Covalently bonded TiO2/graphene interfaces: Interplay between structure, electronic properties, and photocatalytic activity revealed by computational studies

Composites of TiO2 with graphene and other carbon-based nanomaterials are widely used as photocatalysts, since they improve the efficiency of TiO2 photocatalysts by reducing charge recombination and extending TiO2 light-harvesting to the visible range. However, the exact nature of binding at TiO2/graphene interfaces is still unclear. Experimental studies show that Ti–C and Ti–O–C covalent bonds are often present in TiO2/graphene heterostructures. In this study, we used density functional theory to investigate the nature of binding at the interfaces of graphene with the anatase polymorph of TiO2. We investigated graphene binding with both the most stable (101) surface of anatase and the less stable but more photocatalytically active (100) and (001) surfaces, and analysed the electronic properties of these interfaces. We found that pristine graphene binds to anatase surfaces by physisorption, while graphene with carbon vacancies can form covalent bonds to TiO2. The presence of these covalent bonds alters the electronic structure of TiO2/graphene composites, creating hybridised states that may facilitate charge transfer and hinder electron–hole recombination. This study highlights the important role of defects, such as vacancies, in creating interfacial covalent bonds that may be responsible for the high photocatalytic activity of TiO2/graphene heterostructures.