Around the world, research institutions are trying to develop ways to revolutionise the production of graphene sheets of the highest quality. One of the most cost effective ways this is possible is by the reduction of graphene oxide into rGO (reduced graphene oxide). The problem with this technique is the quality of graphene sheets produced, which (with certain methods) displays properties currently below the theoretical potential of pristine graphene compared to other methods such as mechanical exfoliation. However, this doesn't mean that improvements can’t be made, or that this reduced graphene oxide is effectively unusable; far from it, in fact.
Graphite OxideGraphite oxide is a compound made up of carbon, hydrogen and oxygen molecules. It is artificially created by treating graphite with strong oxidisers such as sulphuric acid. These oxidisers work by reacting with the graphite and removing an electron in the chemical reaction. This reaction is known as a redox (a portmanteau of reduction and oxidisation) reaction, as the oxidising agent is reduced and the reactant is oxidised.
The most common method for creating graphite oxide in the past has been the Hummers and Offeman method, in which graphite is treated with a mixture of sulphuric acid, sodium nitrate and potassium permanganate (a very strong oxidiser). However, other methods have been developed recently that are reported to be more efficient, reaching levels of 70% oxidisation, by using increased quantities of potassium permanganate, and adding phosphoric acid combined with the sulphuric acid, instead of adding sodium nitrate.
Graphene oxide is effectively a by-product of this oxidisation as when the oxidising agents react with graphite, the interplanar spacing between the layers of graphite is increased. The completely oxidised compound can then be dispersed in a base solution such as water, and graphene oxide is then produced.
Graphite oxide and graphene oxide are very similar, chemically, but structurally, they are very different. The main difference between graphite oxide and graphene oxide is the interplanar spacing between the individual atomic layers of the compounds, caused by water intercalation. This increased spacing, caused by the oxidisation process, also disrupts the sp2 bonding network, meaning that both graphite oxide and graphene oxide are often described as electrical insulators.
Graphite Oxide to Graphene OxideThe process of turning graphite oxide into graphene oxide can ultimately be very damaging to the individual graphene layers, which has further consequences when reducing the compound further (explanation to follow). The oxidisation process from graphite to graphite oxide already damages individual graphene platelets, reducing their mean size, so further damage is undesirable. Graphene oxide contains flakes of monolayer and few layer graphene, interspersed with water (depending on the base media, the platelet to platelet interactions can be weakened by surface functionality, leading to improved hydrophilicity).
In order to turn graphite oxide into graphene oxide, 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 (almost to levels of full exfoliation), but it can also heavily damage the graphene flakes, reducing them in surface size from microns to nanometres, and also produces a wide variety of graphene platelet sizes. Mechanically stirring is a much less heavy-handed approach, but can take much longer to accomplish.
Graphene Oxide to Reduced Graphene OxideReducing graphene oxide to produce reduced graphene oxide (hitherto referred to as rGO), is an extremely vital process as it has a large impact on the quality of the rGO produced, and therefore will determine how close rGO will come, in terms of structure, to pristine graphene. 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, due to the relative ease in creating sufficient quantities of graphene to desired quality levels.
As you would expect, there are a number of ways reduction can be achieved, though 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 can be complex or time consuming to carry out.
In the past, scientists have created rGO from GO by:
- Treating GO with hydrazine hydrate and maintaining the solution at 100 for 24 hours
- Exposing GO to hydrogen plasma for a few seconds
- Exposing GO to another form of strong pulse light, such as those produced by xenon flashtubes
- Heating GO in distilled water at varying degrees for different lengths of time
- Combining GO with an expansion-reduction agent such as urea and then heating the solution to cause the urea to release reducing gases, followed by cooling
- Directly heating GO to very high levels in a furnace
- Linear sweep voltammetry
Reducing GO by using chemical reduction is a very scalable method, but 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℃ or more creates rGO that has been shown to have a very high surface area, close to that of pristine graphene even.
Unfortunately, the heating process damages the structure of the graphene platelets as pressure between builds up and carbon dioxide is released. This also causes a substantial reduction in the mass of the GO (figures around 30% have been mentioned), creating imperfections and vacancies, and potentially also having an effect on the mechanical strength of the rGO produced.
The final example given above could eventually be the future of large scale production of rGO. Electrochemical reduction of graphene oxide is a method that has been shown to produce very high quality reduced graphene oxide, almost identical in terms of structure to pristine graphene, in fact.
This process involves coating various substrates such as Indium Tin Oxide or glass with a very thin layer of graphene oxide. Then, electrodes are placed at each end of the substrate, creating a circuit through the GO. Finally, linear sweep voltammetry is carried out on the GO in a sodium phosphate buffer at various voltages; at 0.6 volts reduction began, and maximum reduction was observed at 0.87 volts.
In recent experiments the resulting electrochemically reduced graphene oxide showed a very high carbon to oxygen ratio and also electronic conductivity readings higher than that of silver (8500 S/m, compared to roughly 6300 S/m for silver). Other primary benefits of this techniques are that there are no hazardous chemicals used, meaning no toxic waste to dispose of. Unfortunately, the scalability of this technique has come into question due to the difficulty in depositing graphene oxide onto the electrodes in bulk form.
Ultimately, once reduced graphene oxide has been produced, there are ways that we can functionalise rGO for use in different applications. By treating rGO with other chemicals or by creating new compounds by combining rGO with other two dimensional materials, we can enhance the properties of the compound to suit commercial applications.