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Mechanism for spontaneous oxygen and hydrogen evolution on CoO nanoparticles

Abstract

Overall photocatalytic water splitting with high efficiency of ~5% has recently been observed in CoO nanoparticle suspensions in the absence of an applied bias or a co-catalyst. While experimental measurements indicate that the overall photocatalytic water splitting is caused by optimal band edge alignments with respect to the water redox potentials, the mechanism by which H2 and O2 simultaneously evolve on these nanoparticles is unknown. In this study, we use first-principles density functional theory (DFT) calculations to elucidate the mechanisms for charge separation and H2 and O2 evolution on CoO nanoparticles under illumination in an aqueous solution. We demonstrate that electrons are driven to CoO(100) facet and holes are driven to hydroxylated CoO(111) facet (OH*-CoO(111)) as a result of a built-in potential arising from the difference in the band edge positions on the two facets. Furthermore, based on a set of criteria whether 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 of 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 a co-catalyst as well as providing a new strategy for designing novel photocatalysts with high efficiency by controlling the surface configurations and their morphologies.

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Publication details

The article was received on 19 Nov 2018, accepted on 10 Feb 2019 and first published on 12 Feb 2019


Article type: Paper
DOI: 10.1039/C8TA11087E
Citation: J. Mater. Chem. A, 2019, Accepted Manuscript
  • Open access: Creative Commons BY-NC license
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    Mechanism for spontaneous oxygen and hydrogen evolution on CoO nanoparticles

    K. Park and A. Kolpak, J. Mater. Chem. A, 2019, Accepted Manuscript , DOI: 10.1039/C8TA11087E

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