Sol–gel-derived super-hydrophilic nickel doped TiO2 film as active photo-catalyst

Pure and nickel doped TiO2 thin films on soda glass substrates were prepared by sol–gel dip coating process. The resulting films were annealed at 500 °C for 1 h and characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and UV–vis-near IR techniques. AFM reveals that doping of Ni ions suppresses the grain growth of crystal in the TiO2 film. The contact angle with water of these films was measured by sessile drop method. The undoped films could be entirely wetted by water after 1 h UV illuminations, while 0.5 mol% Ni ion-doped films become entirely wetted after 20 min UV illumination. The photo-catalytic activity was characterized by photo-catalytic degradation of aqueous methyl orange under UV radiation. It is found that, with a suitable amount (2–10 mol%), the Ni dopant increases the photo-catalytic activity of TiO2 films. The mechanism can be attributed to these processes: (1) as the anatase grain sizes decrease with Ni doping and the specific surface areas of doped TiO2 films increase, the charge transfer in TiO2 film is promoted; (2) by enhancing the electron–hole pair separation and inhibiting their recombination, the Ni dopant enhances the charge pair separation efficiency for doped TiO2 films.