Surface Crystal Chemistry of Phyllosilicates Using X-Ray Photoelectron Spectroscopy: A Review

Abstract

The characterization of freshly cleaved mica surfaces for surface structure and chemical composition was briefly reviewed and focused on surface crystal chemistry using X-ray photoelectron spectroscopy (XPS) and other surface-sensitive techniques. This paper considers micas, which are useful as a first approximation for the behavior of many clay surfaces. Emphasis was given to phyllosilicate XPS binding energies (''chemical shift''), which were described and used to obtain oxidation state, layer charge, and chemical bonding information from the chemical shifts of different peaks. The chemical shift of the Si2p binding-energy to lower values can result from a negative charge increase because of Si4+ replacement by Al3+ and/or Fe3+. The apparent interlayer coordination number reduction from twelve to eight at muscovite and tetraferriphlogopite (001) surfaces was indicated by the XPS measured K2p binding-energy and is consistent with bond relaxation. Although chemical shifts are valuable to distinguish chemical bonding and oxidation state, chemical shifts usually cannot distinguish between different Al coordination environments where Al is in both tetrahedral and octahedral sites.

MgO−SiO2 Catalysts for the Ethanol to Butadiene Reaction: The Effect of Lewis Acid Promoters

“…All photoelectron peaks were adequately modeled by a single peak of symmetric shape (except the C 1s peak of the CP and WK samples). The binding energies of the Mg, Si, and O core levels for the natural talc sample showed very good agreement with the data found in the literature for talc materials [33–35,39,40] . (Cf.…”

“…Dealing with silicate materials, generally the C 1s line is used for compensation of the charging effect. [40] Accordingly, in the present study the lowest binding energy component of the C 1s spectrum, arising from hydrocarbons at 285.0 eV, was selected as reference point. With this calibration reasonable binding energy values were obtained for both Si 2p electrons around 103 eV and Mg 2p electrons around 50.5 eV.…”

“…As the samples were insulating materials, considerable charging was observed during the measurements. Dealing with silicate materials, generally the C 1s line is used for compensation of the charging effect [40] . Accordingly, in the present study the lowest binding energy component of the C 1s spectrum, arising from hydrocarbons at 285.0 eV, was selected as reference point.…”

“…The binding energies of the Mg, Si, and O core levels for the natural talc sample showed very good agreement with the data found in the literature for talc materials. [33][34][35]39,40] (Cf. Supporting Information).…”

Experimental quantification of the Fe-valence state at amosite-asbestos boundaries using acSTEM dual-electron energy-loss spectroscopy

“…As a second approach, we calculated the expected average valence state assuming that the "shell" has a Fe-valence state of 2.67+ (XPS data), whereas the bulk valence state of amosite, determined by Mössbauer spectroscopy, is 2.08+ (Pollastri 2015). This approach should be valid because the XPS data are usually representative of the first 0.3 to 3 nm (Vansant 1995), and possibly up to 10 nm, of the material starting from the surface (Elmi et al 2016). Both approaches returned similar results, documenting that the near-surface regions of the short fibers are more oxidized than those of the long fibers (Table 2).…”