Authors

B. R. Cuenya; M. A. Ortigoza; L. K. Ono; F. Behafarid; S. Mostafa; J. R. Croy; K. Paredis; G. Shafai; T. S. Rahman; L. Li; Z. Zhang;J. C. Yang

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Abbreviated Journal Title

Phys. Rev. B

Keywords

NEGATIVE THERMAL-EXPANSION; DEBYE-WALLER FACTORS; PLATINUM; NANOPARTICLES; VIBRATIONAL PROPERTIES; MICELLE ENCAPSULATION; CLUSTERS; SURFACE; NANOCRYSTALS; SPECTROSCOPY; TEMPERATURE; Physics, Condensed Matter

Abstract

This study presents a systematic investigation of the thermodynamic properties of free and gamma-Al2O3-supported size-controlled Pt nanoparticles (NPs) and their evolution with decreasing NP size. A combination of in situ extended x-ray absorption fine-structure spectroscopy (EXAFS), ex situ transmission electron microscopy (TEM) measurements, and NP shape modeling revealed (i) a cross over from positive to negative thermal expansion with decreasing particle size, (ii) size- and shape-dependent changes in the mean square bond-projected bond-length fluctuations, and (iii) enhanced Debye temperatures (D-circle minus, relative to bulk Pt) with a bimodal size- dependence for NPs in the size range of similar to 0.8-5.4 nm. For large NP sizes (diameter d > 1.5 nm) D-circle minus was found to decrease toward D-circle minus of bulk Pt with increasing NP size. For NPs < = 1 nm, a monotonic decrease of D-circle minus was observed with decreasing NP size and increasing number of low-coordinated surface atoms. Our density functional theory calculations confirm the size- and shape-dependence of the vibrational properties of our smallest NPs and show how their behavior may be tuned by H desorption from the NPs. The experimental results can be partly attributed to thermally induced changes in the coverage of the adsorbate (H-2) used during the EXAFS measurements, bearing in mind that the interaction of the Pt NPs with the stiff, high-melting temperature gamma-Al2O3 support may also play a role. The calculations also provide good qualitative agreement with the trends in the mean square bond-projected bond-length fluctuations measured via EXAFS. Furthermore, they revealed that part of the D-circle minus enhancement observed experimentally for the smallest NPs (d < = 1 nm) might be assigned to the specific sensitivity of EXAFS, which is intrinsically limited to bond-projected bond-length fluctuations.

Journal Title

Physical Review B

Volume

84

Issue/Number

24

Publication Date

1-1-2011

Document Type

Article

Language

English

First Page

14

WOS Identifier

WOS:000298561800003

ISSN

1098-0121

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