UC Davis

Gallium phosphide is an established photoelectrode material for H₂ or O₂ evolution from water, but particle-based GaP photocatalysts for H₂ evolution are very rare. To understand the reasons, we investigated the photocatalytic H₂ evolution reaction (HER) of suspended n-type GaP particles with iodide, sulfite, ferricyanide, ferrous ion, and hydrosulfide as sacrificial electron donors, and using Pt, Rh_yCr_2-yO₃, and Ni₂P HER cocatalysts. A record apparent quantum efficiency of 14.8% at 525 nm was achieved after removing gallium and oxide charge trapping states from the GaP surface, adding a Ni₂P cocatalyst to reduce the proton reduction overpotential, lowering the Schottky-barrier at the GaP-cocatalyst interface, adjusting the polarity of the depletion layer at the GaP-liquid interface, and optimizing the electrochemical potential of the electron donor. The work not only showcases the main factors that control charge separation in suspended photocatalysts, but it also explains why most known HER photocatalysts in the literature are based on n-type and not p-type semiconductors.