Electrochemical synthesis methods were developed to produce CuBi₂O₄, 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 CuBi₂O₄ at 450 °C in air. Ag-doped CuBi₂O₄ 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 CuBi₂O₄, Ag⁺ ions substitutionally replaced Bi³⁺ ions and increased the hole concentration in CuBi₂O₄. As a result, photocurrent enhancements for both O₂ reduction and water reduction were achieved. Furthermore, while undoped CuBi₂O₄ electrodes suffered from anodic photocorrosion during O₂ reduction due to poor hole transport, Ag-doped CuBiO₄ effectively suppressed anodic photocorrosion. The flat-band potentials of CuBi₂O₄ and Ag-doped CuBi₂O₄ 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 CuBi₂O₄/Ag-doped CuBi₂O₄ 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 CuBi₂O₄ to reduce water was also confirmed by detection of H₂ during photocurrent generation. This study provides new understanding for constructing improved CuBi₂O₄ photocathodes by systematically investigating photocorrosion as well as photoelectrochemical properties of high-quality CuBi₂O₄ and Ag-doped CuBi₂O₄ photoelectrodes for photoreduction of both O₂ and water.

Two n-type W-containing ternary oxides, CuWO₄ and Bi₂WO₆, were prepared as high surface area electrodes and characterized for use as photoanodes in a water-splitting photoelectrochemical cell. The synthesis involved electrochemical preparation of porous WO₃ electrodes and annealing them with Cu²⁺- or Bi³⁺-containing solutions on their surfaces to form the respective electrodes. The resulting CuWO₄ electrode had a bandgap of 2.3 eV, and showed excellent photostability and photocurrent-to-O₂ conversion efficiency (ca. 100%) in 0.1 M borate buffer solution (pH 9). Bi₂WO₆ had a bandgap of 2.8 eV but, regardless of its higher bandgap energy, Bi₂WO₆ showed an earlier photocurrent onset and much higher photocurrent than CuWO₄ due to its more favorable CB edge and flatband potential position for water splitting. Bi₂WO₆ also showed chemical stability over a wide pH range (−0.26 ≤ pH ≤ 9.0). The photocurrent-to-O₂ conversion efficiency of Bi₂WO₆ was in the range of 50–75% and its photocurrent decayed over time, indicating photocorrosion. However, stable photocurrent was obtained when H₂O₂, which has faster oxidation kinetics than water, was introduced into the electrolyte as a hole scavenger. This suggests that the photocorrosion of Bi₂WO₆ can be suppressed when an oxygen evolution catalyst is placed on its surface to improve interfacial hole transfer kinetics. With proper oxygen evolution catalysts and improved charge transport properties, both CuWO₄ and Bi₂WO₆ have the possibility of achieving better photoelectrochemical performances than WO₃.

An electrochemical/chemical route is introduced to deposit both textured and epitaxial films of methylammonium lead iodide (MAPbI₃) perovskites. The perovskite films are produced by chemical conversion of lead dioxide films that have been electrodeposited as either textured or epitaxial films onto [111]-textured Au and [100] and [111] single-crystal Au substrates. The epitaxial relationships for the MAPbI₃ films are MAPbI₃(001)[010]∥PbO₂(100)⟨001⟩ and MAPbI₃(110)[111]∥PbO₂(100)⟨001⟩ regardless of the Au substrate orientation, because the in-plane order of the converted film is controlled by the epitaxial PbO₂ precursor film. The textured and epitaxial MAPbI₃ films both have trap densities lower than and photoluminescence intensities higher than those of polycrystalline films produced by spin coating.