Anomalous lattice dynamics and thermal behavior have been observed for ligand-free, size-, and shape-selected Pt nanoparticles (NPs) supported on nanocrystalline $\gamma {\text{-Al}}_2{\text{O}}_3$ via extended x-ray absorption fine-structure spectroscopy. Several major differences were observed for the NPs with respect to bulk Pt: (i) a contraction in the interatomic distances, (ii) a reduction in the dynamic (temperature-dependent) bond-length disorder and associated increase in the Debye temperature $\left({\theta }_D\right)$, and (iii) an overall decrease in the bond-length expansion coefficient coupled with NP stiffening. The increase in the Debye temperature is explained in terms of the NP size, shape, support interactions, and adsorbate effects. For a similar average size, we observe a striking correlation between the shapes of the NPs and their ${\theta }_D$ values.

• Received 26 August 2010

DOI:https://doi.org/10.1103/PhysRevB.82.155450