Professor Scott Bunch talks to G.P. Thomas about the properties of graphene that make it a useful material in molecular sieving and what applications this may have in the future.
GT: Could you please provide a definition of graphene and a brief history of its discovery?
SB: Graphene is a single atomic layer of graphite and graphite is the material you find in pencil lead. It consists of sheets of carbon atoms covalently bonded in a hexagonal chicken wire lattice where the sheets are held together by weak van der Waals bonds. Individual graphene layers were first isolated in 2004 by a team of scientists at the University of Manchester. Since then, there has been an explosion of work in this field exploring the physical properties of this amazing new material. This culminated in a Nobel Prize in physics to Andre Geim and Kostya Novoselov, 2 of the scientists at University of Manchester that first isolated graphene and carried out a number of pioneering experiments on this new material.
GT: What are the unique properties of graphene that make it such an exciting new material?
SB: Depending on your interest there are many unique properties that make this an exciting new material. For one, it is the thinnest material possible since it consists of just a single layer of atoms. Couple this with its remarkable electrical, mechanical, thermal, and optical properties and you have scientists and engineers busy over the past several years exploring its properties and potential applications. In our case, we are most interested in the mechanical properties. It is the thinnest and strongest material as well as being impermeable to all gases.
GT: You have recently done experimental work relating to graphene – could you please outline the details of this?
SB: Most recently, we found a way to introduce molecular sized pores in this atomically thin material and measure the permeability of gases through these pores. We found that the pores can be created such that they efficiently separate gases based on size exclusion – smaller molecules pass through while bigger ones cannot. You can think of these porous graphene membranes as atomically thin filters for gases.
GT: Could you explain oxidative “etching” and why this was used?
SB: To introduce pores into the material we used an oxidative etching technique. As mentioned above, pristine graphene is impermeable to gases, so a means to introduce pores in the material needed to be developed. To accomplish this, we exposed pristine graphene membranes to UV light of sufficiently low wavelength in the presence of oxygen.
The UV light and the oxygen provide the energy and chemistry necessary to slowly etch small holes in the graphene membranes. We tried several different etching techniques but this particular technique proved to be the most effective. I think this was because the etching process was slow enough so that molecular size pores with high gas selectivity can be created. The main reason we used this technique was because it worked.
GT: What gases were used during the experiment?
SB: We measured the permeability of H2 (hydrogen), CO2 (carbon dioxide), Ar (argon), N2 (nitrogen), CH4 (methane), and SF6 (sulphur hexafluoride). We chose these gases because they cover a wide range of molecular diameters (0.29 nm – 0.49 nm) and were readily available in our lab. Another reason for picking these gases is the practical importance and difficulty of using membranes to efficiently separate them. Two of the most important gases are CH4, which is the primary component in natural gas and CO2 which is emitted from power plants.
GT: What were the findings of this work?
SB: We found that by etching molecular size pores in suspended graphene membranes we were able to separate gases based on their molecular size. Our unique experimental geometry gave us the capability to measure gas permeation and selectivity through a small number of pores in an otherwise pristine membrane.
GT: Why is graphene a superior material for the efficient separation of gas molecules?
SB: The fact that graphene membranes are atomically thin is one of the great promises of this new type of membrane. Having a thin membrane means that you can have a large flux of gas through it. This doesn’t require a lot of energy to push the molecules through so they are extremely energy efficient.
The other advantage is the ability to tune the pore sizes at the atomic scale and not having to worry about the 3rd dimension. In principle, any size pore can be introduced into graphene depending on how many carbon atoms are removed. This provides it with a potentially superior ability to discriminate gas molecules by their molecular diameter and the high fluxes that make these membranes energy efficient.
GT: What would be the environmental benefits of this size-selective sieving of gas?
SB: The environmental benefits are alluded to above. You can have energy efficient membranes from which you can do a number of technologically important separations.
GT: Could the technology currently be used on an industrial scale? What are the current limitations?
SB: Our current membranes had diameters on the micron scale. These membranes would work great if power plants and natural gas pipelines were smaller than the diameter of a human hair, but unfortunately for industrial uses, one needs to filter lots of gas which requires much larger membranes. Fortunately, there are now techniques to grow large area single atomic layers of graphene. One such technique, developed at University of Texas at Austin by Rod Ruoff’s group, uses chemical vapor deposition growth of graphene on copper to obtain large sheets. We, along with several other groups in the world, are now working hard to try to scale graphene membranes up to sizes that would make them amenable to industrial scale processing.
A similar analogy can be made to the first transistor which was the size of the human hand. Through clever engineering and lots of hard work, engineers and scientists figured out a way to miniaturize the transistor such that we can now pack billions of them onto a computer chip. A similar challenge awaits graphene membranes where large sheets with a high density of well-defined molecular sized pores are necessary for industrial applications. This research is still in its infancy but the rapid progress in graphene manufacturing and the promise that porous graphene membranes display keeps us motivated.
GT: Lastly, where can people find more information on your work?
SB: The best place to find out about our work is to check out my group homepage: http://spot.colorado.edu/~jbunch/