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

Abstract

This study presents a systematic investigation of the thermodynamic properties of free and \ensuremath{\gamma}-Al${}_{2}$O${}_{3}$-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 (\ensuremath{\Theta}${}_{D}$, relative to bulk Pt) with a bimodal size-dependence for NPs in the size range of \ensuremath{\sim}0.8--5.4 nm. For large NP sizes (diameter $d$ >1.5 nm) \ensuremath{\Theta}${}_{D}$ was found to decrease toward \ensuremath{\Theta}${}_{D}$ of bulk Pt with increasing NP size. For NPs \ensuremath{\le} 1 nm, a monotonic decrease of \ensuremath{\Theta}${}_{D}$ 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 \ensuremath{\gamma}-Al${}_{2}$O${}_{3}$ 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 \ensuremath{\Theta}${}_{D}$ enhancement observed experimentally for the smallest NPs ($d$ \ensuremath{\le} 1 nm) might be assigned to the specific sensitivity of EXAFS, which is intrinsically limited to bond-projected bond-length fluctuations.