XPS as a powerful tool to investigate the surface properties of simple and mixed metal oxides

XPS is a well established surface sensitive method for the analysis of solid materials, allowing the determination of surface composition and the assessment of oxidation state and chemical environment of the different species. In fact, core level XPS is highly sensitive to the chemical environment, i.e. the binding energy may be strongly dependent on the oxidation state of the investigated ion, as well as on the neighbouring atoms. Oxide-based materials (simple, multicomponent and composites), are a fascinating and technologically appealing class of materials, exhibiting a huge variety of chemical, structural, optical, magnetic, electric and electronic properties. Since in oxides the functional properties are mainly related to the complex interplay among composition, microstructure, shape, characteristic size, charge density, chemical environment, presence of defects etc., the understanding of their actual chemical nature and electronic structure is a primary concern to fully exploit their functionalities in different fields. In this chapter, after a general overview on metal oxides and on their electronic structure and peculiarities, the application of XPS to their investigation will be introduced and discussed and some selected application fields will be considered. The effect of ion sputtering in oxides will be moreover shortly examined. Furthermore, in this contribution, the issues related to the determination of the actual chemical state of metal species in mixed oxides will be also extensively addressed and discussed. The possibility of XPS to evidence the presence of M-O-M’ mixed bonds will be highlighted. In the second part of the chapter, several different selected case studies will be analysed in detail, which deal with different oxide systems: simple oxides, also in mixedvalence state, (ZnO, WO3, CuxOy, VxOy), mixed oxides (e.g. SiO2-HfO2, SiO2-ZrO2, SiO2-TiO2, ZrO2-TiO2), doped oxides (FexOy:TiO2, M(Pt, Ag):TiO2, M (Au, Pt, Pd, Rh):SiO2). In particular, the use of this method for the investigation of the chemical composition of different typologies of surfaces (thin films, powders) will be discussed. A further aspect to be dealt with is the perspective combination of XPS, also at variable pressure, with other site-specific spectroscopic methods (XAS, XEOL) and theoretical tools (e.g. DFT), which represents a valuable tool to unravel and predict issues related to the reactivity and to other functional properties of these materials.