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Boron Buckyballs
August 11, 2014
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, Na
2[B
4O
5(OH)
4]·8H
2O) 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 2s
2 2p
1, and carbon's outer electrons being 2s
2 2p
2. This similarity induced
scientists to look for large
molecules composed of boron atoms; namely, boron
analogues of
Buckminsterfullerene,
colloquially called, buckyballs.
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 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:
- 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.
- 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.
- 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.
- Elizabeth Gibney, "Nature News Blog: First boron 'buckyball' could be used to store hydrogen," Nature, July 13, 2014.
- Researchers discover boron 'buckyball,' Brown University Press Release, July 9, 2014.
Permanent Link to this article
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