Defect-Tolerant TiO₂-Coated and Discretized Photoanodes for >600 h of Stable Photoelectrochemical Water Oxidation

Arrays of GaAs nanowires have been grown by selective-area metal–organic chemical-vapor deposition (MOCVD) onto photoactive planar Si substrates. This tandem, vertical-wire-array-on-planar absorber was then coated by atomic layer deposition (ALD) of an amorphous TiO2 (a-TiO2) stabilization layer, followed by deposition of a NiOx electrocatalyst layer. The tandem planar Si/nanowire GaAs/a-TiO2/NiOx photoanodes exhibited continuous solar-driven water oxidation in 1.0 M KOH(aq) for over 600 h without substantial photocurrent decay. The preservation of the nanowire morphology and structural integrity during >600 h of photoanodic operation confirms the benefits of mitigating and isolating nanoscale defects via the architecture of discretized absorbers on a self-passivating and insulating substrate. Nanoscale morphology and compositions of the photoanode after 600 h of testing were characterized to reveal the self-limiting corrosion behavior. It provides a promising approach to develop efficient but otherwise unstable absorbers such as III–V materials into defect-tolerant, corrosion-resistant photoanodes.