Graphitisation and microstructure transformation of carbon-encapsulated cobalt nanoparticles synthesised by chemical vapour deposition using alumina powder as catalytic support

Abstract

Carbon-encapsulated cobalt nanoparticles (Co@C) are prepared by the chemical vapour deposition of methane at 773–1073 K. This paper investigates the graphitisation and microstructure transformation of Co@C governed by synthesis temperature. The results show that carbon shells consisting of amorphous carbon or small graphite fragments begin forming on Co nanoparticles from 773 to 873 K. The products transform into onion-like carbon-encapsulated cobalt nanoparticles at 973 K. At 1073 K, a higher graphitisation of Co@C is achieved, with the appearance of carbon nanotubes. With a rising synthesis temperature, the carbon shells undergo the transformations from amorphous carbon to graphite fragments and finally to onion-like carbon. The microstructural transformations are demonstrated by scanning electron microscope, transmission electron microscope and Raman characterisations. Therefore, controlling the synthesis temperature is a feasible and easy approach one can take to adjust the structure and graphitisation of Co@C in the chemical vapour deposition process.