Abstract
The size increase of a nanoscale material is commonly associated with the increased stability of its ordered phases. Here we give a counterexample to this trend by considering the formation of the defect-free L11 ordered phase in AgPt nanoparticles, and showing that it is better stabilized in small nanoparticles (up to 2.5 nm) than in larger ones, in which the ordered phase breaks in multiple domains or is interrupted by faults. The driving force for the L11 phase formation in small nanoparticles is the segregation of a monolayer silver shell (an Ag-skin) which prevents the element with higher surface energy (Pt) from occupying surface sites. With increasing particle size, the Ag-skin causes internal stress in the L11 domains which cannot thus exceed the critical size of ~2.5 nm. A multiscale modelling approach using full-DFT global optimization calculations and atomistic modelling is used to interpret the findings.
Highlights
The size increase of a nanoscale material is commonly associated with the increased stability of its ordered phases
As we show in the following, the L11 phase can be obtained in AgPt nanoparticles, as the result of a subtle interplay of different physical effects
For 2.3 × 1015 at cm−2, transmission electron microscopy (TEM) images show an assembly of isolated NPs with mean diameter D = 1.5 nm
Summary
The size increase of a nanoscale material is commonly associated with the increased stability of its ordered phases. 1234567890():,; Increasing the size of a nanoscale material is commonly associated to a widening of the stability domain of its ordered phases at equilibrium[1]. We show that this is not a general rule, since an increase of the system size can have the opposite effect: in small AgPt clusters, up to D ~ 2.5 nm, a wellordered intermetallic phase is stabilized at equilibrium, but, above this size, either the ordered phase breaks down into less-ordered multidomains, or its regular arrangement is interrupted by faults. A crucial point is the catalytic efficiency in terms of durability This implies that the problem of structural evolution of the nanoparticles must be addressed when dealing with applications. In the following we experimentally show that nanoparticles with L11 ordered domains are obtained for sizes up to ~2.5 nm
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