Hierarchical Structure of TiO2 Nano-Aggregates Prepared by Pulsed Laser Ablation in Background Gas

We prepared titanium dioxide TiO2 nanoparticles by pulsed laser ablation of TiO2 target in O2 background gas. Effects of background gas pressure and substrate target distance on the structure of deposited films are clarified. The primary structure, nanocrystals, changes from rutile to anatase phase with increasing background gas pressure or substrate target distance. The porosity of aggregated structure increases with increasing background gas pressure or substrate target distance. These results indicate that confinement of the plume between target and substrate is important for structural formation. The hierarchical micron- and submicron-sized structures were found by the analysis of scanning electron microscope images. The deposits are composed of primary, secondary, and tertiary structures: They are nanoparticles, submicron-sized aggregates, and micron-sized aggregates. The nonequilibrium aggregation processes of nanoparticles in the plume and/or on the substrate are essential for the formation of hierarchical structure.