Careful curve fitting is needed when using XPS to investigate oxidized functional groups on carbon-fibre surfaces. This is important because the core regions often contain a number of overlapping features some of which are of low intensity, and the C Is region is influenced by an asymmetric graphitic feature. The fitting approach used requires that the same content of oxygenated species must be obtained from both the C 1s and 0 Is regions. The approach is used to analyse untreated fibres before and after cleaning.
The chemical bonds of the pentapeptide sequence of elastin ValGlyGlyValGly (VGGVG), both in its monomer and polymer forms, were correlated with their XPS spectra through a well-established curve-fitting procedure. To aid in this correlation, the C1s, O1s, and N1s chemical shifts of the Boc-VGGVG-OEt, were validated by theoretical calculations, performed in the framework of the Koopman approximation of HF/6-31G molecular orbitals, leading to the "preferred" conformation of the protected monomer. Then the same curve-fitting procedure was adopted for interpreting the XPS spectra of the polypentapeptide as a powder, and the XPS results obtained both for monomer and polymer compounds were compared with those obtained by FT-IR. The polymer was then analyzed after deposition onto a silicon substrate, Si(100), either from methanol or water suspensions and the presence of hydrogen bonds was detected at the polymer/substrate interface and between the polymer chains. The "surface rearrangement" that could be inferred from XPS results strongly confirms that derived from AFM images previously obtained under the same experimental conditions. In particular, the observed amyloid conformation is stabilized by hydrogen bonds to water molecules included in the structure while the formation of the beaded string structure observed in deposits from methanolic suspension is probably mediated by hydrogen bonds to the hydrated silicon surface.
In this contribution we report on an XPS study of microporous and mesoporous titanosilicates, in particular microporous titanium silicalite TS-1, ordered mesoporous Ti-MCM-41 and [Ti]-MCM-41 and amorphous mesoporous silica-titania (MST) catalysts. Our aim was to obtain both photoemission and x-ray-excited Auger data for Ti species on these catalysts and use them in a Ti Wagner plot to rationalize the dependence of the local electronic structure on the atomic environment. Isolated Ti(IV) species coordinated to four and six oxygen anions and segregated TiO2 clusters were detected on all catalysts by a curve-fitting procedure of Ti 2p, O 1s and related peaks. The presence of the Si 2p peak excited by an O Kalpha ghost makes the detection of Ti LMM Auger transitions in mesoporous samples impossible due to the low Ti loadings and its homogeneous distribution in the silica matrix. Small TiO2 clusters are eventually segregated within the mesopores of the catalysts and not at their external surface. On TS-1 microporous catalysts with similar Ti loadings to the mesoporous catalysts we were able to detect Ti LMM Auger transitions, and by the Ti Wagner plot we clearly identify the presence of octahedrally coordinated Ti(IV) species. Thus, it is suggested that on TS-1 the in-framework (-O)(4)Ti species are easily changed to (-O)(4)(H2O)(2)Ti species by insertion of water molecules from the atmosphere. Small TiO2 clusters (diameter <5 nm), eventually present on samples with Ti loading >2 wt.%, are segregated at their external surface and present spectroscopic features similar to (-O)(4)(H2O)(2)Ti species. Copyright (C) 2004 John Wiley Sons, Ltd
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