Effect of Fe doping on crystalline phase, structure and photocatalytic properties of TiO2 thin films

Undoped and Fe-doped TiO2 thin films were fabricated on Si substrates using sol -gel method. XRD and Rietveld refinement results show the anatase to rutile transition in the system. SEM and AFM spectra demonstrate that the generation of agglomeration phenomena after Fe doping provides the nucleation point for the phase transition, and the difference in the Gibbs free energies of the two phases provides the driving force for the phase transition. XPS measurements verify that the formation of oxides after Fe doping contributes to the surface thermodynamic equilibrium and the possibility of the surface atomic rearrangements during the anatase to rutile phase transition. The energy band diagram illustrates at the level of electronic structure that the phase transition raises the position of the conduction band minimum of the film and the oxide of Fe that is formed becomes the charge complex center. Finally, the results of degradation of methylene blue solution under simulated visible light irradiation show that Fe doping reduces the photocatalytic ability of TiO2 films. The corresponding mechanism is proposed to explain the photocatalytic activity by which Fe3+ of larger radius replaces Ti4+ to promote the transition of the film from anatase to rutile and to act as a charge complex center.