Preparation And Structure Of Carbon-Encapsulated Fe₃O₄ Nanoparticles

Carbon-encapsulated metal oxide nanoparticles (CEMONs) are a new kind of carbon/metal oxide nanocomposite, in which graphite arrange around metal oxide nanoparticles located in the center to form core-shell structure. Because of the carbon layers preventing metal oxide nanoparticles from environment effection, they have good stability in environment. For the preparation of CEMONs, the present techniques, such as the arc-discharge technique, chemical vapor deposition, liquid-impregnating carbonization of non-graphitizing carbon and etc., show some disadvantages, e.g., the morphology and the structure of the resulting nanocrystals are not easy to control, and it is difficult to produce high-purity nanomaterials in large quantities. Here we proposed a novel method for the preparation of CEMONs by co-carbonization of aromatics with metal compounds, i.e., co-pyrolysis method. It featured in simplicity, low temperature, good controllability and high yield.The research focused on the preparation of CEMONs by co-pyrolysis of 1,3-benzenediol and ferrocene in which 1,3-benzenediol is oxygen source and carbon source, ferrocene is the metal source and carbon source. The effects of synthesis parameters on the formation and transformation of CEMONs were investigated in detail. The morphology, structure and particular properties of the product were characterized by TEM, HREM, XRD, SEM, and TG-DSC measurements. The formation mechanism of CEMONs was elucidated.The results show that large amount of carbon-encapsulated Fe3O4 nanoparticles were obtained by co-pyrolysis of 1,3-benzenediol and ferrocene. The CEMONs with higher purity were prepared in the presence of 33.3 wt.% of 1,3-benzenediol at the reaction temperature of 450℃. With the increase of ferrocene content from 50wt.% to 75wt.% or reaction temperature from 430℃to 495℃, the encapsulated metal core in the product was changed from single Fe3O4 to a-Fe to iron carbides, gradually. Meanwhile, nanoparticles become smaller, and the thickness of corbon layers increases.The above research afforded a novel and effective method for the preparation of carbon-encapsulated metal oxide nanomaterials. By analysis of raw feedstock and the investigation on the relationship among the morphology, structure and the synthesis parameters, it is concluded that the formation of CEMONs by co-pyrolysis method is based on the catalysis and oxidation/reduction of iron nano-cluster for the oxygen-containing aromatic molecules. And this process is based on the dissolution-precipitation, and dynamic equilibrium mechanisms. It is assumed that two essential factors must be met for the formation of the nanoparticles, including proper proportion of iron nano-clusters and oxygen aromatics, and moderate reaction activity in the system.