The effects of vacuum annealing on the structure and surface chemistry of iron:nickel alloy nanoparticles

In order to increase the longevity of contaminant retention on the particle surface, a method is sought to improve the corrosion resistance of bimetallic iron nickel nanoparticles (INNP) used for the remediation of contaminated water, and thereby extend their industrial lifetime. A multi-disciplinary approach was used to investigate changes induced by vacuum annealing (<5 × 10⁻⁸ mbar) at 500 °C on the bulk and surface chemistry of INNP. The particle size was determined to increase significantly as a result of annealing and the thickness of the surface oxide increased by 50%. BET analysis recorded a decrease in INP surface area from 44.88 to 8.08 m² g⁻¹, consistent with observations from scanning electron microscopy (SEM) and transmission electron microscopy (TEM) which indicated the diffusion bonding of previously discrete particles at points of contact. X-ray diffraction (XRD) confirmed that recrystallisation of the metallic cores had occurred, converting a significant fraction of initially amorphous iron nickel alloy into crystalline FeNi alloy. X-ray photoelectron spectroscopy (XPS) indicated a reduction in the proportion of surface iron oxide and a change in its stoichiometry related to annealing-induced disproportionation. This was also evidenced by an increased proportion of Fe(0) and Ni(0) to Fe- and Ni-oxides, respectively. The data also indicated the concurrent development of boron oxide at the metal surfaces, which accounts for the overall increase measured in surface oxide thickness. The improved core crystallinity and the presence of passivating impurity phases at the INNP surfaces may act to improve the corrosion resistance and reactive lifespan of the vacuum annealed INNP for environmental applications.