Photoelectrochemical Properties and Photostabilities of High Surface Area CuBi₂O₄ and Ag-Doped CuBi₂O₄ Photocathodes

Abstract

Here, electrochemical synthesis methods were developed to produce CuBi<sub>2</sub>O<sub>4</sub>, a promising p-type oxide for use in solar water splitting, as high surface area electrodes with uniform coverage. These methods involved electrodepositing nanoporous Cu/Bi films with a Cu:Bi ratio of 1:2 from dimethyl sulfoxide or ethylene glycol solutions, and thermally oxidizing them to CuBi<sub>2</sub>O<sub>4</sub> at 450°C in air. Ag-doped CuBi<sub>2</sub>O<sub>4</sub> electrodes were also prepared by adding a trace amount of Ag+ in the plating medium and codepositing Ag with the Cu/Bi films. In the Ag-doped CuBi<sub>2</sub>O<sub>4</sub>, Ag+ ions substitutionally replaced Bi3+ ions and increased the hole concentration in CuBi<sub>2</sub>O<sub>4</sub>. As a result, photocurrent enhancements for both O<sub>2</sub> reduction and water reduction were achieved. Furthermore, while undoped CuBi<sub>2</sub>O<sub>4</sub> electrodes suffered from anodic photocorrosion during O<sub>2</sub> reduction due to poor hole transport, Ag-doped CuBiO<sub>4</sub> effectively suppressed anodic photocorrosion. The flat-band potentials of CuBi<sub>2</sub>O<sub>4</sub> and Ag-doped CuBi<sub>2</sub>O<sub>4</sub> electrodes prepared in this study were found to be more positive than 1.3 V vs RHE in a 0.1 M NaOH solution (pH 12.8), which make these photocathodes highly attractive for use in solar hydrogen production. The optimized CuBi<sub>2</sub>O<sub>4</sub>/Ag-doped CuBi<sub>2</sub>O<sub>4</sub> photocathode showed a photocurrent onset for water reduction at 1.1 V vs RHE, achieving a photovoltage higher than 1 V for water reduction. The thermodynamic feasibility of photoexcited electrons in the conduction band of CuBi<sub>2</sub>O<sub>4</sub> to reduce water was also confirmed by detection of H<sub>2</sub> during photocurrent generation. This study provides new understanding for constructing improved CuBi<sub>2</sub>O<sub>4</sub> photocathodes by systematically investigating photocorrosion as well as photoelectrochemical properties of high-quality CuBi<sub>2</sub>O<sub>4</sub> and Ag-doped CuBi<sub>2</sub>O<sub>4</sub> photoelectrodes for photoreduction of both O<sub>2</sub> and water.