Low temperature in situ synthesis and the formation mechanism of various carbon-encapsulated nanocrystals by the electrophilic oxidation of metallocene complexes.

The core–shell nanostructures have the advantages of combining distinctive properties of varied materials and improved properties over their single-component counterparts. Synthesis approaches for this class of nanostructures have been intensively explored, generally involving multiple steps. Here, a general and convenient strategy is developed for one-step in situ synthesis of various carbon-encapsulated nanocrystals with a core–shell structure via a solid-state reaction of metallocene complexes with (NH₄)₂S₂O₈ in an autoclave at 200 °C. A variety of near-spherical and equiaxed nanocrystals with a small median size ranging from 6.5 to 50.6 nm are prepared as inner cores, including Fe₇S₈, Ni₃S₄ and NiS, CoS, TiO₂, TiO₂ and S₈, ZrO₂, (NH₄)₃V(SO₄)₃ and VO₂, Fe₇S₈ and Fe₃O₄, MoS₂ and MoO₂. The worm-like carbon shell provides exclusive room for hundreds of nanocrystals separated from each other, preventing nanocrystal aggregation. The synergistic effect of ammonium and a strong oxidizing anion on the electrophilic oxidation of metallocene complexes containing a metal–ligand π bond contributes to the carbon formation at low temperature. It is considered that the cyclopentadienyl ligand in a metallocene complex will decompose into highly reactive straight chain olefinic pieces and the metal–olefin π interaction enables an ordered self-assembly of olefinic pieces on nanocrystals to partially form graphitizable carbon and a core–shell structure. The high capacity, good cycling behavior and rate capability of Fe₇S₈@C and Ni₃S₄ and NiS@C electrodes are attributed to the good protection and electrical conductivity of the carbon shell. [ABSTRACT FROM AUTHOR]