Ge-Doped Cobalt Oxide for Electrocatalytic and Photocatalytic Water Splitting

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 Co 3 O 4 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 Co 3 O 4 to serpentine Co 3 Ge 2 (OH) 5, with the latter transforming to an oxyhydroxide active phase during OER. Combination of the p-type Co 3 Ge 2 (OH) 5 semiconductor as an OEC with an n-type Bi 0.5 Y 0.5 VO 4 semiconductor doubles the photocatalytic OWS activity of the latter, resulting in H 2 and O 2 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 SrTiO 3 . A similar transformation of serpentine Co 3 Ge 2 (OH) 5 to an oxyhydroxide was not observed during OWS, indicating that Ge doping confers distinct advantages for electrochemical OER vs photocatalytic OWS.