Aging study on the structure of Fe⁰-nanoparticles: Stabilization, characterization, and reactivity

Inert/pseudoinert gases, including argon, nitrogen, and carbon dioxide, were utilized to stabilize synthesized Fe⁰-nanoparticles after lyophilization to prevent self-ignition. In addition, the aging effect was investigated for these stabilized Fe⁰-nanoparticles both in humid and dry conditions. Particles' shapes, sizes, and structures were characterized for these fresh and aged Fe⁰-nanoparticles using X-ray diffraction (XRD), X-ray photoelectron spectroscopy, Brunauer-Emmett-Teller surface area and porosity analyzer, transmission electronic microscopy, and energy dispersive X-ray spectroscopy (EDX). Even when aged Fe⁰-nanoparticles were exposed to the atmosphere, the Fe⁰ content in these aged Fe ⁰-nanoparticles did not change significantly, which was confirmed by XRD, EDX, and HCl digestion methods. Reactivity of the fresh Fe ⁰-nanoparticles stabilized using inert/pseudoinert gases for bromate reduction was more than 99% in 20 min, much higher than for microsized ZVI. However, for the aged Fe⁰-nanoparticles, the reactivity decreased as the aging time increased; the reactivity of Fe⁰-nanoparticles stored in humid conditions decreased much more than that of Fe⁰- nanoparticles stored in dry conditions. The observed results revealed that recovery and recrystallization occurred in the aged Fe⁰-nanoparticles at room temperature instead of the traditional theory that recrystallization and annealing occur at a high temperature; in additional, recovery and recrystallization are the real mechanisms of the loss of reactivity for aged Fe⁰-nanoparticles instead of oxidation.