Quantitative Analysis of Size-Dependent Structural Disorder in Ruthenium Nanoparticles by Crystal PDF Full-Space Refinement.

Ruthenium (Ru) nanoparticles (NPs) have attracted significant attention because of their high catalytic activity, which is closely tied to their atomic-scale structural disorder. In this study, we investigate size-dependent structural disorders in both face-centered cubic (fcc) and hexagonal close-packed (hcp) Ru NPs by pair-distribution-function (PDF)-based structural refinement, called crystal PDF full-space refinement. High-energy X-ray total scattering experiments were conducted, and the experimental PDFs were analyzed using an extended unit cell (EUC) model to quantify stacking faults. The results reveal that all Ru NPs, irrespective of their nominal phase, exhibit mixed stacking sequences of fcc and hcp layers. A strong correlation between NP size and stacking fraction was found in fcc-based NPs, whereas hcp-based particles exhibited prominent asymmetry in short-range PDFs, consistent with a higher density of point-defect-like coordination losses. The presence of such defects was supported by peak intensity reduction and lattice contraction trends. Although a direct relationship between point defects and catalytic activity remains unclear, our findings underscore the importance of local atomic arrangements in determining the functionality of Ru NPs. This study demonstrates the utility of crystal PDF full-space refinement for revealing hidden disorders in nanocrystalline systems and lays the groundwork for rational catalyst design through structural control.