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Description of the product/technology that incorporates graphene
Plasma is a partially ionized gas considered as a reactive medium which is constituted by active particles such as electrons and ions, free radicals and high energy photons (UV). The plasma species have enough energy to induce decomposition reactions of the molecules introduced into the discharge. In particular, non-thermal plasmas are characterized by electronic and gas temperature (7000 K and 1200 K, respectively) high enough to decompose organic molecules into their atomic constituents in an effective way (» 99%). These atoms can recombine at the plasma exit forming products different to those initially introduced into the plasma.
Microwave plasmas have been reported as efficient, clean and environmentally friendly catalyst-free method for the synthesis of graphene using alcohol as carbon source. The size of final by-products is limited due to the high electron density which favors the collisions of electrons with the heavy particles avoiding the recombination of the atomic species, coming from the alcohol decomposition, into other products different to the graphene.
Plasma technology is a useful procedure for the synthesis of graphene in powder, substrate-free without needing the participation of metal catalyst to induce the graphene formation and 100 % free of metals. The main advantage of the production of graphene in powder is that it is possible to select the solvents or substrates adequate for each intended application. Besides, the synthesis process can be considered as environmentally friendly since it does not require of metal catalysts or solvents to activate the graphene production.
The plasmas used for this purpose can be generated at atmospheric pressure which results in the reduction of economic costs for the implementation at industrial level. In addition, the experimental setup can be connected in series of more than one device, which is not possible with the other experimental setups.
Currently there are two techniques used for the graphene production: Chemical Vapor Deposition (CVD) which uses hydrocarbons as precursors, and Liquid Phase Exfoliation (LPE) of graphite. However, these two techniques present certain limitations.
On one hand, CVD technique requires the presence of a substrate (SiO2, Al2O3,…), a metal catalyst (Fe, Cu, Ni and their alloys) as well as the addition of a H2 excess to activate the catalyst. In this way, the graphene quality depends on the substrate nature and the catalys participating, limiting the use of the graphene in some applications. Besides, the formation of metal particles which induce the hydrocarbon decomposition and the graphene growing on the substrate occurs in a reactor operated at low pressure (less than atmospheric pressure) and high operational temperature (1000 ºC) to give place to.
On the other hand, LPE method consists in the dispersion of graphite in a solvent as organic acid or others (metal chlorides or bromides, sulphuric acid and ethanol) being submitted to ultrasounds. In this case, the solvent acts separating the graphite flakes. By using this process, large quantities of graphene in dispersion can be obtained but with a significant number of defects due to solvent together to the ultrasound bombardment. In addition, the final material consists of a number of sheets higher than 10.
Indicators
The produced graphene consists of few layer (2-7 layers) sheets without the presence of graphene oxide, carbon nanotubes or graphitic particles. Moreover, it presents a homogeneous structure with low number of defects (similar to graphene synthesize by CVD) and free of amorphous carbon. The synthesis method is reproducible in terms of the graphene quality.
Using plasma technology, the graphene production costs is reduced not needing of vacuum instrumentation, metal catalyst nor chemical solvents. In fact, using ethanol as carbon source the procedure contributes to the revalorization of waste for producing a high-value added product like graphene.
Figure 1: TEM image of a graphene powder sample synthesized by a microwave plasma at atmospheric pressure through ethanol decomposition
- Project: Nanotubos de carbono: síntesis por plasma en ausencia de catalizadores metálicos. Consejería de Economía en Innovación (Junta de Andalucía) 2013-2016 Ref: P11-FQM7489, amount: 131.667 euros
- Publication: Synthesis of multi-layer graphene and multi-wall carbon nanotubes from direct decomposition of ethanol by microwave plasma without using metal catalysts Plasma Sources Science and Technology 24 (2015) 032005
