Non-equilibrium Growth Processes of Porous TiO 2 Nanocrystal-films during Pulsed Laser Ablation
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Non-equilibrium Growth Processes of Porous TiO2 Nanocrystal-films during Pulsed Laser Ablation Ikurou Umezu1, Nobuyasu Yagi1, Akira Sugimura1 and Takehito Yoshida2 1 Department of Physics, Konan University, Kobe 658-8501, Japan 2 Department of Mechanical Engineering, Anan National College of Technology, Anan 774-0017, Japan ABSTRACT We performed pulsed laser ablation of titanium dioxide (TiO2) target in O2 background gas. Effects of background gas pressure and substrate target distance on the structure of deposited films are clarified. The hierarchical structures are observed when we change scale of observation. The film deposited on the substrate is composed of primary nanocrystal and secondary porous-aggregated-nanostructures. The primary nanocrystal changes from anatase to rutile phase with increasing background gas pressure or substrate target distance. The porosity of secondary aggregated structure increases with increasing background gas pressure or substrate target distance. The similarity between the effects of background gas and substrate target distance indicates that confinement of the plume between target and substrate is important for structural formation. The non-equilibrium aggregation processes of nanocrystals in the plume and on the substrate are essential for the hierarchical structure of the nanocrystal film. INTRODUCTION Titanium dioxide (TiO2) has attracted attention, because of its wide application area such as photocatalytic and biomedical functions. It is well known that TiO2 has three crystalline phases. Anatase is efficient than rutile for several applications like catalysis, photocatalysis, and solar cells but it is less stable than rutile. Anatase is low temperature phase and it changes to rutile by thermal annealing. However, the reasons why these different phases are formed are poorly understood. The performances for a given application are strongly influenced not only by the crystalline structure but also by the morphology. The RF sputtering method is widely used to prepare thin films. This technique is able to control the crystal structures and the morphologies of the TiO2 thin films by varying substrate materials, RF powers, substrate temperatures and so on[1-3]. However, the crystal structure of nanoparticles and morphology are not always perfectly controlled. Aggregated nanoparticles have a particular interest because their high surface/volume ratio is one of the advantages for catalyst. Pulsed laser ablation (PLA) in background gas is one of the techniques to prepare highly porous films composed of aggregated nanocrystals under strong non-equilibrium condition[4-6]. Control of both crystal and aggregated structures are desired for catalytic applications. However the effects of background gas on the crystal structure and/or morphology of TiO2 are described in many literatures, the correlation between structure and preparation conditions is discussed rather empirically[7-12]. The formation processes of atomic microstructure and aggregated nanocrystals have been clarified for PLA of
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