Mixing Patterns and Redox Properties of Iron-Based Alloy Nanoparticles under Oxidation and Reduction Conditions

The redox behavior of 5 nm Fe-Me alloyed nanoparticles (where Me = Pt, Au, and Rh) was investigated in situ under H2 and O2 atmospheres by near ambient pressure X-ray photoelectron and absorption spectroscopies (NAP-XPS, XAS), together with ex situ transmission electron microscopy (TEM) and XAS spectra simulations. The preparation of well-defined Fe-Me nanoalloys with an initial size of 5 nm was achieved by using the mass-selected low energy cluster beam deposition (LECBD) technique. The spectroscopic methods permit the direct observation of the surface segregation and composition under different gas atmospheres and annealing temperatures. The ambient conditions were found to have a significant influence on the mixing pattern and oxidation state of the nanoparticles. In an oxidative atmosphere, iron oxidizes and segregates to the surface, leading to the formation of core–shell nanoparticles. This structure persists upon mild reduction conditions, while phase separation and formation of heterostructured bimetallic particles is observed upon H2 annealing at a higher temperature (400 °C). Depending on the noble metal core, the iron oxide shell might be partially distorted from its bulk structure, while the reduction in H2 is also significantly influenced. These insights can be of a great importance in understanding the activity and stability of Fe-based bimetallic nanoparticles under reactive environments