Fundamental aspects of surface engineering of transition metal oxide photocatalysts

Many metal oxide photocatalysts exhibit strong variations in their photocatalytic activities as a function of their crystallographic surface orientation. In this Perspective possible fundamental reasons for the surface dependent photocatalytic properties are discussed. The majority of the data imply that the differing surface activities for photo-oxidation and -reduction reactions are mainly a consequence of electron and hole diffusion towards different surfaces. This is either due to directional charge carrier diffusion as a result of bulk anisotropies or it is a consequence of differing band bending (or flat band potentials) at the interface of the photocatalyst with its environment. Although the fundamental reason for the surface electronic structure must lie in the surface structure, no correlation between surface chemical properties and photochemical properties has yet been unambiguously determined. The importance of surface charge separation and charge trapping at the surface leads to new concepts for engineering surface properties with potentially increased photocatalytic activity. Consequently, enhancement can be achieved by synthesizing photocatalyst powders that expose surfaces with 'naturally' higher photoactivity; or in a proposed approach by modifying surfaces selectively with special surface phases or monolayer heterostructures that enhance charge separation at the surface. In addition, special surface phases with a narrowed band gap may enable visible light activity of otherwise only UV active bulk materials.