Exploring the mechanism of water-splitting reaction in NiOx/β-Ga2O3 photocatalysts by first-principles calculations

Abstract

Experiments found that loading suitable cocatalysts, usually in the form of metal and metal oxide nanoparticles, on the semiconductor surface can remarkably increase the photocatalytic activity of water-splitting reaction. To get insight into the mechanism of experimental observations, we took a NiO_x/β-Ga₂O₃ photocatalytic system as a model and performed detailed density functional theory calculations. Electrochemical computational methods are used to investigate the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Our results show that in the Ni₄O₂/β-Ga₂O₃ system, the reaction sites of HER are on cluster oxygen atoms and Ni atoms in the Ni₂ cluster. Loading the Ni₄O₂ cluster on the β-Ga₂O₃ surface importantly reduces the reaction free energy of HER. On the clean β-Ga₂O₃ surface, water dissociation is energetically unfavorable. After attaching the Ni₄O₂ cluster to the surface, water decomposition becomes thermodynamically favorable. The favorable reaction sites of OER focus on the Ni₄O₂ cluster. The rate-determining step of OER can be changed by adsorbing the Ni₄O₂ cluster. Notable reduction of overpotential (0.87 V) for OER on Ni₄O₂/Ga₂O₃ is found compared with that on the clean Ga₂O₃ surface, which reasonably explains the experimental observation on significant enhancement of activity for generating oxygen after loading NiO_x cocatalysts.