Cobalt Oxide Nanofilms on n-GaN Working Electrodes for Photoelectrochemical Water Splitting

Synthesizing low-cost electrocatalysts using earth-abundant metals is the foremost task in water-splitting techniques. This study mainly discusses the enhancing effects of cobalt oxide (CoOx) on the photocurrent of gallium nitride (GaN) photoelectrodes in a photoelectrochemical (PEC) system for hydrogen generation. The CoOx nanofilm is formed on a GaN epitaxial layer through the thermal evaporation and post-deposition thermal annealing of cobalt metal. The X-ray photoemission spectroscopy spectra confirm that the exposed metal ions in the as-deposited films are Co3+ and Co2+. The chemical composition of the annealed CoOx films is more biased toward Co3+ than that of as-deposited films. This finding indicates that it is suitable for water oxidation during a PEC reaction for water splitting because the films may contain more CoOOH. Besides, a CoOx/n-GaN heterostructure can form a built-in electric field at the interface between n-GaN and CoOx and hence increase the charge separation efficiency. The photocurrent of photoelectrodes made of GaN and decorated with the CoOx nanofilm significantly improves compared with those of bare n-GaN photoelectrodes.