Researchers discover boron 'buckyball'

The discovery 30 years ago of soccer-ball-shaped carbon molecules called buckyballs helped to spur an explosion of nanotechnology research. Now, there appears to be a new ball on the pitch.

Researchers from Brown University, Shanxi University and Tsinghua University in China have shown that a cluster of 40 boron atoms forms a hollow molecular cage similar to a carbon buckyball. It's the first experimental evidence that a boron cage structure—previously only a matter of speculation—does indeed exist.

                                                                  

Researchers have shown that clusters of 40 boron atoms form a molecular cage similar to the carbon buckyball

"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. "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."

Wang and his colleagues describe the molecule, which they've dubbed borospherene, in the journal Nature Chemistry.

Carbon buckyballs are made of 60 carbon atoms arranged in pentagons and hexagons to form a sphere—like a soccer ball. Their discovery in 1985 was soon followed by discoveries of other hollow carbon structures including carbon nanotubes. Another famous carbon nanomaterial—a one-atom-thick sheet called graphene—followed shortly after.

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 high-powered supercomputers.

On the computer, Wang's colleagues modeled over 10,000 possible arrangements of 40 boron atoms bonded to each other. The computer simulations estimate not only the shapes of the structures, but also estimate 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.

The next step is to test the actual binding energies of boron clusters in the lab to see if they match any of the theoretical structures generated by the computer. To do that, Wang and his colleagues used a technique called photoelectron spectroscopy.

Chunks of bulk boron are zapped with a laser to create vapor of boron atoms. A jet of helium then freezes the vapor into tiny clusters of atoms. The clusters of 40 atoms were isolated by weight then zapped with a second laser, which knocks an electron out of the cluster. The ejected electron flies down a long tube Wang calls his "electron racetrack." The speed at which the electrons fly down the racetrack is 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."

The borospherene molecule isn't quite as spherical as its carbon cousin. Rather than a series of five- and six-membered rings formed by carbon, borospherene consists of 48 triangles, four seven-sided rings and two six-membered rings. Several atoms stick out a bit from the others, making the surface of borospherene somewhat less smooth than a buckyball.

As for possible uses for borospherene, it's a little too early to tell, Wang says. One possibility, he points out, could be hydrogen storage. Because of the electron deficiency of boron, borospherene would likely bond well with hydrogen. So tiny boron cages could serve as safe houses for hydrogen molecules.

But for now, Wang is enjoying the discovery.

"For us, just to be the first to have observed this, that's a pretty big deal," Wang said. "Of course if it turns out to be useful that would be great, but we don't know yet. Hopefully this initial finding will stimulate further interest in boron clusters and new ideas to synthesize them in bulk quantities." 

source: Brown University

Nanotube forests drink water from arid air....Rice University lab modifies arrays to capture, hold water for later use

                                                      

If you don’t want to die of thirst in the desert, be like the beetle. Or have a nanotube cup handy.

New research by scientists at Rice University demonstrated that forests of carbon nanotubes can be made to harvest water molecules from arid desert air and store them for future use.

The invention they call a “hygroscopic scaffold” is detailed in a new paper in the American Chemical Society journal Applied Materials and Interfaces.

Researchers in the lab of Rice materials scientist Pulickel Ajayan found a way to mimic the Stenocara beetle, which survives in the desert by stretching its wings to capture and drink water molecules from the early morning fog.

They modified carbon nanotube forests grown through a process created at Rice, giving the nanotubes a superhydrophobic (water-repelling) bottom and a hydrophilic (water loving) top. The forest attracts water molecules from the air and, because the sides are naturally hydrophobic, traps them inside.

                                                                     

“It doesn’t require any external energy, and it keeps water inside the forest,” said graduate student and first author Sehmus Ozden. “You can squeeze the forest to take the water out and use the material again.”

The forests grown via water-assisted chemical vapor deposition consist of nanotubes that measure only a few nanometers (billionths of a meter) across and about a centimeter long.

The Rice team led by Ozden deposited a superhydrophobic layer to the top of the forest and then removed the forest from its silicon base, flipped it and added a layer of hydrophilic polymer to the other side.

In tests, water molecules bonded to the hydrophilic top and penetrated the forest through capillary action and gravity. (Air inside the forest is compressed rather then expelled, the researchers assumed.) Once a little water bonds to the forest canopy, the effect multiplies as the molecules are drawn inside, spreading out over the nanotubes through van der Waals forces, hydrogen bonding and dipole interactions. The molecules then draw more water in.

The researchers tested several variants of their cup. With only the top hydrophilic layer, the forests fell apart when exposed to humid air because the untreated bottom lacked the polymer links that held the top together. With a hydrophilic top and bottom, the forest held together but water ran right through.

But with a hydrophobic bottom and hydrophilic top, the forest remained intact even after collecting 80 percent of its weight in water.

The amount of water vapor captured depends on the air’s humidity. An 8 milligram sample (with a 0.25-square-centimeter surface) pulled in 27.4 percent of its weight over 11 hours in dry air, and 80 percent over 13 hours in humid air. Further tests showed the forests significantly slowed evaporation of the trapped water.

If it becomes possible to grow nanotube forests on a large scale, the invention could become an efficient, effective water-collection device because it does not require an external energy source, the researchers said.

Ozden said the production of carbon nanotube arrays at a scale necessary to put the invention to practical use remains a bottleneck. “If it becomes possible to make large-scale nanotube forests, it will be a very easy material to make,” he said.

Co-authors are postdoctoral researcher Liehui Ge, graduate student Amelia Hart and senior faculty fellow Robert Vajtai, all of Rice; Rice alumnus Tharangattu Narayanan, a scientist at the Central Electrochemical Research Institute, Karaikudi, India; Hyunseung Yang, a graduate student at the Korea Institute of Science and Technology and former visiting scholar at Rice; and Srividya Sridhar, a graduate student at Delhi Technological University, India, and visiting scholar at Rice. Ajayan is Rice’s Benjamin M. and Mary Greenwood Anderson Professor in Mechanical Engineering and Materials Science and of chemistry, and chair of the Department of Materials Science and NanoEngineering.

The U.S. Department of Defense and the U.S. Air Force Office of Scientific Research Multidisciplinary University Research Initiative supported the research.

source : Rice University News & Media

Nanotube composites increase the efficiency of next generation of solar cells

                            

Carbon nanotubes, CNTs, are one dimensional nanoscale cylinders made of carbon atoms that possess very unique properties.  For example, they have very high tensile strength and exceptional electron mobility, which make them very attractive for the next generation of organic and carbon-based electronic devices.

There is an increasing trend of using carbon based nanostructured materials as components in solar cells. Due to their exceptional properties, carbon nanotubes are expected to enhance the performance of current solar cells through efficient charge transport inside the device. However, in order to obtain the highest performance for electronic applications, the carbon nanotubes must be assembled into a well-ordered network of interconnecting nanotubes. Unfortunately, conventional methods used today are far from optimal which results in low device performance.

In a new study, a team of physicists and chemists at Umeå University have joined forces to produce nano-engineered carbon nanotubes networks with novel properties.

For the first time, the researchers show that carbon nanotubes can be engineered into complex network architectures, and with controlled nano-scale dimensions inside a polymer matrix.

 “We have found that the resulting nano networks possess exceptional ability to transport charges, up to 100 million times higher than previously measured carbon nanotube random networks produced by conventional methods,” says Dr David Barbero, leader of the project and assistant professor at the Department of Physics at Umeå University.

                   The high degree of control of the method enables production of highly efficient nanotube networks with a very small amount of nanotubes compared to other conventional methods, thereby strongly reducing materials costs.                                     

                                                   

In a previous study (Applied Physics Letters, Volume 103, Issue 2, 021116 (2013))  the research team of David R. Barbero already demonstrated that nano-engineered networks can be produced onto thin and flexible transparent electrodes that can be used in flexible solar cells. These new results are expected to accelerate the development of next generation of flexible carbon based solar cells, which are both more efficient and less expensive to produce.

so urce:Umeå universitet.

Self-healing carbon nanotube supercapacitors

                                        

               If you ever had problems with the (non-removable) battery in your iPhone or iPad then you well know that the energy storage or power source is a key component in a tightly integrated electronic device. Any damage to the power source will usually result in the breakdown of the entire device, generating at best inconvenience and cost and in the worst case a safety hazard and your latest contribution to the mountains of electronic waste.A solution to this problem might now be at hand thanks to researchers in Singapore who have successfully fabricated the first mechanically and electrically self-healing supercapacitor.Reporting their findings in Advanced Materials ("A Mechanically and Electrically Self-Healing Supercapacitor") a team led by Xiaodong Chen, an associate professor in the School of Materials Science & Engineering at Nanyang Technological University, have designed and fabricated the first integrated, mechanically and electrically self-healing supercapacitor by spreading functionalized single-walled carbon nanotube (SWCNT) films on self-healing substrates.

                                                                 

                   The design and manufacturing process flow of a flexible, electrically and mechanically self-healing supercapacitor. The self-healing composite is composed of hierarchical flower-like TiO2 nanostructures (black spheres) and a supramolecular network (red wires) with a large amount of hydrogen bond acceptors (blue rods) and donors (green rods), which is then compressed under heat to form a self-healing substrate. Carbon nanotube (CNT) films are deposited on the self-healing substrates, which are then assembled to form the sandwiched supercapacitors. b) Optical image of a flexible self-healing substrate on PET sheet after deposition of the CNT film. c) Optical image of an integrated self-healing supercapacitor.

                                           

                               "The successful preparation of this self-healing supercapacitor may provide a way to expand the lifetime of future energy storage devices and empower them with desirable economic and human safety attributes," Che said.

                     Inspired by the biological systems’ intrinsic self-repairing ability, a class of artificial 'smart' materials, called self-healing materials, which can repair internal or external damages have been developed over the past decade (for instance, see our previous Nanowerk Spotlights: "Nanotechnology material, heal thyself" and "Nanocoated fabric is super-repellent and self-healing").Besides restoring mechanical and structural properties, recovery of function after damage has recently been emphasized and successfully achieved in several functional materials (see for instance: "Creating indestructible self-healing circuits"). Nevertheless, further progress in fabrication of integrated functional electronic devices with self-healing attribute remains a challenge.Chen notes that, in order to achieve a functional supercapacitor capable of damage self-healing, the restoration of electrical conductivity after damage is of foremost importance."We worked on the assumption that that the integration of appropriate self-healing materials into supercapacitor devices could prevent the structural fractures of electrode materials, as well as restore the configuration integrity and electrical properties of the devices after mechanical damage," he says.Recently, to meet the requirements of miniaturized portable electronic devices, great efforts have been devoted to flexible and lightweight supercapacitors. Most of these supercapacitors suffer from mechanical fragility – when subjected to practical application, the electrode materials become susceptible to structure fractures under bending or during charge and discharge process, while the polymeric flexible substrates may possibly undergo mechanical damage caused by deformation over time or accidental cutting. This seriously limits the reliability and lifetime of the supercapacitors.

                                                                        

Schematic representation of self-healing capabilities of electrical conductivity of as-prepared SWCNT films spread on self-healing substrates

 source nanowerk