Chemical state

About: Chemical state is a research topic. Over the lifetime, 2378 publications have been published within this topic receiving 78183 citations.

Advanced chemical state studies of oxide films by lab-based HAXPES combining soft and hard X-ray sources

Abstract: The greater information depth provided in Hard X-ray Photoelectron Spectroscopy (HAXPES) enables non-destructive analyses of the chemistry and electronic structure of buried interfaces. Moreover, for industrially relevant elements like Al, Si and Ti, the combined access to the Al 1s, Si 1s or Ti 1s photoelectron line and its associated Al KLL, Si KLL or Ti KLL Auger transition, as required for local chemical state analysis on the basis of the Auger parameter, is only possible with hard X-rays. Until now, such photoemission studies were only possible at synchrotron facilities. Recently however, the first commercial XPS/HAXPES systems, equipped with both soft and hard X-ray sources, have entered the market, providing unique opportunities for monitoring the local chemical state of all constituent ions in functional oxides at different probing depths, in a routine laboratory environment. Bulk-sensitive shallow core-levels can be excited using either the hard or soft X-ray source, whereas more surface-sensitive deep core-level photoelectron lines and associated Auger transitions can be measured using the hard X-ray source. As demonstrated for thin Al2O3, SiO2 and TiO2 films, the local chemical state of the constituting ions in the oxide may even be probed at near constant probing depth by careful selection of sets of photoelectron and Auger lines, as excited with the combined soft and hard X-ray sources. We highlight the potential of lab-based HAXPES for the research on functional oxides and also discuss relevant technical details regarding the calibration of the kinetic binding energy scale.

An investigation of Fe-doped ZnO thin films grown by magnetron sputtering

Abstract: We report on the structural, chemical and optoelectronic properties of Fe-doped ZnO films deposited by dc magnetron sputtering. The effects of sputtering and post-growth thermal annealing conditions on structural and optical properties of ZnO films with different Fe contents were investigated by x-ray diffraction, XPS and photoluminescence techniques. The results are discussed taking into consideration the chemical state of the incorporated iron and altered balance of the intrinsic defect concentration.

Restructuring of the Pt3Sn(111) surfaces induced by atomic and molecular oxygen from first principles

Abstract: The surface restructuring of Pt(3)Sn(111) induced by oxygen chemisorption is examined by means of density-functional theory calculations. Molecular and atomic oxygen chemisorption is investigated on the two available terminations of the bulk alloy--(2 x 2) and (square root(3) x square root(3))R30 degrees--these two surfaces differing by the tin content and the nature of chemical sites. An extensive geometric, energetic, and vibrational analysis is performed including the influence of oxygen coverage in the case of atomic adsorption. For molecular adsorption, regular structures have been obtained for both surfaces with a clear effect of tin on the stability of the adsorption forms. In contrast, for atomic adsorption, two oxygen chemical states are found. In particular, a peculiar surface restructuring, involving the formation of a network of SnO(2) species, appears for large oxygen coverage. However the two terminations present discrepancies for the restructuring mechanism all along the oxygen coverage increase. All these results are supported by a systematic vibrational analysis.

Structural and chemical state of doped and impregnated mesoporous Ni/CeO2 catalysts for the water-gas shift

Abstract: Mesoporous Ni/CeO2 catalysts of variable loadings were prepared using in-situ doping and impregnation synthesis techniques The catalysts were found to exhibit activity for the water-gas shift (WGS) reaction, particularly at temperatures above 250 °C Structural, electronic, and surface chemical characterizations of the materials were carried out using in-situ X-ray diffraction (XRD), in-situ X-ray absorption (XANES), and in-situ infrared (DRIFTS) techniques The effects of metal loading and preparation method on these properties were studied in order to develop a more complete understanding of the design and application of Ni-loaded mesoporous CeO2 catalysts For WGS reaction activity, the in-situ doping method was observed to be superior, and overall activity was observed to increase with increasing metal loadings Simple normalization of activity data to nominal nickel content revealed a trend favoring lower loadings, indicating higher activity per unit nickel The reduction of the catalyst is observed with increasing reaction temperature (Ni2+ → Ni°, Ce4+ → Ce3+) while the active states of all catalysts were identified as a stable, partially reduced ceria fluorite lattice (Ce4+/Ce3+) with Ni2+ and Ni° In Situ DRIFTS showed nearly identical surface chemistry for both doped and impregnated samples, likely involving an associative pathway at lower temperatures and a redox pathway at higher temperatures Structural properties and surface chemistry were observed to depend both on metal loading and preparation method Nickel loadings as low as 1 wt% prepared by in-situ doping were found to display the most favorable metal-support interactions for the WGS reaction