Abbreviated Journal Title
Phys. Rev. B
NEGATIVE THERMAL-EXPANSION; DEBYE-WALLER FACTORS; PLATINUM; NANOPARTICLES; VIBRATIONAL PROPERTIES; MICELLE ENCAPSULATION; CLUSTERS; SURFACE; NANOCRYSTALS; SPECTROSCOPY; TEMPERATURE; Physics, Condensed Matter
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.
Physical Review B
Cuenya, B. Roldan; Ortigoza, M. Alcántara; Ono, L. K.; Behafarid, F.; Mostafa, S.; Croy, J. R.; Paredis, K.; Shafai, G.; Rahman, T. S.; Li, L.; Zhang, Z.; and Yang, J. C., "Thermodynamic properties of Pt nanoparticles: Size, shape, support, and adsorbate effects" (2011). Faculty Bibliography 2010s. 1220.