Phonon confinement effects in the Raman scattering by TiO2 nanocrystals

Nanocrystalline ${\mathrm{TiO}}_{\mathrm{2}}$ has been obtained by a sol-gel process by controlling the crystal size through the water/alkoxide ratio. Raman spectra of anatase nanocrystals with average sizes of 9.5–13.4 nm are reported and the correlation between the Raman band shape (peak position and linewidth) of the main feature at 144 ${\mathrm{cm}}^{\mathrm{-1}}$ and the crystals dimension is discussed. While in this system a minor role is played by nonstoichiometry and pressure effects, a model based on the phonon confinement, which takes into account the size distribution as determined by the transmission electron microscopy images, correctly reproduces the Raman band shape change.