A Reduction in Particle Size Generally Causes Body-Centered-Cubic Metals to Expand but Face-Centered-Cubic Metals to Contract

Abstract

From a careful analysis of existing data as well as new measurements, we show that the size dependence of the lattice parameters in metal nanoparticles with face-centered cubic (fcc) and body-centered cubic (bcc) symmetries display opposite trends: nanoparticles with fcc structure generally contract with decreasing particle size, while those with bcc structure expand. We present a microscopic explanation for this apparently puzzling behavior based on first-principles simulations. Our results, obtained from a comparison of density functional theory calculations with experimental data, indicate that the nanoparticles are capped by a surface monolayer of oxygen atoms, which is routinely detected by surface-sensitive techniques. The bcc- and fcc-based nanoparticles respond in contrasting fashion to the presence of the oxygen capping layer, and this dictates whether the corresponding lattice parameter would increase or decrease with size reduction. The metal-oxygen bonds at the surface, being shorter and stronger than typical metal-metal bonds, pull the surface metal atoms outward. This outward movement of surface atoms influences the core regions to a larger extent in the relatively open bcc geometry, producing a rather large overall expansion of the cluster, compared to the bulk. In case of fcc clusters, on the other hand, the outward movement of surface metal atoms does not percolate too far inside, resulting in either a smaller net expansion or contraction of the cluster depending on the extent of surface oxygen coverage. Our study therefore provides a convincing physicochemical basis for the correlation between the underlying geometry and the nature of change of the lattice parameters under size reduction.

Synthesis of Platinum Nanoparticles by Plasma Sputtering onto Glycerol: Effect of Argon Pressure on Their Physicochemical Properties

“…(111) interplanar distances (average of 0.224 ± 0.005 nm) are slightly lower than the 0.226 nm value corresponding to the Pt bulk face-centered cubic (fcc) phase. This could correspond to a shrinkage of the crystal lattice because of internal stress as it is observed for fcc clusters at the nanoscale because of high surface stress. , …”

“…This could correspond to a shrinkage of the crystal lattice due to internal stress as it is observed for fcc clusters at the nanoscale due to high surface stress. 46,47 Cyclic voltammograms of Pt NPs/C electrodes recorded in N2-purged 0.50 mol L -1 H2SO4 electrolyte between 0.050 V and 1.450 V vs RHE are presented in Figure 7. The signals for the underpotential adsorption of hydrogen and for its desorption from the platinum surface is visible between 0.050 V and 0.400 V vs RHE for all catalysts and are in agreement with other studies performed on Pt catalyst 48,49 .…”

Hexagonal → Cubic Transition in Ag: Prototype for a General Mechanism for Irreversible Solid–Solid Structural Transformations

“…The physical mechanism for the size-induced stabilization of metastable phases may be understood in terms of surface stresses that become appreciable at small enough sizes and may act as a virtual pressure, which is either positive (compressive) or negative (expansive) . Such a size-dependent effective pressure may turn out to be sufficient to stabilize the metastable species.…”

Binary and ternary Pt-based clusters grown in a plasma multimagnetron-based gas aggregation source: electrocatalytic evaluation towards glycerol oxidation

“…38 For such small clusters, interplanar distances of 0.224 +/-0.003 nm, 0.194 +/-0.002 nm and 0.135 +/-0.002 nm for the (111), ( 200) and (110) diffraction planes, respectively, are lower than those determined for bigger clusters, which could indicate a shrinkage of the crystal lattice due to internal stress as it is observed for fcc clusters at the nanoscopic scale due to their high surface stress. 39,40 Regarding PtCu bimetallic clusters, the TEM micrograph (Fig. 6b-left) shows highly aggregated clusters of 4.5 nm average size, and regions with shorter coherence domains compared to pure Pt clusters.…”