Rational Design of Surface Modified TiO2 with Alkaline Earth Oxides (Mg2+, Ca2+) for the Oxygen Evolution Reaction

Photocatalytic O2 evolution reaction was performed TiO2 rutile surface-modified with alkaline earth oxides, namely CaO and MgO. From the structural and surface characterization, modified systems do not show significant differences with bare TiO2, indicating that the integrity of the rutile phase is maintained after modification. However, the charge carrier separation is strongly affected by the presence of small amounts of alkaline-earth. Moreover, the O2 evolution activity is enhanced for Mg2+ modified systems at low loadings. This improved performance may be related to surface features such as higher ion dispersion and surface hydroxylation, and the improved photonic efficiencies observed for low Mg2+ loading. First principles simulations indicate that surface aggregation is more favourable for CaO-modifiers and may explain the greater degree of dispersion of MgO at low loadings. Computed oxygen vacancy formation energies indicate that the modified systems are reducible with moderate energy costs, relative to unmodified rutile, so that Ti3+ ions will be present. A model of photoexcitation shows that modification promotes charge carrier separation; electrons and holes localise at subsurface Ti sites and low-coordinated O sites of the modifiers, respectively. Pathways to water oxidation at interfacial sites of reduced MgO-modified rutile TiO2 are identified, requiring overpotentials of 0.75 V. In contrast, CaO-modified systems required overpotentials in excess of 1 V for the reaction to proceed.