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Boron Buckyballs

August 11, 2014

Location of boron in the periodic table Boron (atomic number 5) sits next to carbon (atomic number 6) in the periodic table. Sitting on the electron-lean side of carbon, boron has one less bonding electron. That's why the oxide of carbon is CO2, but the oxide of boron is B2O3.

Borax (sodium tetraborate, Na2[B4O5(OH)4]·8H2O) is a common compound of boron, being used as a detergent. Readers of my generation might remember the brand, 20 Mule Team Borax, a sponsor of the 1950s and 1960s syndicated television series, Death Valley Days.

The oxide of boron, boron trioxide (a.k.a., boric oxide), is typically produced by reacting borax with sulfuric acid at high temperature to produce the oxide and sodium sulfate. Since molten boric oxide is less dense than sodium sulfate, it floats to the top and is easily removed.

Boric oxide is used often in materials science. It is used as a brazing and welding flux to protect the join from oxidation. Laboratory glassware is made from borosilicate glass, since its low thermal expansion makes it resistant to cracking from heat-shock.

One of the more exotic uses of boric oxide is as an encapsulant for molten gallium arsenide. Crystals of gallium arsenide, used to make various semiconductor devices, are grown from the molten material, but it loses arsenic at high temperatures. Molten boric oxide floats on the surface of the gallium arsenide, and it contains the arsenic in a pressurized vessel. I once did research on this liquid encapsulated Czochralski process.[1]

Since boron and carbon are neighbors in the periodic table, their atoms are comparable in size, and their electrons occupy the same 2s and 2p electron subshells. Each of these atoms has a helium core, with boron's outer electrons being 2s2 2p1, and carbon's outer electrons being 2s2 2p2. This similarity induced scientists to look for large molecules composed of boron atoms; namely, boron analogues of Buckminsterfullerene, colloquially called, buckyballs. A B80 molecular cage

Perhaps Linus Pauling (1901-1994) could have discovered the B80 molecule with a ball-and-stick model.

(Figure one from Tunna Baruah, Mark R. Pederson, and Rajendra R. Zope, "The vibrational stability and electronic structure of B80 fullerene-like cage," arXiv, March 19, 2008.)


In 2008, Boris Yakobson, a materials scientist at Rice University (Houston, Texas) and his colleagues predicted that a fullerene-type molecule of eighty boron atoms should be stable (see figure).[2] Now, a large international team from Shanxi University (Taiyuan, China), Tsinghua University (Beijing, China), Brown University (Providence, Rhode Island), and Fudan University (Shanghai, China) have published their discovery of a 40 atom boron molecule that they call borospherene. Its atoms are bonded in triangles, hexagons, and heptagons.[3-5]

Some other elements have been found to form smaller-sized buckyball-type molecules. These are gold, tin and lead, but only boron has formed a many-atom cage-like structure similar to the sixty carbon atom Buckminsterfullerene (C60).[4] computer simulation of theoretical boron cage structures showed that clusters of forty atoms were unusually stable. More than 10,000 arrangements of forty boron atoms bonded to each other were simulated.[5] There were two stable arrangements. One was a nearly flat molecule, but the other approximated a sphere with simple geometrical facets.[4]

To produce boron clusters, the research team vaporized boron using a laser. The boron vapor was condensed into atom clusters in a jet of helium gas, and then the clusters were analyzed using photoelectron spectroscopy.[3,5] The analysis verified the two predicted forms of B40. Says Lai-Sheng Wang, leader of the research team and a professor of chemistry at Brown University,
"This is the first time that a boron cage has been observed experimentally... As a chemist, finding new molecules and structures is always exciting. The fact that boron has the capacity to form this kind of structure is very interesting."[5]

Borospherene

Borospherene is a cluster for 40 boron atoms forming a hollow, cage-like molecule.

(Brown University Image.)


The particular bonding of the B40 molecule, a mixture of σ- and π- bonds, gives the cage a jagged surface in which some atoms jut out from the structure.[3,5] B40 is also more reactive than C60. This leads to the possibility that B40 can bond with hydrogen for use as a hydrogen storage material.[4-5] This research was supported by the US National Science Foundation and the National Natural Science Foundation of China.[5]

References:

  1. Devlin M. Gualtieri, Edward Porbansky and Mandayam C. Narasimhan, "Non-contacting inductively coupled displacement sensor system for detecting levels of conductive, non-magnetic liquids, and method of detecting levels of such liquids," US Patent No. 4,912,407, March 27, 1990.
  2. Nevill Gonzalez Szwacki, Arta Sadrzadeh, and Boris I. Yakobson , "B80 Fullerene: An Ab Initio Prediction of Geometry, Stability, and Electronic Structure," Phys. Rev. Lett., vol. 98, Document No. 166804, April 20, 2007.
  3. Hua-Jin Zhai, Ya-Fan Zhao, Wei-Li Li, Qiang Chen, Hui Bai, Han-Shi Hu, Zachary A. Piazza, Wen-Juan Tian, Hai-Gang Lu, Yan-Bo Wu, Yue-Wen Mu, Guang-Feng Wei, Zhi-Pan Liu, Jun Li, Si-Dian Li and Lai-Sheng Wang, "Observation of an all-boron fullerene," Nature Chemistry (July 13, 2014), doi:10.1038/nchem.1999.
  4. Elizabeth Gibney, "Nature News Blog: First boron 'buckyball' could be used to store hydrogen," Nature, July 13, 2014.
  5. Researchers discover boron 'buckyball,' Brown University Press Release, July 9, 2014.

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