Abstract
Direct assembling of N-graphene, i.e. nitrogen doped graphene, in a controllable manner was achieved using microwave plasmas at atmospheric pressure conditions. The synthesis is accomplished via a single step using ethanol and ammonia as carbon and nitrogen precursors. Tailoring of the high-energy density plasma environment results in a selective synthesis of N-graphene (~0.4% doping level) in a narrow range of externally controlled operational conditions, i.e. precursor and background gas fluxes, plasma reactor design and microwave power. Applying infrared (IR) and ultraviolet (UV) irradiation to the flow of free-standing sheets in the post-plasma zone carries out changes in the percentage of sp2, the N doping type and the oxygen functionalities. X-ray photoelectron spectroscopy (XPS) revealed the relative extension of the graphene sheets π-system and the type of nitrogen chemical functions present in the lattice structure. Scanning Electron microscopy (SEM), Transmission Electron microscopy (TEM) and Raman spectroscopy were applied to determine morphological and structural characteristics of the sheets. Optical emission and FT-IR spectroscopy were applied for characterization of the high-energy density plasma environment and outlet gas stream. Electrochemical measurements were also performed to elucidate the electrochemical behavior of NG for supercapacitor applications.
Highlights
Beyond its unique set of physico-chemical properties, graphene can be considered as a robust, atomic scale scaffold from which other 2D materials can be derived through the attachment of foreign atoms and functional groups[1,2,3,4]
Optical emission spectroscopy allows the identification of the active carbon/nitrogen species in the plasma that are the “building units” for the synthesis and growth of the nanostructures in the mild plasma zone[39,40,41,42,43]
The emission spectrum of argon/ethanol/ammonia plasma reveals the presence of new molecular and atomic species such as CN, C2 (Swan system, between 450–570 nm, A3Πg → X′3Πu), CH band, the hydrogen Balmer-alpha line Hα (656.3 nm) and several Ar lines. These species are formed as a result of ethanol/ammonia decomposition and intensified chemistry in the “hot” plasma zone
Summary
Beyond its unique set of physico-chemical properties, graphene can be considered as a robust, atomic scale scaffold from which other 2D materials can be derived through the attachment of foreign atoms and functional groups[1,2,3,4]. The substitution of carbon atoms by nitrogen ones influences the atomic charge distribution on the graphene scaffold and creates “active sites” significantly increasing the electrochemical activity of nitrogen-doped graphene, known as N-graphene (NG)[4]. The theoretical studies[4,5] predicted modified electronic and chemical properties of nitrogen-doped graphene, which is being supported by numerous experimental investigations. NG demonstrates better chemical reactivity and sheet-to-sheet separation than the pristine graphene[9] To this end, N-doping is a very promising approach for the development of metal-free carbon-based catalysts with even better performance than commercially available Pt-based electrodes, with prospective impact on fuel cells’ commercialisation. N-doped graphene nanoribbons were tested as conductive host materials in lithium-sulfur (Li-S) batteries, demonstrating stronger fixation of sulfur-containing species and higher stability, as compared with pristine graphene nanoribbons[13]. It was demonstrated that NG can be used to efficiently detect trace amounts of certain molecules
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