Sustainable synthesis of two-dimensional materials via liquid phase exfoliation: graphene and beyond

Thesis

The discovery of graphene, a single layer of graphite, is a game changer that has revolutionised the field of materials science due to the superior properties it owns; ranging from high electrical conductivity, high mechanical strength to large surface area. All these attributes make graphene a promising material for diverse applications in flexible electronic and photonic devices, as well as concrete additives and renewable energy technologies. While the prospects of graphene seem boundless, the challenges for increasing its yield and lowering its production costs persist. These limit its widespread industrial applications. Furthermore, the synthesis processes of graphene often involve the use of toxic chemicals, calling for sustainability concerns. Hence, there is a critical need for the search of a scalable, efficient, and sustainable graphene production method.

Liquid Phase Exfoliation (LPE) synthesis technique emerges as a cost-effective approach, producing graphene dispersions compatible to electronic device printing technologies. However, the reliance on toxic solvent like 1-Methyl-2-pyrrolidinone (NMP) in the LPE processes hampers the progress of LPE. Despite the efforts to replace NMP with greener alternatives like ethanol and isopropanol, graphene yield remains significantly lower than that in NMP. The main advantage of using green solvents with low boiling points and low toxicity, is that they can be easily removed by low temperature evaporation without degrading the graphene quality. The commonly used methods to enhance the graphene yields in green solvents often involve functionalisation of graphene and the use of chemical surfactants/dispersants. These processes degrade graphene quality and require additional treatment steps to remove the surfactants/dispersants.

The present work focuses on the shear mixing technique, one of the LPE techniques used in graphene production. The potential of using green solvents to replace the currently state-of-art NMP is addressed. The main problem associated with low graphene yields in green solvents has been identified and revealed, for the first time, by the NMP-redispersion technique. Subsequently, a pioneering yield enhancement method was developed. This method can achieve a remarkable 90-fold increase in graphene yield, negating the use of dispersants, surfactants, and functionalisation. The efficacy of this technique extends beyond graphene, as may be evident from its ability to enhance the yield of hexagonal Boron Nitride (a layered material sharing the same hexagonal lattice structure as graphite). Each of the experimental chapters unfolds the new dimensions and research insights into the scalability, efficiency, and sustainability of graphene production and of other two-dimensional materials.

Authors: Ng, KL

• DOI: 10.5287/ora-xm6b0pp41
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: NMP; ultrasonication; exfoliation efficiency; hansen solubility parameters; liquid phase exfoliation; green solvents; hexagonal boron nitride; graphene; two-dimensional materials; dispersibility
• Subjects: Synthesis; Nanostructured materials; Graphene