Surface preparation of TiO2 anatase (101): Pitfalls and how to avoid them

TiO2 anatase is a material of high technological importance, yet studies on well-defined model surfaces are scarce. The main impediment to fundamental research of this material is the lack of high-purity single crystals of a sufficient size. Natural anatase crystals always contain impurities, while synthetic crystals are pure but usually very small. We discuss optimum surface preparation procedures that result in clean surfaces under UHV conditions and the best ways to mount small crystals on sample plates. The influence of bulk impurities on surface preparation is discussed. The most troublesome impurity in natural TiO2 crystals is Fe. Upon annealing the crystal in partial O2 pressure Fe segregates, resulting in overgrowth of iron oxide on the surface. Based on the temperature-dependence of O-induced Fe segregation, optimum sample treatment procedures are proposed. Finally, we show that the surface roughness on the anatase (101) surface increases with the number of sputter-anneal cycles. Possible reasons and ways to revert this process are described.