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

TiO2 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 TiO2 (3.9eV) was significantly reduced to 2.9, 3.4 and 3.6eV 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 Ti4+ ions by transition metal ions. Reduction of Ti4+ to Ti3+ state was observed for charge compensation in the crystal structure as a result of foreign ion incorporation. Doping of TiO2 with selected transition metals reduces its band gap significantly. [Display omitted] •TiO2 nanoparticles were doped with Cu, Cr and Ni and the effect of dopant concentration was evaluated.•Direct band gap was assigned to the synthesized TiO2 and doped TiO2.•Absorption spectroscopy displayed a band gap narrowing up to 2.9, 3.4 and 3.6eV for Cu, Ni and Cr doped samples.•Induction of Burstein–Moss effect at higher dopant levels is observed resulting in blue shift of absorption edge.•XPS results established that carrier concentration determines the nature and chemical environment of the doped metal ions.•Reduction of Ti4+ to Ti3+ is reported as a result of incorporation of foreign ions.