Efficient photocatalytic degradation of organic water pollutants using V-N-codoped TiO2 thin films

TiO2, N-doped TiO2, V-doped TiO2, and V-N-codoped TiO2 thin films have been prepared using RF-magnetron sputtering and their photocatalytic activities have been investigated. The codoping strategy was adopted to decrease both the band gap of TiO2 and the recombination rate of the photo-generated electron-hole pairs. Low concentration doping with single element (V or N) preserves the anatase dominated phase in TiO2 film while codoping with V and N produces a mixed phase of nearly equal amount of rutile and anatase as inferred from the XRD and Raman spectroscopy studies. XPS studies reveal that, for N-doped TiO2 elemental N resides at interstitial lattice positions but codoping with V permits N to reside in both substitutional and interstitial sites in TiO2 lattice. UV-vis studies indicate that the band gap of TiO2 (3.2eV) reduced to 3.0eV, 2.8eV and 2.5eV, by N-doping, V-doping and V-N codoping, respectively. The photocatalytic activity of pure, N-doped, V-doped, and V-N codoped TiO2 thin films were tested by examining the degradation of methylene blue, chlorophenol and nitrophenol as a function of time. It was observed that the codoped TiO2 gave the highest photocatalytic activity in comparison to the mono-doped and undoped TiO2 because of high visible light absorption and possible reduction in the recombination of photo-generated charges. Density of states calculated using density functional theory (DFT) showed that the narrowing of band-gap for the codoped TiO2 is obtained by the formation of isolated energy levels of V 3d and N 2p states below the conduction band and above the valence band of pure TiO2, respectively. While for the mono-doped TiO2 the narrowing of the band gap is only contributed by impurity levels formed near any one of the band edges. It is concluded that for the codoped TiO2, high visible light absorption is caused by the formation of impurity energy states near both the band edges which also act as the trapping sites for both the photo-generated charges to reduce the recombination process.