Size effects in the Raman spectra of TiO2 nanoparticles

To investigate the relationship between the particle size and the Raman bands of TiO2 nanoparticles, two different size-selected samples of TiO2 nanoparticles were investigated using transmission electron microscopy (TEM), X-ray absorption near edge structure (XANES), extended X-ray absorption fine structure (EXAFS), and Raman spectroscopy. Analysis of the XANES spectra showed that the nanoparticles in both sample A (particle size ∼30 nm) and sample B (particle size ∼12 nm) have an anatase structure, and that there is no difference between the electronic structure of titanium atoms in TiO2 nanoparticles and that of titanium atoms in bulk TiO2. The EXAFS results indicate that a contraction effect occurs in the anatase TiO2 nanoparticles. In the Raman spectra, both broadening and shifts of the Raman bands with decreasing particle diameter were observed. In this paper, these Raman shifts are attributed to the effects of decreasing particle size on the force constants and vibrational amplitudes of the nearest neighbor bonds.