Preferred Orientation in Sputtered TiO2 Thin Films and Its Effect on the Photo-Oxidation of Acetaldehyde

Abstract

Crystal orientation is not typically considered when investigating the reactivity of thin films. We propose that accounting for the preferred crystallographic orientation may serve as an indirect measure of the active sites along the solid–solid interface that are difficult to measure with direct techniques. The goal of this work is to identify the preferred orientation, examine its evolution as a function of synthesis parameters, and determine its effect on photoreactivity. We examine the effect of substrate radio frequency (RF) bias and reactive gas partial pressure on the structure and photoreactivity of TiO2 films synthesized by reactive direct current (DC) magnetron sputtering. We characterize these films using ellipsometry, scanning electron microscopy (SEM), grazing incidence X-ray diffraction (GIXRD), and pole figure scans, and test their photoreactivity with the degradation of acetaldehyde under 365 nm UV light. We find that, in the parameter space investigated, changes in RF bias strongly influence both film texture and reactivity, and that the orientation of the crystallites is the best predictor of photoreactivity. Under the synthesis conditions tested, we observe an optimum RF bias of −50 V at which the films exhibit biaxial texture with the c-axis parallel to the surface with maximum crystallinity and degree of orientation, corresponding to a maximum in the reactivity as well. Beyond this point a change in the preferred orientation is observed, and the films transition to a fiber texture with the c-axis normal to the film surface and the appearance of small amounts of rutile. The effect of texture on reactivity is discussed.