Physical properties of NxTiO2 prepared by sol-gel route

The compounds NxTiO2(x=0, 0.05, 0.1, 0.2) with the anatase structure have been synthesized by Sol-Gel method using Tri-ethyl Amine as nitrogen source and their optical, electrical and electrochemical properties are investigated. The electrical conductivity and thermoelectric power are measured in the temperature rang 300-600 K. The samples exhibit p-type behavior in contrast to TiO2. The doped-samples exhibit two optical transitions (2.35≤Eh−Vis(eV)≤2.55; 1.97≤El−Vis (eV)≤2.06) directly allowed in the visible region, while only one transition is observed in UV region (EUV∼3.00 eV). Pure TiO2 shows direct band gap transition of 3.17 eV. The results confirm experimentally the calculations of Di. Valentin et al. [42]. The transitions Eh−Vis and El−Vis are attributed respectively to the promotion of electrons from the localized N 2p and π⁎ N-O bond to the conduction band. In all cases, EUV is associated to the forbidden band energy. Though that the conductivity is generally improved by doping process, only N0.05TiO2 and N0.1TiO2 shows an enhanced mobility. The mechanism of conduction takes place by small polaron hopping. The band edge positions of NxTiO2 (x=0, 0.05, 0.1, 0.2) at room temperature is predicted from the obtained physical properties. This study proves experimentally the principal role of nitrogen in doping process and permits the electronic states localization associated with N-impurities in TiO2 anatase.