Titanium oxide is widely employed and investigated as a functional material due to its photocatalytic behavior and to its biocompatibility. A fine control on TiO2 film properties (surface morphology and wettability, structure, oxide phase, chemical termination) is of increasing interest to realize self-cleaning and bio- surfaces with enhanced performances.
We have grown nanostructured titanium oxide films by nanosecond Pulsed Laser Deposition (PLD). PLD in a reactive background gas pressure is an effective method to influence kinetic energy, composition and size of the deposited species. We exploited laser ablation of a Ti target in dry air and Ar/O2 mixture at different pressures (from vacuum to 100 Pa) in order to vary the structural and morphological properties of the deposited films.
Surface morphology was studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM). By increasing gas pressure, the film structure evolves from compact and smooth to nanostructured and porous. The corresponding growth regimes range from atom-by-atom to columnar growth and cluster assembling. Film roughness increases with increasing gas pressure at fixed film thickness and with increasing film thickness at fixed deposition conditions. Surface wettability was investigated through water contact angle measurements. Different surface behaviours, ranging from hydrophobic to highly hydrophilic are observed depending on deposition conditions and film thickness, therein on surface morphology and chemistry.
The structure, the degree of crystallinity and the oxide phase have been investigated by Raman spectroscopy. As-deposited films are generally amorphous while polycrystalline films with different anatase/rutile mixture are obtained after a post-deposition annealing treatment (400°C in air). In particular the oxide phase content, ranging from mainly anatase to mainly rutile, depends on the original nanostructure of the deposited film and can be thus controlled through the deposition parameters (i.e. gas pressure).
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