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2.3.3.2. Cathodic exfoliation Since the anodic exfoliation involves oxidation at the graphite anode, it results in producing graphene either partially oxidised or chemically functionalised. Hence, non- oxidative routes of electrochemical exfoliation at graphite cathode have been realised. Wang et al. have demonstrated the use of Li in propylene carbonate to successfully intercalate the graphite at the cathode (-15 V).122 Subsequent sonication of the GIC in solvents like DMF produced high quality FLG (< 5 layers) as confirmed by XPS and Raman analysis.122 Cooper et al. have used tetraalkylammonium salts to cathodically intercalate (Figure 2.8c-g) HOPG (-2 V) producing FLG (2-5 layers) with no functionalisation and very little O (< 1 at.%, thought to be from exposure to atmosphere).121 Abdelkader et al. have used DMSO-based electrolyte containing Li and triethylammonium ions to electrochemically exfoliate graphene in gram quantities.123 They have suggested that the triethylammonium ions which are dissociated between the graphite layers to give triethylamine and hydrogen gases, that helps in spontaneous exfoliation of the flakes from the graphite electrode. This process has produced FLG with large lateral dimensions (up to 20 μm) with very little defects as analysed by Raman spectroscopy.123 Such electrochemically exfoliated large graphene sheets (20 μm) have shown increased reinforcing ability (even at 20 wt.%) in poly propylene (PP)124 and poly methyl methacrylate (PMMA)125 matrices, compared to smaller graphene sheets (5 μm). 2.3.4. Other top-down methods 2.3.4.1. High-shear exfoliation Recent studies have realised the use of high-shear rotor-stator mixer to produce large-scale exfoliation of graphene dispersions.126, 127 Paton et al. reported the graphene produced in NMP (Figure 2.9a,b,c) by high-shear rotor-stator were predominantly FLG with lateral dimensions between 200-800 nm (TEM analysis) and are unoxidised and free from basal plane defects (XPS and Raman analysis).127 They have shown regardless of laminar or turbulent flow, successful exfoliation can occur when the shear rate exceeds 104 s-1,127 which is mainly at the vicinity of the rotor-stator. Chapter 2 – Graphene: Properties and Production 53PDF Image | PRODUCTION AND APPLICATIONS OF GRAPHENE AND ITS COMPOSITES
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