Liquid Exfoliated Graphene: A Practical Method for Increasing Loading and Producing Thin Films

Abstract

Liquid exfoliation of graphene offers the potential of an easy, simple, and scalable means for the production of high quality graphene sheets. Increasing the loading of exfoliated graphene dispersions is necessary to increase efficiency and make liquid exfoliation a practical method for graphene synthesis. This publication demonstrates the use of NH 3 as an additive to significantly increase the loading of graphene within 1-methyl-2-pyrrolidinone (NMP) dispersions. The simple synthesis of graphene films by addition of the dispersion to water is also demonstrated. The films, made from a disordered collection of graphene flakes, are predominantly constructed from few, <5, layered graphene. © The Author Since its discovery, graphene has been suggested as a new "wonder" material for a variety of applications including transparent electrode materials; 1,2 transistors; 3,4 thermal interfaces; 5 conductive polymer inks;6-8 corrosion resistant coatings 8,9 and bio-coatings; 10,11 and sensors 12-14 among many others. [15][16][17][18] While the electronics sector can benefit from graphene produced by chemical vapor deposition (CVD) for the replacement of indium tin oxide (ITO), 1 graphene produced by CVD may not be as economical for other applications such as reinforcing material in polymer-based composites. 19Liquid exfoliation of graphite to produce graphene is a scalable process to cleave graphite to produce graphene sheets. The process of liquid exfoliation is based on placing graphene within a solvent which expands and helps separate the weakly bonded graphene layers of graphite.19-22 Sonication of the solvent expanded graphite mixture results in the production of single and multi-layered graphene sheets.There have been several publications 19,22 and reviews 20 that describe the sonication and centrifugation dependence of liquid exfoliation on final graphene loading. Sonication produces a dispersion containing sheets of thickness ranging from single layered graphene to multi-layered graphene, including expanded graphite. Centrifugation, at appropriate rates, precipitates the heavier layers and can preserve predominantly single layered graphene within the dispersion, though some multilayered graphene, <5 layers, may persist. [19][20][21][22] This publication reports the inclusion of ammonium hydroxide {NH 4 OH} within the liquid exfoliation process to increase graphene loadings. The inclusion of ammonia {NH 3 } within dispersions has most notably been reported for graphene oxide reduction 23 as a means of preventing sheet aggregation during reduction with hydrazine {N 2 H 4 }. The addition of {NH 4 OH} provides a facile means of including NH 3 in order to prevent aggregation of liquid exfoliated graphene, increase graphene loading and improve dispersion stability. Dripping the highly loaded graphene dispersions into water is demonstrated as a simple method of graphene film formation and provides a qualitative means of comparing dispersion concentrations. The effect of NH 3 on graphene loading and film forming capacity...

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