XRD spectra of fresh iron NPs (a), iron oxide NPs synthesized by air oxidation of Fe0 for: (b) 0.25 h, (c) 0.5 h, (d) 1 h, and aged iron NPs (e). The peaks shown in diffraction patterns correspond as follows: for (a) and (e), Miller indices (110), (200), and (211) of a-Fe (Powder Diffraction File No., PDF#: 00-006-0696); for (b), (c), and (e), Miller indices (110) and (300) of ferrihydrite (PDF#: 01-073-8408). For the rest of the peaks, other iron oxides such as, lepidocrocite (PDF#: 00-003-0079) and wustite (PDF#: 00-006-0615)