Graphene Oxide

Graphene Oxide and Reduced Graphene Oxide are available in powder form, as a dispersion, or as a spin coated film which is available in oxide or reduced forms. Please contact us today to discuss your needs. Some of our Graphene Oxide products are well suited to Ink Jet & 3D printing applications. Please call us to discuss your needs.

Graphite oxide, formerly called graphitic oxide or graphitic acid, is a compound of carbon, oxygen, and hydrogen in variable ratios, obtained by treating graphite with strong oxidizers. The maximally oxidized bulk product is a yellow solid with C:O ratio between 2.1 and 2.9, that retains the layer structure of graphite but with a much larger and irregular spacing.

The bulk material disperses in basic solutions to yield monomolecular sheets, known as graphene oxide by analogy to graphene, the single-layer form of graphite. Graphene oxide sheets have been used to prepare strong paper-like materials, membranes, thin films, and composite materials. Initially, graphene oxide attracted substantial interest as a possible intermediate for the manufacture of graphene. The graphene obtained by reduction of graphene oxide still has many chemical and structural defects which is a problem for some applications but an advantage for some others.

 

Graphene Oxide Properties

Graphene Oxide synthesis methods have been known for decades. Originally called graphite oxide, it is a compound of carbon, oxygen, and hydrogen in variable ratios. It is synthesized by exfoliating graphite with strong oxidizers, rinsed repeatedly until the the rinse water is PH neutral and then freeze dried to preserve solubility.  Many companies try to reduce the rinsing or skip the freeze drying but they are critical to success in using the product. The bulk product is a brownish/yellowish solid material that retains the layer structure of graphite but with a much larger and irregular spacing. Graphene oxide doesn’t require post production functionalization as it consists of graphene sheets with hydroxyl, carboxyl, & epoxide groups. It is highly soluble in Di water, NMP, DMF, THF, Ethanol, and other solvents that behave like water. GO can be reduced using several methods such as laser, microwave, electrochemically, hydrazine vapor treatment, or by annealing at temperatures from 250-400C in a forming gas (95% argon, 5% hydrogen) environment yielding the intrinsically high electrical and thermal conductivity of graphene.

Molecular Structure of Graphene Oxide

GO’s molecular structure is shown above. The functional groups are present on the edges of the flakes and on the top and bottom which helps impart GO with legendary solubility compared to most nanoscale particles. No surfactants are needed when dispersing into typical solvents such as Di Water, NMP, DMF, THF, DCB, or Ethanol.

Graphene Oxide Applications

Graphene Oxide applications are numerous due to its high solubility and the ability to reduce it to near perfect graphene.  This overcomes the well known dispersion problems with other nanomaterials enabling you to get the full benefits of nanoscale additives such as improved mechanical properties as well as enhanced conductivity.

Water purification – In 2016 engineers developed graphene-based films that can filter dirty/salty water powered by the sun. Bacteria were used to produce a material consisting of two nanocellulose layers. The lower layer contains pristine cellulose, while the top layer contains cellulose and graphene oxide, which absorbs sunlight and produces heat. The system draws water from below into the material. The water diffuses into the higher layer, where it evaporates and leaves behind any contaminants. The evaporate condenses on top, where it can captured. The film is produced by repeatedly adding a fluid coating that hardens. Bacteria produce nanocellulose fibers with interspersed graphene oxide flakes. The film is light and easily manufactured at scale

Coating – Optically transparent, multilayer films made from graphene oxide are impermeable under dry conditions. Exposed to water (or water vapor), they allow passage of molecules below a certain size. The films consist of millions of randomly stacked flakes, leaving nano-sized capillaries between them. Closing these nanocapillaries using chemical reduction with hydroiodic acid creates “reduced graphene oxide” (r-GO) films that are completely impermeable to gases, liquids or strong chemicals greater than 100 nanometers thick. Glassware or copper plates covered with such a graphene “paint” can be used as containers for corrosive acids.  Graphene-coated plastic films could be used in medical packaging to improve shelf life.

Related materials –Dispersed graphene oxide flakes can also be sifted out of the dispersion (as in paper manufacture) and pressed to make an exceedingly strong graphene oxide paper. Graphene oxide has been used in DNA analysis applications. The large planar surface of graphene oxide allows simultaneous quenching of multiple DNA probes labeled with different dyes, providing the detection of multiple DNA targets in the same solution. Further advances in graphene oxide based DNA sensors could result in very inexpensive rapid DNA analysis.

Recently a group of researchers, from university of L’Aquila (Italy), discovered new wetting properties of graphene oxide thermally reduced in ultra-high vacuum up to 900 °C. They found a correlation between the surface chemical composition, the surface free energy and its polar and dispersive components, giving a rationale to the wetting properties of graphene oxide and reduced graphene oxide.

Flexible rechargeable battery electrode – Graphene oxide has been demonstrated as a flexible free-standing battery anode material for room temperature lithium-ion and sodium-ion batteries. It is also being studied as a high surface area conducting agent in lithium-sulfur battery cathodes.

Graphene oxide lens – the excellent properties of newly discovered graphene oxide provide novel solutions to overcome the challenges of current planar focusing devices. Specifically, giant refractive index modification (as large as 10^-1), which is one order of magnitude larger than the current materials, between graphene oxide (GO) and reduced graphene oxide (rGO) have been demonstrated by dynamically manipulating its oxygen content using direct laser writing (DLW) method.

 

Reduced Graphene Oxide

Reduced GO first undergoes the typical synthesis process and then it is reduced which removes most of the surface functional groups as well as restores the molecular structure to one much closer to pristine graphene than GO.

There are a number of ways reduction can be achieved and is typically a chemical, thermal or electrochemical process. 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.

Common graphene reduction techniques are:

  • Treating GO with hydrazine hydrate and maintaining the solution at 100c 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
  • Directly heating GO to very high levels in a furnace
  • Directly heating GO in a microwave
  • Electrochemical methods
  • At 400C in a forming has atmosphere 95% argon, 5% hydrogen

Chemical reduction is a highly scalable method, unfortunately the reduced GO produced often has resulted in relatively poor yields in terms of surface area and electronic conductivity. Thermally reducing GO at temperatures of 1000℃ or more creates rGO that has been shown to have a very high surface area but the annealing process damages the structure of the GO when pressure between builds up and carbon dioxide is released. During reduction, there is a substantial reduction in the mass of the GO (figures around 30% have been mentioned) which creates imperfections and voids in the structure and interferes with its unique properties.

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.

Once RGO has been produced, it can be selectively functionalized thus enabling its use in different applications. By treating RGO with other chemicals or by creating new compounds when combining RGO with other two dimensional materials, we can engineer the surface chemistry of the compound to the specific application.

Graphene Oxide Paper

Graphene Oxide Paper is relatively easy to make.  GO is known to disperse very easily due to the type and amount of functional groups on its surface. To make GO paper folks typically disperse the GO in a solvent such as water or an organic solvent and then using a 0.2um membrane filter, they pour the GO solution through a vacuum filtration apparatus and the membrane keeps the particles on top while the solvent is collected below.  When dry, the membrane can be removed leaving a free standing GO paper product. RGO paper can be made by similar methods but will require surfactants to stabilize it so it may be desirable to make the GO paper and then reduce it instead of adding surfactants.

 

Graphene manufacture

Graphite oxide has attracted much interest as a possible route for the large-scale production and manipulation of graphene, a material with extraordinary electronic properties. Graphite oxide itself is an insulator,  almost a semiconductor, with differential conductivity between 1 and 5×10−3 S/cm at a bias voltage of 10 V. However, being hydrophilic, graphite oxide disperses readily in water, breaking up into macroscopic flakes, mostly one layer thick. Chemical reduction of these flakes would yield a suspension of graphene flakes. It was argued that the first experimental observation of graphene was reported by Hanns-Peter Boehm in 1962. In this early work the existence of monolayer reduced graphene oxide flakes was demonstrated. The contribution of Boehm was recently acknowledged by Andre Geim, the Nobel Prize winner for graphene research.

Partial reduction can be achieved by treating the suspended graphene oxide with hydrazine hydrate at 100 °C for 24 hours, by exposing graphene oxide to hydrogen plasma for a few seconds, or by exposure to a strong pulse of light, such as that of a Xenon flash. Due to the oxidation protocol, manifold defects already present in graphene oxide hamper the effectiveness of the reduction. Thus, the graphene quality obtained after reduction is limited by the precursor quality (graphene oxide) and the efficiency of the reducing agent. However, the conductivity of the graphene obtained by this route is below 10 S/cm, and the charge mobility is between 0.1 and 10 cm2/Vs. These values are much greater than the oxide’s, but still a few orders of magnitude lower than those of pristine graphene. Recently, the synthetic protocol for graphite oxide was optimized and almost intact graphene oxide with a preserved carbon framework was obtained. Reduction of this almost intact graphene oxide performs much better and the mobility values of charge carriers exceeds 1000 cm2/Vs for the best quality of flakes. Inspection with the atomic force microscope shows that the oxygen bonds distort the carbon layer, creating a pronounced intrinsic roughness in the oxide layers which persists after reduction. These defects also show up in Raman spectra of graphene oxide.

Large amounts of graphene sheets may also be produced through thermal methods. For example, in 2006 a method was discovered that simultaneously exfoliates and reduces graphite oxide by rapid heating (>2000 °C/min) to 1050 °C. At this temperature, carbon dioxide is released as the oxygen functionalities are removed and explosively separates the sheets as it comes out.

Exposing a film of graphite oxide to the laser of a LightScribe DVD has also revealed to produce quality graphene at a low cost.

Graphene oxide has also been reduced to graphene in situ, using a 3D printed pattern of engineered E. coli bacteria.

 

 

 

 

Leave a Reply

Your email address will not be published.