Exploration and Functionalisation of Plasma-Exfoliated Graphene-Based Materials for Advanced Applications

  • Rachel McLaren

Student thesis: Doctoral Thesis

Abstract

Graphitic materials describe those containing carbon within a honeycomb lattice structure and have attracted vast interest due to their novel and advantageous properties. Whilst large amounts of work have been conducted to investigate the nature of such materials, in the context of graphite, graphene and other carbon allotropes, there are still many voids in our understanding of these materials, extending to their structure, modification, characterisation, and application. Plasma-exfoliation offers a facile approach towards the large-scale synthesis of partially oxidised, multilayered stacks of graphitic layers. Within the scope of this thesis, plasma-exfoliated multilayer graphitic materials (NPs) are investigated in detail using a vast array of characterisation techniques to reveal information regarding their morphology, structure and elemental composition; including X-ray Photoelectron Spectroscopy (XPS), Raman spectroscopy, X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Atomic Force Microscopy (AFM), gas physisorption analysis, Nuclear Magnetic Resonance Spectroscopy (NMR), Fourier-Transform Infrared Spectroscopy (FT-IR), Thermogravimetric Analysis (TGA) and Energy-Dispersive X-ray (EDX) analysis. Various novel functionalisation strategies are also explored to covalently and non-covalently functionalise NPs.

Initially, an aryl boronic acid precursor is utilised as a radical source to provide a facile route towards the covalent functionalisation of NP. Such a methodology serves to identify safer aryl radical precursors as an alternative to commonly employed diazonium salt reagents, which are typically hazardous and undesirable for large-scale application. This functionalisation strategy enables the covalent attachment of 4-(trifluoromethyl)phenyl moieties to the edges of NP1 stacks. A percentage atomic composition (at.%) of 3.5 fluorine incorporation is revealed via XPS and Raman spectroscopy, TGA, FT-IR and EDX provide evidence of successful functionalisation. Brunauer-Emmett-Teller (BET), Barrett-Joyner-Halenda (BJH), SEM and TEM analyses are utilised to provide information relating to the surface area, porosity and morphological changes upon covalent addition. Furthermore, XRD is employed to assess the interlayer spacings and stacking types within the functionalised material and its precursor, revealing the presence of hexagonal and rhombohedral stacking, typical of those found within commercially derived graphene-based materials.

The thesis then moves towards investigating the porous structure of NPs in more detail. It is found that NP materials incorporate extensive slit pores between stacks of partially oxidised graphitic layers. These vary in size across the mesoporous and macroporous regions, where the average slit pore widths range between 6.3 - 10.4 nm. BJH data is utilised to provide an estimation of these slit pore widths within a model NP2 material, which are subsequently correlated with the distance between stacks. Slit pores between 2 to 131.2 nm in size are identified and thus, BJH analysis is shown to be a straightforward technique to offer an insight to the stack separations. This analysis coincides with data obtained using SEM, AFM and XRD techniques. Furthermore, the investigation also explores the surface area and porosity changes of NP material upon oxidation. Upon treatment of NP3 via the modified Hummers’ method, the material suffers a substantial reduction in the BET surface area and porosity, due to the blockage and destruction of slit pores. As such, a novel pillaring strategy utilising the synthetic clay, laponite, as a pillaring device, enables the introduction of slit pores into the oxidised material. This enhances its surface area almost four-fold. The subsequent pillared material is explored utilising a range of characterisation methods, including solid-state NMR and XPS.

Finally, NP4 is non-covalently functionalised with polyelectrolyte and fluorosurfactant moieties to reveal its application within membrane technology. The subsequent composite material exhibits oleophobic properties and is used to synthesise free-standing films and to coat upon various substrates, including Kevlar, carbon fibre, glass fibre, nylon, and stainless-steel mesh. These materials found success in their ability to enable oil/water separation, thus granting them simultaneous oleophobic/hydrophilic properties. The composite structure, free-standing films and coated substrates are comprehensively investigated using various analytical and spectroscopic techniques, whilst contact angle measurements and sliding angle measurements provide evidence of oleophobic behaviour. High oil contact angles of up to 126.4° are obtained. SEM images reveal an interesting porous structure, which provides passage for water through the materials. The applications of the coated substrates are also investigated. It is found that coated carbon fibre can be used as a net to isolate oil from the surface of water, thus, granting it potential in the context of oil-spill clean-up. Meanwhile, coated nylon has properties which lend to its application as a membrane material which enables the passive removal of water from aircraft propellant tanks.

Whilst investigating the structures of NP, as well as exploring modification strategies and applications, many obstacles were encountered during the research. These typically related to the characterisation and processing of the materials. These issues are therefore also reviewed and discussed in the context of the literature, providing an insight into some of the difficulties researchers may encounter whilst working with NPs and analogues materials. As such, the scope of this thesis provides various novel insights to enhance the understanding of NP materials and their modification.
Date of AwardNov 2021
Original languageEnglish
Awarding Institution
  • University of South Wales
SponsorsKESSII & Perpetuus Carbon Technologies
SupervisorGareth Owen (Supervisor) & Christian Laycock (Supervisor)

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