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the π values ranged between Β± 25-65 mV.80 Interestingly, they found the πΆπΊ scaled with square of the π values, meaning πΆπΊ is proportional to the magnitude of the electrostatic potential barrier, which stabilises the surfactant-coated graphene against aggregation.3, 80 Recent studies have shown that addition of alcohols like ethanol to the aqueous surfactant (sodium dodecyl sulfate-SDS) solutions can enhance the πΆπΊ up to 3-10 times.81, 82 Addition of alcohol was shown to reduce the surface tension of the solvent system82 and also decrease the mixing enthalpy with improved stability of surfactants81 thereby enhancing the final πΆπΊ. Biodegradable organic molecules like urea83 and cellulose nanocrystals (CNC)84 were also used as surfactants in producing graphene. High quality FLG with πΆπΊ β 0.15 mg/ml was obtained for urea-assisted exfoliation which is higher yield than exfoliation in DMF under similar experimental conditions.83 It was proposed that the two primary amine groups of urea are responsible for the exfoliation and stability mechanism: an βNH2 group of the urea readily adsorbs on the newly created graphene surface during sonication while the other βNH2 group undergoes reaction with CO2 from air producing β NHCOOH, which ionises in water thereby stabilising the graphene sheets upon electrostatic repulsion.83 CNC-assisted LPE of graphene produced πΆπΊ > 1 mg/ml with 75% of measured flakes are SLG (AFM analysis) and the stability was due to the electrostatic repulsion from adsorbed sulfate groups.84 Due to their higher sulfate content, the πΆπΊ obtained by CNC-assisted exfoliation was > 3 times than SDS-assisted exfoliation under same experimental conditions.84 Aromatic organic molecules like pyrene and its derivatives have been explored for exfoliating graphene in aqueous media. The adsorption of such molecules on graphene surface happens via π β π interactions by sharing their electrons of the π- orbitals through a non-covalent bond. Zhang et al. have produced FLG in aqueous solutions using 1,3,6,8-pyrenetetrasulfonic acid tertasodium salt (Py-(SO3)4) and 1-pyrenemethylamine hydrochloride (Py-NH2) to fabricate transparent conductive films.85 Parviz et al. have explored use of several pyrene derivatives in exfoliating graphene in water. Among all the derivatives tested, 1-pyrenesulfonic acid sodium salt (Py-SO3) was found to be more effective in exfoliation with πΆπΊ of 0.8-1 mg/ml and the HRTEM results reveal 2-4 layers.86 The presence of sulfonyl group had resulted in good stability over wide range of pH values Chapter 2 β Graphene: Properties and Production 43PDF Image | PRODUCTION AND APPLICATIONS OF GRAPHENE AND ITS COMPOSITES
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