Role of substrate crystallographic characteristics on structure and properties of rutile TiO2 epilayers

Abstract

To investigate heterostructures of interest for catalytic applications, we integrated rutile TiO2 epitaxial thin films with Al2O3(0001), Al2O3(101¯0), and Al2O3(011¯2) substrates and studied structure and properties of the epilayers as a function of the crystallographic characteristics of the substrate. The epitaxial relationship across the film/substrate interfaces was established as (100)rutile‖(0001)c-sapphire and [001]rutile‖[101¯0]c-sapphire, (001)rutile‖[101¯0)m-sapphire, and [100]rutile‖[0001]m-sapphire, (101)rutile‖(011¯2)r-sapphire and [010]rutile‖(011¯2)r-sapphire. The origin and the relaxation mechanism of stress and strain for each heterostructure were studied in detail. It was revealed that large lattice misfit strains relax easily even if the primary slip system is not active due to the epitaxial alignment between the film and substrate and orientation of the in-plane stresses. We also showed that even small misfit strains can relax provided that the primary slip system is active. The origin of the residual strains in the epilayers was found to be primarily due to thermal misfit and defect/impurity strains. In addition, the decomposition rate of 4-chlorophenol by the rutile/sapphire heterostructures under ultraviolet illumination was measured. The (001)-plane was found to be the most photoactive face of rutile TiO2, while the (100)-plane showed the lowest photocatalytic activity. The difference in the photochemical characteristics was attributed to the atomic arrangement on different crystallographic surface planes.