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submitted on 21.07.2019 and posted on 22.07.2019by Ana I. Becerro, Stephen Rhatigan, Michael Nolan, Gerardo Colon
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, theO2 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.