Structural, optical, and photoelectrochemical properties of Mn+-TiO2 model thin film photocatalysts

Mn+-TiO2 thin films (M: Cr, V, Fe, Co) with different M/Ti ratios have been prepared by ion beam induced CVD (IBICVD) with the aim of investigating their structural, optical, and photocatalytic properties. These samples can be considered as model systems to describe the photocatalytic behavior of other Mn+-TiO2 materials reported in the literature. In their "as prepared" state the samples were amorphous and the cations were homogeneously distributed within the TiO2 layer. Crystallization of the thin films upon annealing has been investigated with X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR). This latter technique has proved to be a suitable and rapid method of crystal phase assessment of TiO2. After annealing at T > 573 K, TiO2 crystallizes into the anatase or rutile structures depending on the type and amount of cations present in the film. The local environment around the different cations has been ascertained by X-ray absorption spectroscopy (XAS). It has been shown that after annealing, partial segregation of the cations in the form of M2On aggregates takes place. The extent of this segregation depends on the type of cation, being larger for Fe and Co than for V and Cr. Both the doping with a foreign cation and the oxide segregation greatly affect the crystallization behavior of TiO2 thin films favoring the development of the rutile structure even at relatively low temperatures (i.e., T similar to 673 K). The light absorption properties of Mn+-TiO2 systems have been traditionally considered as a hint to determine whether they are photoactive with visible light. In this paper the light absorption properties of the Mn+-TiO2 thin films have been investigated and compared with their photoelectrochemical behavior. As a general result it has been found that incorporation of increasing amounts of a cation M in the original thin films produces a progressive shift of the absorption threshold toward the visible. However, a further shift toward the visible in the light absorption found for the annealed samples is attributed to the segregation of M2On agglomerates. Either for the original or the annealed Mn+-TiO2 samples, no improvement of the photoactivity of the system, but its degradation with respect to pure TiO2 is found. This behavior shows that by itself a shift toward the visible in the light absorption of Mn+-TiO2 materials does not ensure that they have an enhanced photocatalytic efficiency at longer wavelengths.