Characterization of Raman Spectra of Size-Selected TiO2Nanoparticles by Two-Dimensional Correlation Spectroscopy

Abstract

Nanometer-sized materials have intensively studied due to their unique physical and chemical properties as well as their great potential of technological applications. 1-5 Despite many studies on the structure-property relations of nanoparticles using the various characterization techniques, such an understanding is still to be answered. Therefore, we have recently explored the relationship between the size and structure of size-selected TiO 2 nanoparticles, which are prepared via a hydrolysis method with Ti[OCH(CH3)2]4 as the starting material, by X-ray absorption spectroscopy (XAS). 6 Analysis of the XAS of the samples with an average particle size of ~30 nm (sample A), ~12 nm (sample B), and ~7 nm (sample C) demonstrated that samples A and B have an anatase structure, whereas sample C has a structure very similar to that of the TiO 2 II phase. The TiO2 II phase generally arises only under high-pressure conditions. This difference can be attributed to size-induced radial pressure within the smaller nanoparticles, which plays an important role in the phase of TiO 2 nanoparticles in sample C. In addition, we have studied the relationship between the particle size and change of Raman bands of TiO 2 nanoparticles, sample A and sample B. 7 The obtained Raman spectra showed the broadening and shift of Raman bands with decreasing particle diameter. The origin of Raman shifts can be attributed to the effect of smaller particle size and it affects the force constant and vibrational amplitudes of the nearest neighbor bond. In this study, in order to analyze the vibrational modes of size-selected TiO2 nanoparticles and to the shift of Raman bands of TiO2 nanoparticles with particle size, we have applied two-dimensional (2D) correlation spectroscopy to the particle size-dependent Raman spectra of TiO 2 nanoparticles. Generalized 2D correlation spectroscopy has been applied extensively to the analysis of spectral data sets obtained during the observation of a system under some external perturbation. 8-10 Because of the wide range of applications of this technique, it has become one of the standard analytical techniques for interpreting various types of spectroscopic data. The details of this technique are described elsewhere, 8-10 so no further description is given here.