Size-dependent diffusion of supported metal nanoclusters: mean-field-type treatments and beyond for faceted clusters

Nanostructured systems are intrinsically metastable and subject to coarsening. For supported 3D metal nanoclusters (NCs), coarsening can involve NC diffusion across the support and subsequent coalescence (as an alternative to Ostwald ripening). When used as catalysts, this leads to deactivation. The...

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Published inNanoscale horizons Vol. 8; no. 11; pp. 1556 - 1567
Main Authors Lai, King C, Campbell, Charles T, Evans, James W
Format Journal Article
LanguageEnglish
Published England Royal Society of Chemistry 23.10.2023
Royal Society of Chemistry (RSC)
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ISSN2055-6756
2055-6764
2055-6764
DOI10.1039/d3nh00140g

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Summary:Nanostructured systems are intrinsically metastable and subject to coarsening. For supported 3D metal nanoclusters (NCs), coarsening can involve NC diffusion across the support and subsequent coalescence (as an alternative to Ostwald ripening). When used as catalysts, this leads to deactivation. The dependence of diffusivity, D N , on NC size, N (in atoms), controls coarsening kinetics. Traditional mean-field (MF) theory for D N versus N assumes that NC diffusion is mediated by independent random hopping of surface adatoms with low coordination, and predicts that D N ∼ hN −4/3 n eq . Here, h = ν  exp[− E d /( k B T )] denotes the hop rate, and n eq = exp[− E form /( k B T )] the density of those adatoms. The adatom formation energy, E form , approaches a finite large- N limit, as does the effective barrier, E eff = E d + E form , for NC diffusion. This MF theory is critically assessed for a realistic stochastic atomistic model for diffusion of faceted fcc metal NCs with a {100} facet epitaxially attached to a (100) support surface. First, the MF formulation is refined to account for distinct densities and hop rates for surface adatoms on different facets and along the base contact line, and to incorporate the exact values of E form and n eq versus N for our model. MF theory then captures the occurrence of local minima in D N versus N at closed-shell sizes, as shown by KMC simulation. However, the MF treatment also displays fundamental shortcomings due to the feature that diffusion of faceted NCs is actually dominated by a cooperative multi-step process involving disassembling and reforming of outer layers on side facets. This mechanism leads to an E eff which is well above MF values, and which increases with N , features captured by a beyond-MF treatment. Size-dependent diffusion of supported faceted nanoclusters is mediated by disassembly & reassembly of outer layers of facets. A mean-field picture (random independent motion of surface atoms) fails to capture behavior.
Bibliography:https://doi.org/10.1039/d3nh00140g
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USDOE
AC02-07CH11358; FG02-96ER14630
IS-J-11,147
USDOE Office of Science (SC), Basic Energy Sciences (BES). Chemical Sciences, Geosciences & Biosciences Division (CSGB)
ISSN:2055-6756
2055-6764
2055-6764
DOI:10.1039/d3nh00140g