Size and phonon-confinement effects on low-frequency Raman mode of anatase TiO2 nanocrystal

Raman spectra of anatase TiO2 nanocrystals with 2.2-25.5 nm grain sizes were measured at temperature range 83-293 K. The size dependences of the frequency, the bandwidth and the lineshape of the low-frequency mode agree well with those calculated on the basis of the phonon confinement model with the theoretical phonon dispersion relationships. The correlations of both Raman frequency and bandwidth to the grain size L satisfy a L−1.3 law. The contributions of three-phonon anharmonic processes to the frequency and bandwidth at various grain sizes were obtained from the temperature dependence of the Raman spectra, and the results show that the phonon coupling is increased in anatase TiO2 nanocrystal.