The crystal structure of titanium oxide films produced by electrochemical oxidation of titanium can be amorphous or crystalline, depending on the parameters of preparation including: the applied potential, the time of anodising and the composition of the electrolyte. As far as TiO2 nanotube arrays are concerned, as produced, they are usually characterised by an amorphous structure, which can be transformed to anatase at temperatures as low as 300 °C in air.24,25 A further increase in temperature to above 450 °C, results in the formation of an anatase-rutile mixture.25,26 Annealed nanotubes are characterised by a polycrystalline structure. The typical size of crystallites (determined using TEM images or the Scherrer Equation for XRD data) varies in range from 10 to 25 nm. There is a degree of anisotropy in the nanotube array. The preferred orientation of the crystallite is such that plane (101) of anatase is found along the nanotube wall.27
The residual fluoride ions from electrolyte solutions are usually incorporated into amorphous TiO2 nanotubes without formation of a fluoride phase. Annealing of nanotubes, however, leads to an almost complete loss of fluoride ions at 300 °C.27
Sol-gel template-assisted TiO2 nanostructures are also usually characterised by an amorphous structure. Depending on the method of template removal, if pyrolysis is involved, the nanostructures under heat treatment may be converted to anatase or rutile.
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