Boron ‘buckyball’ discovered
July 14, 2014
“This is the first time that a boron cage has been observed experimentally,” said Lai-Sheng Wang, a professor of chemistry at Brown who led the team that made the discovery.
Wang and his colleagues describe the molecule, which they’ve dubbed “borospherene” or “fullerene B40,” in the journal Nature Chemistry.
Discovered in 1985 by by Richard Smalley, Robert Curl, Harold Kroto, James Heath, and Sean O’Brien, carbon buckyballs, a.k.a. fullerene C60, consist of 60 carbon atoms arranged in pentagons and hexagons to form a sphere — like a soccer ball.
After buckyballs, scientists wondered if other elements might form these odd hollow structures.
One candidate was boron, carbon’s neighbor on the periodic table. But because boron has one less electron than carbon, it can’t form the same 60-atom structure found in the buckyball.
The missing electrons would cause the cluster to collapse on itself. If a boron cage existed, it would have to have a different number of atoms.
Wang and his research group have been studying boron chemistry for years. In a paper published earlier this year, Wang and his colleagues showed that clusters of 36 boron atoms form one-atom-thick disks, which might be stitched together to form an analog to graphene, dubbed “borophene.”
Wang’s preliminary work suggested that there was also something special about boron clusters with 40 atoms. They seemed to be abnormally stable compared to other boron clusters. Figuring out what that 40-atom cluster actually looks like required a combination of experimental work and modeling using supercomputers.
Wang’s colleagues modeled more than 10,000 possible arrangements of 40 boron atoms bonded to each other. The computer simulations estimate both the shapes of the structures and the electron binding energy for each structure — a measure of how tightly a molecule holds its electrons. The spectrum of binding energies serves as a unique fingerprint of each potential structure.
Next, they tested the actual binding energies of boron clusters in the lab to see if they matched any of the theoretical structures generated by the computer, using “photoelectron spectroscopy.”
Chunks of bulk boron were zapped with a laser to create vapor of boron atoms. A jet of helium then froze the vapor into tiny clusters of atoms. The clusters of 40 atoms were isolated by weight then zapped with a second laser, which knocked an electron out of the cluster. The ejected electron flew down a long tube Wang calls his “electron racetrack.” The speed at which the electrons flew down the racetrack was used to determine the cluster’s electron binding energy spectrum — its structural fingerprint.
The experiments showed that 40-atom-clusters form two structures with distinct binding spectra. Those spectra turned out to be a dead-on match with the spectra for two structures generated by the computer models. One was a semi-flat molecule and the other was the buckyball-like spherical cage.
“The experimental sighting of a binding spectrum that matched our models was of paramount importance,” Wang said. “The experiment gives us these very specific signatures, and those signatures fit our models.”
Cages for hydrogen storage?
The borospherene molecule is a bit lopsided compared to carbon counterpart. Rather than the neat series of five- and six-membered rings formed by carbon, borospherene consists of 48 triangles, four seven-sided rings,and two six-membered rings, causing several atoms to stick out a bit from the others.
As for possible uses for borospherene, it’s a little too early to tell, Wang says. One possibility, he points out, could be cages for hydrogen storage. Because of the electron deficiency of boron, borospherene would likely bond well with hydrogen, he says.
The theoretical modeling was done with a group led by Prof. Si-Dian Li from Shanxi University and a group led by Prof. Jun Li from Tsinghua University. The work was supported by the U.S. National Science Foundation and the National Natural Science Foundation of China.
Abstract of Nature Chemistry paper
After the discovery of fullerene-C60, it took almost two decades for the possibility of boron-based fullerene structures to be considered. So far, there has been no experimental evidence for these nanostructures, in spite of the progress made in theoretical investigations of their structure and bonding. Here we report the observation, by photoelectron spectroscopy, of an all-boron fullerene-like cage cluster at B40− with an extremely low electron-binding energy. Theoretical calculations show that this arises from a cage structure with a large energy gap, but that a quasi-planar isomer of B40− with two adjacent hexagonal holes is slightly more stable than the fullerene structure. In contrast, for neutral B40 the fullerene-like cage is calculated to be the most stable structure. The surface of the all-boron fullerene, bonded uniformly via delocalized σ and π bonds, is not perfectly smooth and exhibits unusual heptagonal faces, in contrast to C60 fullerene.