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2.3.2.6. Ionic liquid-based LPE Another top-down approach to produce graphene have been investigated using ionic liquids (IL) - which are defined as special salts that remain liquid below 100 oC.112-115 Due to their unique physicochemical characters: chemical and thermal stability, high ionic conductivity, wide electrochemical window, negligible vapour pressure, nonvolatility, IL are proposed as βgreenβ organic solvents.116 2.3.2.6.1. Sonication-assisted IL-based LPE Wang et al. directly exfoliated graphite in 1-butyl-3-methyl-imidazolium bis(trifluoro-methane-sulfonyl)imide ([Bmim]-[Tf2N]) using tip sonication (60 min) and centrifugation at 10000 rpm for 20 min to produce stable graphene dispersions with πΆπΊ - 0.95 mg/ml.112 TEM analysis showed the flakes are in ΞΌm-dimensions and having < 5 layers. The stabilisation mechanisms in ionic liquids were expected to be from the π β π interactions between aromatic cations and the electrostatic interactions between the π electrons of the graphitic planes and the charged ionic species. The XPS analysis of the [Bmim]-[Tf2N] exfoliated graphene showed ~10 at.% of oxygen (O) suggesting some level of oxidation, which are probably due to strong non-covalent interactions (π β π) and/or covalent functionalisation with [Bmim]-[Tf2N] itself.112 2.3.2.6.2. Grinding-assisted IL-based LPE Grinding of the staring graphite with IL has been used in several studies for exfoliating graphene.113-115 Nuvoli et al. have used of 1-hexyl-3-methylimidazolium hexafluorophosphate ([HMIM][PF6]) to exfoliate graphite to graphene.113 Firstly, they grind the graphite flakes with [HMIM][PF6] in a mortar followed by bath sonication for 24 hr and centrifugation at 4000 rpm for 30 min, resulting in stable dispersion with πΆπΊ β 5.33 mg/ml. AFM analysis of the exfoliated flakes show lateral dimensions of 3-4 ΞΌm with average apparent height of 2 nm and Raman spectra confirming the FLG nature. Chloroform was used to wash the IL for producing powdered graphene.113 Shang et al. have developed a method to produce graphene dispersions by using mechanical grinding of small quantities of IL (0.1-0.5 ml) such as 1-butyl-3- methylimidazolium hexafluorophosphate ([BMIM][PF6]) with graphite.114 The graphite-IL mixture was grinded in a mortar for 0.5-4 hr turns into a gel due to the π β π interactions Chapter 2 β Graphene: Properties and Production 50PDF Image | PRODUCTION AND APPLICATIONS OF GRAPHENE AND ITS COMPOSITES
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