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.

Mo-doped BiVO₄ electrodes were prepared by an electrochemical route for use as photoanodes in a photoelectrochemical cell. The purpose of Mo-doping was to improve the electron transport properties, which in turn can increase the electron–hole separation yield. The poor electron–hole separation yield was known to be one of the main limiting factors for BiVO₄-based photoanodes. The electrochemical route provided an effective way of doping BiVO₄, and the optimally doped sample, BiV_0.97Mo_0.03O₄, increased the electron–hole separation yield from 0.23 to 0.57 at 0.6 V vs. RHE, which is a record high separation yield achieved for BiVO₄-based photoanodes. As a result, BiV_0.97Mo_0.03O₄ generated impressive photocurrents, for example, 2 mA cm⁻² at a potential as low as 0.4 V vs. RHE for sulfite oxidation, which has fast oxidation kinetics and, therefore, the loss of holes by surface recombination is negligible. For photooxidation of water, BiV_0.97Mo_0.03O₄ was paired with FeOOH as an oxygen evolution catalyst (OEC) to improve the poor catalytic ability of BiV_0.97Mo_0.03O₄ for water oxidation. The resulting BiV_0.97Mo_0.03O₄/FeOOH photoanodes generated a significantly improved photocurrent for water oxidation compared to previous reported results, but the photocurrent of BiV_0.97Mo_0.03O₄/FeOOH for water oxidation could not reach the photocurrent of BiV_0.97Mo_0.03O₄ for sulfite oxidation. In order to examine the cause, the effects of Mo-doping on the interaction between BiVO₄ and FeOOH and the effects of FeOOH on the electron–hole separation yield of BiV_0.97Mo_0.03O₄ were investigated in detail, which provided new insights into semiconductor–OEC interactions.

Copper nitride nanoparticles supported on a mesoporous superparamagnetic silica microsphere exhibit superior activity toward the Huisgen cycloaddition of azides and alkynes. The nitride catalyst offers significant advantages over homogeneous Cu catalysts.