Low temperaturein situsynthesis and the formation mechanism of various carbon-encapsulated nanocrystals by the electrophilic oxidation of metallocene complexes

Abstract

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.

Highly efficient one-step synthesis of carbon encapsulated nanocrystals by the oxidation of metalπ-complexes

“…When the temperature increases to the reaction temperature, the oxidation reaction takes place rapidly and produces blast water vapour, which can push some reactants to the top of the autoclave (figure 7(b)). Additionally, a large amount of heat is liberated during the reaction, significantly raising the inner temperature of the sealed reactor, which is much higher than the reaction temperature [15]. The high temperature will inevitably result in the cleavage of metal π-complex, and then the generation of different kinds of clusters and radicals, and eventually the formation of a carbon and the metal oxide/sulfide nanocrystals.…”

“…Our previous study proposed a general in situ synthesis approach for different carbon encapsulated oxide/sulfide nanocrystals with a core-shell structure by a solid-state explosive reaction between metallocene complexes and highoxidative ammonium salts in an autoclave at 200 °C [14,15]. Compared to the conventional preparation techniques, the oxidizing agent is indispensable for the formation of carbon [16], implying that the effect of the presence of oxygen in the reaction apparatus on the product can be negligible, and that only a sealed reactor is sufficient for the synthesis.…”

Highly efficient solid-state synthesis of carbon-encapsulated ultrafine MoO2 nanocrystals as high rate lithium-ion battery anode

“…During the explosive reaction, carbon fragments filled the autoclave inhibit the grain growth of MoO 2 , resulting in the formation of ultrafine MoO 2 nanocrystals. And the π interaction between Mo and carbon fragments should be responsible for the creation of core-shell structure (Liu et al 2016). Figure 6a displays the first three CV cycle profiles for MoO 2 @C anode at a scan rate of 0.5 mV s −1 between 0.01-3 V. The wide irreversible peak located at around 0.9 V only appears in the first cycle and can be associated with the formation of solid electrolyte interphase (SEI) film on the carbon shell.…”

Confined space self-propagating room temperature synthesis of carbon-encapsulated Fe3O4 nanocrystals and its lithium storage performance

“…Therefore, ongoing research efforts are still needed to develop a time-effective, low energy consumption and inexpensive method for industrial-scale mass production. In our previous publications, a common oxidation route is proposed for the synthesis of carbon-encapsulated nanocrystals via a moderate detonation reaction of the organometallic compounds with oxidants below 200 °C [6][7][8]. The synthetic approach involves the oxidation removal of hydrogen from the organics and in situ generation of carbon shell.…”