Growth and Morphology Evolution of Plasma-Synthesized Few-Layer Graphene

Graphene flakes are an attractive material due to their unique properties, making them promising to use for a wide variety of applications. Examples of these applications are using them as additives (to alter electrical, thermal, and mechanical properties of other materials e.g., polymer composites) or to improve the electrochemical performance in supercapacitors and batteries. However, these applications require industrial mass-production quantities of graphene powder, which are still challenging to achieve. To overcome the current batch-to-batch drawbacks from the commonly-used exfoliation processes for mass-production of graphene powder, substrate-free gas-phase plasma synthesis has emerged during the last years as an effective method to continuously synthesize freestanding few-layer graphene (FLG). This technique has the advantage to operate in a continuous mode and the potential to be scalable. During the plasma synthesis, a hydrocarbon precursor is pyrolyzed within a high-temperature plasma region. Downstream of this region, the gaseous carbon species nucleate and form FLG, leading to carbon yields as high as 10 wt%.<br/><br/>This work examines the evolution of growth and morphology of few-layer graphene (FLG) synthesized in a microwave-plasma reactor. Aerosol particles were sampled and characterized from various positions downstream of the plasma zone by spatially-resolved thermophoretic sampling and transmission electron microscopy (TEM), Raman spectroscopy, and BET surface area analysis (BET-SSA).<br/><br/>The initial carbon nucleation has been found to commence close to the plasma zone (at less than 12.4 cm downstream from the plasma nozzle) and at a temperature of ≥ 2500 °C. The initially formed flakes show an increasing level of crumpling with increasing distance downstream from the plasma zone. From the TEM images of samples collected at different heights above the plasma nozzle, we observed a growth and self-folding pattern for FLG, providing a hypothesis for their formation from single- to few-layer graphene when the materials are carried downstream the plasma region. This hypothesis consists of the creation of one-layer ovalene-shaped graphene flakes that self-folds from the sides and crumples while it continues to grow, creating wrinkled FLG flakes as these move further away from the plasma nozzle.