Noncrystalline-to-Crystalline Transformations in Pt Nanoparticles

We show that the noncrystalline-to-crystalline
transition of supported
Pt nanoparticles (NPs) in the subnanometer to nanometer size range
is statistical in nature, and strongly affected by particle size,
support, and adsorbates (here we use H2). Unlike in the
bulk, a noncrystalline phase exists and is stable in small NPs, reflecting
a general mesoscopic feature. Observations of >3000 particles by
high-resolution
transmission electron microscopy show a noncrystalline-to-crystalline
transition zone that is nonabrupt; there is a size regime where disordered
and ordered NPs coexist. The NP size at which this transition occurs
is strongly dependent on both the adsorbate and the support, and this
effect is general for late 5d transition metals. All results are reconciled
via a statistical description of particle-support-adsorbate interactions.