Tailoring of structural, optical and electrical properties of anatase TiO2 via doping of cobalt and nitrogen ions

Pure (pristine) anatase and mono–doped and co–doped derivatives of TiO2 having nitrogen (N) and cobalt (Co) as dopants with respective fixed doping concentrations of 0.7 mol.% and 1.0 mol.% were synthesized using auto–combustion sol–gel technique. The doping effects at corresponding non–metal and transition metal sites of TiO2 on the basis of the structural, optical and electrical properties have been investigated. X–ray diffraction (XRD) measurement confirms the formation of pure anatase phase of TiO2 for all samples having I41/amd space group of tetragonal structure which has been also verified by the Raman spectroscopy measurement. Various crystallographic parameters have been calculated by performing Rietveld refinement of XRD data including average crystallite size that has been observed in the range of 10–15 nm. Pure anatase phase indicates the incorporation of Co2+ into TiO2 lattice which assists the substitution of N in place of oxygen in co–doped TiO2. The band gap tuning towards the visible region from 3.2 to 2.1 eV has been achieved with mono–doping and co–doping of the N and Co in TiO2 lattice. This can be described in terms of the formation of localized levels of N–2p and Co–3d states in mono–doping and an isolated intermediate band formation in co–doping case. Electrical properties have been investigated in details and explained as the synergetic effects of structural and inherent ionic characters of various dopants. The observed band gaps of all doped samples lie within the visible region which makes them pertinent as the solar energy harnessing materials for photocatalytic and photovoltaic applications.