Sheets of graphene oxide can undergo spontaneous crosslinking reactions, which essentially glue the sheets together, as a result of common drying procedures. The finding broadens scientists’ understanding of basic graphene chemistry and sounds a cautionary note about processing this 2D form of carbon, which is a common starting material for graphene-based products (Chem 2022, DOI: 10.1016/j.chempr.2022.05.016).
Graphene oxide (GO) is an inexpensive material easily prepared by oxidizing and exfoliating graphite. The ultrathin sheets are easily dispersed and processed in water, and readily reduced to graphene chemically or via other methods, making GO a convenient reagent from which to make fibers and films for applications in energy storage, electronics, and other areas.
The oxidation reaction used for preparing GO leaves the sheets covered with hydroxyl, carboxyl, and other oxygen-containing species. Many researchers have functionalized those groups to attach various types of molecules to the GO surface, often through esterification reactions. Given how well-known those reaction are, “it’s somewhat surprising that no one seems to have studied direct esterification between graphene oxide sheets,” says Jiaxing Huang of Westlake University, Hangzhou.
Huang’s group ended up studying the crosslinking process as a result of an unexpected observation. As the story goes, during a COVID-caused research lull, Haiyue Huang, a student in J. Huang’s group, unintentionally left a GO sample in a freeze dryer for a week or so. When she tried to redisperse the material in water, it didn’t behave like GO. Even after extensive stirring, the material did not form a stable suspension.
By conducting a series of experiments and analyzing GO samples with spectroscopy and microscopy, the team figured out what was going on. GO’s functional groups attract water molecules that remain in place even in air-dried samples. The molecules protect the sheets from spontaneous crosslinking, leaving them easily dispersed in water. Thoroughly drying GO with heat, vacuum, spray-drying, or other methods commonly used for processing GO, however, drives off the water molecules, enabling the sheets to react with each other. “This is very simple chemistry,” J. Huang acknowledges, “but it fundamentally alters what we know about GO sheets and suggests we should be mindful when drying them for applications.”
Sang Ouk Kim, a materials specialist at Korea Advanced Institute of Science and Technology, agrees. This is an interesting systematic study of how common processing steps affect the way GO redisperses, he says Kim suggests that the possibility of self-crosslinking has been overlooked in the graphene community, ,probably because it’s common knowledge that esterification typically requires an elevated temperature. He adds that the work clarifies how to control redispersion of GO after dehydration and it suggests ways to exploit the self-crosslinking behavior to customize the material.