Anharmonic phonon coupling and phonon confinement in nanocrystalline anatase TiO2

A temperature-dependent Raman scattering study of the Eg(1) mode was carried out at 83 to 723K for nanocrystalline anatase TiO2 with different sizes of 19486 and 56nm. By means of a combined model of anharmonic coupling and phonon confinementthe Eg(1) Raman spectra were fitted and calculated. The results show thatthe temperature-dependent lattice vibration fundamentals are essentially the same for the three samples. The blue shift of Raman frequencies mainly comes from the contribution of three-phonon processes. In order to get good fittingsboth the three- and four-phonon processes need to be considered. With increasing temperaturethe four-phonon process becomes more important and counteracts the effect of the three-phonon process. The anharmonic-decay-related phonon lifetime increases as nanocrystalline size decreasesand the smaller nanoparticles have slower anharmonic decay. At low temperaturesthe phonon confinement is responsible for a much shorter phonon lifetime for the 56-nm nanocrystals. The phonon confinement induces a blue shift of the Eg(1) mode and an asymmetrical broadening at high frequencies. It has a relatively larger effect on the linewidth broadening than on the peak shift.