Solubility and precipitation of Fe on reduced TiO2(001)

The solubility of Fe in reduced rutile TiO2 crystals and the followed precipitation on the (001) surface have been studied using X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) in ultra-high vacuum. The first step includes dissolving Fe in reduced TiO2 at 1073 K by the means of thermal diffusion and as a result the saturated solid solution is formed. Then, it undergoes fast cooling which leads to obtaining a supersaturated solid solution. When this supersaturated crystal is annealed at low temperatures of about 500 K, Fe starts to precipitate on the (001) surface forming spherical Fe-containing nanoparticles. The fast migration of Fe cations to the surface and their precipitation at relatively low temperatures are caused by high diffusion anisotropy and the reduction of the TiO2. At about 900 K, the size of nanoparticles increases and they are transformed into nanocrystals with clearly visible facets. Simultaneously, the number of the nanocrystals substantially decreases. The partial oxidation of Fe is also observed around 900 K, which is related to strong metal support interaction between Fe and reduced TiO2(001). The XPS and STM results suggest that the nanocrystals are mostly composed of mixed Fe/Ti oxides like FeTiO3 of ilmenite structure. Above 973 K, the nanocrystals disappear which is explained by the restored solubility of Fe cations in the reduced TiO2. The process of the nanoparticle precipitation at lower temperatures is repeatable and the precipitation and disappearance of Fe-containing nanocrystals on TiO2(001) are also a fully reversible phenomenon easily controlled by annealing temperature.