Chemicals to Conduct and Generate Electricity
What's Hot in Chemistry, September/October 2010
By John Emsley
Papers about graphene occupy seven positions in the current Hot Ten. The only new one among them is paper #10, by Yongchao Si and Edward Samulski of the University of North Carolina at Chapel Hill. It describes the synthesis of water-soluble single sheets of graphene. To a chemist this almost seems a contradiction in terms, but by careful manipulation and with simple chemistry they have managed to achieve this remarkable feat.
Graphite comes in layers of carbon atoms stacked one on top of the other and held together by van der Waals energy. What makes this form of carbon a rarity among non-metals is its ability to conduct electricity like a metal. One way to separate these layers of graphene is to oxidize the graphite with acid, but the resulting graphite oxide will have lost its ability to conduct electricity. Restoring this property to single sheets of graphene is what #10 is all about.
First, Si and Samulski reacted the graphite oxide with sodium borohydride to remove most of the oxygen atoms. Then they lightly sulfonated it to make it soluble in water. Finally, they treated it with hydrazine to remove the remaining oxygens, but leaving enough sulfonic acid groups on the final product to prevent the graphene sheets from sticking together again by mutual attraction.
Read a Fast Moving Front Commentary with Younan Xia. Figures &
descritpions are included with his comentary.
From Xia's commentary, an illustration showing how a near-infrared light is
used to trigger the conformational change for a polymer coating on a gold
nanocage and thus the release of encapsulated drug.
The material they obtained was checked for electrical conductivity, and at 1250 S/m it is only four times less conducting than graphite itself. The implication of this conductivity is that these single sheets of graphene have delocalized sp2 bonding just as in graphite. Soluble graphene offers all kinds of commercial opportunities in such areas as displays, transistors, and ultra-sensitive chemical detectors.
Another kind of oxygen reduction is the subject of paper #9 from Washington University, St Louis, Missouri. The work of a team headed by Younan Xia (who was featured in Science Watch in the September/October 2006 issue), the paper offers an alternative catalyst for fuel cells where hydrogen oxidation and oxygen reduction are the driving force. As yet, the widespread use of fuel cells to power vehicles is inhibited by the high costs of the platinum-based catalysts that are needed. Paper #9 offers a different type of platinum.
Xia has devised nano-sized clusters of platinum branches grown on the surface of tiny (9 nm) crystals of palladium. These serve as his alternative catalyst for the rate-determining step of oxygen reduction in proton-exchange membrane fuel cells.
Uniform crystals of palladium were produced by reducing a solution of Na2PdCl4 with L-ascorbic acid. These were then used to seed a solution of K2PtCl4 which was again reduced using L-ascorbic acid. The metallic platinum which was released then grew as branches from the surfaces of the palladium crystals, and these nano-dendrites were on average 23.5 nm in size but with relatively large surface areas.
Compared to the behavior of their commercial counterparts, platinum/carbon and platinum black, the new catalyst performed much better than either, even through countless discharges. Xia postulates that his new catalyst might well have applications beyond fuel cells, and in that respect he is almost certainly right.
Also in the current Hot Ten is paper #8 from a group at the University of California, Los Angeles, headed by Yang Yang. This reports the synthesis of a new type of polymer with possible use in solar cells because of its low band gap. Polymer solar cells have great potential because of their light weight, high flexibility, and low cost. Fullerene-type polymer materials are already widely used but have the disadvantage of poor absorption of energy in the visible spectrum.
Benzodithazole polymers are better because they cover more of the spectrum, but replace one of their carbons with a silicon group, as Yang has done, and they now cover the whole of the visible spectrum. Not only that, but his new polymer is thermally stable up to 250° C and without needing an inert atmosphere to protect it, and the all-important band-gap remains more or less the same. The result was a power efficiency of around 5%, ahead of other devices of this kind.
Dr. John Emsley is based at the Department of Chemistry, Cambridge University, U.K.
What's Hot in Chemistry | |||
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Rank | Paper |
Cites This Period Mar-Apr 10 |
Rank Last Period Jan-Feb 10 |
1 | C. Lee, et al., "Measurement of the elastic properties and intrinsic strength of monolayer graphene," Science, 321(5887): 385-8, 18 July 2008. [Columbia U., New York, NY] *327FB | 43 | 3 |
2 | A. Reina, et al., "Large area, few-layer graphene films on arbitrary substrates by chemical vapor deposition," Nano Letters, 9(1): 30-5, January 2009. [MIT, Cambridge] *395IZ | 40 | 7 |
3 | X.S. Li, et al., "Large-area synthesis of high-quality and uniform graphene films on copper foils," Science, 324(5932): 1312-4, 5 June 2009. [U. Texas, Austin] *453TF | 37 | † |
4 | D.C. Elias, et al., "Control of graphene’s properties by reversible hydrogenation: Evidence for graphane," Science, 323(5914): 610-3, 30 January 2009. [U. Manchester, U.K.; Inst. Microelectronics Tech., Chernogolovka, Russia; Cambridge U., U.K.; U. Nijmegen, Netherlands] *400JB | 34 | † |
5 | L.Y. Jiao, et al., "Narrow graphene nanoribbons from carbon nanotubes," Nature, 458(7240): 877-80, 16 April 2009. [Stanford U., CA] *433CS | 34 | † |
6 | C. de la Cruz, et al., "Magnetic order close to superconductivity in the iron-based layered LaO1-xFx FeAs systems," Nature, 453(7197): 899-902, 12 June 2008. [6 U.S. and China institutions] *311WV | 31 | † |
7 | D.V. Kosynkin, et al., "Longitudinal unzipping of carbon nanotubes to form graphene nanoribbons," Nature, 458(7240): 872-6, 16 April 2009. [Rice U., Houston, TX] *433CS | 31 | † |
8 | J.H. Hou, et al., "Synthesis, characterization, and photovoltaic properties of a low band gap polymer based on silole-containing polythiophenes and 2,1,3-benzothiadiazole," J. Am. Chem. Soc., 130(48): 16144-5, 3 December 2008. [U. Calif., Los Angeles; Solarmer Energy, El Monte, CA] *406UG | 28 | † |
9 | B. Lim, et al., "Pd-Pt bimetallic nanodendrites with high activity for oxygen reduction," Science, 324(5932): 1302-5, 5 June 2009. [Washington U., St. Louis, MO; Brookhaven Natl. Lab., Upton, NY] *453TF | 28 | † |
10 | Y. Si, E.T. Samulski, "Synthesis of water soluble graphene," Nano Letters, 8(6): 1679-82, June 2008. [U. North Carolina, Chapel Hill] *313LL | 27 | † |
SOURCE: Thomson Reuters Hot Papers Database. Read the Legend. |