Surface Modification of Rutile TiO₂ with Alkaline-Earth Oxide Nanoclusters for Enhanced Oxygen Evolution

The oxygen (O2) evolution reaction (OER) is accepted as the bottleneck in the overall water splitting and has seen intense interest. In this work, we prepared rutile TiO2 modified with nanoclusters of alkaline-earth metal oxides for the OER. Photocatalytic OER was performed over rutile TiO2 surface-modified with alkaline-earth oxide nanoclusters, namely, CaO and MgO. The O2 evolution activity is notably enhanced for MgO-modified systems at low loadings and a combination of characterization and first-principles simulations allows interpretation of the role of the nanocluster modification in improving the photocatalytic performance of alkaline-earth-modified rutile TiO2. At such low loadings, the nanocluster modifiers would be small, and this facilitates a close correlation with theoretical models. Structural and surface characterizations of the modified systems indicate that the integrity of the rutile phase is maintained after modification. However, charge-carrier separation is strongly affected by the presence of surface nanoclusters. This improved performance is related to surface features such as higher ion dispersion and surface hydroxylation, which are also discussed with first-principles simulations. The modified systems are reducible so that Ti3+ ions will be present. Water dissociation is favorable at cluster and interfacial sites of the stoichiometric and reduced modified surfaces. Pathways to water oxidation at interfacial sites of reduced MgO-modified rutile TiO2 are identified, requiring an overpotential of 0.68 V. In contrast, CaO-modified systems required overpotentials in excess of 0.85 V for the reaction to proceed.