Electro- and Photochemical Water Oxidation on Ligand-free Co<sub>3</sub>O<sub>4</sub> Nanoparticles with Tunable Sizes

Splitting of water to hydrogen and oxygen on colloidal catalysts is a promising method for future energy and chemistry cycles. The currently used high-performance oxides containing expensive elements (Ru, Ir) are progressively being replaced by more sustainable ones, such as Co<sub>3</sub>O<sub>4</sub>. Although the size of the nanoparticles determines their catalytic performance, the control over the particles’ diameter is often synthetically difficult to achieve. An additional obstacle is the presence of stabilizing agent, an organic molecule that blocks accessible surface-active centers. Herein, we present how precise control over size of the cobalt oxide nanoparticles (Co<sub>3</sub>O<sub>4</sub> NPs), their colloidal stability, and the ligand-free surface affect overall performance of the photocatalytic oxygen evolution. We accordingly correlated the photochemical results with the electrochemical studies, concluding that accessibility of the active species on the particles’ surface is crucial parameter in water oxidation.