By Alessandro Pirolini
Lai-Sheng Wang (middle) and graduate students Wei-Li Li (left) and Zachary Piazza (right) holding a model of "Borospherene" - Credit: Wang Lab/Brown University
Nanotechnology experienced a huge surge 30 years ago when a formation of carbon atoms shaped like a soccer ball was created, which became known as a buckyball. The discoverers were awarded the Nobel Prize in 1996 and the windfall of publicity that followed helped nanotechnology explode into technologists' interests across the world. Now researchers from Brown University (USA), Shanxi University (China) and Tsinghua University (China) have taken a small step to the left in the periodic table and developed a buckyball made from boron.
Led by Lai-Sheng Wang, a professor of chemistry at Brown University, the research team have shown that a cluster of 40 boron atoms can be be combined, forming a structure they have named "borospherene". It is similar to the carbon buckyball and is a hollow molecular cage that had only been previously hypothesized.
The discovery of the carbon buckyball in 1985 led to a number of other hollow carbon structure discoveries. This includes carbon nanotubes which has already imposed significant changes on the microelectronics industry, but has potential to be used in solar cells, energy storage, medicine and a host of other applications. Researchers involved were awarded the Nobel Prize for this breakthrough and the team responsible for the boron "buckyball" will surely be very excited about the potential of their finding too.
The Path to Discovery
Shortly after the carbon buckyball was found, scientists almost immediately began to ponder the possibility of other elements, the possibility that other elements, which display similar characteristics to carbon, may also be able to form this unique structure.
Among the main group of elements in the periodic table, boron can form strong bonds with itself, second only to carbon. This can be seen from the high melting temperature of solid boron, compared to graphite. Hence we believed, boron would be the most promising element to form fullerene-like structures.
Lai-Sheng Wang - Professor of Chemistry, Brown University
One such prospect was carbon's neighboring element boron. However, because boron has one less electron than carbon, the 60 atom carbon buckyball would be unstable if replicated using boron. For this element to be adopted in this type of structure, the number of atoms would have to be altered.
The research group led by Wang has vast experience in the study of boron chemistry having been involved in its research for a number of years. In fact earlier this year, they managed to combine 36 atoms of boron into a one atomic-layer thick disk similar to that of graphene that they dubbed borophene, and consequently published a paper on their findings. The step from this to the development of a hypothesized buckyball was not far away after that.
Preliminary work carried out by Wang suggested that a cluster of 40 atoms of boron may be the golden number required to form the elusive buckyball structure and this number proved to be unusually stable compared to other numbers of boron.
The hard work began once this number had been determined and Wang's team used supercomputers to model over 10,000 different arrangements for the bonding of the 40 atoms of boron. The binding energy, the measure of the strength of the atoms' hold on its electrons, can be determined using their supercomputers, as well as all of the potential structures for the cluster.
After all of the different combinations were modelled, the testing of the most suitable arrangements began. This was in order to examine the theorized structures that the computer had developed and their respective binding energies. Photoelectron spectroscopy was employed by Wang's team to carry this kind of experimentation out. This involves using a laser to heat small pieces of boron to create a gaseous vapor of boron atoms and then helium to freeze these atoms into small clusters.
Once the clusters have been created, they were separated based on their weight and then a second laser was employed to split off single electrons from them. The detached electron is then transported down a tube, refererred to by Wang as his "electron racetrack", at great speed which determines that particular cluster's binding energy, which essentially acts like a structural fingerprint.
From this experiment, two structures were observed to form from the 40-atom clusters both with specific binding spectra. These happened to be an identical match to two of the modelled structures from the supercomputer. One shaped like a semi-flat molecule, and the other a hollow spherical cage similar to a buckyball.
How Does it Compare to a Carbon Buckyball?
The carbon buckyball contains rings formed of carbon that consist of 5 or 6 atoms. Borospherene differs to this slightly, as it is made up of 48 triangles, four seven-sided rings and two six-membered rings, all forming together to create a large hollow sphere.
There are also some imperfections - several atoms protrude out from others in the structure, making it slightly less smooth than its carbon counterpart.
We must explore possible methods to synthesize large quantities of borospherene and find out what forms this material may take and what properties it may have before we can discuss possible applications. We speculate that such a material may be good for H2 storage or even as new battery materials because of the light weight and electron deficiency of boron.
Lai-Sheng Wang - Professor of Chemistry, Brown University
As far as applications go, it is too soon to accurately predict what borospherene could be used for. However, Wang does say that one possible application could be hydrogen storage. This is due to boron's electron deficiency and the likelihood that borospherene would easily bond with hydrogen. Therefore, the hollow borospherene cages could be used to contain hydrogen molecules.
Speaking exclusively to AZoNano, Professor Wang discussed the next steps in the research of borospherene, "there are a number of directions for further research," he said "first, we would like to know if there is a whole family of borospherenes, like the fullerenes. Second, we would like to continue to investigate the molecular properties of the B40 borospherene. Third, we would like to explore possible methods to synthesize large quantities of borospherene and find out what forms this material may take and what properties it may have."
As with all discoveries, the potential of borospherene will likely be inflated at the early stages of development. In time it will become clear where it can be employed most effectively and which industries can best exploit the technology. But for now, it's a brand new nanostructure that technologists and scientists across the world can begin to experiment with and discuss.