Graphene OxideNov 21, 2017 | ACS MATERIAL LLC
Commercializing graphene has been a main interest in research and technology, but its high cost and some difficulty in producing large quantities of the material remains to be a topic of discussion. However, graphene oxide is exceptionally feasible in creating a scalable production; in fact, it has been an attractive precursor material for a large-scale production of graphene-based materials. Here, we discuss more about graphene oxide, its synthesis and how it can be an excellent material for various applications.
Introduction to Graphene Oxide
Graphene is a two-dimensional, crystalline allotrope with a hexagonal lattice structure made from pure carbon atoms. They are best known for its unique properties containing high optical transparency, the best heat conductivity at room temperature and the ability to be flexible all within a strong, nano-sized material. Graphene was first discovered by the mechanical exfoliation of 3D graphite crystals and peeling away a single layer of graphene with Scotch tape. Since then, graphene has gained recognition for its attributes and different methods have been tested to find the best way to produce large-scale amounts with low costs but it is still challenging to manufacture with these specifications.
Graphene oxide (GO) happens to be a great precursor to obtaining graphene with higher yields and lower costs. To obtain GO, graphite oxide is first produced by utilizing graphite crystals that have been oxidized with strong oxidizing agents, such as sulfuric acid. Through sonication, graphite adopts oxygen-containing functional groups that allow the material to be dispersible in water while increasing interlayer distance.1 Then, graphite oxide can be exfoliated into either single or multilayers of oxygen-functionalized graphene oxide (GO). The difference between graphite oxide and GO are based on their different structures but chemical composition remains alike. GO is a single-layered material made of carbon, hydrogen and oxygen molecules, which ultimately becomes inexpensive yet abundant.2 However, due to the disruption of its sp2 hybridization, GO tends to be described as an electrical insulator rather than a conductor. To counteract this disruption, GO can be reduced to form reduced graphene oxide (rGO) to retrieve its hexagonal lattice structure and produce graphene-like sheets by removing a large portion of oxygen groups to closely resemble graphene.
Figure 1. The graphene oxide molecular structure consists of carbon, hydrogen and oxygen.
One of the most important traits of GO is that it can be produced using graphite (since it is inexpensive) using different chemical methods, yielding a high production with exceptional cost-efficiency. The second characteristic is that GO is very dispersible in water and can form stable aqueous colloids in order to assemble macroscopic structures with cheaper solution processes. Although the surface of these GO sheets carries some defects, the overall size of the unit cells remains very similar to graphene.2 Thus, GO is an oxidized version of graphene that comprises of oxygen-containing groups. Due to the presence of different functional groups, GO has lower elasticity and its Young’s modulus is dependent on the functionalization and molecular structure of the functional groups.3
Currently, GO can be synthesized in different ways such as the Modified Hummer’s method and Staudenmaier method. Both methods involve the oxidation of graphite but differ in mineral acids, oxidizing agents, preparation time and type of washing/drying processes.4 In the original Hummers method, GO was synthesized by using KMnO4 and NaNO3 in concentrated H2SO4. Typically, for the Modified Hummer’s method, Hummer’s reagents with the addition of NaNO3 was used. GO is produced from pure graphite powder that has been gradually added (along with NaNO3) into a hot concentration of H2SO4 solution that would be cooled in an ice bath. KMnO4 has to be slowly added to keep the reaction temperature under 20°C to prevent overheating and explosions. To complete the reaction with KMnO4, the suspension would then be treated with an H2O2 solution and washed with HCl and H2O. After filtration and drying, GO sheets would be obtained.5, 6 This modified method happens to be very common and reliable when producing high yields of GO.
The Staudenmaier method is another chemical synthesis of GO that improved an already existing method (Brodie, 1859) using KClO3 to a slurry of graphite in fuming HNO3; with the improvised version, it includes an additional concentrated H2SO4 and HNO3 as oxidizing agents.7 An additional KClO3 would slowly be added over a period of one week during the procedure.8 The small changes provided a simple procedure to produce highly oxidized GO.
The large, convenient production of GO has led to its emergence as a precursor for fabricating transparent conductive films (TCFs).9 Since GO is hydrophilic, they can make stable and homogeneous colloidal suspensions in aqueous or polar organic solvents that allows for an easy dispersion process in order to produce TCFs on a substrate. Using monolayer GO increases the transparency of thin films due to the lower concentration of GO in the dispersion which enables thin films to possibly obtain higher transparency and conductivity.10 In fact, TCFs made from GO can even have potential to replace indium tin oxide transparent conductors.11
Meanwhile, functionalized GO can be used as fluorescence and photoluminescent means in cellular imaging. In fact, research was conducted using squaraine dyes that had been loaded inside mesoporous silica nanoparticles where nanoparticle surfaces were wrapped with ultrathin GO sheets. This process can efficiently protect the loaded dye from a nucleophilic attack and is significant for applications regarding fluorescence imaging in vitro.12 Other applications have been applied to bio-sensing, detection and drug-carrier materials based on its fluorescence.
Obtaining graphene with a scalable production and low cost is viable to commercialize the material. GO is an excellent precursor for graphene as it possesses distinctive traits and can be manipulated to have graphene’s molecular structure after a reduction process. Utilizing both of the Modified Hummer’s and Staudenmaier methods have been very feasible and common when producing high yields of GO effectively. As a derivate of graphene, GO has proven to have unique qualities and many capabilities of being used in different applications.
ACS Material Products:
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