Effects of surface disorder on EXAFS modeling of metallic clusters

Abstract

Small ͑1-5 nm͒ metal clusters may undergo significant surface relaxation under the influence of ligands, adsorbates, and substrate-induced stress. As a result, the nearest-neighbor distance between surface atoms can be reduced by up to 10% relative to those in the cluster core, enhancing the disorder in the interatomic distances. Accordingly, the pair distribution function extracted from EXAFS data under the standard assumption that the distribution function of nearest-neighbor bonds is quasi-Gaussian yields systematic errors. Here we analyze the surface disorder effects with emphasis on their impact on the accuracy of the size and shape determination of nanocatalysts.

A theoretical and experimental examination of systematic ligand-induced disorder in Au dendrimer-encapsulated nanoparticles

“…In this case, the interplay of theory and experiment provides a means to extract more accurate structural information from experimental EXAFS signals and to reduce the effects of tting artifacts that arise in disordered systems. 1,3 For example, one important limitation of EXAFS analysis is that it provides averaged information about the structure and dynamics of the nearest environment of all absorbing atoms in the nanoparticle ensemble, and this can lead to erroneous structural determinations if an "average" conguration is assumed to be the actual conguration. Specically, if the distribution of sizes, shapes, crystal structures, and states of order is broad, 4-6 then the notion of a "representative" nanoparticle is unhelpful at best, or, as recently shown, can lead to misinterpretation.…”

“…[1][2][3] One analytical method that is particularly amenable to combined theoretical and experimental studies is extended X-ray absorption ne structure (EXAFS) spectroscopy, and particularly its application to the analysis of small metallic nanoparticles. In this case, the interplay of theory and experiment provides a means to extract more accurate structural information from experimental EXAFS signals and to reduce the effects of tting artifacts that arise in disordered systems.…”

Structural and Magnetic Properties of Amorphous Co–W Alloyed Nanoparticles

“…However, a quantitative determination of this W metal fraction was prevented given the intrinsic difficulties of fitting EXAFS signals for amorphous systems, as mentioned above. 50,51 In spite of it, W L 3 EXAFS results show interesting trends as shown in Fig 10(a), where it is noticeable that the shoulder corresponding to the metallic W contribution increases as the amount of W increases in the sample (a series). A similar trend was also observed in their fitting results and is well in agreement to what was observed by HRTEM.…”

“…A reduction from N = 12 in the metallic Co to N = 6.1 in the first coordination shell of the bare Co nanoparticles was obtained from this fit. However, recent works have demonstrated that such a strong reduction in the number of first neighbors could be an artifact of EXAFS analysis, 50 due to effects of surface distortions on the nanoparticles, producing an underestimation of the average coordination number.…”

“…Moreover, as it was mentioned above, strong structural disorder in the NPs systems can lead to erroneous interpretation of the EXAFS analysis. 50,51 Then, fitting of the Co-W NPs EXAFS signal was performed on the R space in a range from 1.0-3.0Å using a hanning window function, so that it covered the Co first coordination shell in the alloy (see Fig. 6).…”