GaN-coated III–V photoelectrocatalyst with oxide-free interface and ambipolar charge transport

III–V semiconductors are promising light absorbers for solar photochemical synthesis, but their surfaces tend to corrode and form defect-rich interphases under photochemical conditions. Here, we report a plasma-enhanced atomic layer deposition (ALD) GaN coating for InP(100), a model high-efficiency absorber, that forms an oxygen-nearly-free, atomically abrupt interface at low temperatures (<350°C). This coating reduces the population of deleterious surface states to below one-thousandth of those at a benchmark TiO2/InP interface. The resulting Pt/GaN/InP photocathode exhibits a barrier height of ~1.2 eV, delivering a ~0.82 V photovoltage, ~97% external quantum efficiency, and over 160 hours of stability in a pH 0 electrolyte. Additionally, the remaining states support ambipolar electron and hole transport through a ~10-nm-thick GaN layer, thereby facilitating photocatalytic H2 evolution and reversible redox-mediator oxidation. Our study demonstrates electronically benign and chemically robust III–V/liquid interfaces for efficient and stable photochemical charge separation and solar energy conversion, enabled by conformal, isotropic, acid-stable GaN coatings.