Structural and surface characterizations of nanocrystalline Pt-Ru alloys prepared by high-energy ball-milling

摘要

The bulk and surface characteristics of Pt-Ru alloysprepared by high-energy ball-milling were studied over the wholecompositional range. These materials were prepared in their as-milled and Mg-dispersed forms. The latter form is obtained bymilling Mg with nanocrystalline Pt-Ru alloy prepared in a first stepand then leaching out Mg in an acidic solution. The structure ofthe various alloys was determined by X-ray diffraction.Quantitative structural information was extracted from the X-rayhistograms through Rietveld refinement analysis. In the as-milledform, a nanocrystalline fcc or a hcp structure is obtained aftermilling (40 h) of an initial powder mixture whose compositionfalls into the Pt- or Ru-rich side of the compositional range,respectively. For mixtures whose composition falls into theimmiscibility gap, a metastable amorphous phase is formed, whichco-exists with a fcc or a hcp structure, depending on which side ofthe immiscibility gap the composition is. The structure of Mg-dispersed Pt-Ru alloys is identical to that of the as-milledmaterials. The chemical composition and structure of the alloysurface was determined by X-ray photoelectron spectroscopy.From measurements of the Ru 3p and Pt 4f core level peaks, it isshown that the surface composition of both as-milled and Mg-dispersed alloys closely follow their bulk composition, with only aslight enrichment in Pt. There is also no change in the surfacecomposition resulting from the extra milling step with Mg and itssubsequent leaching. The separation between the maximum of theRu 3d _(5/2) and Pt 4f _(7/2) core level peaks varies with thecomposition of the alloy, indicating that a true surface alloy isformed between Pt and Ru.