Copper and Nitrogen co-doped TiO2 photocatalyst with enhanced optical absorption and catalytic activity

Cu-N-codoped TiO2 photocatalyst is synthesized by sol-gel method to obtain enhanced optical absorption in the visible region. Optimum concentrations of Cu and N were obtained by maximizing the photocatalytic activity for the monodoped (Cu or N) TiO2. These optimized concentrations were used for synthesizing (Cu, N)-codoped TiO2. The photocatalysts were characterized using XRD, micro-Raman, SEM, XPS, BET surface area analyzer, UV-vis diffuse reflectance spectroscopy and photoluminescence. XPS study suggests the incorporation of Cu2+ into TiO2 lattice, which assists N to substitutionally replace oxygen in codoped TiO2, while maintaining the anatase phase even after doping. N-doping creates minor variation in the energy band gap of TiO2 reducing it upto 3.0 eV, while Cu doping was able to narrow the band gap to 2.2 eV mainly due to the localized levels of Cu-3d states and shifting of Ti-3d states to lower energy (due to oxygen vacancies) in the band-gap as deduced from XPS data and confirmed by the DFT calculation. In (Cu, N)-codoped TiO2, visible light absorption is higher than the other TiO2 samples, a feature that is mainly attributed to the formation of an isolated intermediate band (IB) occurring due to the strong hybridization between Cu 3d and N 2p orbitals. This LB contributes to visible light absorption by two step optical transition with the first transition from valence band (VB) to IB and the second from IB to conduction band (CB). The dopant species may also act to reduce the charge carrier recombination by acting as the trapping sites for photogenerated charges. (Cu, N)-codoped TiO2 was able to degrade Methylene Blue dye and p-Nitrophenol solution under light irradiation with significantly better rate in comparison to monodoped and undoped TiO2. High photocatalytic activity is attributed to the presence of IB in the energy band gap of TiO2, which creates the synergic effect by higher visible light absorption and lower recombination of photogenerated charges.