Effect of the Chromium Ion Adsorption on the Photoluminescent Properties of Titanium Dioxide

We have studied the photoluminescence (PL) of nanocrystalline titanium dioxide (TiO2), synthesized by the thermal hydrolysis in the form of anatase (A) or rutile (R), the surface of which has been modified by adsorption of chromium ions (Cr3+). The samples were characterized by X-ray diffraction, X-ray fluorescence, Raman spectroscopy and optical absorption. PL spectra were excited by a nitrogen UV laser. It was found that the processes of the PL emission and optical absorption near the band gap edge of anatase and rutile TiO2 occur with the same electronic transitions. In the Cr3+-doped R/TiO2, the process of nonradiative Auger recombination leads to the quenching of the PL intensity. The Cr3+ ion doping in A/TiO2 leads to short-wave and long-wave shifts of the PL peaks due to the Burstein-Moss effect and due to the contribution of radiation "tails" of the electron density of states, respectively. PL intensity of the Cr3+-doped A/TiO2 at low concentration of Cr3+ (up to 0.5 at. %) increases in comparison with the undoped A/TiO2 due to the formation of additional centers of radiative recombination of carriers. With increasing concentration of Cr3+ (similar to 1.0 at. %), the TiO2 PL intensity decreases due to the concentration quenching.