Surface Structure of V₂O₃(0001): A Combined I/V-LEED and STM Study

Using I/V-LEED and scanning tunneling microscopy, we have investigated the surface structure of ∼100 Å thick V₂O₃(0001) films on Au(111). Both methods clearly show that the surface is terminated by a layer of vanadyl groups. I/V-LEED quantitative structure determination applied to differently prepared films always leads to a Pendry <i>R</i>-factor for the VO termination close to 0.11 while the <i>R</i>-factor for a reconstructed O₃ termination is always larger than 0.2 and increases with increasing data set size. These results are at variance with a recent publication by Window et al. [Phys. Rev. Lett. 2015, 114, 216101] in which the authors propose that the V₂O₃(0001) surface is terminated by a reconstructed O₃ structure. Surface oxidation experiments also contradict the conclusions of Window et al. since oxidation leads to a previously identified structure with a (√3 × √3)<i>R</i>30° LEED pattern which is not expected for oxidation of an O3-terminated surface. In the course of the I/V-LEED calculations the individual Debye temperatures of the surface atoms were determined as part of the structural optimization procedure. We show that this approach is superior to the kinematical analysis of temperature-dependent LEED measurements

Model building analysis : a novel method for statistical evaluation of Pt L3-edge EXAFS data to unravel the structure of Pt-alloy nanoparticles for the oxygen reduction reaction on highly oriented pyrolytic graphite

Extended X-ray absorption fine structure (EXAFS) is a powerful tool to determine the local structure in Pt nanoparticles (NP) on carbon supports, active catalysts for fuel cells. Highly oriented pyrolytic graphite (HOPG) covered with Pt NP gives samples with flat surfaces that allow application of surface science techniques. However, the low concentration of Pt makes it difficult to obtain good quality EXAFS data. We have performedin situhighly sensitive BCLA-empowered Back Illuminated EXAFS (BCLA + BI-EXAFS) measurements on Pt alloy nanoparticles. We obtained high quality Pt L-3-edge data. We have devised a novel analytical method (model building analysis) to determine the structure of multi-component nanoparticles from just a single absorption edge. The generation of large numbers of structural models and their comparison with EXAFS fits allows us to determine the structures of Pt-containing nanoparticles, catalysts for the oxygen reduction reaction. Our results show that PtCo, PtCoN and AuPtCoN form a Pt-shell during electrochemical dealloying and that the ORR activity is directly proportional to the Pt-Pt bond length

Development of Surface Fluorescence X‐Ray Absorption Fine Structure Spectroscopy Using a Laue‐Type Monochromator

Surface fluorescence X-ray absorption fine structure (XAFS) spectroscopy using a Laue-type monochromator has been developed to acquire structural information about metals with a very low concentrate on a flat highly oriented pyrolytic graphite (HOPG) surface in the presence of electrolytes. Generally, surface fluorescence XAFS spectroscopy is hindered by strong scattering from the bulk, which often chokes the pulse counting detector. In this work, we show that a bent crystal Laue analyzer (BCLA) can efficiently remove the scattered X-rays from the bulk even in the presence of solution. We applied the technique to submonolayer (similar to 10(14) atoms cm(-2)) Pt on HOPG and successfully obtained high signal/noise in situ XAFS data in combination with back-illuminated fluorescence XAFS (BI-FXAFS) spectroscopy. This technique allows in situ XAFS measurements of flat electrode surfaces to be performed in the presence of electrolytes