Computational insights into crystal plane dependence of thermal and photoresponse of pure and palladium-substituted titania

Abstract

Shape effects have caught attention of researchers worldwide with attempts to synthesize catalytic materials with well-defined exposed facets to obtain high activity being the central idea. In this study, we have dedicated our attention to develop structure-property relationships in TiO2-based solid solutions with particular reference to net oxygen activation. Density functional theory calculations were implemented on (001) and (100) planes of anatase TiO2 with anionic vacancy defects (TiO2-δ), cationic substitution (Ti1−xPdxO2) and solid solutions with anionic vacancy defects (Ti1−xPdxO2−δ). (100) plane was found to show higher oxygen activation with smaller rigidity towards surface relaxations. Vacancy formation energy was found to decrease with an introduction of cationic defects with the net oxygen activation reaching up to 26% in Ti1−xPdxO2. Different vacancy sites on the two planes were observed to show different oxygen activations. Formation of surface hydroxyl groups, envisaged as dissociative adsorption of H2 over oxidized catalyst or dissociative adsorption of H2O over reduced catalyst, was found to be influenced by cation substitution with the stability of surface hydroxyls being compromized by substitution. Synergism of surface hydroxyl groups and lattice oxygen activation for the water-gas shift reaction was identified. Photocatalytic response was found to be higher for TiO2-δ catalysts as it narrowed the band gap via formation of mid-gap states and shifting the valence band maximum closer to the conduction band minimum.