Size-dependent crystallinity of nano-Pt/$\gamma $-Al$_{2}$O$_{3}$

Metallic platinum nanoparticles (NP) on $\gamma $-Al$_{2}$O$_{3 }$powders were synthesized with a size range from sub- to several nanometers. High-resolution transmission electron microscopy (HRTEM) studies revealed a size-dependent crystallinity of the Pt NPs, where Pt NPs with size $<$1 nm had a disordered structure, Pt NPs with size $>$2.5 nm all showed a crystalline structure. For Pt NPs with sizes between 1.1 and 2.4 nm, a transition zone exists in which $\sim $85{\%} of NPs appeared disordered and $\sim $15{\%} ordered. X-ray absorption spectroscopy (XAS) measurements support this result where increasing-disorder distribution of Pt-Pt bond lengths was noted with decreasing Pt nanoparticle size. A search for ground state structure of Pt37/$\gamma $-Al$_{2}$O$_{3}$ (100) with density functional theory (DFT) showed that the disordered structure is energetically more favorable than the ordered close-packed structures by 1.53 eV at the size of 1.1 nm.