Buckyballs (or Buckminsterfullerene), the soccer ball-like structures of 60 carbon atoms, have a new playmate. Previously only theorized, researchers from Brown University in the US and Shanxi and Tsinghua Universities in China have been the first to experimentally observe a boron "buckyball."
Since the discovery of buckyballs in 1985, it had been speculated that other elements might also be able to form hollow molecular structures, with boron considered a prime candidate. Sitting next to carbon on the periodic table, boron has one less electron than its neighbor, meaning any boron cage would have a different number of atoms.
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Lai-Sheng Wang, a professor of chemistry at Brown, and his team had previously created one-atom-thick disks made up of 36 boron atoms that shows potential for being stitched together to form a graphene analog, dubbed borophene. This work suggested that clusters of 40 boron atoms were abnormally stable when compared to boron clusters made up of a different number of atoms.
Using high-powered supercomputers, Wang's colleagues modeled over 10,000 possible arrangements of 40 boron atoms bonded to each other. As well as estimating the shapes of the structures, the computer simulations estimate the electron binding energy of each structure, with the spectrum of binding energies serving as a unique fingerprint for each potential structure.
The team then moved onto experimental work using photoelectron spectroscopy to test whether the binding energies of boron clusters in the lab matched any of the potential structures generated by the computers.
This involved zapping the chunks of bulk boron with a laser to create a boron atom vapor. Using a jet of helium, this vapor is then frozen into tiny clusters of atoms, with the clusters of 40 atoms isolated by weight before being zapped with a second laser. This knocks an electron out of the cluster that flies down a long tube with the speed at which it travels used to determine the electron binding energy spectrum of the cluster.
The experiments showed that 40-atom-clusters form two structures with distinct binding spectra, which were a match for the spectra of two structures generated by the computer models. The first was a semi-flat molecule, while the second was the buckyball-like spherical cage that the team has dubbed borospherene.
Where buckyball molecules feature 20 hexagons and 12 pentagons of carbon atoms arranged in a way that provides a smooth spherical surface, borospherene consists of 48 triangles, four seven-sided rings and two six-sided rings. This results in a shape that is a little bumpier, with several atoms sticking out from the sides.
Wang says it is too early to say what applications the molecule may lend itself to. However, due to boron's electron deficiency it is expected that borospherene would bond well with hydrogen. For this reason, Wang believes one potential application would be hydrogen storage, with the tiny boron cages serving as safe houses for the hydrogen molecules.
The new molecule is described in the team's paper that is published in the journal Nature Chemistry.
Source: Brown University