Extension of Rietveld Refinement for Benchtop Powder XRD Analysis of Ultrasmall Supported Nanoparticles

We
present a method for characterizing ultrasmall (<2 nm) supported
crystallites with benchtop XRD. Central to the method is an understanding
of the intensity effects at play; these intensity effects and their
corrections are discussed in depth. Background subtractionlong
considered one of the main barriers to ultrasmall crystal characterizationis
solved by correcting the diffractogram of a separately measured support
for the relevant intensity effects. Rietveld refinement is demonstrated
to be an adequate analysis method for the general characterization
of simple nanosystems. A 4.8% Pt/SiO2 sample (1.3 nm, volume-weighted
average) is used as a case study; it is found that the Pt spontaneously
oxidizes under ambient conditions and consists of a metallic core
surrounded by a PtO2 shell. Both phases have average dimensions
smaller than 1 nm. The XRD results also suggest lattice expansion
of the Pt core as compared to bulk Pt.