Effect of hydrogen concentration on graphene synthesis using microwave-driven plasma-mediated methane cracking

Gas phase graphene forms as an aerosol in a microwave plasma among other carbon forms. Consisting of 2-6 sheets per stack with dimensions between 100 and 500 nm, it is referred to as nanographene (NG). Surprisingly, increasing H/C ratio in the feedstock increases the relative graphitic content of the product. Dependence of the different carbon forms upon H/C ratio of the gas feed mixture is shown across multiple analytical characterizations. Attributes of (a) phase quality (pristine nature of NG) and (b) phase quantity (how much NG forms relative to other carbon sp(2) phases) are addressed. Phase identification of the forms is performed via transmission electron microscopy with quantification by thermogravimetric analysis, assessing their respective oxidative reactivity benchmarked to commercially available similar carbon products applied as standards. X-ray diffraction differentiates these forms based on varied extent of graphitic structure. Electron energy loss spectroscopy assesses graphitic content by the ratio of sp(2)/sp(3) bonding. Raman spectroscopy supports the observed shift in relative proportions of the carbon forms towards preferential graphitic content with increasing H/C. Selected area diffraction illustrates this for NG. Fringe analyses of nanostructure quantifies this shift for carbon particles. Infra-red spectroscopy reveals complementary C-H bonding as a measure of graphitic quality. (C) 2018 Elsevier Ltd. All rights reserved.