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 TiO 2 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.

Synthesis of phase pure iron oxide polymorphs thin films and their enhanced magnetic properties

“…Owing to different surface energies along different plane or orientations [26], the properties and stability of a thin film depend upon the degree of preferred orientation [25]. Therefore, the study of preferred orientation factor is needed.…”

Titanium Dioxide Thin Films Deposited by Electric Field‐Assisted CVD: Effect on Antimicrobial and Photocatalytic Properties**

“…TiO 2 photocatalytic activity may depend on several parameters; the crystal orientation, for instance, has been reported to have a marked effect, as it can influence the process of electron‐hole separation through surface arrangement . In particular it was shown that crystallite alignment of the (004) orthogonal to the film surface favors a higher reactivity rate …”

Tuning the Photocatalytic Activity of Anatase TiO2 Thin Films by Modifying the Preferred <001> Grain Orientation with Reactive DC Magnetron Sputtering

“…There are much fewer studies on sputter deposited TiO 2 films. It has been reported that sputtered TiO 2 films on biased substrates alter the texturing and preferred orientation depending on bias voltage, leading to an improved photodegradation rate of acetaldehyde as a function of increasing orientation [24]. Other reports show that changing total pressure [25], and O 2 partial pressure [26] may also affect film growth and preferential orientation of anatase films.…”