Covalent functionalization of polydisperse chemically-converted graphene sheets with amine-terminated ionic liquidw Huafeng Yang, Changsheng Shan, Fenghua Li, Dongxue Han, Qixian Zhang and Li Niu* Received (in Cambridge, UK) 12th March 2009, Accepted 11th May 2009 First published as an Advance Article on the web 5th June 2009 DOI: 10.1039/b905085j A facile method to obtain polydisperse chemically-converted graphene sheets that are covalently functionalized with ionic liquid was reported—the resulting graphene sheets, without any assistance from polymeric or surfactant stabilizers, can be stably dispersed in water, DMF, and DMSO. Graphene-based materials, which were first created in 2004, 1 are of great interest because of their excellent mechanical and electrical properties. 1–4 As with carbon nanotubes, a key challenge in the synthesis and processing of bulk-quantity graphene sheets is the prevention of aggregation. To date, individual graphene or chemically-modified graphene sheets have been prepared by several techniques including the Scotch tape method, 1 non-covalent and covalent functionalization of reduced graphene oxide (GO), 5–9 and chemical reduction of suspensions of graphene oxide. 10–12 However, the lack of an efficient approach to produce polydisperse and long-term stable graphene sheets in different solvents has been a major obstacle to their exploitation in most of the proposed applications. Ionic liquids (ILs) seem well positioned to address this challenge. Due to their wide solubility, and by introducing a surface charge, modification with ILs should enable the preparation of long-term stable and polydisperse chemically- converted graphene sheets (p-CCG) that can be dispersed in various matrices. To date, investigations into the covalent attachment of an ionic material to graphene surface have been not carried out. In this communication, we report a convenient method to obtain polydisperse chemically-converted graphene (p-CCG) sheets that are functionalized with 1-(3-aminopropyl)- 3-methylimidazolium bromide (IL-NH 2 ). GO has been suggested to contain plentiful and reactive epoxy groups. 13,14 Therefore, a nucleophilic ring-opening reaction between the epoxy groups of GO and the amine groups of an amine-terminated ionic liquid, when catalysed by potassium hydroxide (KOH), should easily occur. Thus, the cations of the amine-terminated ionic liquid (IL-NH 2 ) would be introduced to the graphene sheets, contributing to a stabilization of graphene dispersions via electrostatic repulsion. In addition, the resulting introduction of charge and the widely soluble ionic liquid units to the graphene plane should result in a well dispersible graphene-based material. The preparation of chemically-converted graphene sheets functionalized by IL-NH 2 is illustrated in Scheme 1. Details of the synthesis can be found in the ESI.w It was found that the resulting composites could be well dispersed into water, N,N-dimethylformamide (DMF), and dimethyl sulfoxide (DMSO) at various concentrations, forming long-term stable and homogeneous dispersions after ultrasonic treatment, respectively (see Fig. 1A–C). By way of comparison, unfunctionalized chemically-converted graphene (u-CCG) sheets were also prepared and a poor dispersibility of u-CCG was clearly observed, as shown in Fig. 1. In short, the attachment of IL-NH 2 to the graphene plane improved the dispersibility of graphene in a wide range of solvents and it is believed that p-CCG sheets were successfully obtained in this work. Moreover, polydispersibility in water and several organic solvents will make graphene sheets an ideal candidate for various applications. Fig. 2 is a typical AFM image of a p-CCG dispersion in water (0.25 mg mL 1 ) after deposition on a freshly cleaved mica surface through drop-casting. The AFM analysis reveals that the average interlayer spacing for exfoliated p-CCG sheets obtained in this work was ca. 1.49 nm. When compared with well exfoliated GO sheets, with a spacing of ca. 0.96 nm (Fig. S1w), the distance between p-CCG sheets is greater, as would be expected. This is due to both the presence of the ionic liquid chains grafted onto both sheet sides, as well as the electrostatic repulsion between the p-CCG sheets. Moreover, it is unavoidable that some solvent molecules are still trapped between the p-CCG sheets after drying (at ambient conditions for 24 h) and these molecules will have also contributed to the measured interlayer spacing. Similarly, an X-ray diffraction (XRD) analysis (shown in Fig. S2w) was carried out in order to investigate and compare the exfoliation of GO and p-CCG. The (002) diffraction peak of graphite (Fig. S2aw) appears at Scheme 1 Illustration of the preparation of p-CCG. State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, and Graduate University of the Chinese Academy of Sciences, Chinese Academy of Sciences, Changchun 130022, P. R. China. E-mail: lniu@ciac.jl.cn; Fax: +86 431 85262800; Tel: +86 431 85262425 w Electronic supplementary information (ESI) available: Details of synthetic procedure; FTIR spectra and TGA curves of graphite, GO, and p-CCG; XPS spectra, and AFM image of GO; 1 H NMR of IL-NH 2 . See DOI: 10.1039/b905085j/ 3880 | Chem. Commun., 2009, 3880–3882 This journal is  c The Royal Society of Chemistry 2009 COMMUNICATION www.rsc.org/chemcomm | ChemComm