Dr Karl Coleman, Durham University
Ben - Using sticky tape is just one way of getting some graphene and it is not likely to be much use, as Andrea was saying, for Samsung making television screens. Exactly how graphene is made may well depend on what it is that you intend to use it for. So to explain more, weíre joined by Dr. Karl Coleman. Heís from the Department of Chemistry at Durham University. Karl, thank you ever so much for joining us.
Karl - Pleasure.
Ben - How do we make graphene in larger quantities? Sticky tape is all well and good for doing physics on graphene but how do we make it in bigger quantities, in bulk?
Karl - Yes. I mean, as you can imagine using sticky tape is a bit labour intensive if you wanted a large volume of the material. There are a number of ways to make it and we categorise those I guess into two camps. A top down approach, the sticky tape method would be a top down, so that's starting from graphite to the thin layers and then a bottom up approach. So top down, you can actually use a chemical method to really replace the using of the sticky tape. So there are people out there who basically take graphite and they do a rapid thermal expansion in the presence of an oxidising acid such as sulphuric acid. That breaks away, breaks apart those layers, and that also gives you the single sheets. So thatís another way of producing it and of course, you can sort of imagine that being a little bit more scalable.
" alt="Explosive exfoliation of graphite oxide by heating." />Ben - So effectively, by superheating a block of graphite, rather than peeling layers off, you're causing the layers to break apart spontaneously.
Karl - Exactly. So, just give it a big kick in energy, through oxidising acid, split those layers apart and then you'll get some single sheets. Itís a little bit more difficult to separate of course if you have lots of graphitic impurities in there as well. So thereís lots of processing that's necessary, so lots of centrifugation, dispersing in solvents. There are other hurdles to overcome using that method. If you go for more of the bottom up approaches, which you can potentially see as more scalable, then we are talking more or so CVD, that's chemical vapour deposition approach. And there, you can actually decompose pretty much any hydrocarbon containing gas that you like; methane, ethane, propane, et cetera. People tend to prefer methane, but you can use a mixture of methane to hydrogen, and literally flow it over the top of a metal catalyst, and what the researchers in graphene like to do at the moment is use nickel or copper Ė copper being perhaps the most prominent. And you can grow a graphene film by the decomposition of the methane hydrogen on top of the copper, and I believe that's what Samsung are using, the Korean scientists, to grow very large tens of centimetre graphene films. So thatís a little bit more scalable and easy to use than sticky tape.
Ben - And rather than using something that's becoming relatively rare as it gets used too much, the indium that we are seeing in current screens, using copper which is quite common and still relatively cheap, and something like methane or any hydrocarbon, of which the prices are slowly going up, but we do still have plenty.
Karl - We do still have plenty. I mean, thatís another issue about using fossil fuels, I guess. But yes, we do have plenty of natural gas kicking around. So using natural gas, you need hydrogen in there, you need a reducing atmosphere, and thatís the key. And copper is perhaps the most useful because the solubility of carbon in copper is quite low. This is one of the reasons why you tend to grow only one or perhaps two layers on top of the copper substrate. Itís not without its own difficulties, of course, because you'd have to remove the copper because the whole idea of replacing indium tin oxide, ITO of course, is to have a transparent electrode and copper isnít transparent, so you'd need to remove that copper and so itís usually an etch to remove the copper and then you can transfer the graphene film onto your plastic to assemble your plastic electronics.
Ben - So, there are still fairly complicated methods that we have to create it. Is there one great way of doing it and several not so good ways or do they each have advantages over the other, depending on what product you actually want?
Karl - Yeah. It really depends on what you want to do and that will really dictate what method you want to use. The micromechanical methods, i.e. the scotch tape, sticky tape is very good for physicists, they need a small little chunk of the graphene, or a small sheet and they can study it, measure it, and do lots of different things. The display is obviously a little bit different so you are going to need something like a large piece of copper foil to grow your film on, but then you have to transfer it from that copper onto your plastic and remove the copper. So yeah, I mean if you want the ITO replacement, that would be the method you would use. But if you were to use it in say, composite materials, thereís a lot of work going on at the moment about using graphene in composites so as a nanofiller. Then of course, neither of those methods would be suitable and so we need another method to make the graphene. There are variations on CVD to do that, so where you don't actually use a copper foil, you use other metal catalysts or you use a spray pyrolysis type approach to get basically graphene powder that you would then be able to disperse in your polymer matrix to make your composite material. So it does really depend on what you want to do with it.
Ben - Composites are relatively new, we havenít been using them for that long, but they've made huge waves in the aerospace industry. Weíve had a question from David Whalley who got in touch via Twitter. He wants to know if graphene is likely to be used in spacecrafts. I would imagine itís composites weíd be looking at there.
Karl - There definitely would be composites. On its own, it would be difficult to use in aerospace. So, it would be incorporated into some sort of polymer matrix. So yes, thereís lots of carbon based nanofillers. So of course, we are all familiar with carbon fibre which is perhaps the most common one thatís out there at the moment, and carbon nanotubes which have been making some headway, but of course, there are some problems associated with that. I think really, people have been looking there at these one dimensional, high aspect ratio materials as fillers. And then graphene has come along and challenged carbon nanotubes in the prospect of being a carbon fibre replacement. So, you'd have to get it into some sort of polymer matrix, so a resin or a thermo plastic, and really, the difference I guess with the nanofillers is you require much less of the nanofillers compared to say, carbon fibre. And so, that opens up a whole new game for the guys dealing in composites, and how they process the materials, how they mould complex shapes, et cetera. So it opens up new doors perhaps that you wouldnít get with carbon fibre, but you'd still get the high strength:weight, at a ratio that you would need from a composite material.