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Posted: Jan 18, 2012

Simple green production of high quality graphene nanosheets and quantum dots in bulk amounts

(Nanowerk Spotlight) A University of Ulster laboratory has found a simple, low cost and environmentally friendly way to turn common graphite flakes into bulk amounts of either high quality graphene nanosheets or quantum dots. Such structures could lead to new nanoelectronics and energy conversion technologies.
Our lab of carbon based nanomaterials, in collaboration with colleagues in China, UK, and Ireland, has discovered a simple process, which is quicker and environmentally friendlier than currently established techniques for making high quality graphene (single sheet of graphite) nanosheets and quantum dots at an industrial scale. The results have been published online this month in the Royal Chemical Society's journal Chemical Communications ("Controllable selective exfoliation of high-quality graphene nanosheets and nanodots by ionic liquid assisted grinding").
Quantum dots are tiny islands of electrons, which can be used as building blocks for controlling the flow of electrons at the single electron level. Graphene quantum dots can be created by cutting sheets of graphene into small islands of the desired shape. Past attempts to create high quality graphene quantum dots, have involved sophisticated equipment or expensive raw materials, which have resulted in low yields. On the other hand, up to now, solution processes to produce graphene nanosheets and quantum dots in high yields have involved the use of strong acids or prolonged sonication, which introduce defects on the graphene nanocrystal.
AFM image of graphene quantum dots dots
AFM image of graphene quantum dots dots, produced with a grinding time of 4 hours. The nanodots present a monodispersion in height of only 13 nm (few layer graphene) and in diameter of only 10 nm. (a) and (b) XPS spectra of graphene nanosheets and starting graphite, respectively. . This demonstrates that the graphene sheets are clean and free of any impurities and contaminations from the chemicals used, except for a small amount of oxygen inherited from the starting graphite material. (Figure: Prof P. Papakonstantinou, University of Ulster)
Our team came up with a simple solution to produce high quality graphene nanosheets and dots. We ground cheap graphite flakes with a small quantity of ionic liquid to produce a gel and subsequently cleaned the ionic liquid. The grinding in ionic liquid helps to simultaneously fragment and exfoliate graphite flakes into graphene nanosheets. Their size could be tailored by applying different the grinding times. When longer grinding times are used, graphene quantum dots with an average diameter of 10 nm and a thickness of 2 to 5 graphene layers are the dominant products.
The most important attribute of the produced graphene nanosheets and quantum dots compared to those reported in the literature is that they are clean from any solvent contamination and possess a low concentration of oxygen, which is inherited from the starting graphite powder. The X-ray photoelectron spectra of the figure above illustrate that the graphene products possess the same amount of oxygen as that found in the starting graphite flakes. Supported by other microstructural investigations, this suggests that the center of graphene nanosheets and quantum dots is free of defects and therefore it would be possible to maintain very high mobilities suitable for nanoelectronic devices.
Our procedure is mild and relies on pure shear forces to detach the graphene layers from the graphite flakes. Therefore, in contrast to other techniques reported so far, severe defect formation on the crystalline plane of graphene is avoided. No acids or prolonged sonication are used, resulting in high quality material. Moreover our method has the potential to be applied to other layered materials such as MoS2 or BN in addition to graphite.
Dr Nai-Gui Shang, a researcher at UU, comments: "Grinding is a Chinese traditional way of making ink for calligraphy and painting for over two thousand years, where the ink is produced by grinding the ink stick in a ink slab, mixed with a small amount of water. We thought why not try it with graphite flakes? Here, ionic liquid used as a novel green grinding agent, plays a critical role in the both good quality and high yield of graphene nanostructures. We believe that graphene nanostructures produced in this way can be applied successfully to inkjet printing of nanoelectronics."
Armed with valuable experience on how to produce controllably graphene nanosheets and quantum dots, we are currently exploiting the large amount of catalytic edges for energy conversion applications.
The research was supported by the INVEST Northern Ireland (Proof of Concept Award POC114), Royal Academy of Engineering/Leverhulme Trust Senior Research Fellowship (to PP), EPSRC funded facility access to HRTEM at the University of Nottingham and University of St Andrews, and the Tyndall National Access Programme, NAP supported by SFI.
By Dr. Pagona Papakonstantinou, Professor of Advanced Materials, Nanotechnology and Integrated Bioengineering Centre, School of Engineering, University of Ulster and principal investigator of the graphene team.

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