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

Abstract

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 [1–5]. In the past decades, much effort has been made to improve the overall energy efficiency of PEC devices [6–9]. 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, BiVO₄ has attracted research attention because it has a deep valance band position for the oxygen evolution reaction [11]. Also, BiVO₄ is relatively stable in neutral aqueous environments (pH 7–9) [12, 13]. Over 100-h PEC water oxidation has been reported using crystalline BiVO₄ photoanodes [14]. One of the remaining challenges for BiVO₄ 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 BiVO₄ 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 BiVO₄ to make stand-alone, photocatalytic water-splitting catalysts.

In this work, we report a facile synthesis of BiVO₄ 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)₂CO₃) and vanadium ions (Fig. S1 in the supporting information) allows nucleation of stichometrical BiVO₄ during calcination. Also, bismutite hydrate decomposes at temperatures > 500 °C and releases CO₂; the synthesized BiVO₄ is thus free of contamination. Using Na₂SO₃ as a sacrificial reagent, stable photoelectrochemical H₂ generation was realized over 6 h of water splitting. The present study shows a promising solution-based process for the preparation of BiVO₄ 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 Bi₂O₃-CO₂-H₂O [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 Bi₄O₄(OH)₂CO₃ thin films on ITO glass via Kolbe electrolysis with the presence of Bi ions, following the Eq. 1:

$${\text{CH}}_3\text{COONa}+\text{Bi}{({\text{NO}}_3)}_3+{\text{H}}_2\text{O}\stackrel{+2.3V_{\text{Ag}/\text{AgCl}}}{⟶}{(\text{BiO})}_4{(\text{OH})}_2{\text{CO}}_3+{\text{NaNO}}_3$$ 1

Figure 1 shows the anodic plating of amorphous Bi₄O₄(OH)₂CO₃ films on ITO substrates using NaCOOH and Bi(NO₃)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 CO₂ on the electrode surfaces. With the presence of Bi³⁺ in the solution, Bi₄O₄(OH)₂CO₃ precipitated on the electrodes. We also found that Bi metals precipitated on the cathode. This is because the reduction potential of Bi³⁺ 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 Bi₄O₄(OH)₂CO₃ films are provided on the right panel in Fig. 1, which shows the change of color at the different time of the plating.

Fig. 1.

Schematic of the anodic plating for depositing (BiO)₄(OH)₂CO₃ films on ITO glasses

To understand anodic plating details, Bi₄O₄(OH)₂CO₃ 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 Bi₄O₄(OH)₂CO₃ films were scratched from ITO/glasses for TGA with a temperature rise from 30 to 500 °C in an N₂ atmosphere (N₂ was used to avoid adsorption of CO₂ from the air). As shown in Fig. 2a, three steps of endothermic decomposition were obtained in a TGA run for Bi₄O₄(OH)₂CO₃. A continuous weight loss of 5.6% below 180 °C was observed, likely attributed to absorbed solvent and water [17]. Bi₄O₄(OH)₂CO₃ 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(CO₃)_0.94 of the anodically plated films, which agreed with the predicted products of Bi₄O₄(OH)₂CO₃. As a reference, we also analyzed Bi₂O₂CO₃ 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 Bi₂O₂CO₃ compared to that of plated Bi₄O₄(OH)₂CO₃ films was likely due to the crystalline and amorphous nature of the two materials. This result suggested that amorphous Bi₄O₄(OH)₂CO₃ films were anodically plated on ITO.

Fig. 2.

a TG and DTA of a plated (BiO)₄(OH)₂CO₃ film. b TG and DTA of the commercial (BiO)₂CO₃ (99.5%). c XRD pattern of the as-plated (BiO)₄(OH)₂CO₃, hydrothermal-treated (BiO)₄(OH)₂CO₃, ITO and Bi₂O₃ calcinated from (BiO)₄(OH)₂CO₃. d UV–vis diffuse reflectance spectra of the as-plated (BiO)₄(OH)₂CO₃, hydrothermal-treated (BiO)₄(OH)₂CO₃, ITO and Bi₂O₃ calcinated from (BiO)₄(OH)₂CO₃

Fig. 3.

a Optical images of the solution-based process for the synthesis of BiVO₄ films on ITO glass (12 pieces of the 2 cm × 2 cm samples). b Schematic diagram of  a proposed reaction mechanism. c XRD patterns of the BiVO₄/ITO synthesized at 520 °C

Figure 3a shows the optical images of the synthesis process of BiVO₄ 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 BiVO₄ 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 BiVO₄ thin films with similar morphology. The Bi precursor materials were calcined in the air at 520 °C (Fig. 3c) for 2 h. The fabricated BiVO₄ films were in a monoclinic structure, which agreed with the previous report [8].

The scanning electron microscopy (SEM) images of the plated Bi₄O₄(OH)₂CO₃ film are present in Fig. S3 in the supporting information. The 7-min anodically plated amorphous Bi₄O₄(OH)₂CO₃ 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 BiVO₄ film was made of large BiVO₄ particles with an in-plane diameter of ~ 500–1000 nm and a thickness of ~ 300 nm. Such large BiVO₄ crystalline likely decreased the number of boundaries between particles. Therefore, improved photoelectrochemical performance was realized.

Fig. 4.

SEM images of BiVO₄. a The cross-sectional view of BiVO₄. b–d The top view of BiVO₄

Finally, we tested the photoelectrochemical performance of the fabricated BiVO₄ films in 0.1 M Na₂SO₃ solution. As shown in Fig. 5a, a quick raise of the photocurrent density was observed with BiVO₄ films with the increase of the positive potential. At 0.9–1.2 V_RHE, the photocurrent density reached a plateau of ~ 5 mA/cm² 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).

Fig. 5.

Photoelectrochemical performance of BiVO₄ film in a 0.1 M potassium phosphate (KPi) and 0.1 M Na₂SO₃ solution. a Photocurrent densities at 0–1.2 V_RHE; b evolved H₂ in 6 h

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 BiVO₄ was fabricated on ITO substrates. The photocurrent density reached ~ 4–5 mA/cm² at 0.9–1.2 V_RHE in Na₂SO₃-containing electrolytes under simulated solar illumination. The developed electrochemical deposition and thermal calcination may offer a new pathway for the synthesis of photocatalytic materials.

Experimental

Anodic-plating of Bi₄O₄(OH)₂CO₃ film on ITO glass

A 0.1 M Bi(NO₃)₃ solution was prepared by dissolving Bi(NO₃)₃·5H₂O in 25 ml acetic acid solution (99.7% mass ratio, Wako). The prepared solution was mixed with a 10 mL pH 5 sodium acetate aqueous solution. 5 M NaOH solution was used to adjust the pH of the mixed solution to 4.8. Anodic plating was performed at 2.3 V_Ag/AgCl at room temperature using a three-electrode cell with ITO working electrodes, a platinum counter electrode and an Ag/AgCl reference electrode. A potentiostat (CHI630e) was used for anodic plating and subsequent electrochemical measurements. The optimum plating time was 7 min.

Fabrication of BiVO₄ thin film on ITO glass

First, ~ 300 nm amorphous Bi₄O₄(OH)₂CO₃ 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 Bi₄O₄(OH)₂CO₃ 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 Bi₄O₄(OH)₂CO₃ 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 BiVO₄ 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 Na₂SO₃ 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:

$$V_{\text{RHE}}=V_{\text{Ag}/\text{AgCl}}+0.059\text{pH}+V_{\text{Ag}/\text{AgCl}}^0$$ 2

$$V_{\text{Ag}/\text{AgCl}}^0=0.199\text{V}\text{at}25{}^{\circ }\text{C}$$ 3

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 N₂ and O₂ gases in the cell. The H₂ evolution was measured in under simulated sunlight illumination for 6 h at 0.9 V_RHE. The theoretical amounts of evolved H₂ were estimated from the passed charges on the assumption that faradaic efficiency was unity.

Abbreviations

ITO

Indium tin oxide

PEC

Solar-driven photoelectrochemical

SEM

Scanning electron microscopy

XPS

X-ray photoelectron spectroscopy

XRD

X-ray diffraction

TGA

Thermogravimetric analysis

Author contributions

M.Z. supervised the project. M.Z. and H.J. conceived the idea and designed the experiments. H.J. performed the synthesis, characterizations, and performance tests. M.Z., H.J. and Y.X. wrote the manuscript. All authors discussed the results and assisted during manuscript preparation.

Funding

This work was supported by the National Key Research and Development Program of the Ministry of Science and Technology of China (No. 2020YFA0406102), the National Natural Science Foundation of China (Grant Number 22272078, 91963121), and the Frontiers Science Center for Critical Earth Material Cycling of Nanjing University, “Innovation & Enterpreneurship Talents” plan of Jiangsu Province.

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.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.