Atomic-Scale Structural Evolution and Characterization of Molybdenum Carbide Nanoparticles Grown from MoO3 and MoS2: High-Resolution In Situ ETEM and EELS

The transformation mechanism of different types of molybdenum compounds into molybdenum carbide was explored by a classical temperature-programmed reduction carbonization method. The structure, morphology, and chemical state of molybdenum carbide nanoparticles during high-temperature growth were studied using in situ characterization at the atomic level, based on which the nucleation mechanism and transition behavior of molybdenum carbide nanoparticles were investigated. It was found that when MoO3 and MoO2 were used to prepare molybdenum carbide nanoparticles, the reaction processes of MoO2 (locally formed MoCxOy) into MoOx, Mo, and Mo2C were successively observed from the microscopic point of view. However, under the same preparation conditions, both Mo2C and MoC crystal forms were obtained using molybdenum sulfide as a precursor, and a small amount of unreacted molybdenum sulfide remained in the product. The low melting point of molybdenum oxide was more conducive to the gas phase transport of atoms, which can form molybdenum carbide crystal nucleus through the deoxidation and carburization process. However, molybdenum sulfide with larger spacing between atomic layers can be directly replaced by a C atom isocrystalline, and the initial product Mo2C can be further carburized to form MoC with higher carbon content.