Liquid Phase Exfoliation of 2D Materials under Ambient Conditions
Citation:Boland, John Brendan, Liquid Phase Exfoliation of 2D Materials under Ambient Conditions, Trinity College Dublin. School of Physics, 2021
A technique developed by the Coleman group in 2008 called liquid phase exfoliation (LPE) has been shown to be effective for the delamination of any material exhibiting a layered structure. Molybdenum dioxide (MoO2) was identified as promising candidate for exfoliation, and a natural successor to extensive work carried out previously on molybdenum trioxide (MoO3). This material shows promise for a number of applications in the electrochemical energy storage arena. Mostly studied as a bulk layered material, MoO2has not previously been exfoliated in large quantities.Chapter 3 of this thesis describes the work performed with MoO2, the primary goal of which was to use the increased surface area of the exfoliated material to maximise lithium storage capacity. After preliminary research the objectives were also revised to include achieving an understanding of the oxidation process of MoO2in ambient conditions and identifying a means to slow this process to facilitate battery fabrication.Initial tests of MoO2dispersions indicated a layered structure, contradicting the general perception of the material. Exfoliation conditions such as sonication time and subsequent centrifugation rates were optimised, and isopropanol (IPA) was identified as a suitable solvent for dispersion of the nanosheets.It was found that when dispersed in IPA under ambient conditions, MoO2nanosheets are gradually oxidized to higher oxides such as MoO3over a period of days. Conversely, if the nanosheets are processed into dried films immediately after exfoliation, and before oxidation has had a chance to progress, the nanosheets are relatively stable under ambient conditions, remaining unoxidised unless the films are heated. It was found that MoO2nanosheets could be size selected by controlled centrifugation and showed size-dependent optical properties, which allowed the proposal of spectroscopic metrics for concentration and size-estimation from extinction spectra. Liquid-exfoliated nanosheets were used to produce lithium ion battery anodes with capacities up to 1140 mAhg-1.Thepropensity of many 2D materials to oxidizein ambient conditions can complicate production and limit applications potential. Described in Chapter 4 is ambient liquid phase exfoliation of germanium sulfide (GeS), a layered material known for its chemical instability.The goal of this research was to gain an understanding of the degradation process and to potentially identify a means of alleviating the problem.The previously optimized parameters from the work on MoO2were utilised to easily identify the ideal exfoliation conditions for this material. Ambient exfoliation in organic solvents such as N-methyl-pyrrolidone yields good quality multi-layer GeS nanosheets. Although oxidation appears to occur with a time constant of ~10 days, the data suggests it to be limited to nanosheet edges leaving the basal plane intact. The rate of oxidation is slow enough to allow processing of the dispersions. For example, it was possible to size-select GeS nanosheets and characterize the size-dependence of nanosheet optical properties, leading to the observation of significant changes in bandgap with nanosheet thickness. Additionally, it is possible to incorporate the nanosheets into lithium ion battery anodes using carbon nanotubes as both binder and conductive additive. These electrodes were relatively stable and displayed near-theoretical capacity of 1523 mAh/g. Chapter 5 of this thesis entails the preliminary work in the extension of LPE to a new application –light emission. Light-emitting electrochemical cells (LECs) were first demonstrated in 1995 and have since been lauded as a potential successor to OLEDs for solid state artificial lighting due to their simpler architecture and lower production cost when compared to state of the art OLEDs. Solution processable luminescent material is ideally required for the fabrication of these devices and given that LPE dispersions can readily be spray-deposited, they potentially provide a largely untapped source of light-emitting material to compete with those currently in circulation.
Author: Boland, John Brendan
Publisher:Trinity College Dublin. School of Physics. Discipline of Physics
Type of material:Thesis
Availability:Full text available