Extension of Rietveld Refinement for Benchtop Powder XRD Analysis of Ultrasmall Supported Nanoparticles
Jeremiah Lipp, Ritubarna Banerjee, Md. Fakhruddin Patwary, Nirmalendu Patra, Anhua Dong, Frank Girgsdies, Simon R. Bare, John R. Regalbuto
Abstract
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.