Ni/NiOx core-shell nanoparticles were used to improve the water splitting efficiency and stability of an n-GaN photoanode. Nanoparticles were fabricated by thermal annealing and oxidizing a Ni thin film deposited on an n-GaN substrate. The performance of water splitting was investigated by varying Ni film thickness and oxidation time, which led to different nanoparticle distribution and NiOx thickness in the core-shell structure. Nano particles fabricated from a thinner Ni film led to a smaller particle-to-particle spacing. It allowed holes in n-GaN to diffuse to nanoparticle catalysts in a shorter distance for water splitting reaction, thereby providing higher photocurrent and better protection against oxidation corrosion. Ni/NiOx core-sell nanoparticles obtained from partial oxidation showed higher photocurrent than sole Ni or fully oxidized NiOx nanoparticles. It is attributed to the optimization between having more NiOx for reaction centers and keeping NiOx thin for easy hole transport from n-GaN across NiOx to NiOx/electrolyte interface for reaction. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.