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Plasma Enabled and Graphene Allowed Synthesis of Unique nano Structures

Periodic Reporting for period 3 - PEGASUS (Plasma Enabled and Graphene Allowed Synthesis of Unique nano Structures)

Reporting period: 2020-05-01 to 2022-05-31

Considering the exceptional properties of graphene and graphene-based 2D materials and their entry into industry, it becomes extremely important to develop means to mass produce high-quality graphene and derivatives in all related product segments. Even considering recent developments in the graphene market, with around 250 companies offering graphene and derivatives in various forms, applications that rely on large-scale production have not taken off due to the very low quality of commercially available graphene and derivatives. Developing a new method providing high-quality graphene and derivative products tailored for specific applications and at the gram scale, while ensuring repeatability, consistent end-product quality and competitive cost, is a major challenge and crucial to drive large-scale applications. However, going from lab-scale to pilot-scale is not a matter of elementary mathematics, as scaling up the synthesis process faces enormous hurdles to overcome.

To this end, the project's ambitious goal is to translate the unique properties of plasma into exceptional material characteristics and create new forms of matter by using multiple specific plasma mechanisms to control energy and matter transfer processes at nanoscale. The primary objective of PEGASUS is the design and construction of a proof-of-concept operating machine for the gram-scale production of nitrogen-doped graphene (N-graphene) and graphene/N-graphene hybrids containing metal oxides via plasma-based single-step method under atmospheric pressure conditions. Considering the extension of N-graphene properties into the third dimension, the development of graphene-based 3D architectures, such as unique vertical N-graphene arrays grown on metal substrates and their hybrids, is also being pursued.
Within PEGASUS project a disruptive plasma technology and corresponding laboratory prototype (TRL ~ 3-5) of a versatile microwave plasma-based machine for a gram-scale fabrication of high-quality graphene and derivatives have been developed. Plasma based machine is a versatile, simple to use, provides high level of customization. It enables conversion of cheap carbon-based precursors (ethanol, CH4, etc.) into high-valued, high-quality graphene derivatives with tailored properties. Using the same machine different structures, can be fabricated, such as pure graphene sheets, N-graphene, hybrid nano-materials using graphene as a highly conductive matrix to incorporate metal oxides nanoparticles. The plasma machine provides high level of control and customisation via the application of special protocols, resulting in a controllable level of N-doping, oxygen impurities and sheets size (~300 – 400 nm). The type and level of doping is controlled by the position where N-precursor is injected into plasma environment and by the type of precursor used. Controllable, continuous fabrication of graphene/N-graphene at a gram scale ensuring high-level single layer selectivity (~50 %), high production rate (~30 mg/min) and repeatability while using ethanol, methane, acetonitrile as starting materials has been achieved. The fabricated graphene/N-graphene sheets possess high quality as evidenced by comprehensive physicochemical analyses made (for pure graphene C/O ratio: > 50; sp2% ~ 70%; for N-graphene C/O ratio: > 40; sp2% >60%).
Moreover, a novel method for a single-step, microwave plasma driven controllable assembly of nanocomposites comprising N-graphene sheets decorated with metal oxide nanoparticles at atmospheric pressure that circumvent drawbacks associated with conventional methods was developed. The plasma-based method enables reduction of micron sized metal oxides particles into nanoparticles as well as formation of new phases of metal oxides.
Furthermore, vertically aligned N-graphene structures were fabricated using low-pressure radio frequency inductively coupled cold nitrogen plasma treatment, and a high doping level (~8-12 at %N) was attained. It is also identified that the bonding configurations of the nitrogen groups can be adjusted by varying plasma parameters. Based on the experimental evidences, a nitrogen incorporation mechanism in graphene is established. A fast and facile technique for decorating vertical graphene and vertical carbon structures with metal nanoparticles using plasma sputtering was also elaborated. Exploiting capacitively coupled RF plasma where temperatures go as low as 480°C fast growth (minutes) of vertical graphenes/N-graphenes was achieved for a large variety of substrates. Using a hybrid microwave-DC plasma reactor vertically oriented carbon nanostructures were successfully grown on Ni-foam in a very short time (~ 8 min) at atmospheric pressure conditions as well.
Taking advantage of the exceptional electrical conductivity and extensive electrochemically active surface area of the graphene produced within the project, two supercapacitor demonstrators were developed. Aimed at IoT applications, a graphene-based supercapacitor as well as a smoothing AC-DC converter supercapacitor with the potential to replace tantalum electrolytic capacitors have been developed.
In addition to the above energy storage applications, some of the graphene-based vertical nanostructures produced during the PEGASUS period have been shown to have excellent energy storage properties for battery systems, suggesting the extension of the project to various research areas related to energy.
The main advantage of the used plasma-based approach is the achievement of a very high and extremely controllable energy density in the plasma reactor, which allows effective control over the energy and material fluxes towards growing nanostructures at the atomic scale via proper reactor design and tailoring of the plasma environment in a synergistic way. The method allows synthesis-by-design and represents a significant leap from the state-of-the-art methods that rely on cumbersome batch procedures involving harsh chemistry and energy intensive processes. It is cost-effective because the free-standing graphene/N-graphene sheets and derivatives are assembled through a single-step process in readily dispersive form, without the need for cleaning or any other post processing. The end-result is a high-quality product, obtained in a reproducible manner, with the desired morphological, structural, and functional properties. Moreover, comparison with N-graphene from commercial reference supplier has demonstrated superior quality regarding crystalline structure, presence of graphene and level of oxygen impurities.
The PEGASUS project opens up a new way for custom-made 2D carbon materials and their 3D architectures. The impact is definitely a breakthrough, as it combines Europe's most ambitious goals in nanomaterials as key technologies: destructive properties through cheap green roads that allow existing materials to be replaced by new, cost-effective and more efficient one. The development in this field of plasma technologies is expected to have a great influence in the applications of energy storage and conversion devices, conductive inks, nanocomposites, membranes, sensors, metamaterials, etc.

Strategic achievements:
• Formation of an ever-growing user community of scientists to explore the new physics and application opportunities related with graphene/N-graphene and derivatives.
RFCC plasma for the production of vertically aligned graphene structures
Pegasus supercapacitor cell used in the project as a working prototype.
SEM image of grown 3D vertically aligned graphenes/N-graphenes on substrate.
Scanning Electron Microscopy image of N-graphene sheets
1 gram of N-graphene sheets as synthesized
The illumination of the RF plasma with a laser sheet reveals the clouds of levitating nanoparticles.
Analyzing the samples at BESSY II, Berlin
Microwave plasma delivering free-standing N-graphene sheets