Scalable synthesis of BiVO4 thin films via anodic plating and thermal calcination

Fabrication of high-quality semiconductor thin films has long been a subject of keen interest in the photocatalytic field. Here, we report a facile, solution-based anodic plating and calcination for large-scale synthesis of BiVO4 thin films on indium tin oxide coated glass for use as photoanodes in solar water splitting. Using Na2SO3 as a sacrificial reagent, continuous solar H2 production with 94% Faradaic efficiency was obtained over 6 h of photoelectrochemical water splitting. Supplementary Information The online version contains supplementary material available at 10.1186/s11671-023-03774-z.


Introduction
Solar-driven photoelectrochemical (PEC) water splitting is a promising route for the large-scale production of renewable hydrogen fuel from water [1][2][3][4][5]. In the past decades, much effort has been made to improve the overall energy efficiency of PEC devices [6][7][8][9]. In terms of the photocathodes, high photocurrent densities with low overpotentials have been realized using p-type solar-cell materials in combination with hydrogen-evolution co-catalysts. However, the improvement in photoanodes remains limited [10].
Among a range of photoanodic materials, BiVO 4 has attracted research attention because it has a deep valance band position for the oxygen evolution reaction [11]. Also, BiVO 4 is relatively stable in neutral aqueous environments (pH Supplementary Information The online version contains supplementary material available at https:// doi. org/ 10. 1186/ s11671-023-03774-z. [7][8][9] [12,13]. Over 100-h PEC water oxidation has been reported using crystalline BiVO 4 photoanodes [14]. One of the remaining challenges for BiVO 4 is to increase the photocurrent density under photocatalytic conditions without applying any external electrical potential. To this end, Choi et al. reported the synthesis of nanoporous BiVO 4 photoanodes in a two-step process using BiOI nanoplates as the precursor [15]. Nanostructure certainly improves the charge separation; however, it also presents a difficulty for fabricating a p-n junction that is able to cover the entire BiVO 4 to make standalone, photocatalytic water-splitting catalysts. In this work, we report a facile synthesis of BiVO 4 thin films on transparent, conductive indium tin oxide (ITO) substrates using anodic plating and thermal calcination. A homogeneous mixture of the anodically deposited bismutite hydrate ((BiO) 4 (OH) 2 CO 3 ) and vanadium ions (Fig. S1 in the supporting information) allows nucleation of stichometrical BiVO 4 during calcination. Also, bismutite hydrate decomposes at temperatures > 500 °C and releases CO 2 ; the synthesized BiVO 4 is thus free of contamination. Using Na 2 SO 3 as a sacrificial reagent, stable photoelectrochemical H 2 generation was realized over 6 h of water splitting. The present study shows a promising solution-based process for the preparation of BiVO 4 thin films for use in water-splitting applications.

Results and discussion
Layer-structured bismutite and its hydrate were first reported in 1943 [16] and systematically studied in 1984 [17]. In mineralogy, bismutite is a well-established solid carbonate in the system Bi 2 O 3 -CO 2 -H 2 O [16] with a natural color of yellow to brown. In the laboratory, the synthesis of bismutite has only been reported using the hydrothermal method and with the products in the form of particles [17,18]. In the present study, we found that anodic plating can also synthesize amorphous Bi 4 O 4 (OH) 2 CO 3 thin films on ITO glass via Kolbe electrolysis with the presence of Bi ions, following the Eq. 1: Figure 1 shows the anodic plating of amorphous Bi 4 O 4 (OH) 2 CO 3 films on ITO substrates using NaCOOH and Bi(NO 3 ) 3 solutions at pH ~ 5. The applied potential is + 2.3 V vs Ag/AgCl (V Ag/AgCl ). After 7-min anodic plating, ~ 300-nm-thick film was plated. Likely, the NaCOOH was oxidized at anodic potentials (Eq. 1) to form CO 2 on the electrode surfaces. With the presence of Bi 3+ in the solution, Bi 4 O 4 (OH) 2 CO 3 precipitated on the electrodes. We also found that Bi metals precipitated on the cathode. This is because the reduction potential of Bi 3+ to Bi was + 0.2 V RHE , which is more positive than 0 V RHE of the hydrogen evolution reaction. To suppress the Bi precipitation, p-benzoquinone can be added to the electrolyte. The cathodic reaction then mainly shifts to the reduction of p-benzoquinone to 1,4-hydroquinone with a reduction potential of ~ + 0.6 V RHE (Fig. S2). The optical images of the anodically plated amorphous Bi 4 O 4 (OH) 2 CO 3 films are provided on the right panel in Fig. 1, which shows the change of color at the different time of the plating.
To understand anodic plating details, Bi 4 O 4 (OH) 2 CO 3 films were characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), UV-Vis diffuse-reflectance analyses and thermogravimetric analysis (TGA) (Figs. 2, 3 and Figs. S3, S4 in the supporting information). To evaluate constituent compositions, the plated Bi 4 O 4 (OH) 2 CO 3 films were scratched from ITO/glasses for TGA with a temperature rise from 30 to 500 °C in an N 2 atmosphere (N 2 was used to avoid adsorption of CO 2 from the air). As shown in Fig. 2a, three steps of endothermic decomposition were obtained in a TGA run for Bi 4 O 4 (OH) 2 CO 3 . A continuous weight loss of 5.6% below 180 °C was observed, likely attributed to absorbed solvent and water [17]. Bi 4 O 4 (OH) 2 CO 3 decomposition often occurs in two stages [17]: major loss of hydrate with a small loss of carbon dioxide at 180-240 °C and major loss of carbon dioxide at 240-500 °C. The calculation of weight losses in each step yielded an empirical formulation of (BiO) 4 (OH) 1.01 (CO 3 ) 0.94 of the anodically plated films, which agreed with the predicted products of Bi 4 O 4 (OH) 2 CO 3 . As a reference, we also analyzed Bi 2 O 2 CO 3 powder (Wako, 99.5%) by TGA under the same conditions. Decomposition to release carbon dioxide was observed at 240-500 °C with a weight loss of 9.6 wt%, close to the theoretic value of 8.6 wt%. A slight shift of decomposition onset temperature of Bi 2 O 2 CO 3 compared to that of plated Bi 4 O 4 (OH) 2 CO 3 films was likely due to the crystalline and amorphous nature of the two materials. This result suggested that amorphous Bi 4 O 4 (OH) 2 CO 3 films were anodically plated on ITO. Figure 3a shows the optical images of the synthesis process of BiVO 4 thin films on ITO substrates. In brief, we deposited amorphous Bi precursors on ITO substrates and calcinated them with the vanadium source at 520 °C. After the reaction, we washed the surface residual vanadium chemicals using 1 M NaOH solution. This process was similar to our previous report, in which the BiVO 4 was fabricated via three steps: precursor deposition, pre-calcination of the deposited films in the air at 200 °C, and calcination with a vanadium source at 490-530 °C. We used two-step fabrication, which was able to fabricate BiVO 4 thin films with similar morphology. The Bi precursor materials were calcined in the air at 520 °C (Fig. 3c) for 2 h. The fabricated BiVO 4 films were in a monoclinic structure, which agreed with the previous report [8]. The scanning electron microscopy (SEM) images of the plated Bi 4 O 4 (OH) 2 CO 3 film are present in Fig. S3 in the supporting information. The 7-min anodically plated amorphous Bi 4 O 4 (OH) 2 CO 3 film was ~ 300 nm thick on the ITO substrate (Fig. S3). Bi was detected on the surface and in the bulk of the film (Fig. S4). As shown in cross-sectional SEM images in Fig. 4, the BiVO 4 film was made of large BiVO 4 particles with an in-plane diameter of ~ 500-1000 nm and a thickness of ~ 300 nm. Such large BiVO 4 crystalline likely decreased the number of boundaries between particles. Therefore, improved photoelectrochemical performance was realized.
Finally, we tested the photoelectrochemical performance of the fabricated BiVO 4 films in 0.1 M Na 2 SO 3 solution. As shown in Fig. 5a, a quick raise of the photocurrent density was observed with BiVO 4 films with the increase of the positive potential. At 0.9-1.2 V RHE , the photocurrent density reached a plateau of ~ 5 mA/cm 2 under simulated solar light irradiation. We used a micro gas chromatography (micro-GC) to analyze the hydrogen evolution, which was stable over 6-h photoelectrochemical water splitting at 0.9 V RHE (Fig. 5b).

Conclusions
In this work, anodic plating was reported for the fabrication of Bi precursors on the indium tin oxide (ITO) substrates. Following high-temperature calcination with vanadium sources, crystalline BiVO 4 was fabricated on ITO substrates. The photocurrent density reached ~ 4-5 mA/cm 2 at 0.9-1.2 V RHE in Na 2 SO 3 -containing electrolytes under simulated solar illumination. The developed electrochemical deposition and thermal calcination may offer a new pathway for the synthesis of photocatalytic materials.

Fabrication of BiVO 4 thin film on ITO glass
First, ~ 300 nm amorphous Bi 4 O 4 (OH) 2 CO 3 film was anodically plated on an ITO glass in a bismuth nitrite and sodium acetate aqueous solution. Drop-casting vanadyl acetylacetonate (VO(acac) 2 ) organic solutions onto the plated Bi 4 O 4 (OH) 2 CO 3 films allowed homogeneous mix of the V and Bi species. The mixed samples were annealed at different temperatures. Finally, the obtained film samples were washed with NaOH to remove impurities. In detail, 0.075 mL 0.2 M VO(acac) 2 dimethyl sulfoxide solution was dropped on the Bi 4 O 4 (OH) 2 CO 3 films (2 cm × 2 cm) and then calcined in a furnace at 520 °C for 2 h in air. Remained VO x on top of the BiVO 4 films was washed in 1 M NaOH solution for 10 min with gentle stirring.

Measurements
The scanning electron microscopic (SEM) images were obtained by a Hitachi SU8020. The XRD diffraction spectra were performed using the smart lab XRD of Rigaku, Japan. The XPS analyses were performed using Mg Kα (1253.6 eV) photon energy. During XPS depth profile studies, Ar bombardment with an etching speed of several tens nm/time was used. Binding energy peak shifts due to any charging were normalized with C 1s peak set to 284.8 eV and Fermi energy position. The TGA analyses were conducted with a differential thermogravimetric analyzer (Rigaku, Japan). The PEC performances were measured by a three-electrode electrochemical configuration with a 0.1 M Na 2 SO 3 solution under simulated sunlight illumination (SAN-EI electronic, XES40S1, AM 1.5G, 100 mW cm −2 ). An Ag/AgCl electrode was used as a reference electrode, and a Pt coil was used as a counter electrode. The measured potentials were all converted to the reversible hydrogen electrode according to the Nernst equation: The PEC cell was connected to a vacuum pump and a micro-GC (Agilent 990 micro). Before measurement, the PEC cell was pumped to a low vacuum, and Ar gas was used to purge out the N 2 and O 2 gases in the cell. The H 2 evolution was measured in under simulated sunlight illumination for 6 h at 0.9 V RHE . The theoretical amounts of evolved H 2 were estimated from the passed charges on the assumption that faradaic efficiency was unity.
Acknowledgements The authors acknowledge Prof. Kazunari Domen and his group for assistance in the material fabrication and characterization.

Data availability
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Declarations
Ethics approval and consent to participate Not applicable.

Competing interests
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