Synthesis of graphene materials in a plasma stream from an up to 40 kW direct current (DC) plasma torch is investigated. These materials are created by means of the conversion of hydrocarbons under the pressure 350–710 Torr without using catalysts, without additional processes of inter-substrate transfer and the elimination of impurities. Helium and argon are used as plasma-forming gas, propane, butane, methane, and acetylene are used as carbon precursors. Electron microscopy and Raman imaging show that synthesis products represent an assembly of flakes varying in the thickness and the level of deformity. An occurrence of hydrogen in the graphene flakes is discovered by X-ray photoelectron spectroscopy, thermal analysis, and express-gravimetry. Its quantity depends on the type of carrier gas. Quasi-one-dimensional approach under the local thermodynamic equilibrium was used to investigate the evolution of the composition of helium and argon plasma jets with hydrocarbon addition. Hydrogen atoms appear in the hydrogen-rich argon jet under higher temperature. This shows that solid particles live longer in the hydrogen-rich environment compared with the helium case providing some enlargement of graphene with less hydrogen in its structure. In conclusion, graphene in flakes appears because of the volumetric synthesis in the hydrogen environment. The most promising directions of the practical use of graphеne flakes are apparently related to structural ceramics.

Results of experimental study of the one-step plasma-based process of the synthesis of unsupported graphene and hydrogenated graphene are presented. A direct current (DC) plasma torch is used, the pressure is held at 350 Тоrr, and the flow rates of plasma forming gas (helium) and carbon source (propane-butane mixture) are kept constant. An influence of reactor geometry on the properties of synthesized product is investigated. Graphene and hydrogenated graphene were synthesized in an appreciable rate in the plasma jet volume under equal conditions using cylindrical and conical reactors accordingly. Synthesized graphene materials are characterized using electron microscopy, Raman spectroscopy, x-ray, and XPS analysis, confirming the existence of graphene and of hydrogenated graphene (graphane). In order to examine an influence of input parameters on the process of the synthesis of graphene materials, the quasi-1D numerical flow model is used to calculate the distributions of temperature and velocity within the reactor channel. The key role of the temperature distribution within the reactor in the synthesis of graphene materials is established. Cylindrical flow channel provides higher temperatures compared with the conical channel. It affects the flow composition at the outlet. Under lower temperature, the flow contains in addition to condensed carbon a great amount of hydrocarbons CH, which is favorable for the production of hydrogenated graphene. Under higher temperature, the pure graphene is synthesized, since the outlet flow has the carbon mainly in the condense phase, the quantity of CH being insignificant.

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The possibility of graphene synthesis (the bottom-up approach) in plasma and the effective control of the morphology and electrical properties of graphene-based layers were demonstrated. Graphene flakes were grown in a plasma jet generated by a direct current plasma torch with helium and argon as the plasma-forming gases. In the case of argon plasma, the synthesized graphene flakes were relatively thick (2–6 nm) and non-conductive. In helium plasma, for the first time, graphene with a predominance of monolayer flakes and high conductivity was grown in a significant amount using an industrial plasma torch. One-dimensional (1D) flow modeling shows that the helium plasma is a less charged environment providing the formation of thinner graphene flakes with low defect density. These flakes might be used for a water-based suspension of the graphene with PEDOT:PSS (poly(3,4-ethylenedioxythiophene): polystyrene sulfonate) composite to create the structures employing the 2D printing technologies. Good structural quality, low layer resistance, and good mechanical strength combined with the ability to obtain a large amount of the graphene powder, and to control the parameters of the synthesized particles make this material promising for various applications and, above all, for sensors and other devices for flexible electronics and the Internet of things ecosystem.