Unusual effects of vacuum annealing on large-area Ag₃PO₄ microcrystalline film photoanode boosting cocatalyst- and scavenger-free water splitting

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.