Characterization of the structures of size-selected TiO₂ nanoparticles using X-ray absorption spectroscopy

To investigate the relationship between the size and structure of TiO₂ nanoparticles, three size-selected samples of TiO₂ nanoparticles were prepared via a hydrolysis method that uses Ti[OCH(CH₃)₂]₄ as the starting material. The structures of the nanoparticles were characterized using powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray absorption spectroscopy (XAS). Analysis of the XRD patterns and of the TEM images showed that the samples were dispersed, with an average particle size of ∼30 nm (sample A), ∼12 nm (sample B), and ∼7 nm (sample C). Their X-ray absorption spectra indicate that samples A and B have an anatase structure, whereas sample C has a structure very similar to that of the TiO₂ II phase, which generally arises only under high-pressure conditions. This difference can be attributed to size-induced radial pressure within the smaller nanoparticles, which plays an important role in the phase of TiO₂ nanoparticles in sample C.