Structure and luminescence of sol-gel synthesized anatase nanoparticles

Two samples of mesoporous anatase nanoparticles, prepared by the sol-gel method, were characterised by Cs-corrected high resolution transmission electron microscopy (HRTEM), X-ray powder diffraction (XRD) and Raman spectroscopy. Statistical evaluation of TEM data showed an average diameter of these crystallites of 8.8 nm and 11.1 nm, respectively. Optical spectroscopy by cathodoluminescence (CL) in a scanning electron microscope (SEM) showed free exciton transitions related to the direct and the indirect band gap of anatase TiO2. From the analysis of the excited states of the free excitons an exciton binding energy of 10 meV and a Bohr radius of 2.35 nm is obtained. The small Bohr radius could explain the absence of quantum confinement in the particles presented in this study.