The influence of the interplay between central (Q C ) and secondary (Q S ) channel gas flow, as well as delivered microwave power (P MW ), during graphene nanosheet synthesis in a dualchannel electrode configuration of a microwave plasma torch at atmospheric pressure by ethanol decomposition was investigated. In the dual-channel configuration, plasma discharge can be sustained, even at high flow rates of ethanol, due to the separation of argon working and carrier gas. The plasma discharge instability was mainly influenced by an increase in the central channel flow, and a minor influence of secondary channel flow was also observed. With respect to the dependence on experimental conditions, the synthesized nanopowder consisted of amorphous carbon and nanocrystalline diamond nanoparticles, defective carbon nanosheets or few-layer graphene nanosheets. The synthesized nanosheets are rectangular in shape with a lateral size of several hundreds of nanometres and a few graphene layers thick, as shown by electron microscopy. Raman and x-ray photoelectron spectroscopy analysis of the synthesized nanosheets showed a good degree of graphitization, low oxygen content and increasing quality of graphene nanosheets with increasing microwave power. The number of defects in the synthesized nanosheets could be decreased by elongation of the graphene nanosheet assembly zone. An increase in the C 2 /C emission line intensity ratio correlated with a decrease in the number of defects in the graphene nanosheet structure. The achieved conversion yield of ethanol into graphene nanosheets was 8.3%, without negatively affecting the nanosheet quality.

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Fourier‐transform infrared spectroscopy and proton‐transfer‐reaction–mass spectrometry are used in a complementary way to study gas‐phase processes during decomposition of ethanol in a microwave plasma torch. Decomposition products (C, C2 and simple hydrocarbons) reassemble into higher hydrocarbons and graphene nuclei and further grow into graphene nanosheets (GNS). Depending on microwave power, ethanol flow rate and molecular gas admixture, the material structure changes from amorphous to crystalline. The presence of C2n + 1H y species was found to be responsible for the formation of defects in the GNS structure. O2 and H2 admixtures change the gas temperature axial profile and consequently modify reaction pathways influencing growth and production rate of GNS. Determination of reaction pathway selectivity enables us to predict whether high‐quality or defective GNS are produced.

This paper experimentally investigates the processes governing the single-and multi-filament regimes in an atmospheric pressure microwave (MW) torch operated in argon. Optical emission spectroscopy and spectral imaging are the principal diagnostics techniques which are employed. MW power is found to be the main parameter controlling the number of filaments. The single-filament regime exhibits many properties typical for surface wave discharges, e.g. a linear decrease in electron density along the axis or the existence of a central dip in the radial/lateral emission profiles. Simple geometric quantities, such as the length or thickness of the filament(s), vary almost linearly with the input MW power, and exhibit discontinuities at successive filament splitting events. These take place at similar values of filament maximum thickness, and may be due to skin-depth limited power transfer. The presence and chemistry of a low-emission intensity plasma shell surrounding the filament(s) is also investigated. The gas temperature is estimated from the OH band and complemented by Schlieren imaging, which revealed that a much larger cone of gas is being heated by filaments than is their diameter.

High temperature stability of nanomaterials plays an important role for their application in the field of nanocomposites, batteries, and sensors. Few-layer graphene nanosheets prepared by microwave plasma based decomposition of ethanol exhibited high thermal stability in the oxidation atmosphere in dependence on controlled formation of structural disorder. Analysis of differential thermogravimetry (DTG) curve profile showed three temperature regions, around 345 °C, 570 °C and above 700 °C, related to amorphous phase with a carbon–oxygen functional groups, small defective nanostructures and highly crystalline structure of graphene nanosheets, respectively. Raman spectroscopy and x-ray photoelectron spectroscopy (XPS) analysis of the nanosheets showed an increase of D/G Raman band ratio as well as increasing of sp3 phase content, from 6.1 at% to 15.2 at%, for highly crystalline and highly disordered structure of the nanosheets. Thermal annealing under synthetic air was used to investigate the variation in D/G and 2D/G Raman band ratio of the samples and to estimate activation energy of oxidation and disintegration process of graphene nanosheets. The highest oxidation resistance exhibited sample with high 2D/G band ratio (1.54) and lowest oxygen content of 1.7 at%. The synthesis process led to stabilization of nanosheet structure by formation of curved edges and elimination of free dangling bonds. The nanosheets prepared in microwave plasma exhibited high surface area, over 350 m2 g−1, and superior thermal stability with defect activation energy in an oxidation atmosphere higher than 2 eV. Heat release rate during the oxidation process was in correlation with the amount of disorder in the samples. Fast and easy to use technique based on high power Raman spectroscopy was developed for assessment of nanomaterial oxidation resistance.