Local ordering of nanostructured Pt probed by multiple-scattering XAFS

Abstract

We present detailed results of a multiple-scattering (MS) extended x-ray absorption fine structure (EXAFS) data analysis of crystalline and nanocrystalline platinum. Advanced MS EXAFS analysis has been applied to raw x-ray absorption data including the background, using the expansion of the absorption cross section in terms of local two-body and three-body configurations. Present EXAFS results on bulk Pt are found to be in agreement with previous structural and vibrational data, and has been used as a reference for reliable structural refinement of nanosized systems. EXAFS structural refinement of Pt nanoparticles has been performed in combination with electron microscopy and x-ray diffraction, showing the importance of considering the actual size distribution and morphology of the samples. Present samples were unsupported and supported Pt nanocrystalline systems with size distributions showing clusters of quasispherical shape in the 1-7 nm range. In particular, EXAFS spectra have been analyzed accounting for the reduction of the coordination number and degeneracy of three-body configurations, resulting from the measured size distribution and expected surface atom contributions. The importance of a correct account of the reduction of the number of neighbors for calculating MS contributions is emphasized in the paper. EXAFS results have been found compatible with x-ray diffraction and transmission electron microscopy investigations. We estimate that EXAFS could be used to study cluster shapes only for sizes below 2 nm using present methods and quality of the experimental data. We have also shown that the local distribution of distances and angles probed by EXAFS is broader than in bulk Pt, with first-neighbor bond length variance and asymmetry increasing upon reducing the particle size. Methods and results presented in this paper have been found to be successful for a robust structural refinement of monatomic nanocrystalline systems and represents a solid starting point for analyzing subtle structural and dynamical local changes occurring during in situ experiments involving nanomaterials for specific applications like supported nanocatalysts

Characterization of Nanomaterials in Electrochemistry

“…Results obtained by TEM and XRD demonstrate that the Pt-Co/C sample can be treated as constituted by a homogeneous ensemble of Pt 3AEd Co fcc nanocrystalline particles with a mean diameter of~5 nm. Moreover, the average local structure of Pt-Co/C can be assumed very similar to the bulk crystalline Pt 3 Co (as for Pt nanoparticles [37]) confirmed by XAS, TEM, and XRD results.…”

“…Moreover, nanoparticle size affects also the total CNs, so reduction (first neighbor CN is~11 instead of 12 for particles with a mean size about 5 nm) due to the finite size effect is taken into account using other simulations performed for fcc nanoparticles [37]. All of these constraints guarantee a robust structural refinement for double-edge EXAFS data analysis of the nanocrystalline Pt 3AEd Co/C sample, using a total of 15 structural parameters over around 1000 experimental points and including all of the MS signals up to the fourth coordination shell [24].…”

Local structure relaxation in nanosized tungstates

“…The unit-cell volume compression is common for metal nanoparticles [2,3], whereas its expansion occurs in most nanosized metal-oxides [4,5]. Since physical properties of nanoparticles are determined by their structure, an understanding of nanoscale lattice modifications is advantageous for properties tuning but is a challenging task [6,7,8,9].…”

Temperature and potential-dependent structural changes in a Pt cathode electrocatalyst viewed by in situ XAFS

“…Details of the analytical procedure, presented and discussed in Ref. [19], may be summarized as follows.…”

“…In situ measurements in the transmission mode have demonstrated the possibility of obtaining high-quality XAFS spectra for a wide range of X-ray energies and metal loading [16,17]. Detailed multiple-scattering (MS) XAFS analysis can be applied to low-noise data of nanosized systems [18,19] in order to obtain information about local average geometry.…”