Local Structure, Electronic Behavior, and Electrocatalytic Reactivity of CO-Reduced Platinum–Iron Oxide Nanoparticles

A series of platinum–iron oxide nanoparticles was synthesized using a “clean” CO-reduction method that employed different ratios of Pt−Fe precursor salts in oleylamine at elevated temperatures. High-resolution transmission electron microscopy (HRTEM) and energy-dispersive X-ray spectroscopy (EDS) studies revealed that nearly monodisperse (i.e., with relative standard deviations of less than 15%) nanoparticles with mean diameters of 3.5–4.4 nm and varied elemental compositions (Pt 54 Fe 46 Pt 70 Fe 30, and Pt 87 Fe 13 ) were obtained. X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements at the Pt L 3 - and Fe K-edges revealed that these nanoparticles all consisted of a Pt core with amorphous iron oxide on the surface. Furthermore, it was observed that the local structure (e.g., Pt–Pt bond distance and coordination number) and electronic behavior of the Pt–FeO nanoparticles (e.g., Pt d electron density and Fe valence state) are dependent on the Pt−Fe precursor ratios used in their synthesis. Quantum mechanical ab initio calculations were employed to interpret the results from X-ray spectroscopy and help elucidate the relationships between local structure and electronic properties in the nanoparticle samples. Finally, the surface reactivity of these nanoparticles in the oxygen reduction reaction (ORR) was explored, demonstrating higher electrocatalytic activity for all three platinum–iron oxide samples in comparison with a commercial Pt catalyst. The surface reactivity was also found to be sensitive to the Pt−Fe ratios of the nanoparticles and could be correlated with their local structure and electronic behavior.