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Cobalt oxides find widespread application in energy materials as oxygen evolution catalysts (OECs) in the oxygen evolution reaction (OER) and photocatalytic overall water splitting (OWS) reaction. However, the nature of the active cobalt species, their role in these reactions, and possible commonalities remain poorly understand. Here, the impact of (redox-inert) germanium dopants on the physicochemical properties of Co3O4 nanoparticles was investigated in electrochemical oxygen evolution and photocatalytic OWS reactions. A significant enhancement in OER performance on doping is attributed to the restructuring of spinel Co3O4 to serpentine Co3Ge2(OH)5, with the latter transforming to an oxyhydroxide active phase during OER. Combination of the p-type Co3Ge2(OH)5 semiconductor as an OEC with an n-type Bi0.5Y0.5VO4 semiconductor doubles the photocatalytic OWS activity of the latter, resulting in H2 and O2 productivities of ∼175.7 and ∼90.1 μmol/h, respectively. Formation of the composite semiconductor induces an intense internal electric field across the p–n junction, facilitating separation of photogenerated carriers and increased OWS activity, which is also validated in alternative photocatalyst, Al-doped SrTiO3. A similar transformation of serpentine Co3Ge2(OH)5 to an oxyhydroxide was not observed during OWS, indicating that Ge doping confers distinct advantages for electrochemical OER vs photocatalytic OWS.

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