Understanding graphene part 6: Graphene Oxide
Graphene is made of carbon so its oxide should be a fairly straightforward thing shouldn’t it? Everyone has heard of carbon dioxide and its sister gas carbon monoxide, so far so simple.
Graphene Oxide (GO) has more variations because the sheet of carbon atoms offers more ways for oxygen to engage with one or more carbon atoms. It gets a bit more complicated because hydrogen is involved too. Have a look at the picture of GO and you’ll see what I mean.
How to make graphene oxide
Strong oxidising agents and concentrated acids do the job of creating graphene oxide directly from graphite using an improved Hummer’s Method I would not recommend trying this at home.
The resulting graphene oxide has a variety of different functional groups attached to the hexagonal carbon sheet as you can see in the above graphic. These are Carboxylic acids (-COOH) Hydroxyl (-OH) and Epoxy (-O-) and others. The effect of this oxidation is to change the properties of graphene in a number of ways. Let’s focus on just two of the ways GO differs from graphene. Electrical conductivity and the way it interacts with water.
Electrical properties of graphene and graphene oxide
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Graphene oxide does not conduct electricity very well, whereas graphene is one of the best electrical conductors known. This is due to the nature of the bonding, sp2 in graphene and sp3 in graphene oxide. You can find out more about sp bonding here.
You’ll now need another piece of information. The opposite of oxidation is called reduction. Graphene oxide can be turned back into graphene with a reducing agent and heating. This makes it conduct electricity again.
Imagine how this might be used. A layer of graphene oxide can be printed with microscopic patterns using ink containing a reducing agent such as ascorbic acid. Heat the layer and conductive graphene forms wherever the reducing ink has been printed. We now have the ability to make microscopic electronic circuits that can form the basis of all sorts of useful electronic devices. This is the type of work that Dr Vivek Pachauri has been doing in Germany to create tiny sensors that can detect health conditions in blood samples. This means saving time, money and reducing the stress in patients who do not have to wait long for their results.
Graphene and graphene oxide affinity with water and oils
Graphene is hydrophobic / oleophilic (it repels water and attracts oils). Graphene Oxide is hydrophilic / oleophobic (it attracts water and repels oils). Put graphene platelets into water and they will not mix and float on the surface. Do the same with graphene oxide platelets and they will disperse in the water.
This interaction with water is what makes graphene oxide the material of choice for desalination membranes that can filter drinking water from seawater.
Reverse some of the oxidation and you can create a material that is both hydrophilic and oleophilic. This is called Janus Graphene. So what? Well, graphene sponges can be made that can mop up oil spills on the surface of the ocean and other bodies of water. These sponges can deal with both water-in-oil and oil-in-water emulsions that are normally very difficult to treat.
So, graphene oxide is useful stuff. It has complementary properties to its sister compound graphene. It is already finding practical applications with clear benefits. It is this connection of cutting edge research with clear customer benefits that is taking graphene and its derivatives from the lab to the market. Industrial scale commercialisation will follow in the coming years.
Adrian Nixon began his career as a scientist and is a Chartered Chemist and Member of the Royal Society of Chemistry. As a scientist and ... <Read more about Adrian Nixon>