Mechanism for spontaneous oxygen and hydrogen evolution reactions on CoO nanoparticles

Abstract

Overall photocatalytic water splitting with a high efficiency of ∼5% has recently been observed for CoO nanoparticle suspensions in the absence of an applied bias or co-catalyst. Although experimental measurements indicate that the overall photocatalytic water splitting is caused by optimal band edge alignments with respect to the redox potentials of water, the mechanism by which H₂ and O₂ simultaneously evolve on these nanoparticles is unknown. In this study, we used first-principles density functional theory (DFT) calculations to elucidate the mechanisms for the charge separation and H₂ and O₂ evolution on CoO nanoparticles under illumination in aqueous solution. We demonstrated that electrons are driven to the CoO(100) facet and holes are driven to the hydroxylated CoO(111) facet (OH*–CoO(111)) as a result of the built-in potential arising from the difference in the band edge positions on the two facets. Furthermore, based on a set of criteria, depending on if the photoexcited electrons and holes have sufficient energy to overcome the kinetic barrier along the H₂ and O₂ evolution reaction pathways, respectively, on the relevant surface facet, we show that H₂ evolution preferentially occurs on the CoO(100) facet, while O₂ evolution occurs on the OH*–CoO(111) surface. Our understanding of the overall water splitting mechanism on CoO nanoparticles provides a general explanation for the experimentally observed overall water splitting phenomena on a variety of self-standing photocatalysts, including γ-Ga₂O₃, Cu₂O, and KTaO₃, without an external driving potential or co-catalyst. In addition, we provide a new strategy for designing novel photocatalysts with high efficiency by controlling their surface configurations and morphologies.