Evaluation of sputtered nickel oxide, cobalt oxide and nickel–cobalt oxide on n-type silicon photoanodes for solar-driven O₂(g) evolution from water

Abstract

Thin films of nickel oxide (NiO_x), cobalt oxide (CoO_x) and nickel–cobalt oxide (NiCoO_x) were sputtered onto n-Si(111) surfaces to produce a series of integrated, protected Si photoanodes that did not require deposition of a separate heterogeneous electrocatalyst for water oxidation. The p-type transparent conductive oxides (p-TCOs) acted as multi-functional transparent, antireflective, electrically conductive, chemically stable coatings that also were active electrocatalysts for the oxidation of water to O₂(g). Relative to the formal potential for water oxidation to O₂, E^(o′)(O₂/H₂O), under simulated Air Mass (AM)1.5 illumination the p-TCO-coated n-Si(111) photoanodes produced mutually similar open-circuit potentials of −270 ± 20 mV, but different photocurrent densities at E^(o′)(O₂/H₂O), of 28 ± 0.3 mA cm⁻² for NiO_x, 18 ± 0.3 mA cm⁻² for CoO_x and 24 ± 0.5 mA cm⁻² for NiCoO_x. The p-TCOs all provided protection from oxide growth for extended time periods, and produced stable photocurrent densities from n-Si in 1.0 M KOH(aq) (ACS grade) under potential control at E^(o′)(O₂/H₂O) for >400 h of continuous operation under 100 mW cm−2 of simulated AM1.5 illumination.