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Graphene Production

May 26, 2014

There are several ways to make scrambled eggs. The method I prefer is when the eggs are whipped in a bowl before cooking, the resulting cooked portion being a very uniform blend of yoke and white. Another method is when the cracked eggs are added to the hot frying pan, where they are mixed, in situ, but not very well. Some people may prefer the later result, but I think that if you're going to mix something, you should do it well, or not at all.

Blending of powders and other granular materials is an important industrial processes for everything from foodstuffs to pharmaceuticals. A common apparatus for doing such blending is the double-cone "V" blender, as shown in the simplified diagram, below.

Slow rotation of the "V" shaped container shifts the contents from the apex of the "V" to the two prongs. At each rotation, the contents are divided in half, so the process is a lot like shuffling cards. A large number of rotations ensures that the contents are well blended, just as a large number of shuffles will randomize a deck of cards. Blending time can be as long as fifteen minutes.

Schematic of a V blenderSimplified diagram of a "V" blender.

(Illustration by the author using Inkscape.)

This process looks good on paper, but there might be problems when mixing some materials. The card shuffling analogy assumes that all the cards are identical, and no cards stick together. Materials having different sized and shaped particles might be a problem, as would static electricity, generated by the granules' rubbing together, holding particles together.

The slow mechanical action of a "V" blender contrasts sharply with the rapidly rotating blades in a kitchen blender. Early electric motor technology was fairly primitive, so the first blenders required a heavy motor, which was mounted in a base. A jar with the rotating blades at its bottom was inverted onto the motor. Modern blenders are smaller, handheld devices that can be immersed into most containers.

Kitchen blenders can be used to chop solids, such as bread crumbs and coffee beans, but they're typically used for creating purées or liquid mixtures such as pancake batter. Kitchen blenders, and their industrial cousins, are often seen in chemistry laboratories, since they are used there for similar non-food mixing tasks. A team of more than 25 scientists from Ireland and the UK has just published its discovery that a laboratory blender can be used to exfoliate graphene sheets from graphite.[1-4]

The research team of chemists, physicists and materials scientists from Trinity College Dublin (Dublin, Ireland), Thomas Swan and Company Limited (Consett, UK), STFC Daresbury Laboratories (Warrington, UK) and Oxford University (Oxford, UK) subjected mixtures of graphite flake in liquid solvents to the mixing action of a Silverson Machines Ltd. LSM high shear laboratory mixer in a demonstration of an industrially scalable process for production of large quantities of defect-free graphene.[1-2]

Figure captionCaution advised.

Jacob Lanphere, a Ph.D. candidate at the The University of California, Riverside, holding a solution of graphene oxide.

A UCR study found that graphene oxide nanoparticles are very mobile in waterways, and they would have a negative environmental impact if released.

(University of California, Riverside, photograph.)

The 250 watt motor of the Silverson Machines mixer rotates a 50 mm mixing head at 6000 rev/min under load.[2] As Count Rumford proved in his cannon boring experiments, mechanical action such as this generates heat. For that reason, the mixing vessel is placed in a water bath at 15°C.[2]

The team identified the processing parameters under their control. These included mixing time, mixing speed, mixing volume, rotor diameter, graphite concentration, rotor-stator gap, rotor-stator position in the liquid volume, the presence/number/configuration of baffles, graphite pre-treatment, graphite type, and solvent type.[2]

Solvents were selected based on their surface energy (or solubility parameter) close to that of graphene itself. This results in a low mixing energy, which leads to stabilization of the exfoliated graphene flakes against reaggregation.[2] organic solvents used in this study were N-methyl-2-pyrrolidone and N-cyclohexyl-2-pyrrolidone. Also suitable are aqueous surfactant solutions, such as sodium cholate in water, and some polymer solutions.[2]

The research team demonstrated that high-shear mixing of graphite in such solvents results in dispersions of exfoliated graphene nanosheets in liquid volumes as small as a few hundred of milliliters up to hundreds of liters.[1,3] Exfoliation was found to occur when the local shear rate exceeds 104 s−1. These flakes were verified as unoxidized and free of basal-plane defects by X-ray photoelectron spectroscopy and Raman spectroscopy.[1]

Funding for this project was provided jointly by Thomas Swan Ltd. and the Science Foundation Ireland.[3] Thomas Swan Ltd. has commercialized this discovery with two products, Elicarb® Graphene Powder and Elicarb® Graphene Dispersion.[3] This process can be applied to exfoliate boron nitride, molybdenum disulfide (MoS2) and other layered crystals.[1]

After you place graphene on a substrate, the next step would be its modification to produced electronic devices such as transistors. That's the topic of research published in a recent issue of Nature Materials by scientists from the University of Arizona (Tucson, Arizona), the Massachusetts Institute of Technology (Cambridge, Massachusetts), Harvard University (Cambridge, Massachusetts), the US Army Research Laboratory (Adelphi, Maryland), the National Institute for Materials Science (Tsukuba, Japan), and the Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC, Madrid, Spain).[5-6]

Working with something as small as graphene requires a special tool, and that's the scanning tunneling microscope (STM), which is also used to image graphene surfaces. Although the bonding within graphene sheets is very strong, the inter-sheet bonding is weak, so it's possible to slide one layer on another. carbon atoms in three stacked graphene layers can arrange themselves in two different configurations, Bernal and rhombohedral. In the Bernal stacking, the carbon atoms of the top layer are directly above the atoms in the lower layer. In rhombohedral stacking, they're aligned with the holes in the bottom layer.[6]

Stacking of hexagonal graphene sheetsTrilayers of graphene sheets can stack in two different configurations, which can co-exist in a single flake with a transition region in between.

(Pablo San-Jose ICMM-CSIC image, via University of Arizona.)

(Click for larger image)

These exhibit very different electronic properties, the Bernal stacking leading to a semiconductor, while the rhombohedral forms an insulator.[5] Both stacking configurations can coexist on a single graphene flake, and the region between them, called a domain wall, accommodates the strain with a modified carbon–carbon bond distance.[5-6] The research team found that the applied electric field of a scanning tunneling microscopy tip can move this domain wall.[6]

Study coauthor, Brian LeRoy, an associate professor in the University of Arizona Department of Physics, explains that "It is extremely rare for a material to change its crystal structure just by applying an electric field... Making trilayer graphene is an exceptionally unique system that could be utilized to create novel devices."[6]

The research team found that the free energy difference between the two stacking states scales with the second power of the electric field, so that the rhombohedral stacking is favored at higher electric fields.[5] They speculate that use of a wide, knife-edged electrode, rather than the single point, might allow movement of a domain over larger areas.[6]

Scanning tunneling microscopy tip modification of graphene.The metal tip of a scanning tunneling microscope can be used to move the domain border between different graphene configurations in the same flake.

Pablo San-Jose ICMM-CSIC image, via (University of Arizona.)

References:

  1. Keith R. Paton, Eswaraiah Varrla, Claudia Backes, Ronan J. Smith, Umar Khan, Arlene O'Neill, Conor Boland, Mustafa Lotya, Oana M. Istrate, Paul King, Tom Higgins, Sebastian Barwich, Peter May, Pawel Puczkarski, Iftikhar Ahmed, Matthias Moebius, Henrik Pettersson, Edmund Long, João Coelho, Sean E. O'Brien, Eva K. McGuire, Beatriz Mendoza Sanchez, Georg S. Duesberg, Niall McEvoy, Timothy J. Pennycook, et al., "Scalable production of large quantities of defect-free few-layer graphene by shear exfoliation in liquids," Nature Materials, April 20, 2014, doi:10.1038/nmat3944.
  2. Supplementary Information for ref. 1.
  3. AMBER in world-first Graphene Innovation, Centre for Research on Adaptive Nanostructures and Nanodevices Press Release, April 22, 2014.
  4. Rachel Courtland, "Graphene You Can Whip Up In A Blender," IEEE Spectrum, April 21, 2014.
  5. Matthew Yankowitz, Joel I-Jan Wang, A. Glen Birdwell, Yu-An Chen, K. Watanabe, T. Taniguchi, Philippe Jacquod, Pablo San-Jose, Pablo Jarillo-Herrero andBrian J. LeRoy, "Electric field control of soliton motion and stacking in trilayer graphene," Nature Materials, advance online publication, April 28, 2014, doi:10.1038/nmat3965.
  6. Daniel Stolte, "Playing Pool with Carbon Atoms," University of Arizona Press Release, April 30, 2014.

Permanent Link to this article

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