Experimental study proposes mechanism for crumpled graphene formation and growth
Plasma synthesis of free-standing, few-layer graphene (FLG) was first reported in 2007. Since then, the process has undergone further development and refinement, which has led to overall improvements in yield. However, there has been a lack of experimental studies into the specifics of FLG synthesis – namely, the underlying details of FLG formation and structural growth. Numerous theoretical and computational investigations have proposed a ‘stacking’ mechanism for graphene sheets, but this has yet to be confirmed experimentally. In addition, no experimental studies have explained the ever-present crumpling observed in free-standing FLG structures.
A group of researchers from the University of Duisburg-Essen in Germany set out to fill that information gap. Writing in the latest issue of Carbon [DOI: 10.1016/j.carbon.2023.118732], they report on a conceptual model they have developed, via experiment, to accurately describe what happens during the initial steps of FLG formation.
Their experimental setup – first described in a paper published in December 2023 – consisted of a microwave-plasma reactor with a gas inlet, a thermophoretic sampling device, and a fibre filter for product harvesting. The reactant vapour was ethanol, which is known to produce high-quality crumpled FLG. A mix of argon and hydrogen was used as the sheath gas. Ethanol vapor was injected into the centre of the microwave plasma zone at a fixed flow rate. Downstream of that, hot gases carrying the atomic and molecular decomposition products cooled, forming FLG via nucleation and growth. In this system, FLG formation began at <12 cm from the plasma nozzle.
The synthesized particles were harvested from the post-plasma flow via thermophoretic sampling. This involved rapidly inserting a transmission electron microscopy (TEM) grid to specific positions in the reactor; in this case, at 12.4, 14.4, 19.4, and 24.4 cm from the plasma nozzle. The exposure period at each position was ~10 ms. Each grid was inserted at the same position 20 times to increase the amount of collected material. To ensure that the TEM grid material had no influence on the morphology of the FLG, samples were collected on both lacey carbon TEM grids and closed carbon-film-supported copper grids. This approach allowed the researchers to get ‘snapshots’ of the graphene formation process.
TEM analysis of these samples revealed a progressive pattern of morphology evolution for single- and few-layer graphene. Firstly, carbonaceous species nucleate and grow to form rounded, single-layer graphene sheets. These sheets continue to grow in stage two, while retaining their shape. In the absence of any interactions with a substrate, and beyond a sheet diameter of ≤370 nm, each laterally-growing graphene sheet becomes unstable, and begins to change shape. The team found that this change follows one of two pathways – the graphene sheet starts to crumple randomly, or the graphene self-folds as an envelope and/or it curls onto itself. Their results suggest that at different nozzle distances, curled, self-folded, and crumpled flake structures co-exist.
An additional finding was that when an FLG flake folds ‘neatly’, it forms sharp edges, at which no further graphene forms. Evidence of two consecutive self-folding events was also seen, and it led to the creation of ~90° flake edges. However, reactive positions (i.e., dangling bonds or hydrogen bonds) form away from those terminated edges, which is what allows growth to continue. It’s this growth that leads to the final form of arbitrarily crumpled FLG.
The authors write that, “All the observations from this work align with the most recent literature published using MD [molecular dynamics] simulation approaches, yet showing for the first time the FLG evolution using an experimental approach.
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Claudia-F. López-Cámara, Paolo Fortugno, Markus Heidelmann, Hartmut Wiggers, Christof Schulz. “Graphene self-folding: Evolution of free-standing few-layer graphene in plasma synthesis,” Carbon 218 (2024) 118732. DOI: 10.1016/j.carbon.2023.118732