Cox, Kathleen Marie; (2018) Characterisation of two dimensional nanomaterials produced via spontaneous liquid exfoliation. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Developing scalable nanomaterial production methods is necessary for realising nanomaterial commercialisation. In principle, production via Liquid Phase Exfoliation satisfies this need. However, techniques reliant on energy input damage the material via mechanical stress, yielding suspensions of multi-layer stacks, stable only for days, and necessitating centrifugation for manipulation. An alternative, emerging technique relies upon the charging of material to allow spontaneous dissolution of pristine 2d nanomaterials. Here this is explored for pnictogen chalcogenide layered materials, focusing on unanswered questions relating to the practicality of the method. In this thesis, ion intercalated Bi2Te3 and Sb2Te3 were dissolved within the aprotic solvents: N-methyl-2-pyrrolidone (NMP) and dimethylformamide (DMF). Successful exfoliation of undamaged, hexagonal 2d nanomaterials was confirmed. A range of complementary experimental techniques were used including TEM, AFM, and SAXS. From the analysis of thousands of nanosheets it was found that gradual diffusion of nanosheets, as a result of their spontaneous exfoliation, lead to fractionation of nanosheets of differing lateral width throughout the liquid volume without need for centrifugation. Nanosheet lateral dimension was also controlled by stoichiometry of the intercalant metal, with an optimum intercalant stoichiometry of 0.1 < x < 1.5 for Kx. Bi2Te3 for production of pristine nanomaterial. The chemical stability of the solution was investigated in relation to exposure to air, water, and heating, with a focus on tellurium impurities. Using SEM and TEM it was shown that tellurium impurities resulted from the presence of alkali metal polytellurides, which could be minimised by optimising the Kx. Bi2Te3 stoichiometry. However, the existence of nanosheets in a 16 month old solution demonstrates stability of these liquids when handled under inert conditions. Together these results demonstrate that this scalable method allows material manipulation and tailoring of nanosheet dimensions, whilst also giving weight to the argument that the liquids can be described as true thermodynamic solutions.

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