Monitoring interface interactions by XPS at nanometric tin oxides supported on Al2O3 and Sb2Ox

Two model systems consisting of nanometric SnO and SnO2 particles deposited by evaporation on Al2O3 and Sb2O3 or Sb2O, substrates (x means a mixture of Sb3+ and Sb5+ oxidation states) are studied by X-ray photoelectron spectroscopy to account for the interactions that develop at the interfaces. The conclusions derived from this study are relevant for the characterization by this technique of nanostructured materials formed by nanoparticles and/or very thin films of an oxide deposited on another oxide support. Electronic parameters such as the binding energy (BE) of the Sri 3d(5/2) photoemission peak and the Auger parameter (alpha') of the metal cation of the deposited tin oxides shift systematically from a nanometric size of the deposited moieties to a thick layer of these materials. The changes in the electronic parameters between the thickest layers of the deposited oxides and the first deposition situations (i.e., DeltaBE, Deltaalpha', both in eV) are as follows for the different systems: SnO/Al2O3 (DeltaBE = 1.0, Deltaalpha' = -1.6), SnO/Sb2O3 (0.6, -0.7), SnO2/Al2O3 (0-9, -1.2), and SnO2/Sb2Ox (0.6, -0.6). These variations are stronger than those existing between Sn2+ and Sn4+ species in their respective bulk oxides (i.e., approximately 0.5 eV for both DeltaBE and Deltaalpha'). These changes are systematized within the concept of the "Chemical state vector" and explained on the basis of a model that considers both the development of specific interactions between the substrates and tin oxide nanoparticles and the influence of different polarizabilities by the surrounding media on the relaxation energy of the photoholes. The changes in the electronic parameters for the initial states of deposition are correlated with the size and shape of the nanoparticles deposited, as determined by XPS peak shape analysis using the Tougaard's method. The analysis of the spectra shows that the tin oxides grow on both substrates in the form of small particles that do not cover completely the substrate before similar to10 Angstrom of material is deposited. It is found that when similar to2 Angstrom of material is deposited, small clusters of approximately similar to8 Angstrom (i.e., about two Sn-O monolayers) cover similar to18% of the surface. Changes in the width of the photoemission peaks are also detected. Removal of surface charging when depositing SnO is responsible for part of these changes, although very likely local inhomogeneities in coordination around the cations also contribute to broadening the peaks. This is particularly important for the smallest nanoparticles where the degree of interface interactions with the substrate is high. Finally, a shift of the valence band edge toward higher binding energies is observed at the initial stages of deposition of tin oxide nanoparticles. This shift is related with particle size effects and/or the development of specific interactions at the interface between the nanoparticles and the support.