(Invited) Microwave Plasmas Applied for Synthesis of Free-Standing Carbon Nanostructures at Atmospheric Pressure Conditions

Abstract

A microwave plasma-assisted method for synthesis of advanced carbon nanostructures is presented. The method is based on the injection of gas/liquid carbon precursors into a surface-wave sustained argon plasma environment, where the decomposition of molecules takes place. Gas-phase carbon atoms and molecules created in the "hot" plasma diffuse into colder zones of plasma reactor and aggregate into solid carbon nuclei. The main part of the solid carbon is gradually withdrawn from the "hot" plasma region in the outlet plasma stream where flowing carbon nanostructures assemble and grow. Selective synthesis of free-standing graphene sheets and diamond-like nanostructures was achieved. The engineering of the graphene sheets’ structural qualities and the control of the number of atomic layers per sheet is achieved via synergistic tailoring of the "hot"plasma environment and thermodynamic conditions in the post-discharge zone. The produced sheets (1-5 atomic layers) are stable and present much better qualities than those of graphene assembled via conventional methods, while being synthesized without the need of metal/crystal substrates. The synthesized structures have been analyzed by Raman spectroscopy, scanning electron microscopy, high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy. It is worth noting that milligrams of high quality self-standing graphene sheets in a readily dispersive form can be obtained within a minute, through a single step process at atmospheric pressure conditions. The method is widely open for scale-up by using large-scale configurations of wave driven plasmas. Acknowledgements: This work was supported by Fundação para a Ciência e a Tecnologia, under 2015 Strategic Project.