X-ray Absorption Spectroscopic Studies of the Penetrability of Hollow Iron Oxide Nanoparticles by Galvanic Exchange Reactions

Abstract

Hollow Fe oxide nanoparticles have many applications in catalysis, drug delivery, and energy storage. Hollow Fe oxide shells can also be used for preventing the sintering of catalytically active cores and for magnetic recovery of bimetallic nanoparticles. However, more studies are required under real reaction conditions on the availability of the interior surface or active cores in hollow nanoparticles. Herein, we introduce a simple approach to study the penetrability of hollow Fe oxide shells by attempting galvanic exchange reactions between the remaining Fe(0) core within the hollow Fe oxide shell and Pd(II) salts. First, in situ high-temperature Fe K-edge XANES was used to monitor the formation of hollow Fe oxide nanoparticles from Fe nanoparticles. Core-void-shell Fe-Fe oxide nanoparticle intermediates were captured at different time intervals and then reacted with Pd(II). The reduction of Pd(II) was characterized by in situ Pd L3-edge XANES spectra. The results show that the core-void-shell nanoparticles had Fe3O4 shells, which were found to be impenetrable to Pd(II) salts when the thickness of the shell was more than 2 nm. However, the core could be accessed using a high-temperature etching strategy for the shell, which then allowed for galvanic reactions with Pd.