Low temperature synthesis and formation mechanism of carbon encapsulated nanocrystals by electrophilic oxidation of ferrocene

A novel method is developed for low temperature synthesis of carbon encapsulated spherical Fe7S8, equiaxed Fe3O4 and spherical porous FeOOH nanocrystals with a core–shell structure via a solid–solid reaction of ferrocene with (NH4)2S2O8, (NH4)2Cr2O7 and NH4ClO4 in an autoclave, respectively. Samples were characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. It is found that the median size of Fe7S8, Fe3O4 and FeOOH nanocrystals is about 28.99, 52.68 and 25.66 nm, respectively. The hollow worm-like carbon shell provides exclusive rooms for tens to hundreds of nanocrystals separated from each other. The cooperative effect of ammonium and strong oxidizing ion on electrophilic oxidation of ferrocene contributes to the in situ formation of carbon shell on the nanocrystals. The metal–π interaction of iron atom with cyclopentadienone oligomers enables ordered self-assembling of carbon rings, resulting in the formation of graphitizable carbon shell with roughly parallel fringes at low temperature.