Effect of carrier concentration on the optical band gap of TiO2 nanoparticles

Abstract TiO 2 nanoparticles doped with various concentrations of selected transition metals (2–8% Cu, Ni and Cr) were synthesized by sol–gel method. These nanoparticles were characterized using UV–Visible, fluorescence, transmission and scanning electron microscopies to investigate their optical and morphological properties. Direct band of pure TiO 2 (3.9 eV) was significantly reduced to 2.9, 3.4 and 3.6 eV for Cu, Ni and Cr doped samples respectively. Higher dopant concentrations induced the widening of the band gap according to the Burstein–Moss phenomenon. X-ray diffraction (XRD) spectroscopy established the retention of anatase phase in the synthesized doped materials. X-ray photoelectron spectroscopy (XPS) was used to determine the oxidation state and chemical environment of each kind of dopant ions in the crystal structure. XPS results confirmed the substitution of Ti 4 + ions by transition metal ions. Reduction of Ti 4 + to Ti 3 + state was observed for charge compensation in the crystal structure as a result of foreign ion incorporation.