Graphene Oxide as a Two-dimensional Surfactant
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Graphene Oxide as a Two-dimensional Surfactant Andrew R. Koltonow, Jaemyung Kim, Laura J. Cote, Jiayan Luo and Jiaxing Huang Department of Materials Science and Engineering, Northwestern University, Evanston, USA Abstract Graphene oxide (GO) is a nonstoichiometric two-dimensional material obtained from the chemical oxidation and exfoliation of graphite, which has recently attracted intense research interest as a precursor for bulk production of graphene. GO has long been believed to be hydrophilic due to its dispersibility in water. Recent work in our group, however, has found that GO is actually a two-dimensional amphiphile; the edge of the sheet-like material is hydrophilic, while the basal plane of the material contains more hydrophobic graphitic nanodomains. To prove the concept, we demonstrate GO’s surface activity at an air-water interface, as well as its utility in dispersing insoluble aromatic materials such as toluene, graphite, and carbon nanotubes in water. As a colloidal surfactant which can be converted to a conducting material, GO presents unique possibilities for aqueous solution processing of organic electronic materials. Introduction Graphene oxide (GO) is a nonstoichiometric two-dimensional carbon material which has recently attracted intense research interest as a precursor for bulk production of graphene. Typically GO is synthesized by treating graphite powders to strong oxidizing agents such as KMnO4 in concentrated sulfuric acid.1,2 This oxidizing treatment exfoliates the [002] planes of graphite atoms into water-dispersable single layers derivatized by carboxylic acids at the edges and phenol hydroxyl, and epoxide moieties mainly at the basal plane.3,4 While extensive functionalization of the conjugated network renders GO insulating, conductivity can be partially restored through reduction by chemical,5,6 thermal,7,8 photothermal,9,10 and electrochemical means,11,12 giving reduced GO (r-GO, a.k.a. chemically modified graphene). While r-GO is more defective and less conductive than pristine graphene produced by mechanical exfoliation, the comparatively easy synthesis and processing of GO make it an attractive precursor for large scale production of graphene-based materials.13-15
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Figure 1. (a) Structural model of GO depicting ionizable hydrophilic edges (orange) and hydrophobic, unoxidized polyaromatic patches on the basal plane (blue). (b) Aberration-
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corrected TEM image of the basal plane of a GO sheet, showing graphitic nanopatches (blue) surrounded by a matrix of oxidized material. (adapted from [20]). GO is often described in the literature as hydrophilic due to its ability to disperse in water.14, 16-19 However, while the edges of GO are hydrophilic, the basal plane still contains unoxidized ʌ-conjugated islands nanometers in diameter, as highlighted in figure 1.20 Therefore, GO can be better described as an amphiphile, largely hydrophobic at the basal plane and hydrophilic at the edges. Armed with this hypothesis, we consider the various ways its amphiphilic behavior might be
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