Graphene growth gets greener
It is not often that recycled materials perform better than new ones, but that is the case with a newly developed method for growing graphene that uses a copper catalyst. When the copper film of the catalyst is reused to grow more graphene the properties of the resulting graphene is steadily improved. This is important when it comes to a sustainable way of producing flat panel displays and touch screens.
Transparent conductors everywhere…
We are surrounded by flat panel displays and touch screens – on our desks, in our bags, and in our pockets – and the demand for such devices is growing. They all incorporate a material that is both transparent and conductive – typically indium tin oxide. The indium needed to make such materials is only available at a few tens of parts per billion in the Earth’s crust (and becoming rarer) and demand is outstripping supply.
Graphene is a carbon-based alternative which is just a single layer of atoms thick but highly conductive and with a transparency of 97.7%. It might be the transparent conductor of the future but only if we can make enough of it… and cheaply.
Sustainable growth of graphene
Today large areas of graphene can be produced by passing carbon-containing gases such as methane or acetylene over hot metal catalyst surfaces leading to the formation of a single layer of graphene. The process is called chemical vapour deposition. The quality of the graphene produced depends critically on the properties of the metal surface such as the purity, flatness and temperature.
Copper foil, about 50 µm thick, is the usual material of choice for growing graphene. Once the graphene is grown the copper layer is typically chemically dissolved and destroyed. Associate Professor Tim Booth explains that “this usually means that for every gram of graphene produced more than 2kg of copper is dissolved. There is also the cost of manufacturing and distributing the copper and of disposing of the copper contaminated waste liquids – so it is easy to see that this is not a sustainable process”.
There are two ways to reduce the amount of copper needed for graphene growth. Firstly: We can use less copper, and secondly: We can reuse it.
Reduce, reuse, recycle… improve?
Researchers from DTU Nanotech and collaborators at DTU Danchip, DTU Energy, Columbia University USA and Aixtron Ltd, UK have done both. They minimised the amount of copper needed for growth from 50 µm thick foils to 100 nm thick layers supported by silicon wafers (although in principle any flat surface can be used as long as it can withstand the high temperatures needed during growth). This means they started with only 0.2% of the originally needed amount of copper.
To transfer the graphene, they employed a novel electrochemical method in a liquid electrolyte. In this method, oxygen from the atmosphere dissolved in a liquid electrolyte creeps in between the copper and graphene layer in a process similar to the rusting of iron. By applying a reducing potential, the oxidised copper surface is changed back to copper without dissolving it, and at the same time releasing the graphene. This works well because graphene does not stick well to copper oxide, and reducing the copper oxide results in a volume decrease which helps further to release the graphene from the catalyst surface.
When turned into electronic devices, the graphene layers produced by this new method show better properties than devices made by dissolving the copper. Tim Booth says: “Furthermore, it turns out that each time the copper layer is reused, its surface improves – it becomes flatter and more pure – so in this instance reusing less material gives us a real advantage”.
In reducing the amount of copper needed from 2 kg per gram of graphene to effectively zero, graphene growth has become much less expensive, and far more sustainable. The time previously needed to clean and optimise the copper layer by heat treatment has been substantially reduced, saving energy. Finally, only non-toxic salt water or some other simple electrolyte is needed to remove the graphene from the catalyst.
The patented process is now being used routinely by the researchers, and is a key technology in the EU FP7 Gladiator project, where DTU Nanotech leads activities in large area growth and transfer of graphene, with the goal of producing large organic light emitting diodes in collaboration with partners from across Europe.
The technology could also be applied to make ‘smart glass’, incorporating heads up displays in car windscreens, windows that can be made opaque or transparent or with the ability to display information, transparent panels that could light up a room and other applications.
This work appears in the article by F. Pizzocchero et al. in the April 2015 issue of Carbon, doi:10.1016/j.carbon.2014.12.061