User:Ennegma/Graphene oxide

Graphene oxide is a single layer of graphite oxide. Compared to pristine graphene, graphene oxide is highly oxygenated, containing epoxide and hydroxyl groups on the surface of the sheets and carbonyl and carboxyl groups on the edges. Due to the presence of these functional groups, it is strongly hydrophilic and can be readily dispersed in water. As the functional groups can be transformed via organic chemistry reactions into a range of different species, graphene oxide broadens the field of potential applications for graphene, which in its pristine state cannot be chemically functionalized. Its reduction is commonly used for the large-scale production of graphene sheets (reduced graphene oxide, rGO).

Exfoliation of graphite oxide
Graphene oxide is most commonly produced by exfoliation of graphite oxide. This may be done by heating graphite oxide in an inert atmosphere above 550 °C, causing the hydroxyl and epoxide groups throughout the material to decompose into gaseous products, which lead to rapid expansion and splitting of the layers. Although the amount of oxygen-containing groups is reduced because of the decomposition of these species, some are retained after the expansion. Alternatively, graphene oxide sheets may be produced by ultrasonication of graphite oxide in water, which leads to more complete exfoliation and aqueous dispersions consisting almost entirely of monolayer sheets. The size of the graphene oxide sheets varies based on the degree of oxidation of the initial graphite oxide, the sonication time, and any additional mechanical energy input such as shaking.

Electrochemical exfoliation of graphite
An alternative to exfoliation of graphite oxide for the preparation of graphene oxide is the direct electrochemical exfoliation of graphite in an electrolyte of either ionic liquids, acids or aqueous solutions of inorganic salts , which takes only a few minutes compared to the several days required to produce graphite oxide from graphite and its subsequent exfoliation. Graphite is used as the working electrode in an electrochemical cell with a counter electrode such as platinum. A direct current (DC) voltage (usually +10V) is applied to the graphite electrode, causing graphene oxide flakes to break away from it and become dispersed in the electrolyte. This is due to oxidation at the edges and grain boundaries of the graphite which causes the layers to expand, and the subsequent intercalation of gaseous species between the expanded graphite layers which exerts enough force to separate the layers. The exfoliated material is then collected by filtration or centrifugation and washed to remove any contaminants. It can be kept as a powder or dispersed in a solvent. Depending on the conditions, the degree of oxidation varies and the products are sometimes called graphene rather than graphene oxide.

Using inorganic salts as the electrolyte, electrochemically exfoliated graphene was produced with a good yield (>85%) of few layer sheets (≤3 layers). The yield of monolayers has since been improved using a combination of electrochemical exfoliation in acetonitrile and microwave-triggered expansion of the sheets. Initially the procedure could only be performed using a continuous graphite starting material, such as graphite rods or highly oriented pyrolytic graphite (HOPG), which are expensive. However, more recently, cheap graphite flakes were exfoliated in this way by compression and mechanical containment to maintain an electrical contact between them and the electrode.

Microwave-assisted oxidation of graphite
Another way of producing graphene oxide directly from graphite is by irradiating the reactants involved in the Hummers' method for the production of graphite oxide (a mixture of graphite powder, sulfuric acid, nitric acid and potassium permanganate) with microwaves at a power of 300W for 30 seconds.

Preparation of reduced graphene oxide
Graphene oxide sheets may be reduced to form graphene by various routes. Chemical reduction of aqueous suspensions of graphene oxide has been achieved using hydrazine, hydroquinone, sodium borohydride, and ascorbic acid. However, as the oxygen-containing groups are removed, the sheets become less hydrophilic and tend to aggregate and precipitate into multi-layered graphitic structures. To overcome this, methods such as light sulfonation of the graphene oxide sheets before complete reduction have been successfully employed, leading to stable dispersions of reduced graphene oxide sheets that are held apart by electrostatic repulsion. Graphene oxide may also be thermally reduced in a nitrogen atmosphere at temperatures of up to 2000 °C, although significantly lower temperatures (e.g. 150 °C) have been achieved with the addition of other chemical agents. In an alternative approach, graphene oxide films spray-coated onto substrates have been reduced by an electrochemical procedure, applying a potential of -0.9V for 5000 seconds.