(Invited) Microwave Plasmas Applied for a Large-Scale Production of Graphene & Derivatives

Abstract

As graphene&derivatives penetrate the industry, it becomes crucial to develop the means to mass produce high quality across all related product segments. Research efforts meet formidable challenges in the marketplace because of the lack of discernible protocols to fabricate customized products at increasingly higher yields while maintaining its quality. Notably, a recent study on the commercial graphene purchased from 60 companies worldwide has found that the majority of companies are selling a product containing less than 10% graphene content and that none of them supplied samples containing more than 50% graphene [1]. To find pathways to broad commercialization of graphene&derivatives the constant efforts must be focused on disruptively improved material synthesis.In the present report we provide substantial evidence that microwave plasma based technologies can be used as a competitive and disruptive alternative to existing chemical methods in the production of free-standing flakes of graphene and N-graphene at a large scale. Prioritizing operational flexibility, microwave plasma induced by surface waves (SW) at atmospheric pressure conditions has been employed [2]. The surface wave sustained plasma produced by the field of a traveling wave has an extended active zone outside the wave launcher because the plasma is sustained by a wave that simultaneously propagates and creates its own propagation structure. In this way, large microwave power densities can be injected into processing area and high population densities of active species of interest can be achieved. The reactor can be virtually separated in two different zones. The first one is the "hot" plasma zone. Here, the wave power is absorbed primarily by plasma electrons, which transfer the power to heavy particles via elastic and inelastic collisions resulting in gas temperatures up to 4000 K. The gas temperature gradually decreases moving away from the SW launcher and drops to about 500 K in the "mild" plasma zone. The carbon precursor mixed with background argon gas is injected in the "hot" high energy density zone where decomposition processes of the injected carbonaceous molecules take place as a result of particles collisions and intensified chemistry. The created radicals become precursors of chain reactions leading to creation of the main building blocks, i.e. carbon atoms and C2 radicals. The transport of plasma generated carbon atoms/molecules into colder zones of the reactor results in formation of solid carbon nuclei that are gradually withdrawn in the outlet plasma stream where kinetic processes of assembly and growth of “flowing” carbon nanostructures take place. Therefore, by tailoring the temperature gradients, the density of carbon molecules and atoms as well as their residence time, selective synthesis of graphene/N-graphene (nitrogen doped graphene) sheets in a narrow range of operational parameters can be achieved. Two different experimental strategies to deliver the nitrogen precursors to the plasma environment, i.e., top-to-down and down-to-top injection, have been applied. The top-to-down approach provides injection of both the carbon and the nitrogen precursors mixed with argon gas into the “hot” plasma zone, while down-to-top approach provides injection of nitrogen precursor into the “mild” plasma zone against the main flux.In this report we demonstrate an operational plasma-based proof-of-concept machine for a controllable, continuous fabrication of graphene and N-graphene at a gram scale ensuring high-degree of single layer selectivity (~50 %), high yield (~30 mg/min) and repeatability while using cheap starting materials (e.g., ethanol, methane, acetonitrile, etc.). The plasma machine provides high level of control and customization via the application of special protocols, resulting in a controllable level of N-doping, oxygen impurities and sheets size (~300 – 400 nm). The fabricated graphene/N-graphene flakes possess high quality as evidenced by comprehensive physicochemical analyses made (for pure graphene C/O ratio: > 50; sp2 carbons~ 70%; for N-graphene C/O ratio: > 40; sp2 carbons >60%).The microwave plasma driven synthesis is controllable at a single step and atmospheric pressure conditions, allows continuous production in an environmentally friendly manner.