Elena Tatarova

Rechargeable aluminum batteries are promising for next‐generation energy storage owing to the low cost and abundance of aluminum. Graphene is an attractive cathode due to its high surface area and electrical conductivity, but its performance is limited by difficult production and sheet aggregation, which restrict electrolyte accessibility and ion transport. This study employs a plasma‐enabled synthesis approach to produce both pristine graphene and nitrogen‐doped graphene (N‐graphene), followed by an ice templating strategy to directly construct a three‐dimensional porous N‐graphene electrode. The effects of nitrogen doping and porous electrode architecture are systematically investigated by comparing dense and porous electrodes. Nitrogen‐doping increases the specific capacity by ∼50%, from 36.24 to 54.4 mAh/g at a current density of 0.1 A/g. Furthermore, introducing porous structure provides an additional ∼20% improvement, achieving a capacity of 65.5 mAh/g under the same conditions. This enhanced performance of porous electrode is attributed to its interconnected pores and partially aligned graphene sheets, which improve electrolyte accessibility and facilitate the adsorption and transport of AlCl ₄ ⁻ anions. The robust porous structure of N‐graphene ensures excellent cycling stability, with no noticeable capacity decay over 5000 cycles. Coupling nitrogen doping with porous electrode architecture provides an effective strategy to enhance graphene‐based aluminum batteries.

Here, a plasma-enabled strategy is presented for the controllable fabrication of ultralight nanocomposites with absorption-dominated EM-shielding performance across the 5–70 GHz frequency range. The fabricated nanocomposites exhibit average microwave absorption exceeding 60% across the 5–25 GHz frequency range and 70% across the 25–70 GHz range while maintaining a constant thickness of 2 mm and a fixed filler content of 15 wt%. Plasma-driven nanoscale engineering and multiphase hybridization synergistically enhance dielectric and magnetic loss mechanisms, resulting in effective absorption bandwidths in the C-band (2.1 GHz), Ku-band (3.4 GHz), Ka-band (8 GHz) and V-band (4 GHz). The plasma process produces free-standing nitrogen-doped graphene sheets uniformly decorated with Fe_xO_y, MnO_x, and TiO_x nanoparticles, which are subsequently incorporated into a polymer matrix. The microwave plasma environment simultaneously enables graphene nanosheet synthesis, metal oxide nanoparticles formation, surface functionalization, multiphase generation, and the anchoring of nanoparticles onto the graphene scaffold. Using low-cost precursors such as acetonitrile and micron-sized metal oxides, the process achieves production rates of up to 50 mg min⁻¹. Overall, this one-step plasma-enabled approach provides a low-cost, scalable, and efficient route for manufacturing lightweight, high-performance EM wave absorbers.

Graphene-based composite materials have attracted much attention for a range of applications in various fields, including electronics, sensing, catalysis, energy storage and conversion. Single-step large-scale microwave plasma synthesis of graphene and nitrogen-doped graphene (N-graphene) composite materials has been demonstrated. The developed atmospheric pressure plasma method allows continuous synthesis of different graphene-based hybrids in a controllable and environmentally friendly manner. Control over the synthesis process, i.e., size, uniformity, surface distribution of the nanoparticles and graphene/N-graphene quality, was provided by adjusting plasma parameters and injection configuration. Protocols for the production of particular composites, i.e., graphene-MnO, N-graphene-MnO, N-graphene-MnS, and N-graphene-Fe_xO_y, have been established using methane and acetonitrile as precursors. A comprehensive physicochemical characterization of the produced composites was conducted using high-resolution transmission electron microscopy, scanning transmission electron microscopy, Raman spectroscopy, X-ray diffraction, and near-edge X-ray-absorption fine-structure and X-ray photoelectron spectroscopies.

The large-scale microwave plasma synthesis of graphene and nitrogen-doped graphene with tailored structural properties, crucial for their successful usage applications, has been demonstrated. The developed atmospheric pressure plasma method offers several advantages, including the continuous production of high-quality, free-standing graphene without the use of chemicals, solvents, catalysts, or additional heating. This non-toxic process eliminates the need for vacuum systems while achieving high temperatures. The method enables the precise control over graphene’s properties, such as the layer number, defects, sheet size, uniformity, and functionality, as well as the doping type and configuration, by adjusting the plasma parameters. Protocols for the synthesis of specific nanostructures with a controlled structural quality, production rate, and chemical composition have been established using methane and methylamine as precursors. The comprehensive physicochemical characterization of the graphene and nitrogen-doped graphene was carried out using scanning electron microscopy, high-resolution transmission electron microscopy, Raman spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy.

Materials with tunable negative electromagnetic performance, i.e., where dielectric permittivity becomes negative, have long been pursued in materials research due to their peculiar electromagnetic (EM) characteristics. Here, this promising feature is reported in materials on the case of plasma‐synthesized nitrogen‐doped graphene sheets with tunable permittivity over a wide (1–40 GHz) frequency range. Selectively incorporated nitrogen atoms in a graphene scaffold tailor the electronic structure in a way that provides an ultra‐low energy (0.5–2 eV) 2D surface plasmon excitation, leading to subunitary and negative dielectric constant values in the Ka‐band, from 30 up to 40 GHz. By allowing the tailoring of structures at atomic scale, this novel plasma‐based approach creates a new paradigm for designing 2D nanomaterials like nanocarbons with controllable and tunable permittivity, opening a path to the next generation of 2D metamaterials.

Considering the increasing demand for advanced energy materials for future energy-related applications, designing promising materials at a low cost is critical. Given the importance of structural design and morphological features of the designed material in energy applications, fabricating materials at the nanoscale with controlled morphology and orientation is important. Recently 2-dimensional graphene-based materials have emerged as a potential candidate for next-generation energy applications. However, conventional chemical and physical routes for producing high-quality graphene have certain limitations either due to the cost or the processing time. Therefore, an advanced technique for designing and processing graphene structures at the atomic scale is needed to produce high-quality materials. In this regard, safe and clean environmentally-friendly plasma-enabled techniques have been explored as a potential method to tailor different structures at the nanoscale. As a synthesis approach, plasma assembles the nanostructures from gaseous into a solid form. Therefore, this paper suggests the advantages of atmospheric pressure plasma-enabled approaches to design and engineer graphene-based materials at the nanoscale with high structural quality and controllability with hybrid morphologies. Here, a novel, single-step microwave plasma-enabled approach at atmospheric conditions used to design hybrid high-quality graphene-based nanostructures is presented. The plasma techniques allow the synthesis of high-quality N-graphene (nitrogen-doped graphene) metal-based nanostructures at one of the fastest production rates of ∼ 19 mg/min. The graphene production is carried out in the high energy density zone of microwave plasma, and the growth of N-graphene sheets occurred in the afterglow region. Spraying metal particle-containing gases into this zone allows the formation of hybrid N-graphene structures anchored with metal oxide/sulphide nanoparticles. Structural and morphological analysis of these hybrids using different microscopic and spectroscopic techniques confirmed the high structural quality and distribution of metal-based nanostructures on N-graphene sheets. This fast and facile approach is expected to provide a significant impact on designing high-quality graphene hybrids, which can be used for sustainable energy storage applications.

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. References [1] A.P. Kauling et al. "The Worldwide Graphene Flake Production". Adv. Mater. 30 (2018) 1803784. [2] E. Tatarova, et al.^"Towards large-scale in free-standing graphene and N-graphene sheets" Scientific Reports 7 (2017) 10175. Acknowledgments This work was performed under de framework of the PEGASUS (Plasma Enabled and Graphene Allowed Synthesis of Unique nano-Structures) project, funded by the European Union's Horizon research and innovation program under grant agreement No 766894. Work partially funded by Portuguese FCT, through Eager project (PTDC/NAN-MAT/30565/2017).