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2.3.2.2. Surfactant-based LPE The use of surfactant molecules, like small organic molecules or polymers, can effectively exfoliate graphite to graphene both in aqueous and in organic solvents. Such surfactants have a high affinity towards the hydrophobic graphene surface and stabilise the graphene sheets against aggregation by means of electrostatic repulsion or steric stabilisation. 2.3.2.2.1. LPE in aqueous media The use of water is ideal especially for biological studies due to its non-toxic nature. However due to grapheneโs hydrophobic nature, LPE of graphite in water is challenging. This can be overcome by use of surfactants to stabilise the graphene in aqueous media. Lotya et al. have shown successful exfoliation of graphite in aqueous ionic surfactants like sodium dodecylbenzenesulfonate (SDBS)77 and sodium cholate (SC).60 The ๐ถ๐บ obtained for SDBS-based dispersion are 0.002-0.05 mg/ml depending on starting graphite concentration (sonication 30 min, centrifugation 500 rpm for 90 min) and by increasing the sonication time to 400 hrs ๐ถ๐บ โ 0.3 mg/ml was achieved for SC- based dispersion. Unlike for NMP exfoliated graphene, the flake sizes of graphene doesnโt reduce significantly on longer sonication time.59 Although the monolayer fraction obtained is lower than the solvent exfoliated graphene, the flakes were found to be relatively defect and oxide free.60, 77 Hersam et al. have also used SC to exfoliate graphene in water using intense tip sonication with ๐ถ๐บ โ 0.09 mg/ml and the SC-coated graphene sheets were separated based on thickness by density gradient ultracentrifugation.78 Up to ~80% monolayer can be separated using this process, and it was shown to improve the electrical and optical performance of transparent conductive films prepared from these flakes. Sun et al. optimised the sonication parameters (including starting surfactant and graphite concentrations, sonication time etc.) and demonstrated by using sodium taurodeoxycholate (TDOC) high ๐ถ๐บ โ 7.1 mg/ml with 8 % monolayer and 82 % < 5 layers (TEM) can be achieved.79 In such ionic surfactant systems, the stability against aggregation is mainly due to the electrostatic repulsion between the surfactant-coated graphene sheets, and can be characterised by the ๐ values. Smith et al. studied wide range of surfactants and found Chapter 2 โ Graphene: Properties and Production 42PDF Image | PRODUCTION AND APPLICATIONS OF GRAPHENE AND ITS COMPOSITES
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