Characterisation Protocol for Liquid- Phase-Synthesised Graphene

Graphene, a two-dimensional honeycomb sp2 carbon lattice has received enormous attention because of the potential for various applications such as the electrodes of photovoltaic devices and batteries, next-generation flexible electronics and even antibacterial coatings. Interest in the application of graphene is mainly due to its unique physical and chemical properties, flexibility, and tuneability of the properties in graphene-based materials. However, while promising applications of graphene are being discussed, the term ‘graphene’ is often misused, and the difficulties in large-scale production of true two-dimensional graphene have further limited its applications. Methods such as top-down solution-processed exfoliation were developed to overcome the obstacles for large-scale graphene production, but these approaches do not yet produce completely delaminated and homogeneous graphene. To monitor and optimise the graphene production process, the development of a fast, standardised and reliable characterisation protocol for large-scale solution-processed graphene is therefore desirable. Among the many characteristics of graphene flakes, the nano-structural features including the lateral dimension, crystal imperfections and the thicknesses of graphene are the most important factors that affect the various properties of graphene. However, though many of the analytical techniques have continuously been improved, methods to obtain and quantify these graphene nano-structural features are still limited. This is owing to the difficulties of visualising the ultra-thin nano-flakes and the fact that many of the properties of graphene are still unknown to be used to identify the material. In this study, a characterisation protocol was proposed to quantify the fundamental nano- structural features of graphene. In all cases, the nano-structural feature was initially characterised by using the most precise technique based on direct imaging from transmission electron microscopy (TEM), the results were being used as benchmarks for the other fast but less direct methods that based on photon-probe techniques. To integrate and assess different characterisation techniques, quantification and statistical analysis of results have been used. By utilising the method proposed, it was found that the lateral dimension distribution of graphene can be rapidly obtained by Dynamic Light Scattering (DLS), especially for flakes smaller than 1000 nm. The crystalline imperfections within graphene can be obtained and quantified by conventional Raman spectroscopy, in which a simple method based on linear correlation and random sampling was proposed to indicate the source of disorder in graphene samples. The result was compared to the TEM study, and the differences were assigned to the uneven distribution of the defects in graphene flakes. The thickness of graphene was characterised via various techniques. Several empirical equations were derived in order to can be rapidly obtained the thickness of graphene. However, it may not be feasible at this stage to develop a method to accurately determine graphene thickness for large-scale characterisation. It was found that the level of graphitic character could be obtained utilising the variation of Raman 2D (G’) band, which is rather more important, and can be used to improve the graphene synthesis process. In summary, the proposed graphene characterisation protocol offers a practical method to integrate and evaluate different characterisation techniques. Also, the protocol development method can be used as a reference point, which can be applied to other materials for developing material-specific characterisation protocols. Nevertheless, it has been shown that such a graphene characterisation protocol has the ability to quantify and differentiate between inhomogeneous solution-processed graphene samples and can be used for optimising the graphene synthesis processes.