PT Unknown AU Fortugno, P TI Gas-phase synthesis of graphene in a microwave plasma reactor PD 08 PY 2024 DI 10.17185/duepublico/82221 LA en DE graphene, microwave plasma AB The present work investigates the growth of carbon nanoparticles – mainly few‑layer graphene (FLG) – produced from gaseous hydrocarbons in a microwave plasma reactor. In addition to the analysis of the produced carbon particles, different in situ and inline methods are applied to characterize the synthesis process. The plasma is analyzed using optical emission spectroscopy (OES) and the concentration of molecular products formed in the exhaust gas is analyzed quantitatively and semi-quantitatively using infrared absorption and gas chromatography. The methods are additionally compared with reaction kinetic and equilibrium simulations of gas phase chemistry based on combustion mechanisms for interpretation. The growth of carbon particles for FLG is investigated using spatially-resolved thermophoretic sampling. The first part of this work examines the structure and morphology formation of the growing carbon particles along the reactor via thermophoretic sampling. It is observed that the particles change their morphology along the reactor axis. In accordance with the general theory for the growth of carbon particles, the formation of monolayer graphene for the present process is demonstrated by microscopic analysis. The first FLG particles obtained close to the plasma are almost flat. A comparison of samples taken at different points along the reactor shows that the FLG particles fold more strongly along the reactor axis and form denser structures. This process is confirmed by time‑resolved laser induced incandescence and explains the typical morphology of the obtained samples of crumpled FLG sheets. The second part of the work deals with the investigation of the influence of different precursor mixtures and concentrations on the formed carbon particle product. The precursors used are ethanol, ethylene, and various ethylene‑water mixtures. In particular, the influence of oxygen in the mixture and the resulting C/O ratio is considered and discussed. The results show in accordance with literature that low C/O ratios are favorable for the formation of FLG, whereas higher ratios lead to mixtures with graphitic or soot-like particles. At the same time, oxygen‑free precursors show the same trend when their applied concentration is reduced. The yield of carbon particles scales with the concentration of precursor used and the amount of water added. The addition of less precursor or more water decreases the particle yield. OES results indicate that the plasma temperatures in the process are the same for all experimental cases and thus different temperature conditions for particle formation can probably be excluded. The recorded FTIR spectra semi-quantitatively show changes in the concentrations of the gas molecules formed when water is added. With increasing water content, more and more CO is formed and the amount of small hydrocarbons such as C2H2 decreases. These observed trends can be reproduced with reaction kinetics simulations. Based on the results of the applied in situ methods and simulation, the influence of oxygen can be explained by a change in concentration. The oxygen present reacts with carbon to form CO and thus reduces the concentration of growth species available for the formation of carbon particles. The third part of the work attempts to transfer the knowledge gained for water to other oxygen carriers. Water is substituted by nitrous oxide (N2O), carbon dioxide (CO2) and molecular oxygen (O2). Based on simulations, a comparable reduction of particle formation is expected for all oxygen carriers. A validation of the simulations using gas chromatography shows a maximum relative deviation of ~20% for CO and C2H2. The yield of carbon particles is reduced for all oxygen carriers in line with expectations. However, the analysis of the produced particles shows that not all oxygen carriers suppress the formation of soot‑like particles. Experiments with low C/O ratios with the addition of CO2 continue to lead to the formation of different carbon particles other than FLG. Samples obtained with thermophoretic sampling already show partial differences in composition close to the plasma, i.e., close to the onset of particle formation and growth. Distinctive decomposition products of the oxygen carriers are detected using OES. In contrast to N2O and O2, CO2 spectra are characterized by the unexpected formation of C2 and C. The temperature determined from C2 luminescence is significantly higher than the other estimated temperatures and therefore indicates thermal non‑equilibrium for this species. These additional carbon species may be able to influence the first nucleation of solid particles, leading to the formation of additional soot‑like particles. ER