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Ultrafine, nanometer-sized anatase particles were prepared by the hydrolysis of titanium isopropoxide. Their Raman spectra are discussed in view of the finite size effects. The crystallite size D estimated by the X-ray diffraction peaks (Scherrer method) was found to be correlated with the Raman peak width Γ. The relationship could be expressed in the empirical equation, Γ = Γ0 + kD−α with the scaling exponent α of 1.5. Because the anatase crystal spans a three-dimensional network, the result demonstrates that 2× α is equal to the dimensionality of the sample. In addition, the anatase powders prepared by different hydrolysis schemes regressed into curves with different constant terms γ0 and k. The amount of organic residue was mainly affected by the hydrolysis scheme, and the organic residue caused the formation of defects in the crystalline particle by the following heat treatment. Therefore, it is obvious that the variation in Γ0 and k has a close relation to the defect density of the crystalline particles. The defect densities were estimated by using the correlation between the X-ray diffraction and the Raman data.

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The phase transformation from a gel of titanium alkoxide to crystalline anatase was observed in situ by anti-Stokes Raman scattering. Utilization of anti-Stokes scattering eliminates interference from the fluorescence of decomposing organic residuals during the pyrolysis process. The anti-Stokes Raman spectra suggest a highly disordered structure during the pyrolysis process. The structure of the titania gel, which is characterized by broad Raman peaks around 440 and 620 cm−1, was transformed into the amorphous structure by increasing the temperature up to 200–300 °C. This amorphous structure is supported by a flat Raman spectrum lacking specific features in the wavenumber region from 200 to 1000 cm−1. A partially crystallized gel also exhibits the further progress of amorphization during the pyrolysis process, which is explained by the disordering of the local structure of the gel due to the generation of pyrolyzed organic species.