Issue 11, 2023

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

Abstract

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, DN, on NC size, N (in atoms), controls coarsening kinetics. Traditional mean-field (MF) theory for DNversus N assumes that NC diffusion is mediated by independent random hopping of surface adatoms with low coordination, and predicts that DNhN−4/3neq. Here, h = ν exp[−Ed/(kBT)] denotes the hop rate, and neq = exp[−Eform/(kBT)] the density of those adatoms. The adatom formation energy, Eform, approaches a finite large-N limit, as does the effective barrier, Eeff = Ed + Eform, 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 Eform and neqversus N for our model. MF theory then captures the occurrence of local minima in DNversus 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 Eeff which is well above MF values, and which increases with N, features captured by a beyond-MF treatment.

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

Supplementary files

Article information

Article type
Communication
Submitted
14 Apr 2023
Accepted
31 Jul 2023
First published
01 Aug 2023
This article is Open Access
Creative Commons BY-NC license

Nanoscale Horiz., 2023,8, 1556-1567

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

K. C. Lai, C. T. Campbell and J. W. Evans, Nanoscale Horiz., 2023, 8, 1556 DOI: 10.1039/D3NH00140G

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