Impact of surface hydroxylation in MgO-/SnO-nanocluster modified TiO₂ anatase (101) composites on visible light absorption, charge separation and reducibility

Surface modification with metal oxide nanoclusters has emerged as a candidate for the enhancement of the photocatalytic activity of titanium dioxide. An increase in visible light absorption and the suppression of charge carrier recombination are necessary to improve the efficiency. We have studied Mg₄O₄ and Sn₄O₄ nanoclusters modifying the (101) surface of anatase TiO₂ using density functional theory corrected for on-site Coulomb interactions (DFT + U). Such studies typically focus on the pristine surface, free of the point defects and surface hydroxyls present in real surfaces. We have also examined the impact of partial hydroxylation of the anatase surface on a variety of outcomes such as nanocluster adsorption, light absorption, charge separation and reducibility. Our results indicate that the modifiers adsorb strongly at the surface, irrespective of the presence of hydroxyl groups, and that modification extends light absorption into the visible range while enhancing UV activity. Our model for the excited state of the heterostructures demonstrates that photoexcited electrons and holes are separated onto the TiO₂ surface and metal oxide nanocluster respectively. Comparisons with bare TiO₂ and other TiO₂-based photocatalyst materials are presented throughout.