Monitoring Interface Interactions by XPS at Nanometric Tin Oxides Supported on Al₂O₃ and Sb₂O_x

Abstract

Two model systems consisting of nanometric SnO and SnO 2 particles deposited by evaporation on Al 2 O 3 and Sb 2 O 3 or Sb 2 O x substrates (x means a mixture of Sb 3+ and Sb 5+ 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 Sn 3d 5/2 photoemission peak and the Auger parameter (R′) 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., ∆BE, ∆R′, both in eV) are as follows for the different systems: SnO/Al 2 O 3 (∆BE ) 1.0, ∆R′ ) -1.6), SnO/Sb 2 O 3 (0.6, -0.7), SnO 2 /Al 2 O 3 (0.9, -1.2), and SnO 2 /Sb 2 O x (0.6, -0.6). These variations are stronger than those existing between Sn 2+ and Sn 4+ species in their respective bulk oxides (i.e., approximately 0.5 eV for both ∆BE and ∆R′). 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 ∼10 Å of material is deposited. It is found that when ∼2 Å of material is deposited, small clusters of approximately ∼8 Å (i.e., about two Sn-O monolayers) cover ∼18% 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.

Study of formation and thermal stability of the Fe/ZnO(000‐1) interface

“…It has been reported in the literature that the Auger parameter of the substrate rises 0.2 eV during deposition of two layers of SnO on Al 2 O 3 and then remains at a constant value for thicker films. [27] In our case, the parameter rises α = 0.5(±0.2) eV ( Fig. 7(a)) with respect to the clean crystal surface and then remains constant until approximately 8 ML Fe are deposited.…”

Surface hydrophilization of SU‐8 by plasma and wet chemical processes

“…The relatively high binding energy of the Sb 3d 3/2 peak (540.6 eV for the longest exposure time) indicates, that the antimony is mainly in the Sb V+ oxidation state. [46,47] …”

Promoting effect of tin oxides on alumina-supported gold catalysts used in CO oxidation

“…The binding energy of Sn 3d 5/2 electrons is higher than that corresponds to bulk oxides. It can be explained by the low tin content [29][30][31] and the effect of the greater heterogeneity in the chemical environment of the nanoparticles. This correlates well with the results of Mö ssbauer spectroscopy, namely with the presence of two kind of tin(IV) species and with XRPD results that showed that there is no crystalline tin oxide and gold phase on the support.…”