Lattice vibration fundamentals in nanocrystalline anatase investigated with Raman scattering

A detailed nano-size and temperature-dependent Raman scattering study of the Eg(1) mode was carried out from 83 to 723 K for nanocrystalline anatase with sizes of 5.6, 8.6, and 19.4 nm. The Eg(1) Raman spectra were fitted and calculated on the basis of a combined model of both anharmonic coupling and phonon confinement. The temperature-dependent lattice vibration fundamentals are essentially the same for all these three nanocrystallites. Both the three- and four-phonon processes are needed to obtain excellent fitting, and the main contribution comes from the three-phonon processes. The anharmonic-decay-related phonon lifetime increases with decreasing nanocrystallite size, and the smaller nanoparticles have slower anharmonic decay. The phonon confinement has a comparative effect on both the linewidth broadening and the peak frequency shift, and it is responsible for a much shorter phonon lifetime at low temperatures for the 5.6 nm nanocrystallite.