Thin-Film Epitaxy and Enhancement of Photocatalytic Activity of Anatase/Zirconia Heterostructures by Nanosecond Excimer Laser Treatment

We present a novel method to improve the photocatalytic efficiency of epitaxial c-axis anatase TiO2 thin films by a factor of 2 by using nanosecond laser annealing. The anatase films were epitaxially grown by pulsed laser deposition on Si(001) substrates, where a tetragonal yttria-stabilized zirconia (t-YSZ) buffer was used to effectively remove the native SiOx layer from the substrates prior to deposition of anatase. With the information from X-ray and TEM diffraction patterns, the epitaxial relationship across the interfaces was shown to be: (001)[110](anatase)parallel to(001)[110](t-YSZ)parallel to(001)[001](silicon). Performing high-temperature XRD, we observed that the anatase epilayers were stable up to 1100 degrees C, far beyond the normal anatase-to-rutile transition temperature (approximately 600-700 degrees C). The samples were subsequently laser-annealed in air by a single pulse of KrF excimer laser beam at an energy density of similar to 0.3 J.cm(-2). On the basis of the detailed HRTEM studies, the interface between the laser annealed and the pristine region as well as the anatase/t-YSZ interface were crystallographically continuous. The XPS results revealed the presence of point defects, and the XRD patterns showed some reduction and broadening after laser treatment. Photocatalytic activity of the pristine and the laser-annealed heterostructures was assessed by measuring the decomposition rate of 4-chlorophenol under UV light. The photocatalytic reaction rate constants were determined to be 0.0077 and 0.0138 min(-1) for the as-deposited and the laser-treated samples, respectively. Such an appreciable enhancement was attributed to the formation of point defects, in particular, oxygen vacancies, near the surface of the anatase thin films. Oxygen vacancies facilitate adsorption of 4-chlorophenol, hydroxyl, and water molecules to the titania surface. In addition, these defects trap the photogenerated electrons, giving rise to charge separation and, hence, improvement of the photocatalytic efficiency. The nanosecond photochemical enhancement may lead to fabrication of smart photocatalytic materials to address the needs of future photochemical devices.