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TiO 2 , N-doped TiO 2 , V-doped TiO 2 , and V–N-codoped TiO 2 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 TiO 2 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 TiO 2 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 TiO 2 elemental N resides at interstitial lattice positions but codoping with V permits N to reside in both substitutional and interstitial sites in TiO 2 lattice. UV–vis studies indicate that the band gap of TiO 2 (3.2 eV) reduced to 3.0 eV, 2.8 eV and 2.5 eV, by N-doping, V-doping and V–N codoping, respectively. The photocatalytic activity of pure, N-doped, V-doped, and V–N codoped TiO 2 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 TiO 2 gave the highest photocatalytic activity in comparison to the mono-doped and undoped TiO 2 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 TiO 2 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 TiO 2 , respectively. While for the mono-doped TiO 2 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 TiO 2 , 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. |
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