Shallow and deep trap emission and luminescence quenching of TiO2 nanoparticles on Cu doping

TiO2 nanoparticles with 2 and 4 % Cu are synthesized by sol-gel method. The crystalline phase and size of the nanoparticles are investigated with X-ray diffraction and transmission electron microscope. Cu-doped TiO2 has an extended absorption ranging from UV to visible region. Doping of Cu disturbs the arrangement of oxygen ions around Ti4+ and generates oxygen vacancies. These oxygen vacancies capture electrons and form some ionized oxygen vacancy centers or F centers. These F centers form subband states extending from shallow to the deep level in the band gap of TiO2. The visible emission peaks of pure and doped TiO2 are mainly associated with self-trapped excitons (STEs) and F centers. We have observed that Auger type nonradiative recombination is responsible for the quenching of the UV and STE emission peak in the doped samples. The intense visible emission peaks in pure TiO2 are due to shallow type centers whereas deep trap emission is predominant in doped samples. The intensity of UV and visible emission peaks are quenched with the increase in the doping level of Cu. Defects, Cu d-states, band structure of TiO2 and low mobility of the carriers are responsible for the quenching of the emission peaks.