1356 ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 1356–1364 ’ 2012 ’ Vol. 45, No. 8 Published on the Web 06/04/2012 www.pubs.acs.org/accounts 10.1021/ar300047s & 2012 American Chemical Society Two Dimensional Soft Material: New Faces of Graphene Oxide JAEMYUNG KIM, LAURA J. COTE, AND JIAXING HUANG* Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States RECEIVED ON FEBRUARY 12, 2012 CONSPECTUS G raphite oxide sheets, now called graphene oxide (GO), can be made from chemical exfoliation of graphite by reactions that have been known for 150 years. Because GO is a promising solution-processable precursor for the bulk production of graphene, interest in this old material has resurged. The reactions to produce GO add oxygenated functional groups to the graphene sheets on their basal plane and edges, and this derivatization breaks the π-conjugated network, resulting in electrically insulating but highly water-dispersible sheets. Apart from making graphene, GO itself has many intriguing properties. Like graphene, GO is a two-dimensional (2D) sheet with feature sizes at two abruptly different length scales. The apparent thickness of the functionalized carbon sheet is approximately 1 nm, but the lateral dimensions can range from a few nanometers to hundreds of micrometers. Therefore, researchers can think of GO as either a single molecule or a particle, depending on which length scale is of greater interest. At the same time, GO can be viewed as an unconventional soft material, such as a 2D polymer, highly anisotropic colloid, membrane, liquid crystal, or amphiphile. In this Account, we highlight the soft material characteristics of GO. GO consists of nanographitic patches surrounded by largely disordered, oxygenated domains. Such structural characteristics effectively make GO a 2D amphiphile with a hydrophilic periphery and largely hydrophobic center. This insight has led to better understanding of the solution properties of GO for making thin films and new applications of GO as a surfactant. Changes in pH and sheet size can tune the amphiphilicity of GO, leading to intriguing interfacial activities. In addition, new all-carbon composites made of only graphitic nanostructures using GO as a dispersing agent have potential applications in photovoltaics and energy storage. On the other hand, GO can function as a 2D random diblock copolymer, one block graphitic and the other heavily hydroxylated. Therefore, GO can guide material assembly through πÀπ stacking and hydrogen bonding. Additionally, the selective etching of the more reactive sp 3 blocks produces a porous GO network, which greatly enhances interactions with gas molecules in chemical sensors. With their high aspect ratio, GO colloids can readily align to form liquid crystalline phases at high concentration. As single-atomic, water-dispersible, soft carbon sheets that can be easily converted to a conductive form, this 2D material should continue to inspire many curiosity-driven discoveries and applications at the interfaces of chemistry, materials science, and other disciplines. Introduction Graphite oxide is an old material that has been known for more than 150 years. 1 It is typically synthesized by reacting graphite powder (Figure 1a) with strong oxidizing agents such as KMnO 4 in concentrated sulfuric acid. 2 The oxidized graphene sheets, now termed graphene oxide (GO), can readily exfoliate to form stable, light brown colored, single layer suspensions in water and other common polar solvents. As a result of the functionalization, the apparent thickness of GO increases to about 1 nm, as measured by atomic force microscopy (AFM) imaging of single layers (Figure 1b) and X-ray diffraction of the spacing between stacked GO sheets. 3 It has been proposed 4 and recently confirmed by high-resolution transmission electron micro- scopy (HRTEM) (Figure 1c) 5 that the graphite oxidation reactions break the extended 2D π-conjugation of the