Supramolecular Approaches to Graphene: From Self‐Assembly to Molecule‐Assisted Liquid‐Phase Exfoliation

Abstract

Graphene, a one-atom thick two-dimensional (2D) material, is at the core of an ever-growing research effort due to its combination of unique mechanical, thermal, optical and electrical properties. Two strategies are being pursued for the graphene production: the bottom-up and the top-down. The former relies on the use of covalent chemistry approaches on properly designed molecular building blocks undergoing chemical reaction to form 2D covalent networks. The latter occurs via exfoliation of bulk graphite into individual graphene sheets. Amongst the various types of exfoliations exploited so far, ultrasound-induced liquid-phase exfoliation (UILPE) is an attractive strategy, being extremely versatile, up-scalable and applicable to a variety of environments. In this review, we highlight the recent developments that have led to successful non-covalent functionalization of graphene and how the latter can be exploited to promote the process of molecule-assisted UILPE of graphite. The functionalization of graphene with non-covalently interacting molecules, both in dispersions as well as in dry films, represents a promising and modular approach to tune various physical and chemical properties of graphene, eventually conferring to such a 2D system a multifunctional nature.

Liquid-Phase Exfoliation of Graphite into Single- and Few-Layer Graphene with α-Functionalized Alkanes

“…1,11 Liquidphase exfoliation (LPE) of graphite enables the production of graphene in different solvents. [12][13][14][15] Typically, graphite can be exfoliated into single-and few-layer graphene (SLG and FLG, respectively) in a solvent having a surface tension (γ) close to 40 mJ m −2 , [12][13]15 which favours an increase in the total area of graphite crystallites. 14 Therefore, solvents like N-methyl-2-pyrrolidone (NMP), 14 ortho-dichlorobenzene, 5,16 dimethylformamide, 17 and a few others 15 are commonly chosen as a dispersion media.…”

“…12 Centrifugation can then be used to separate SLG and FLG from unexfoliated material. [12][13]15 To promote the exfoliation, organic molecules, in particular those having a high energy of adsorption on the graphene surface, [20][21][22] can be employed. In order to adsorb on the exfoliated graphene sheets, these dispersion-stabilizing agents (DSAs) need to have adsorption energy higher than the solvent interacting with graphene.…”

“…24 However, by adding charged DSAs, such as pyrenes [25][26][27][28] or polymers [29][30][31][32][33] , exfoliation in water can be accomplished. 13,15 Nevertheless, waterbased dispersions may not be suitable for all applications. For example, water remaining at the 4 interface with a dielectric substrate can result in charge-trapping, which is detrimental for application in electronics.…”

“…15 However, a full understanding of the DSA role, needed for identifying the best DSAs to enhance the LPE process, is still lacking. Refs.…”

Water Dispersible Few Layer Graphene Stabilized by a Novel Pyrene Derivative at Micromolar Concentration

“…Polycyclic aromatic hydrocarbons such as pyrene derivatives have been studied for the exfoliation and stabilization of graphene and FLG in water. [6,7] The adsorption of pyrene derivative molecules onto the graphene surface occurs via π-π interactions between the planar π-conjugated surfaces while hydrophilic functional groups attached to the pyrene moiety allow their stabilization in aqueous media. Compounds such as pyrenebutyrate (PB -) and pyrene-sulfonic acid salt, as well as a pyrene-terminated poly (2-N,N'-(dimethyl amino ethyl acrylate) (PDMAEA) and poly(acrylic acid) (PAA) have been investigated for use in solar cells, electrochemical and composites applications.…”