Understanding Graphene Inception and Growing by Investigating Early-Stage Particle Formation

Microwave-assisted plasma synthesis is a promising pathway for large-scale production of few-layer graphene flakes. One of the challenges associated to the advancement of this process, is to develop a better understanding of the stages of inception and growth of graphene, as well as of the connection between the synthesis conditions, the reaction conditions in the gas phase, and the quality of the material obtained.<br/>In a microwave-assisted plasma reactor, the carbon-bearing precursor enters the plasma region and gets decomposed into smaller reactive fragments, such as C, H, H₂, C₂, C₂H₂, and CO. These reactive fragments then leave the plasma region, where they form nuclei that rapidly grow into graphene¹. The literature suggests connections between the type of structure formed and the temperature gradient, residence time, and carbon concentration in the nucleation and growing zones². Further, the use of low precursor content is common to prevent or reduce soot formation.<br/>In this work, ethanol and ethylene are used as precursor to study graphene formation. The first one is a common precursor used in the literature, and the second one is a known key intermediate species during graphene formation. Key species formed during precursor decomposition which later become building blocks for graphene formation downstream have been analyzed in situ and through exhaust gas sampling.<br/>Optical emission spectrometry (OES) has been performed at different heights above the nozzle of the reactor, and light collection with a plano-convex lens was used to provide good spatial resolution. Following the approach described in Ref. ³, several wavelength regions of key species light emission were selected, and for each wavelength region the measurement conditions were fixed for proper comparison of signals at different heights above the nozzle (HAN). Spectra corresponding to C and C₂ appeared at higher HAN. When increasing HAN, light emission corresponding to CH first increases and then decreases in intensity for both precursors. Emission from the H_β Balmer, important for determination the electron density, could be well distinguished from background radiation. With ethylene as precursor, C, CH, C₂, and H emissions were stronger than with ethanol. Analysis of collected material by TEM show characteristic folded layers of graphene. With ethylene as precursor graphene and soot were formed in parallel, while ethanol led to the formation of graphene only. FTIR, light scattering, and thermophoretic sampling from the post-plasma region will provide further information about particle nucleation and growth.<br/>References<br/>1. Dato, A. Graphene synthesized in atmospheric plasmas - A review. J. Mater. Res. 34, 214–230 (2019).<br/>2. N. Bundaleska, D. Tsyganov, A. Dias, E. Felizardo, J. Henriques, F.M. Dias, M. Abrashev, J. Kissovski, A. Tatarova Microwave plasma enabled synthesis of free-standing carbon nanostructures at atmospheric pressure conditions. Phys. Chem. Chem. Phys. 20, 13810–13824 (2018).<br/>3. P. Fortugno, S. Musikhin, X. Shi, H. Wang, H. Wiggers and C. Schulz, Synthesis of freestanding few-layer graphene in microwave plasma: The role of oxygen. Carbon 186, 560–573 (2022).