Ag₃PO₄ as a novel photoanode material, despite its arguably highest photoactivity, suffers for its poor light absorption and stability for photoelectrochemical (PEC) water oxidation. In this work, 4.5 × 4.5 cm² Ag₃PO₄ microcrystalline films are grown via a room-temperature solution process, and vacuum annealing is proposed to solve the stability and light absorption issues. It is found that the major process below 400 ℃ of vacuum annealing is the |Recovery| process for Ag₃PO₄ microcrystals, when lattice defects and Ag⁰ surface species get reduced. Next, |Recrystallization| stage occurs at >400 ℃. The recovery of native defects of silver vacancies, with both density functional theory calculation and experimental results, could simultaneously improve the light absorption and catalytic activity of Ag₃PO₄. The 400 ℃-annealed Ag₃PO₄ photoanode, with enhanced light harvesting and crystal quality, exhibits 88% increase in (J_light − J_dark) value (1.94 mA cm⁻²) than non-annealed photoanode (1.03 mA cm⁻²). Moreover, it retains >99% current density after a 4000-s stability test. These results suggest that vacuum annealing can substantially improve the PEC performance of Ag₃PO₄ microcrystalline film photoanodes due to mitigated effects of native defects, improved light harvesting, and inhibited Ag₃PO₄ decomposition during water oxidation reaction.

Copper oxide nanowires with varying oxidation states are prepared and their activity for water oxidation is studied. The nanowires with a CuO phase are found to be the most active, and their degree of crystallinity is important in achieving efficient water oxidation. For the crystalline CuO nanowires in a weakly basic Na₂CO₃ electrolyte, a Tafel slope of 41 mV/decade, an overpotential of approximately 500 mV at ~ 10 mA/cm² (without compensation for the solution resistance), and a faradaic efficiency of nearly 100% are obtained. This electrode maintains a stable current for over 15 h. The low overpotential of 500 mV at 10 mA/cm², small Tafel slope, long-term stability, and low cost make CuO one of the most promising catalysts for water oxidation. Moreover, the evolution of the CuO nanowire morphology over time is studied by electron microscopy, revealing that the diffusion of Cu ions from the interior of the nanowires to their surface causes the aggregation of individual nanowires over time. However, despite this aggregation, the current density remains nearly constant, because the total electrochemically active surface area of CuO does not change.