Local structural and chemical ordering of nanosized Pt3±δCo probed by multiple-scattering x-ray absorption spectroscopy

Abstract

International audienceThis work reports a detailed investigation of the local structure and chemical disorder of a Pt3±δCo thin film and Pt3±δCo nanoparticles. We have used a combination of techniques including x-ray absorption spectroscopy (XAS), x-ray diffraction (XRD), and high-resolution transmission electron microscopy (TEM). High-quality XAS spectra at the Co K edge and Pt L3 edge have been analyzed using double-edge multiple-scattering data analysis. Structural extended x-ray absorption fine structure (EXAFS) refinements have been performed accounting for the reduction of the coordination numbers and degeneracy of three-atom configurations, resulting from the measured size distribution and stoichiometry. The important effect of chemical ordering on pair and three-atom configurations has been studied using computer simulations based on a simple model accounting for substitutional disorder, defined by an order parameter s. It has been found that individual EXAFS signals related to the minority species (Co) are extremely sensitive to substitutional disorder so their intensities, especially those of the collinear three-atom configurations, can be used as a measure of the ordering level. The thin film has been found to be chemically disordered (s 0.4), in agreement with previous estimates. The Pt3±δCo nanoalloy has been found to be partially ordered (s = 0.6 ± 0.1) while the local structure around Co atoms is characterized by a higher level of structural disorder as compared to the bulk-like thin film. The robust approach for nanomaterial characterization used in this work combining different techniques can, in principle, be applied for structural refinements of any binary nanocrystalline functional system

Evolution of the nanostructure of Pt and Pt–Co polymer electrolyte membrane fuel cell electrocatalysts at successive degradation stages probed by X-ray photoemission

“…Whereas, the obtained during analysis a relative content of Co II equaled 38(2)% (Table 3). This difference shows that small Co (or Co-rich alloy) particles and/or crystalline cobalt oxide are present in studied pristine material, consistent with previous TEM image analysis showing that about 5% of all particles in pristine powder had a diameter smaller than 2 nm [18].…”

“…Other parameters presented: D ave e particles mean diameter as extracted from XRD and TEM data [19,22] (I, II indices e mean value of the first and second peak, respectively, in "bimodal" particles size distribution profile), s e chemical order parameter for a bimetallic alloy (define in Ref. [18], s ¼ 1 indicates perfect chemical order) and calculated on the base of XAFS data [19].…”

Element-specific quantitative determination of the local atomic order in CoPt alloy nanoparticles: Experiment and theory

“…This is the signature of a significant dispersion of the NN distances. 40 From best fits [see Fig. 3(a)] we find that in all cases d CoCo and d CoPt increase upon annealing (see Table I).…”

Operando Time-Resolved X-ray Absorption Fine Structure Study for Surface Events on a Pt3Co/C Cathode Catalyst in a Polymer Electrolyte Fuel Cell during Voltage-Operating Processes

“…These bond distances elucidated that these bonds were attributed to those in the Pt− Co alloy particles and not in discrete Pt or Co particles and agreed with the Pt-skin Pt−Co alloy structure proposed before. 14,36,48,75 The Pt−Co/Co−Pt bond distance implied that the composition of the Pt−Co alloy core was similar to PtCo alloy, and the average observed bond distance of Pt−Pt (0.272 nm >0.267 nm (Pt−Co)) seemed to be obtained from both the PtCo alloy core and the Pt-rich skin.…”