Dispersion and Stabilization of Exfoliated Graphene in Ionic Liquids.

Emilie Bordes,Jean-Michel Andanson,Bishoy Morcos,Anass Benayad,Pierre-Olivier Bussière,Catherine C. Santini,David Bourgogne,Agílio A. H. Pádua,Margarida Costa Gomes

doi:10.3389/fchem.2019.00223

Abstract

The liquid-phase exfoliation of graphite is one of the most promising methods to increase production and commercial availability of graphene. Because ionic liquids can be easily obtained with chosen molecular structures and tuneable physicochemical properties, they can be use as media to optimize the exfoliation of graphite. The understanding of the interactions involved between graphite and various chemical functions in the solvent ions will be helpful to find liquids capable of dissociating and stabilizing important quantities of large graphene layers. After a step of sonication, as a mechanical precursor, samples of suspended exfoliated graphene in different ionic liquids have been characterized experimentally in terms of flake size, number of layers, total concentration and purity of the exfoliated material. Nine different ionic liquids based on imidazolium, pyrrolidinium and ammonium cations and on bis(trifluoromethylsulfonyl)imide, triflate, dicyanamide, tricyanomethanide, and methyl sulfate anions have been tested. UV-vis, Raman and X-ray photoelectron in addition to high resolution transmission electron and atomic force microscopy have been selected to characterize suspended exfoliated graphene in ionic liquids. The number of layers in the flakes exfoliated, the size and concentration depend of the structure of the ionic liquid selected. In order to obtain large flake sizes, ionic liquids with bis(trifluoromethylsulfonyl)imide anions and a cation with an alkyl chain of medium length should be selected. Smaller cation and anion favors the exfoliation of graphene. The exfoliation caused the formation of C-H bonds and the oxidation of the graphitic surface.

Highlights

Since its isolation in 2004 (Novoselov, 2004), graphene has showed superior mechanical (Lee et al, 2008), electrical (Mayorov et al, 2011), and thermal (Mak et al, 2010) properties (Novoselov et al, 2012)
In order to have a first assessment of the solvent ability of ionic liquids (ILs), after the sonication and the centrifugation step, we have measured the absorbance of the supernatant suspensions containing exfoliated graphite
In the case of exfoliated graphite in [C2C1im][Otf]: more than two every three carbons are not graphitic. This result can be correlated with the number of graphene layers determined by atomic force microscopic (AFM) measurements in Figure 6d. [C2C1im][Otf] produced the most exfoliated graphite among the ILs we studied and it is known that one method to produce graphene form graphite is to oxidize the graphite (Zhu et al, 2010), so the high content of graphene in [C2C1im][Otf] could be due to oxydation at the surface of graphene

Summary

Introduction

Since its isolation in 2004 (Novoselov, 2004), graphene has showed superior mechanical (Lee et al, 2008), electrical (Mayorov et al, 2011), and thermal (Mak et al, 2010) properties (Novoselov et al, 2012). Two approaches are Exfoliation of Graphene in Ionic Liquids known to produce graphene: in “bottom-up” methods, such as chemical vapor deposition (CVD), graphene is synthesized, whereas in “top-down” methods, for example liquid-phase exfoliation, graphene sheets are separated from bulk material. The first approach produces low quantities with high quality and large flakes. The second method (top-down) using graphite is low in cost and yields a high concentration of suspended flakes but fabricates limited-size sheets with a low yield of mono-layer graphene. The development of technologies to produce large quantities of high-quality exfoliated graphene is important. One way to achieve a large scalable production is to improve methods of the liquid-phase exfoliation of graphite

Methods

Results

Conclusion