Low temperature electrical transport in microwave plasma fabricated free-standing graphene and N-graphene sheets

Funding Information: This work was performed under the 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 - Fundação para a Ciência e a Tecnologia, through EAGER project (PTDC/NAN-MAT/30565/2017) and under projects UIDB/50010/2020 and UIDP/50010/2020. The authors would like to thank S. Russev for the SEM images. M A, E V, K K and Zh K thank the European Regional Development Fund within the Operational Programme ‘Science and Education for Smart Growth 2014–2020’ under the Project CoE ‘National center of mechatronics and clean technologies ‘BG05M2OP001-1.001-0008’. Publisher Copyright: © 2023 The Author(s). Published by IOP Publishing Ltd.In this paper, the electrical transport in free-standing graphene and N-graphene sheets fabricated by a microwave plasma-based method is addressed. Temperature-dependent resistivity/conductivity measurements are performed on the graphene/N-graphene sheets compressed in pellets. Different measurement configurations reveal directional dependence of current flow—the room-temperature conductivity longitudinal to the pellet’s plane is an order of magnitude higher than the transversal one, due to the preferential orientation of graphene sheets in the pellets. SEM imaging confirms that the graphene sheets are mostly oriented parallel to the pellet’s plane and stacked in agglomerates. The high longitudinal electrical conductivity with values on the order of 103 S/m should be noted. Further, the current flow mechanism revealed from resistivity-temperature dependences from 300K down to 10K shows non-metallic behavior manifested with an increasing resistivity with decreasing the temperature d ρ / d T < 0 usually observed for insulating or localized systems. The observed charge transport shows variable range hopping at lower temperatures and thermally activated behaviour at higher temperatures. This allows us to attribute the charge transport mechanism to a partially disordered system in which single graphene sheets are placed predominantly parallel to each other and stacked together.publishersversionpublishe

Prospects for microwave plasma synthesized N-graphene in secondary electron emission mitigation applications

PTDC/NAN-MAT/30565/2017 D01-284/2019 (INFRAMAT) IBB BASE 2020-2023 UID/FIS/00068/2019.The ability to change the secondary electron emission properties of nitrogen-doped graphene (N-graphene) has been demonstrated. To this end, a novel microwave plasma-enabled scalable route for continuous and controllable fabrication of free-standing N-graphene sheets was developed. High-quality N-graphene with prescribed structural qualities was produced at a rate of 0.5 mg/min by tailoring the high energy density plasma environment. Up to 8% of nitrogen doping levels were achieved while keeping the oxygen content at residual amounts ( 1%). The synthesis is accomplished via a single step, at atmospheric conditions, using ethanol/methane and ammonia/methylamine as carbon and nitrogen precursors. The type and level of doping is affected by the position where the N-precursor is injected in the plasma environment and by the type of precursors used. Importantly, N atoms incorporated predominantly in pyridinic/pyrrolic functional groups alter the performance of the collective electronic oscillations, i.e. plasmons, of graphene. For the first time it has been demonstrated that the synergistic effect between the electronic structure changes and the reduction of graphene $-plasmons caused by N doping, along with the peculiar “crumpled” morphology, leads to sub-unitary (textless 1) secondary electron yields. N-graphene can be considered as a prospective low secondary electron emission and plasmonic material.publishersversionpublishe

Photocatalytic splitting of water.

The use of photocatalysis for the photosplitting of water to generate hydrogen and oxygen has gained interest as a method for the conversion and storage of solar energy. The application of photocatalysis through catalyst engineering, mechanistic studies and photoreactor development has highlighted the potential of this technology, with the number of publications significantly increasing in the past few decades. In 1972 Fujishima and Honda described a photoelectrochemical system capable of generating H2 and O2 using thin-film TiO2. Since this publication, a diverse range of catalysts and platforms have been deployed, along with a varying range of photoreactors coupled with photoelectrochemical and photovoltaic technology. This chapter aims to provide a comprehensive overview of photocatalytic technology applied to overall H2O splitting. An insight into the electronic and geometric structure of catalysts is given based upon the one- and two-step photocatalyst systems. One-step photocatalysts are discussed based upon their d0 and d10 electron configuration and core metal ion including transition metal oxides, typical metal oxides and metal nitrides. The two-step approach, referred to as the Z-scheme, is discussed as an alternative approach to the traditional one-step mechanism, and the potential of the system to utilise visible and solar irradiation. In addition to this the mechanistic procedure of H2O splitting is reviewed to provide the reader with a detailed understanding of the process. Finally, the development of photoreactors and reactor properties are discussed with a view towards the photoelectrochemical splitting of H2O

On the plasma-based growth of 'flowing' graphene sheets at atmospheric pressure conditions

The article of record as published may be found at http://dx.doi.org/10.1088/0963-0252/25/1/015013A theoretical and experimental study on atmospheric pressure microwave plasma-based assembly of free standing graphene sheets is presented. The synthesis method is based on introducing a carbon-containing precursor (C₂H₅OH) through a microwave (2.45 GHz) argon plasma environment, where decomposition of ethanol molecules takes place and carbon atoms and molecules are created and then converted into solid carbon nuclei in the ‘colder’ nucleation zones. A theoretical model previously developed has been further updated and refined to map the particle and thermal fluxes in the plasma reactor. Considering the nucleation process as a delicate interplay between thermodynamic and kinetic factors, the model is based on a set of non-linear differential equations describing plasma thermodynamics and chemical kinetics. The model predictions were validated by experimental results. Optical emission spectroscopy was applied to detect the plasma emission related to carbon species from the ‘hot’ plasma zone. Raman spectroscopy, scanning electron microscopy (SEM), and x-ray photoelectron spectroscopy (XPS) techniques have been applied to analyze the synthesized nanostructures. The microstructural features of the solid carbon nuclei collected from the colder zones of plasma reactor vary according to their location. A part of the solid carbon was deposited on the discharge tube wall. The solid assembled from the main stream, which was gradually withdrawn from the hot plasma region in the outlet plasma stream directed to a filter, was composed by ‘flowing’ graphene sheets. The influence of additional hydrogen, Ar flow rate and microwave power on the concentration of obtained stable species and carbon−dicarbon was evaluated. The ratio of sp³/sp² carbons in graphene sheets is presented. A correlation between changes in C₂ and C number densities and sp³/sp² ratio was found.Portuguese FCT--Fundação para a Ciência e a Tecnologia, under Project UID/FIS/50010/2013, Project INCENTIVO/FIS/LA0010/2014, and grant SFRH/BD/52413/2013 (PD-F APPLAuSE)

Low total electron yield graphene coatings produced by electrophoretic deposition

Stainless steel and copper technical substrates have been coated by free standing graphene using electrophoretic deposition technique, with the final goal to obtain chemically inert low secondary electron emission surfaces. This class of materials is of utmost interest in future accelerators in order to increase further the flux of accelerated charged particles in the beam. The measured maximum total electron yield of pristine graphene, which has been previously characterized by electron microscopies and X-ray photoelectron spectroscopy, appears to be about 1.0. The deposition parameters have been optimized in order to obtain surfaces with lowest total electron yield, but also composition and morphology close to that of pristine graphene. When applying these optimized deposition parameters graphene coatings on two substrates, i.e. stainless steel and copper, the maximum total electron yield of about 1.04 was obtained. The composition of coatings and the relative amounts of sp2 bonds are slightly worse than those of the pristine graphene, while the surface morphology appears to be the same. Annealing at 150 °C for 64 h in high vacuum, showed that vacuum baking did not affect considerably the electron emission properties of the coatings. © 2019 Elsevier B.V

Simultaneous Synthesis and Nitrogen Doping of Free-Standing Graphene Applying Microwave Plasma

An experimental and theoretical investigation on microwave plasma-based synthesis of free-standing N-graphene, i.e., nitrogen-doped graphene, was further extended using ethanol and nitrogen gas as precursors. The in situ assembly of N-graphene is a single-step method, based on the introduction of N-containing precursor together with carbon precursor in the reactive microwave plasma environment at atmospheric pressure conditions. A previously developed theoretical model was updated to account for the new reactor geometry and the nitrogen precursor employed. The theoretical predictions of the model are in good agreement with all experimental data and assist in deeper understanding of the complicated physical and chemical process in microwave plasma. Optical Emission Spectroscopy was used to detect the emission of plasma-generated &lsquo;&lsquo;building units&rsquo;&rsquo; and to determine the gas temperature. The outlet gas was analyzed by Fourier-Transform Infrared Spectroscopy to detect the generated gaseous by-products. The synthesized N-graphene was characterized by Scanning Electron Microscopy, Raman, and X-ray photoelectron spectroscopies

N-Graphene-Metal-Oxide(Sulfide) hybrid Nanostructures: Single-step plasma-enabled approach for energy storage applications

Hybrid graphene-based nanostructures are considered promising materials for energy storage applications. However, the synthesis of high-quality hybrid graphene nanostructures at high yields is challenging. In the present work we propose a novel, single-step microwave plasma-enabled approach to synthetize customizable hybrid graphene-based nanostructures at high-yield while preserving their quality. Hybrid N-graphene (nitrogen-doped graphene) metal-based nanostructures, for instance, can be produced at a rate of ∼ 19 mg/min. The high energy density region of a microwave plasma provides sufficient energy and “building particles” fluxes towards the low-energy density plasma afterglow for the processes of assembly and growth of N-graphene sheets. Simultaneously, a controlled jet of metal-oxide(-sulfide) microparticles is sprayed into the plasma afterglow region where they bind to N-graphene sheets. Methane/methylamine are used as carbon and nitrogen precursors, combined with micron-sized MnO2 and oxy-MnS particles to synthesize the hybrid structures. As a result, nano-sized (∼10–30 nm) MnOx particles decorated N-graphene (4.6 at. N%) and oxidized metal sulfide anchored N-graphene sheets (3.1 at. N%) are produced at atmospheric conditions. High structural quality and distribution of metal-based nanostructures on N-graphene sheets are revealed using transmission and scanning electron microscopes and other advanced spectroscopic techniques. Finally, an electrode for supercapacitor based on the N-graphene-metal-oxide(sulfide) hybrid nanostructures is developed with promising specific capacitances (∼273 F.g−1 at 0.5 A.g−1). The described chemically engineered process is one of the fastest approaches reported for designing the high-quality hybrid nanostructures produced at a high-yield, and as such, is expected to provide a high impact on the design of electrode materials for sustainable energy storage systems.publishedVersio

Towards large-scale in free-standing graphene and N-graphene sheets

Abstract One of the greatest challenges in the commercialization of graphene and derivatives is production of high quality material in bulk quantities at low price and in a reproducible manner. The very limited control, or even lack of, over the synthesis process is one of the main problems of conventional approaches. Herein, we present a microwave plasma-enabled scalable route for continuous, large-scale fabrication of free-standing graphene and nitrogen doped graphene sheets. The method’s crucial advantage relies on harnessing unique plasma mechanisms to control the material and energy fluxes of the main building units at the atomic scale. By tailoring the high energy density plasma environment and complementarily applying in situ IR and soft UV radiation, a controllable selective synthesis of high quality graphene sheets at 2 mg/min yield with prescribed structural qualities was achieved. Raman spectroscopy, scanning electron microscopy, high resolution transmission electron microscopy, X-ray photoelectron spectroscopy and Near Edge X-ray-absorption fine-structure spectroscopy were used to probe the morphological, chemical and microstructural features of the produced material. The method described here is scalable and show a potential for controllable, large-scale fabrication of other graphene derivatives and promotes microwave plasmas as a competitive, green, and cost-effective alternative to presently used chemical methods

N-Graphene-Metal-Oxide(Sulfide) hybrid Nanostructures: Single-step plasma-enabled approach for energy storage applications

Hybrid graphene-based nanostructures are considered promising materials for energy storage applications. However, the synthesis of high-quality hybrid graphene nanostructures at high yields is challenging. In the present work we propose a novel, single-step microwave plasma-enabled approach to synthetize customizable hybrid graphene-based nanostructures at high-yield while preserving their quality. Hybrid N-graphene (nitrogen-doped graphene) metal-based nanostructures, for instance, can be produced at a rate of ∼ 19 mg/min. The high energy density region of a microwave plasma provides sufficient energy and “building particles” fluxes towards the low-energy density plasma afterglow for the processes of assembly and growth of N-graphene sheets. Simultaneously, a controlled jet of metal-oxide(-sulfide) microparticles is sprayed into the plasma afterglow region where they bind to N-graphene sheets. Methane/methylamine are used as carbon and nitrogen precursors, combined with micron-sized MnO2 and oxy-MnS particles to synthesize the hybrid structures. As a result, nano-sized (∼10–30 nm) MnOx particles decorated N-graphene (4.6 at. N%) and oxidized metal sulfide anchored N-graphene sheets (3.1 at. N%) are produced at atmospheric conditions. High structural quality and distribution of metal-based nanostructures on N-graphene sheets are revealed using transmission and scanning electron microscopes and other advanced spectroscopic techniques. Finally, an electrode for supercapacitor based on the N-graphene-metal-oxide(sulfide) hybrid nanostructures is developed with promising specific capacitances (∼273 F.g−1 at 0.5 A.g−1). The described chemically engineered process is one of the fastest approaches reported for designing the high-quality hybrid nanostructures produced at a high-yield, and as such, is expected to provide a high impact on the design of electrode materials for sustainable energy storage systems