Structures of Pd-Fe(0) bimetallic nanoparticles near 0.1 nm resolution

Palladized nanoscale zero-valent iron (Pd-nZVI) has attracted significant attention due to its enhanced capability to transform halogenated hydrocarbons, which are among the most frequently detected contaminants in groundwater. In this study, the structural evolution of Pd-nZVI (1.0% Pd) was examined using spherical aberration-corrected scanning transmission electron microscopy (Cs-STEM) in combination with X-ray energy dispersive spectroscopy (EDS). This method with sub-nano spatial resolution was applied to map the location and distribution of Pd on the nZVI surface, and generated the most detailed images of Pd-nZVI. Results show that fresh Pd-nZVI consists of a metallic iron core and a thin oxide shell, with Pd scattered as discrete clusters of 2-4 nm particles on the nZVI surface. High-magnification images further confirm that the Pd nanoparticles have a highly ordered core enclosed by a disordered outer layer. With the progress of iron oxidation, the Pd clusters disappear from the surface while new deposits of iron oxides with varied morphologies appear. The bright cores in the HAADF images, which correspond to metallic iron, become smaller while the outer shell expands with time. After aging in water for 120 h, Pd-nZVI particles are depleted with metallic iron, totally oxidized and recrystallized. A physical model of Pd-nZVI is presented.