Application and Uses of Graphene Oxide and Reduced Graphene Oxide

Sekhar C. Ray,    Department of Physics, College of Science, Engineering and Technology, University of South Africa, Florida Park, Johannesburg, South Africa

Graphene oxide (GO) is a unique material that can be viewed as a single monomolecular layer of graphite with various oxygen-containing functionalities such as epoxide, carbonyl, carboxyl, and hydroxyl groups. When the GO is reduced with a suitable process, the reduced graphene oxide (rGO) formed resembles graphene but contains residual oxygen and other heteroatoms, as well as structural defects. GO and rGO have been used in nanocomposite materials, polymer composite materials, energy storage, biomedical applications, and catalysis, and as a surfactant with some overlaps between these fields. In this chapter, their different uses and applications are described.

Keywords

Graphene oxide (GO); reduced graphene oxide (rGO); synthesis of GO/rGO; properties of GO/rGO; applications of GO/rGO

2.1 Introduction

Graphite oxide has a layered structure similar to that of graphite, but the plane of carbon atoms in graphite oxide is heavily decorated by oxygen-containing groups, which not only expand the interlayer distance but also make the atomic-thick layers hydrophilic. These oxidized layers could exfoliate in water under ultrasonication. If the exfoliated sheets contain only one or a few layers of carbon atoms like graphene, then these sheets are named graphene oxide (GO) (Novoselov et al., 2004). So, GO is a single-atomic-layered material comprising carbon, hydrogen, and oxygen molecules by the oxidation of graphite crystals, as shown in Figure 2.1 (Stergiou et al., 2014), which are inexpensive and abundant. It is dispersible in water and easy to process. Most importantly, the GO can be (partly) reduced to graphene-like sheets by removing the oxygen-containing groups and with the recovery of a conjugated structure. The reduced GO (rGO) sheets are usually considered one kind of chemically derived graphene and are known as rGO. Some other names have also been given to rGO, such as functionalized graphene, chemically modified graphene, chemically converted graphene, or reduced graphene (Eda et al., 2010). GO has two important characteristics: (i) it can be produced using inexpensive graphite as the raw material and by using cost-effective chemical methods with a high yield and (ii) it is highly hydrophilic and can form stable aqueous colloids to facilitate the assembly of macroscopic structures by simple and cheap solution processes. The graphene sheet consists of only trigonally bonded sp2 carbon atoms and is perfectly flat (Lui et al., 2009), apart from its microscopic ripples. The heavily decorated GO sheets consist partly of tetrahedrally bonded sp3 carbon atoms, which are displaced slightly above or below the graphene plane (Schniepp et al., 2006). Because of the structure deformation and the presence of covalently bonded functional groups, GO sheets are atomically rough (Paredes et al., 2009; Mkhoyan et al., 2009). Several researchers (Paredes et al., 2009; Kudin et al., 2007; Gomez-Navarro et al., 2007, 2010) have studied the surface of GO and observed highly defective regions, probably due to the presence of oxygen, and other areas are nearly intact. A report shows that the graphene-like honeycomb lattice in GO is preserved, albeit with disorder (i.e., the carbon atoms attached to functional groups are slightly displaced), but the overall size of the unit cell in GO remains similar to that of graphene (Pandey et al., 2008). Hence, GO can be described as a random distribution of oxidized areas with oxygen-containing functional groups combined with nonoxidized regions where most of the carbon atoms preserve sp2 hybridization. GO and rGO are hot topics in the research and development of graphene, especially regarding mass applications of graphene.

2.2 Preparation/Synthesis of GO/rGO

Graphite is a three-dimensional (3D) carbon-based material comprising millions of graphene layers, whereas graphite oxide is a little different. By oxidation of graphite with strong oxidizing agents, oxygenated functionalities are introduced in the graphite structure, which not only expand the layer separation but also make the material hydrophilic (meaning that they can be dispersed in water).

This property enables the graphite oxide to be exfoliated in water using sonication, ultimately producing single-layer graphene or graphene with a few layers, known as GO. Many modern procedures for the synthesis of GO are based on the method first reported by Hummers in which graphite is oxidized by a solution of potassium permanganate in sulfuric acid (Hummers et al., 1958; Kim et al., 2010). Hydrazine is generally used for the reduction of GO (Gilje et al., 2007). However, hydrazine is highly toxic and can potentially functionalize GO with nitrogen heteroatoms (Shin et al., 2009); because of these issues, alternatives to hydrazine including NaBH4 (Lightcap et al., 2013), ascorbic acid (Fernández-Merino et al., 2010), and HI (Moon et al., 2010; Pei et al., 2010), among others, have been used for the reduction of GO. GO can be reduced to a thin film or in an aqueous solution. GO is effectively a by-product of this oxidization because when the oxidizing agents react with graphite, the interplanar spacing between the layers of graphite is increased. The completely oxidized compound can then be dispersed in a base solution such as water, and GO is then produced. Graphite oxide and GO are very similar chemically, but structurally they are very different. The main difference between graphite oxide and GO is the interplanar spacing between the individual atomic layers of the compounds, which is caused by water intercalation. This increased spacing, caused by the oxidization process, also disrupts the sp2 bonding network, meaning that both graphite oxide and GO are often described as electrical insulators. GO is a poor conductor but its treatment with light, heat, or chemical reduction can restore most properties of the famed pristine graphene.

To turn graphite oxide into GO, a few methods are possible. The most common techniques are by using sonication, stirring, or a combination of the two. Sonication can be a very time-efficient way of exfoliating graphite oxide, and it is extremely successful at exfoliating graphene; however, it can also heavily damage the graphene flakes, reducing them in surface size from microns to nanometers, and it also produces a wide variety of graphene platelet sizes. The main difference between graphite oxide and GO is the number of layers. Although graphite oxide is a multilayer system in GO dispersion, a few layers of flakes and a monolayer of flakes can be found.

Reducing GO to produce rGO is an extremely vital process because it has a large impact on the quality of the rGO produced; therefore, it will determine how close rGO will come in terms of structure to pristine graphene (Chuang et al., 2014). In large-scale operations where scientific engineers need to utilize large quantities of graphene for industrial applications such as energy storage, rGO is the most obvious solution because of the relative ease in creating sufficient quantities of graphene with desired quality levels. There are a number of ways reduction can be achieved, although they are all methods based on chemical, thermal, or electrochemical means. Some of these techniques are able to produce very high-quality rGO, similar to pristine graphene, but they can be complex or time-consuming to perform.

In the past, scientists have created rGO from GO by:

Reducing GO by using chemical reduction is a very scalable method; unfortunately, the rGO produced has often resulted in relatively poor yields in terms of surface area and electronic conductibility. Thermally reducing GO at temperatures of 1000°C or more creates rGO that has been shown to have a very high surface area, close to that of pristine graphene. The heating process damages the structure of the graphene platelets as pressure builds and carbon dioxide is released. This also causes a substantial reduction in the mass of the GO, creating imperfections and vacancies, and potentially also has an effect on the mechanical strength of the rGO produced. Electrochemical reduction of GO is a method that has been shown to produce very high-quality rGO, almost identical in terms of structure to pristine graphene. This...