(328b) Crumpling and Unfolding Behavior of Spray Dried Pristine Graphene Sheets
In the present work, we report crumpling of pristine graphene sheets using a scalable industrial spray drying process. Aqueous dispersions of pristine graphene sheets stabilized by pyrene derivatives were atomized in a spray dryer. The morphology transition from a 2D sheet to a 3D crumpled particle was investigated by collecting samples at various heights inside the spray dryer. During drying, a shell of graphene sheets form at the droplet surface. As the droplet shrinks, the capillary forces induce deformation of the shell surface. Initially, the pristine graphene shell folds inward at the edges; where the stiffness is less. Further increase in surface tension forces results in bending of the graphene sheets and buckling of the shell. Graphene oxide (GO) sheets which were spray dried using the same procedure showed a different morphology; these particle surfaces were highly wrinkled, and less buckling was observed. This difference in final morphology is attributed to differences in surface chemistry of pristine graphene and GO sheets. Spray drying parameters such as atomizer pressure, drying temperature and concentration of sheets in the dispersions were varied in order to observe their effect on the final morphology of crumpled particles. Also, other stabilizers such as polymers and surfactants were applied to assess the role of second component in the final morphology of crumpled pristine graphene sheets. Unfolding behavior of crumpled graphene and GO sheets was also studied by redispersing the spray-dried back into various solvents. While pristine graphene sheets immediately unfolded in the water due to the high affinity of pyrene derivatives for water, GO particles preserved their crumpled morphology in water. This can be explained by hydrogen bonds formed between GO sheets in crumpled particles which prevent the unfolding in the solvent.