Two Dimensional Soft Material: New Faces of Graphene Oxide

Abstract

Graphite 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 π-π 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.

Structural Effects of Residual Groups of Graphene Oxide on Poly(ε-Caprolactone)/Graphene Oxide Nanocomposite

“…The introduction of oxygen-containing groups (such as hydroxyl, epoxy, carbonyl and carboxyl groups) [5] can increase the hydrophilic ability of GO. With large content of functional groups, GO could form stable single-layer suspensions in water or other common polar solvents [6].…”

Chemically converted graphene: scalable chemistries to enable processing and fabrication

“…In this regard, the use of highly dispersed liquid crystalline GO (LCGO), a graphene precursor that can be readily processed and fabricated into a wide variety of structures using a number of different 3D fabrication methods, offers significant advantages. 4,28 The LCGO nanoplatelets are more than 10 times larger than the typical GO nanoplatelets affording highly ordered structures in solution and thus more viscous solutions for processing and fabrication. These larger nanoplatelets result from oxidation of natural graphite.…”

Composite Membrane Based on Graphene Oxide Sheets and Nafion for Polymer Electrolyte Membrane Fuel Cells

“…[10][11][12] They have been studied for fuel cell applications as an additive to proton exchange membranes by several groups since 2012. Choi et al prepared GO/Nafion membranes for DMFC applications and found that the transport properties of Nafion are favorably modified by the incorporation of GO, which greatly enhanced fuel cell performance.…”