A contribution to the surface characterization of alkali metal sulfates

The analytical characterization of surfaces of sulfur-bearing samples that present sulfides, polysulfides and/or elemental sulfur as reaction products can be difficult by simply relying on the binding energy of the S2p X-ray photoelectron signals, due to the small chemical shifts. In such cases the Auger parameter concept can be used to distinguish among different chemical states, but this requires a model to curve fit complex Auger SKLL signals in order to resolve the contributions arising from sulfur in different chemical states on the surface. With this scope a detailed X-ray photoelectron spectroscopy (XPS) and X-ray induced Auger electron spectroscopy (XAES) surface analytical study of the group IA sulfates is presented in this paper. Sulfates were chosen as model compounds for curve fitting the X-ray induced SKLL spectra since in these compounds sulfur is present in a unique chemical state. For the first time the multicomponent SKLL spectra are fitted with model functions consisting of an intense D-1 and a low intensity S-1 contribution with constant energy difference of 8 eV. It was found that the kinetic energy of the SK2,L-3(2,3) (D-1) line increases from 2105.1 +/- 0.1 to 2107.5 +/- 0.2 eV whereas the corresponding S2p(3/2') binding energy decreases from 169.5 +/- 0.1 eV for Li2SO4 to 167.8 +/- 0.1 eV for Cs2SO4. Shifts to lower binding energy values are observed also for S2p, S2s and O1s peaks. In the chemical state plot the alkali sulfates fall on a line with an Auger parameter of ca. 2275 eV whereas elemental sulfur is found at 2277 eV. A shift of 0.6 eV in the calculated Auger parameter alpha' is observed from 2274.7 +/- 0.1 eV for Li to 2275.3 +/- 0.2 eV for Cs sulfate. These results are discussed with respect to the influence of the cation using fundamental data and concepts such as ion radius, oxidation state and ionization energy, and separating intra-atomic and interatomic effects. All the information show a relatively small interaction between the sulfate anion and the cations as a consequence of the strong group localization of the valence states over the sulfate tetrahedron. The sulfur SKLL spectrum of sulfates and elemental sulfur consisting of an intense D-1 and a low intensity S-1 contribution with constant energy difference of 8 eV can be considered a first step toward curve fitting of multicomponent sulfur SKLL spectra and the interpretation of complex mechanisms of oxidation and dissolution that involve sulfur. (C) 2014 Elsevier B.V. All rights reserved.