Formation of TiO₂ Thin-Films Through Laser-Induced Oxidation of a TiS₂ Precursor

In this work, crystalline thin-films of anatase and rutile TiO₂ are formed within an amorphous TiS₂ precursor (400 nm thick) through laser-induced oxidation. This was achieved by scanning a continuous wave, 514 nm laser beam across the precursor film surface in the presence of oxygen (ambient air and pure O₂). The 20 µm (1/e² diameter) focal spot allowed for the oxidation of many localized regions at varying intensities and scan speeds. These regions were characterized under Raman and ultraviolet-visible spectroscopy to determine the crystalline phase and band gaps of the laser-oxidized regions. After identifying the conditions needed to produce varying anatase/rutile mixtures in the intensity/scan speed phase space, large regions of crystalline TiO₂ were then produced by raster-scanning the laser repeatedly. Raman mapping was used to determine the spatial variation in the crystallinity of these larger regions, while X-ray diffraction analysis was used to estimate the anatase/rutile mass fraction present. Multiple whole-sample (2 in² area) thin-films of varying antase/rutile mixtures were subsequently created in the same manner, with each sample produced using different laser-processing conditions. Finally, the photocatalytic activity of each sample was determined by measuring the relative change in concentration of various NO_x species in a flow reactor during exposure of the film to simulated sunlight. Mechanisms driving the reaction kinetics of the TiS₂ oxidation/crystallization as well as the photocatalytic activity of the films are discussed.