A contribution to the surface characterization of alkali metal sulfates

作者：Fantauzzi Marzia; Rigoldi Americo; Elsener Bernhard; Atzei Davide; Rossi Antonella^*

来源：Journal of Electron Spectroscopy and Related Phenomena, 2014, 193: 6-15.

DOI：10.1016/j.elspec.2014.01.007

• X-ray photoelectron spectroscopy
• X-ray induced Auger electron spectroscopy
• Auger parameter
• Sulfur
• Polarization energy
• SKLL curve fitting

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. %26lt;br%26gt;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%26apos;) 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%26apos; is observed from 2274.7 +/- 0.1 eV for Li to 2275.3 +/- 0.2 eV for Cs sulfate. %26lt;br%26gt;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. %26lt;br%26gt;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.

• 出版日期2014-3