Graphene Oxide

This chapter introduces a peculiar organic macromolecule previously named as graphitic acid, graphite oxide (GO), or more recently, graphene oxide (GO). It is basically a wrinkled two-dimensional carbon sheet with various oxygenated functional groups on its basal planes and peripheries, with the thickness around 1 nm and lateral dimensions varying between a few nanometers to several microns. It was first prepared by the British chemist B. C. Brodie in 1859 and became very popular in the scientific community during the last decade, simply because it was believed to be an important precursor to graphene (a single atomic layer of graphite, the discovery of which won Andre Geim and Konstantin Novoselov the 2010 Nobel Prize in Physics). Several strategies have been introduced to reduce GO back to graphene; however, in this chapter we will mainly focus on GO itself and, more relevantly, its synthesis, chemical structure, and general characterizations. We emphasize here that, despite its strong relevance to graphene, GO also has its own scientific significance as a basic form of oxidized carbon and technological importance as a platform for all kinds of derivatives and composites that have already demonstrated various interesting applications.

Graphene oxide (GO) is an important material that provides a scalable approach for obtaining chemically derived graphene. Its optical and electrical properties are largely determined by the presence of oxygen-containing functionalities, which decorate its basal plane. This chemical derivatization results in useful properties such as the existence of a band gap as well as emission spanning both the visible and near infrared. Notably, GO’s optical and electrical properties can be altered through reduction, which proceeds through the removal of these oxygen-containing functional groups. However, widely variable behavior has been observed regarding the evolution of GO’s optical response during reduction. These discrepancies arise from the different reduction methods being used and, in part, from the fact that nearly all prior measurements have been ensemble studies. Consequently, detailed mechanistic studies of GO reduction are needed which can transcend the limitations of ensemble averaging.

In this chapter, we show the spectroscopic evolution of GO’s optical properties during photoreduction at the single-sheet level. Laser-induced reduction, in particular, offers a unique and potentially controllable method for producing reduced GO (rGO), a material with properties similar to those of graphene. However, given the complexity of GO’s photoreduction mechanism, microscopic monitoring of the process is essential to understanding and ultimately exploiting this approach.

In this chapter, we discuss a variety of chemical reactions introduced for GO. Among all studies on the chemistry of GO, the largest portion focused on the reduction of GO back to graphene, mainly due to its high relevance to graphene and the gold rush of graphene research over the last decade. However, doping, functionalization and cross-linking of GO are equally, if not more, interesting to chemists, since GO is a giant model compound of polycyclic aromatic hydrocarbon (PAH) oxides. Here, we start with a thorough comparison between various reducing recipes for GO, and follow with some theoretical simulations and predictions on its convertibility toward graphene. In addition to that, we elaborate on extended chemical modifications (covalent and non-covalent), cross-linking, and doping recipes for this macromolecule shown in literature. After all, we intend to show you that GO became a relatively hot research topic, not only due to its relevance to graphene, but also for its high chemical activity and tunability, which enabled the prosperity of its research in various fields led by chemists, materials scientists, biologists, physicists, as well as engineers. It is a perfect paradigm for young researchers as an important subject thrived in interdisciplinary research. After all, when real-life problems come, potential solutions do not impose boundaries between disciplines. All relevant disciplines can offer their input, and contribute together to the final solutions, in which cases communications and collaborations between different researchers need to be encouraged and appreciated.

Recently, GO/rGO has become promising platforms for advanced materials in energy related technologies. Extensive applications of rGO have been explored in a variety of electrochemical energy storage and conversion technologies (e.g., fuel cells, metal–air batteries, supercapacitors, and water splitting devices). In particular, GO/rGO was studied as efficient components in catalysts in fuel cells and metal–air batteries for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), one pair of the most important electrochemical reactions. The promising applications are primarily due to their unique physical and chemical properties, such as high surface area, access to large quantities, tunable electronic/ionic conductivity, unique graphitic basal plane structure, and the easiness of modification or functionalization. Chemical doping with heteroatoms (e.g., N, B, P, or S) into graphitic domains can tune the electronic properties, provide more active sites, and enhance the interaction between carbon structures and oxygen molecules. Meanwhile, GO/rGO has demonstrated excellent performances in electric double-layer capacitors (EDLCs, also known as supercapacitors or ultracapacitors) with excellent volumetric and gravimetric capacitance densities as well as feasibility in design and fabrications. Additionally, rGO holds great promise to be high-performance anode materials in lithium-ion batteries due to its favorable interactions with Li. In this chapter, we will discuss the uses of GO/rGO derivatives in these energy conversion and storage technologies, providing insights and guidance for further optimization and design of multifunctional materials for energy applications.

Graphene oxides feature much richer structural and physiochemical properties than the two-dimensional crystal graphene they are derived from. The stacked structures as well as functional groups and defects in the monatomic layers lead to a porous microstructure and engineerable channels for selective transport of water, ions, and gases across the graphene oxide membranes. Additional merits include their facile fabrication, low cost, and flexibility. Recent efforts in exploring the structure–property relationship of these materials and environment and energy-related applications not only demonstrate excellent balance between the permeability and selectivity of fluid transport through the graphene oxide membranes, but also deepen our understanding of the molecular transport mechanisms down to the nanoscale. In this chapter we review some of the major theoretical and experimental advances in this field, along with our perspectives for the future development.