Physical and chemical characterisation of exfoliated layered nanomaterials

Liquid phase exfoliation (LPE) is a versatile and scalable production technique for two-dimensional nanomaterials, such as graphene and molybdenum disulfide (MoS2). Solution processing enables a wide range of applications, many of which are sensitive to nanosheet microscopic properties, including size, thickness and functionalisation. Yet these nanosheets remain poorly characterised with the lack of standardisation. A method to sonochemically edge functionalise MoS2 in acetone is detailed here;away of producing stable dispersions over extended periods of time (over one year) at high concentrations. By using a range of techniques, it is shown that this stabilisation is achieved through a self-limiting oxidation of MoS2 at the edges. The method results in enhanced catalytic performance for MoS2 and potentially other sulfur containing layered materials. In addition, a general method to reconstruct nanosheets size and thickness distributions based on Raman spectroscopic metrics is demonstrated with graphene and MoS2. This is essential for any research that relies on quantifying the influences of size and thickness on applications, such as mechanical reinforcement, electrical conductivity, sensing, and catalysis. A new metric for characterising layer number of MoS2 nanosheets is developed using an intensity ratio of resonant Raman modes. Raman spectroscopy is less time consuming and less dependent on sample preparation when compared to microscopic characterisation techniques that yield the same information. The method presented here is more robust than current literature metric as it does not rely on mode positions, which shift depending on factors inherent to the sample such as strain, doping, and defect density. The metric was developed for LPE nanosheets but it can also be applied to mechanically exfoliated sheets. The first proposed metric for LPE nanosheet length was developed using the main Raman modes of MoS2 for resonant spectra, showing excellent agreement with microscopic measurements. It is anticipated this combination of mapping and metric analysis can be extended to other materials, paving the way for a much-needed standardisation for industry and laboratory research applications of layered nanomaterials.