Stabilization of n-cadmium telluride photoanodes for water oxidation to O₂(g) in aqueous alkaline electrolytes using amorphous TiO₂ films formed by atomic-layer deposition

Although II-VI semiconductors such as CdS, CdTe, CdSe, ZnTe, and alloys thereof can have nearly ideal band gaps and band-edge positions for the production of solar fuels, II-VI photoanodes are well-known to be unstable towards photocorrosion or photopassivation when in contact with aqueous electrolytes. Atomic-layer deposition (ALD) of amorphous, "leaky" TiO₂ films coated with thin films or islands of Ni oxide has been shown to robustly protect Si, GaAs, and other III-V materials from photocorrosion and therefore to facilitate the robust, solar-driven photoelectrochemical oxidation of H₂O to O₂(g). We demonstrate herein that ALD-deposited 140 nm thick amorphous TiO₂ films also effectively protect single crystalline n-CdTe photoanodes from corrosion or passivation. An n-CdTe/TiO₂ electrode with a thin overlayer of a Ni-oxide based oxygen-evolution electrocatalyst produced 435 ± 15 mV of photovoltage with a light-limited current density of 21 ± 1 mA cm^-2 under 100 mW cm^-2 of simulated Air Mass 1.5 illumination. The ALD-deposited TiO₂ films are highly optically transparent and electrically conductive. We show that an n-CdTe/TiO₂/Ni oxide electrode enables the stable solar-driven oxidation of H₂O to O₂(g) in strongly alkaline aqueous solutions, where passive, intrinsically safe, efficient systems for solar-driven water splitting can be operated.