous BiVO4 photoanodes with high charge separation efficiency for oxygen evolution

Spray pyrolysis is a convenient method for fabricating BiVO4 photoanondes from a precursor solution. As the precursor solution of spray pyrolysis can significantly influence the nanostructure and the amount of oxygen vacancies of the final films, modifying the precursor solution is an efficient strategy for improving the photoelectrochemical (PEC) performance of BiVO4 photoanodes. Herein, an ultraviolet and ultrasonic treatment for modifying a vanadium precursor solution of spray pyrolysis was developed to produce efficient nanoporous BiVO4 photoanodes. By the simple treatment, the AM 1.5 photocurrent density of the photoanode reached 1.76 mA/cm2 at 1.23 V vs the reversible hydrogen electrode (RHE) for water oxidation, which is 12.3 times higher than the untreated BiVO4 photoanode. The good PEC performance was mainly attributed to the excellent charge separation that reached approximately 94.2% at 1.23 V vs RHE. Systematic studies revealed that the treatment for the precursor solution could tune the nanoporous structure and increase the amount of oxygen vacancies in the final films. This finding offers a facile and effective approach for fabricating efficient photoelectrodes for PEC water splitting.Spray pyrolysis is a convenient method for fabricating BiVO4 photoanondes from a precursor solution. As the precursor solution of spray pyrolysis can significantly influence the nanostructure and the amount of oxygen vacancies of the final films, modifying the precursor solution is an efficient strategy for improving the photoelectrochemical (PEC) performance of BiVO4 photoanodes. Herein, an ultraviolet and ultrasonic treatment for modifying a vanadium precursor solution of spray pyrolysis was developed to produce efficient nanoporous BiVO4 photoanodes. By the simple treatment, the AM 1.5 photocurrent density of the photoanode reached 1.76 mA/cm2 at 1.23 V vs the reversible hydrogen electrode (RHE) for water oxidation, which is 12.3 times higher than the untreated BiVO4 photoanode. The good PEC performance was mainly attributed to the excellent charge separation that reached approximately 94.2% at 1.23 V vs RHE. Systematic studies revealed that the treatment for the precursor solution could tune the nanop...


I. INTRODUCTION
Photoelectrochemical (PEC) water splitting is a promising method to convert solar energy to chemical fuels. [1][2][3][4][5][6][7][8][9][10] Monoclinic BiVO 4 was shown as a fascinating photoanode material for water splitting, owing to the suitable bandgap energy (∼2.4 eV) and appropriate band edge positions. [8][9][10] The theoretical solar-to-hydrogen (STH) efficiency of BiVO 4 is about 9.2% (maximum photocurrent density is about 7.5 mA/cm 2 ). 11,12 However, most reported STH efficiencies of BiVO 4 photoanodes are far below the expected value because the material suffers from severe charge recombination. [12][13][14][15] The total photocurrent densities (J ph ) of the BiVO 4 photoanodes are mainly determined by light absorption efficiency (η abs ), charge separation efficiency (ηsep), and charge transfer efficiency (η transfer ), that is, J ph = Jmax × η abs × ηsep × η transfer . 16 Charge separation efficiency is limited by the charge recombination in the bulk. Charge transfer efficiency (η transfer ) is limited by the surface carrier recombination, which is determined by water oxidation kinetics. 14,17 Since the light absorption is determined with the bandgap of the semiconductor and sluggish water oxidation kinetics can be eliminated significantly by loading proper oxygen evolution cocatalysts (OECs), enhancing the charge separation efficiency (ηsep) by suppressing the bulk carrier recombination is an important aspect for improving the PEC performance of BiVO 4 photoanodes. 17,18 The nanoporous structure was considered as one of the promising strategies for producing high charge separation BiVO 4 photoanodes. [19][20][21][22] The introduction of the nanoporous structure to BiVO 4 photoanodes can reduce the bulk carrier recombination by increasing the volume of depletion and shortening the carrier transport distance. 19 Notably, Choi et al. reported that nanoporous BiVO 4 photoanodes produced by an electrochemical synthesis route manifested a high charge separation efficiency, which was over 90% at 1.23 V vs reversible hydrogen electrode (RHE). 20 However, the strategy of the electrochemical synthesis route requires complex fabrication techniques and is hard to produce on a large scale. Since the spray synthesis route is convenient and has potential to scale up, fabricating nanoporous BiVO 4 photoanodes by spray pyrolysis is particularly attractive and still needs to be explored. Moreover, increasing the density of oxygen vacancies in nanoporous BiVO 4 electrodes is an effective strategy to further enhance the charge separation efficiency. [22][23][24] Oxygen vacancies act as n-type donors without introducing foreign elements, which effectively increase the carrier density and enhance the charge transport property. [25][26][27] The density of oxygen vacancies can be increased by the reduction of BiVO 4 via hydrogenation, 22,28 chemical reduction, 29 and electrochemical treatment. 25,30 However, the methods of creating oxygen vacancies by the reduction of BiVO 4 would produce V 4+ , which is harmful to the PEC performance of BiVO 4 photoanodes, because V 4+ with larger radius acts as a scattering center and reduces the diffusion length of holes. 25,26 Thus, creating oxygen vacancies without V 4+ in nanoporous BiVO 4 photoanodes remains a challenge.
Herein, we developed a facile spray pyrolysis synthesis route to produce nanoporous BiVO 4 photoanodes with high charge efficiency by adding an ultraviolet and ultrasonic treatment to the vanadium precursor solution. As a result, the BiVO 4 photoanodes fabricated with our method exhibited 94.2% charge separation efficiency. The AM 1.5 photocurrent densities of the photoanode were 1.76 mA/cm 2 for water oxidation and 4.16 mA/cm 2 for sulfite oxidation at 1.23 V vs RHE. The precursor treatment well tuned the nanoporous structure of the final films and effectively increased the density of oxygen vacancies without producing V 4+ . The strategy of increasing oxygen vacancies in nanoporous BiVO 4 photoanodes can significantly enhance the charge separation by reducing bulk carrier recombination. The nanoporous BiVO 4 photoanodes with high charge separation efficiency can be achieved by the simple spray pyrolysis synthesis route.

A. Preparation
The nanoporous BiVO 4 thin films were prepared by spray pyrolysis. The bismuth solution was prepared by dissolving 0.004 mol Bi(NO 3 ) 3 5H 2 O (99.999%, Sigma-Aldrich) in 5 ml acetic acid (98%, Sigma-Aldrich). The vanadium solution was prepared by adding an equimolar amount of vanadium in the form of VO(acac) 2 (98%, Aladdin) dissolved in 100 ml dimethyl sulfoxide (DMSO) (99.9%, Alfa Aesar). Before mixing bismuth and vanadium solution, the vanadium solution was subjected to ultraviolet (365 nm, 2 mW/cm 2 ) and ultrasonic treatment (500 W) for 4 h by using an XH-300UL ultrasonic synthesis machine (Xianghu Science and Technology Development Limited Company, Beijing). The BiVO 4 films (named UV-US BiVO 4 ) were synthesized via the spray pyrolysis method on FTO substrates with the mixed precursor solution of bismuth and vanadium solution that was subjected to ultraviolet and ultrasonic treatment. The substrate temperature was maintained at 425 ○ C, and the nozzle-substrate distance was kept at 20 cm. Each spray cycle consisted of 10 s spray time and 3 s delay time. 2 ml precursor solution was used for preparing one film. The BiVO 4 photoanodes were obtained after calcination at 450 ○ C for 2 h. Meanwhile, the UV BiVO 4 photoanode (fabricated with the precursor solution only subjected to ultraviolet treatment), the US BiVO 4 photoanode (fabricated with the precursor solution only subjected to ultrasonic treatment), and the pristine BiVO 4 photoanode (fabricated with the precursor without any treatment) were also prepared via the same method.

B. Characterizations
The morphology of the BiVO 4 thin films was examined by using a field-emission scanning electron microscope (FE-SEM, FEI Quanta 250 FEG). X-ray photoelectron spectroscopy (XPS) was performed by using a VGESCA-LAB MKII instrument with an Mg Kα ADES (hν = 1253.6 eV) source. The UV-vis absorption was measured by using a Perkin Elmer UV WinLab spectrophotometer with an integrating sphere. X-ray diffraction (XRD) patterns were measured by using a Rigaku D/max-2500 diffractometer. Raman spectra measurement was conducted by using a Jobin Yvon HR 800 Raman microscope.

C. Photoelectrochemical measurements
The photoelectrochemical performance of the BiVO 4 photoanodes was characterized by using a PARSTAT 2273 electrochemical system with a typical three-electrode configuration, a Pt counter electrode, and a Ag/AgCl (3.5M KCl) reference electrode. 0.5M Na 2 SO 4 aqueous solution and 0.5M Na 2 SO 3 aqueous solution were used as the electrolyte. A solar simulator (Newport 370-RC) was used as the light source. An optical meter (PM 100D, Thorlabs) was used to adjust the light intensity. J-V curves were recorded with a scan rate of 10 mV/s. The Mott-Schottky analysis of BiVO 4 electrodes was measured at a frequency of 1 KHz in dark with a scan rate of 20 mV/s.

III. RESULTS AND DISCUSSION
The preparation process of the UV-US BiVO 4 electrodes reported in this study was illustrated in Fig. 1. UV-US BiVO 4 films were grown on fluorine-doped SnO 2 (FTO) substrates by the spray pyrolysis method, followed by calcination in air atmosphere at 450 ○ C for 2 h. Before spraying, the vanadium precursor solution was subjected to ultraviolet and ultrasonic treatment (named UV-US treatment) for 4 h. The UV-US BiVO 4 photoanode with the most suitable thickness of about 600 nm was selected to serve as the working electrode. For comparison, the pristine BiVO 4 electrode with the same thickness fabricated by the precursor without UV-US treatment was also investigated. The crystal structure can be confirmed by x-ray diffraction. Along with FTO signals, the peaks of UV-US BiVO 4 and pristine BiVO 4 films were indexed to be monoclinic BiVO 4 (Fig. S1). The Raman spectra of UV-US BiVO 4 and pristine BiVO 4 also exhibited the characteristics of monoclinic BiVO 4 (Fig. S2). Figure 2(a) shows the J-V curves of UV-US BiVO 4 and pristine BiVO 4 photoanodes measured in the electrolyte of 0.5M Na 2 SO 4 /0.5M Na 2 SO 3 aqueous solution under AM 1.5 G illumination. In all cases, the values of dark current densities could be neglected. The AM 1.5 photocurrent density of UV-US BiVO 4 measured in the electrolyte of Na 2 SO 4 aqueous solution was 1.76 mA/cm 2 at 1.23 V vs RHE, which was about 12.3 times higher than that of pristine BiVO 4 . The photocurrent densities of UV-US BiVO 4 and pristine BiVO 4 measured in the Na 2 SO 3 aqueous solution were used to calculate the charge separation efficiency (ηsep) and charge transfer efficiency (η transfer ) by using the following equation: J abs is the photon-absorption rate expressed as current density, which can be estimated by using the absorption and the value of 2.4 eV bandgap of BiVO 4 31,32 (Fig. S3). The J abs of UV-US BiVO 4 was calculated to be 4.42 mA/cm 2 , and the J abs of pristine BiVO 4 was 4.44 mA/cm 2 . As shown in Fig. 2(c), the η transfer of pristine BiVO 4 was only 13.2% at 1.23 V vs RHE, while the value of UV-US BiVO 4 was increased to be 41.9% at 1.23 V vs RHE. The UV-US treatment also boosted the charge separation efficiency (ηsep) of UV-US BiVO 4 to be 94.2% at 1.23 V vs RHE, which was about 3.9-fold that of the pristine BiVO 4 [ Fig. 2(b)].

ARTICLE scitation.org/journal/apm
of different morphologies, we inferred that a combustion reaction was induced by the UV-US treatment, which could result in the nanoporous structure of the BiVO 4 film. 33 To bring out a combustion reaction, an oxidizer and fuel are required. 33 In our method, Bi(NO 3 ) 3 and VO(acac) 2 were used as feedstock to prepare the precursor solution. NO 3 could act as an oxidizer, which was from Bi(NO 3 ) 3 . The fuel might be acetylacetone generated by the decomposition of VO(acac) 2 during the process of UV-US treatment. Previous studies have shown that metal acetylacetone complexes can be decomposed under the ultraviolet irradiation. 34,35 Hence, VO(acac) 2 could be decomposed and produce acetylacetone under the UV-US treatment. To confirm the inference, the BiVO 4 film was prepared by using the precursor solution with 0.5% acetylacetone added and the morphology of the BiVO 4 films was confirmed to be nanoporous (Fig. S4).
In order to investigate the surface chemical composition and the change in the oxidation states of BiVO 4 , x-ray photoelectron spectrometry (XPS) analysis was carried out.  Figure 4(b) shows the peaks of V 2p of the pristine BiVO 4 electrode and UV-US BiVO 4 electrode, which coincide with the reported values of monoclinic BiVO 4 . In previous reports, V 5+ species appeared at a binding energy of about 0.7-1.2 eV that was higher than for V 4+ species; 22,37 the signal of V 4+ cannot be found in the XPS spectra for both types of BiVO 4 , and all peaks were attributed to V 5+ species.
In comparison with pristine BiVO 4 , a small shift of 0.2 eV to low energy for the V 2p 3/2 peak is found in UV-US BiVO 4 . The shift of binding energy originated from the reduction of V 5+ to lower oxidation states with the corresponding oxygen vacancies, 22,38 indicating that the density of oxygen vacancies was increased in UV-US BiVO 4 electrodes.
By using the UV-US treatment for the vanadium solution, the density of oxygen vacancies was successfully increased. Oxygen vacancies acted as an effective n-type doping that enhanced the carrier density, which was proved by the Mott-Schottky analysis (Fig. 5). The electrochemical surface area of UV-US BiVO 4 and pristine BiVO 4 films was estimated by the electrochemical active surface area (ECSA) measurement. The results suggested that the surface area of UV-US BiVO 4 was about 2.4 times that of pristine BiVO 4 (Fig. S6). According to the Mott-Schottky equation, and the electrochemical surface area, the N d of UV-US BiVO 4 , could be estimated to be about 1.4 times larger than the pristine BiVO 4 , which was attributed to the increasing amount of oxygen vacancies. Moreover, the V FB of UV-US BiVO 4 exhibited 0.06 V negative than pristine BiVO 4 , reflecting that the Fermi level of UV-US BiVO 4 was closer to its conduction band (CB) edge.
Increasing the density of oxygen vacancies is an efficient strategy for enhancing the PEC performance of the nanoporous BiVO 4 electrode. The traditional methods such as hydrogenation, chemical reduction, and electrochemical treatment 22,25,26 were conducted to create oxygen vacancies via reducing BiVO 4 . However, these traditional methods would produce V 4+ , which is harmful to charge transport. V 4+ acted as scattering centers and reduced the effective diffusion length of holes because the radius of V 4+ is larger. 25,26 Recent investigations demonstrated that the density of oxygen vacancies in BiVO 4 films could be increased by optimizing the process of fabrication. 16,39 In our study, a simple UV-US treatment for the precursor solution was used for increasing the density of oxygen vacancies in the final film. By comparing the previous study and our UV-vis absorbance spectra of vanadium precursor solutions (Fig. S7), 40 we inferred that an oxidation reaction occurred and incomplete oxidation of V 4+ to V 5+ occurred during the process of the UV-US treatment. It is known that gaseous oxygen is consumed to convert V 4+ into V 5+ in the final BiVO 4 photoanode during the process of annealing. [41][42][43][44] Incomplete oxidation of V 4+ to V 5+ could lead to the formation of oxygen vacancies to meet the charge balance in the final BiVO 4 . 41 In addition, the combustion reaction induced by the UV-US treatment can cause local higher temperature, 33 which can also lead to the generation of oxygen vacancies in the nanoporous UV-US BiVO 4 . XPS demonstrated that the density of oxygen vacancies was increased and no V 4+ species existed on the surface of the final BiVO 4 film. Hence, by the simple solution route, an increase in the density of oxygen vacancies without excess V 4+ was achieved in the nanoporous BiVO 4 photoanode, which can significantly enhance the charge separation efficiency.
The UV BiVO 4 photoanode and the US BiVO 4 photoanode were prepared for comparative investigation. Both UV BiVO 4 and US BiVO 4 exhibited significantly enhanced photocurrent densities compared with pristine BiVO 4 (Fig. S8). The porosity of the final films can be enhanced by both the single ultraviolent treatment and the single ultrasonic treatment (Fig. S9). The US BiVO 4 film shows higher porosity than the UV BiVO 4 film [Figs. S9(c) and S9(d)], indicating that the US treatment played a more significant role in generating the structure of nanoporous photoanodes. According to the Mott-Schottky analysis, the V FB of UV BiVO 4 is more negative than the V FB of US BiVO 4 , indicating that the electronic density of UV BiVO 4 is higher, which is attributed to the larger amount of oxygen vacancies (Fig. S10). (Although the electronic density of UV BiVO 4 is higher, it is reasonable that the Mott-Schottky plot of UV BiVO 4 is steeper as the surface area of the US BiVO 4 film is larger.) Hence, the ultraviolet treatment played a more significant role in generating oxygen vacancies. Combining the ultraviolet treatment and ultrasonic treatment, the UV-US BiVO 4 photoanode with the nanoporous structure and rich oxygen vacancies exhibited a more significantly enhanced photocurrent density.

IV. CONCLUSION
In summary, we demonstrated a facile UV-US treatment for the vanadium precursor solution of spray pyrolysis to prepare highly efficient BiVO 4 photoanodes for photoelectrochemical water splitting. The UV-US treatment for the precursor solution controlled the nanostructure of the films and created oxygen vacancies without producing V 4+ . By combining the strategies of nanoporous morphology and increasing the amount of oxygen vacancies, the photoanode showed good PEC performance, which exhibited an AM 1.5 photocurrent density of 1.76 mA/cm 2 in Na 2 SO 4 and 4.16 mA/cm 2 in Na 2 SO 3 at 1.23 V vs RHE. The charge separation of the BiVO 4 photoanode was dramatically enhanced, which reached 94.2% at 1.23 V vs RHE. Modifying the precursor solution with the UV-US treatment is an efficient strategy for preparing highly efficient BiVO 4 photoanodes. Moreover, we believed that the spray synthesis route can be applied to the synthesis of other metal oxides and provide a new opportunity in the development of solar water splitting.

SUPPLEMENTARY MATERIAL
See the supplementary material for additional figures, analysis, and discussion.