Anomalous Vibrational Properties Induced by Surface Effects in Capped Pt Nanoparticles

Abstract

The Debye temperature of Pt nanoparticles (NP) capped in PVP-K30 with an average diameter of 3 nm was determined from extended X-ray absorption fine structure measurements between 20 and 373 K. The size distribution of the NP was determined by high-resolution transmission electron microscopy, high-angle annular dark field, and small-angle X-ray scattering. We have experimentally established that the Debye temperature of truncated cuboctahedron, capped Pt NP is 70 K higher than that of bulk Pt metal. The rather high value of the Debye temperature of nanosized-capped Pt crystals is related to surface Pt−Pt contraction effects in conjunction with capping interactions. This finding evidences that the vibrational behavior of nanostructure materials differs markedly from that of materials in bulk form.

Anomalous lattice dynamics and thermal properties of supported size- and shape-selected Pt nanoparticles

“…agreement with literature reports. 19 Surprisingly, significantly higher Debye temperatures ͑262-284 K͒ were observed for samples S1, S2, and S4 ͑ϳ1 nm in diameter͒. However, a similar D value to that of the bulk Pt was obtained for the sample with the smallest NPs ͑S3, ϳ0.8 nm, D = 250Ϯ 5 K͒.…”

“…In free clusters, enhanced surface energy and the concomitant increase in the surface stress causes a lattice contraction at the NP surface, softening of interatomic force constants, and a suppression of D and melting temperatures. 1 On the other hand, the increased D observed for certain systems has been attributed to structural stiffening and inhomogeneous internal stress correlated with the NP size, surroundings ͑e.g., adsorbates/ surface ligands, 15,16,19 a matrix encapsulating the NPs or a support 12,13 ͒, and to the presence of structural defects and multiple grains in large NPs. 14 It is evident that the understanding of thermal, structural, and electronic properties of these systems is hindered by their complexity, due to the multiple competing factors that define their behavior, and any effort in sorting out these influences will be helpful for developing first-principles theories.…”

Thermodynamic properties of Pt nanoparticles: Size, shape, support, and adsorbate effects

“…For larger NPs (> 200 atoms), monotonic size-dependent trends in T m were observed. 11 For a given material system, the specific thermal behavior was found to be drastically affected by environmental influences such as the presence of a support, an encapsulating matrix, the internal defect density within a NP, the structural and chemical nature of the NP/support interface, and the presence of ligands or surface adsorbates [12][13][14][15][16][17] .…”

Hierarchy of bond stiffnesses within icosahedral-based gold clusters protected by thiolates

“…In other words, a crucial factor that governs thermal behaviours is the hierarchy of the bonding within NPs; metal–metal bonds on the surface are generally softer than those within the core 3 7 8 . The thermal behaviours of metal NPs are not only affected by their size, but also by the atomic packing of the core 3 7 8 9 10 11 12 13 14 15 16 and the interaction with the environment, such as an organic ligand 17 or solid support 3 18 19 . For example, the vibrational spectrum of a metal NP with an icosahedral (Ih) structure has a component with a vibrational frequency that is higher than those of other structures such as cuboctahedra and decahedra, which suggests the formation of stiff bonds within the NPs 7 .…”

“…For example, the vibrational spectrum of a metal NP with an icosahedral (Ih) structure has a component with a vibrational frequency that is higher than those of other structures such as cuboctahedra and decahedra, which suggests the formation of stiff bonds within the NPs 7 . The influence of surface adsorbates on the thermal properties of metal NPs has been ascribed to a change in the bond stiffness; Pt–Pt bonds of small Pt NPs are softened by hydrogen adsorption, but stiffened by oxidation 19 or capping with a polymer 17 . However, an atomic-level understanding of how the bond stiffnesses are affected by a variety of structural parameters has not been attained to date because of the experimental difficulties in defining the atomic packing of a metal NP and the interfacial structure with the surrounding environment.…”