Stable aqueous dispersions of graphitic nanoplatelets via the reduction of exfoliated graphite oxide in the presence of poly ( sodium 4-styrenesulfonate )

Graphite nanoplatelets have recently attracted considerable attention as a viable and inexpensive filler substitute for carbon nanotubes in nanocomposites, given the predicted excellent in-plane mechanical, structural, thermal, and electrical properties of graphite. As with carbon nanotubes, full utilization of graphite nanoplatelets in polymer nanocomposite applications will inevitably depend on the ability to achieve complete dispersion of the nano-filler component in the polymer matrix of choice. In this communication, we describe a method for preparing watersoluble polymer-coated graphitic nanoplatelets. We prepare graphite nanoplatelets via the chemical reduction of exfoliated graphite oxide nanoplatelets. Graphite oxide is produced by the oxidative treatment of graphite. It still possesses a layered structure, but is much lighter in color than graphite due to the loss of electronic conjugation brought about during the oxidation. The basal planes of the graphene sheets in graphite oxide are decorated mostly with epoxide and hydroxyl groups, in addition to carbonyl and carboxyl groups, which are located at the edges. These oxygen functionalities alter the van der Waals interactions between the layers of graphite oxide and render them hydrophilic, thus facilitating their hydration and exfoliation in aqueous media. As a result, graphite oxide readily forms stable colloidal dispersions of thin graphite oxide sheets in water. From these stable dispersions, thin ‘‘graphitic’’ nanoplatelets can be obtained by chemical deoxygenation, e.g., removal of the oxygen functionalities with partial restoration of the aromatic graphene network. It is possible that even single graphite sheets (i.e., finite-sized graphene sheets) can be accessed via graphite oxide exfoliation and a subsequent solution-based chemical reduction. In practice, reduction of water-dispersed graphite oxide nanoplatelets results in a gradual decrease in their hydrophilic character, which eventually leads to their irreversible agglomeration and precipitation. However, stable aqueous dispersions of reduced graphite oxide nanoplatelets can be prepared if the reduction is carried out in the presence of an anionic polymer. A stable water dispersion of graphite oxide nanoplatelets, prepared by exfoliation of the graphite oxide (1 mg mL) via ultrasonic treatment (Fisher Scientific FS60, 1 h), was reduced with hydrazine hydrate at 100 uC for 24 h. As the reduction proceeds, the brown-colored dispersion of exfoliated graphite oxide turns black and the reduced nanoplatelets agglomerate and eventually precipitate. This precipitated material could not be re-suspended even after prolonged ultrasonic treatment in water in the presence of surfactants such as sodium dodecylsulfate (SDS) and TRITON X-100, which have been found to successfully solubilize carbon nanotubes. Elemental analyses, coupled with Karl Fisher titration (Galbraith Laboratories), of both graphite oxide and the reduced material indicate that there is a considerable increase in C/O atomic ratio in the reduced material (10.3) compared to that in the starting graphite oxide (2.7). Hence, the reduced material can be described as consisting of partially oxidized graphitic nanoplatelets, given that a fair amount of oxygen is retained even after reduction. The black color of the reduced materials suggests a partial re-graphitization of the exfoliated graphite oxide, as observed by others. In addition to the decrease in the oxygen level, reduction of graphite oxide is accompanied by nitrogen incorporation from the reducing agent (C/N = 16.1). Attempts to reduce graphite oxide in the presence of SDS and TRITON-X100 also failed to produce a stable aqueous dispersion of graphitic nanoplatelets. However, when the reduction was carried out in the presence of poly(sodium 4-styrenesulfonate) (PSS) (Mw = 70000, Sigma-Aldrich, 10 mg mL , 10/1 w/w vs. graphite oxide), a stable black dispersion was obtained. This dispersion can be filtered through a PVDF membrane (0.2 mm pore size, Fisher Scientific) to yield PSS-coated graphitic nanoplatelets that can be re-dispersed readily in water upon mild sonication, forming black suspensions (Fig. 1). At concentrations lower than 0.1 mg mL, the dispersions obtained after a 30-minute ultrasonic treatment appear to be stable indefinitely— samples prepared over a year ago are still homogeneous to date. More concentrated dispersions would develop a small amount of precipitate after several days. However, they never fully settle, even upon months of standing. Elemental analysis of the PSS-coated platelets indicates that it contains y40% polymer as judged by its sulfur content (graphite oxide reduced without any PSS contains no sulfur at all). Its comparatively high oxygen and hydrogen Department of Mechanical Engineering, Northwestern University, 2145 Sheridan Rd., Evanston, IL 60206-3133, USA. E-mail: r-ruoff@northwestern.edu; Fax: +1(847)491-3915; Tel: +1(847)467-6596 Keck Interdisciplinary Surface Science Center, NUANCE, 2220 Campus Drive, #2036, Northwestern University, Evanston, IL 60208, USA. Fax: +1(847)491-5429; Tel: +1(847)491-5505 Department of Chemistry, 2145 Sheridan Rd., Evanston, IL 60208-3133, USA. E-mail: stn@northwestern.edu; Fax: +1(847)491-7713; Tel: +1(847)467-3347 COMMUNICATION www.rsc.org/materials | Journal of Materials Chemistry