Supramolecules get time to shine: Technique reveals interactions between nanotubes, photoluminescent materials

What looks like a spongy ball wrapped in strands of yarn -- but a lot smaller -- could be key to unlocking better methods for catalysis, artificial photosynthesis or splitting water into hydrogen, according to Rice University chemists who have created a platform to analyze interactions between carbon nanotubes and a wide range of photoluminescent materials.

The microscopic particles assembled in the lab of Angel Martí, an assistant professor of chemistry and bioengineering, combine single-walled carbon nanotubes with porous silicate materials that can absorb various molecules -- in this case, a ruthenium complex.

Martí, graduate student and lead author Avishek Saha and their colleagues reported their results July 12 in the Royal Society of Chemistry journal Chemical Science.

The ability to immobilize individual carbon nanotubes on a solid surface is interesting enough, but combining supramolecular systems with nanomaterials to produce hybrids is unique, they said.

"This can be used as a general platform to study the interaction of not only ruthenium complexes, but most photoactive molecules can be encapsulated within these porous silicates in a very simple way without chemical modification, without anything," Marti said.

Saha endured trial and error at every step in bringing the new particles to fruition, first figuring out the best way to keep long, single-walled carbon nanotubes produced by the Rice-born HiPco process from aggregating into bundles while allowing them to adhere to the particles.

The solution suggested by co-author Matteo Pasquali, a Rice professor in chemical and biomolecular engineering and in chemistry, involved dissolving the bundles in chlorosulfonic acid, which added protons -- and thus a positive charge -- to each nanotube.

That was the key to making nanotubes attractive to the three types of silicate particles tested: a commercial version of MCM-41, a mesoporous material used as a molecular sieve; another version of MCM-41 synthesized at Rice by Saha, and microporous Zeolyte-Y.

"We don't fully understand the mechanism, but the truth is they have a very strong affinity to silicon oxide networks," said Marti, describing the nanotube-wrapped particles. "Once they're protonated, they just bind."

But that wasn't enough to create a proper platform because protonated nanoparticles are no longer photoluminescent, a quality the researchers required to "see" such tiny structures under a spectroscope. "Protonated nanotubes are cool, but we want to have pristine nanotubes," Martí said.

"We were stuck there for a while. We tried a lot of things," he said. Acetone, ammonia, chloroform and other substances would deprotonate the nanotubes, but would also release them from the silicate sponges and allow them to clump. But vinylpyrrolidone (VP) did the trick by giving the nanotubes a polymer-like coating while returning them to their pristine states.

"This becomes interesting not only from the standpoint of getting individualized nanotubes on top of a surface, but also because we got fluorescence of nanotubes not from a solution, but from a solid material," Martí said.

The experiment went one critical step further when the researchers introduced ruthenium molecules to the mix. The silicates absorbed the ruthenium molecules, putting them into close proximity with an array of nanotubes. Under a spectroscope, the ruthenium complexes would photoluminesce, but they saw something unexpected in the interaction.

"Basically, we found out that if you put a photoactive species (ruthenium) there and excite it with light, two different processes happen. If it has carbon nanotubes close by, it will transfer an electron to the nanotubes. There's a charge transfer, and we knew that would happen," Martí said. "What we didn't expect when we analyzed the spectrum was seeing two different species of ruthenium complexes, one with a very short photoluminescence lifetime and one very long."

The researchers theorized that ruthenium in the center of the sponge was too far from the nanotubes to transfer electrons, so it retained its standard luminescence.

The research leads to some interesting possibilities for materials science, Saha said. "MCM itself has many applications (as a mesoporous sieve in fuel refineries, for instance), and carbon nanotubes are wonderful materials that many people are interested in. We're just combining these two into a hybrid material that might have the virtues of both."

While pore sizes in zeolites are locked by their crystalline structure at 0.7 nanometers, pores in MCM can be customized, as Saha has done, to absorb specific materials. "There are many things we can do to tune the system that we haven't explored," he said; combining metal molecules or even quantum dots with MCM and nanotubes might lead to interesting results.

Martí said putting charged nanotubes on the surface of a solid also opens the door to use them as catalysts in solar-energy conversion. "You need that driving force, that charge separation, for artificial photosynthesis," he said.

Co-authors of the paper are Rice graduate students Saunab Ghosh and Natnael Behabtu.

The Welch Foundation supported the research.

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by Rice University.

Journal Reference:

Avishek Saha, Saunab Ghosh, Natnael Behabtu, Matteo Pasquali, Angel A. Mart. Single-walled carbon nanotubes shell decorating porous silicate materials: A general platform for studying the interaction of carbon nanotubes with photoactive molecules. Chemical Science, 2011; DOI: 10.1039/C1SC00323B

'Amplified' nanotubes may power the future

Rice University scientists have achieved a pivotal breakthrough in the development of a cable that will make an efficient electric grid of the future possible. Armchair quantum wire (AQW) will be a weave of metallic nanotubes that can carry electricity with negligible loss over long distances. It will be an ideal replacement for the nation's copper-based grid, which leaks electricity at an estimated 5 percent per 100 miles of transmission, said Rice chemist Andrew R. Barron, author of a paper about the latest step forward published online by the American Chemical Society journal Nano Letters.

A prime technical hurdle in the development of this "miracle cable," Barron said, is the manufacture of massive amounts of metallic single-walled carbon nanotubes, dubbed armchairs for their unique shape. Armchairs are best for carrying current, but can't yet be made alone. They grow in batches with other kinds of nanotubes and have to be separated out, which is a difficult process given that a human hair is 50,000 times larger than a single nanotube.

Barron's lab demonstrated a way to take small batches of individual nanotubes and make them dramatically longer. Ideally, long armchair nanotubes could be cut, re-seeded with catalyst and re-grown indefinitely.

The paper was written by graduate student and first author Alvin Orbaek, undergraduate student Andrew Owens and Barron, the Charles W. Duncan Jr.-Welch Professor of Chemistry and a professor of materials science.

Amplification of nanotubes was seen as a key step toward the practical manufacture of AQW by the late Rice professor, nanotechnology pioneer and Nobel laureate Richard Smalley, who worked closely with Barron and Rice chemist James Tour, the T.T. and W.F. Chao Chair in Chemistry as well as a professor of mechanical engineering and materials science and of computer science, to lay out a path for its development.

Barron charged Orbaek with the task of following through when he joined the lab five years ago. "When I first heard about Rice University, it was because of Rick Smalley and carbon nanotubes," said Orbaek, a native of Ireland. "He had a large global presence with regard to nanotechnology, and that reached me.

"So I was delighted to come here and find I'd be working on nanotube growth that was related to Smalley's work."

Orbaek said he hasn't strayed far from Barron's original direction, which involved chemically attaching an iron/cobalt catalyst to the ends of nanotubes and then fine-tuning the temperature and environment in which amplification could occur.

"My group, with Smalley and Tour's group, demonstrated you could do this -- but in the first demonstration, we got only one tube to grow out of hundreds or thousands," Barron said. Subsequent experiments raised the yield, but tube growth was minimal. In other attempts, the catalyst would literally eat -- or "etch" -- the nanotubes, he said.

Refining the process has taken years, but the payoff is clear because up to 90 percent of the nanotubes in a batch can now be amplified to significant lengths, Barron said. The latest experiments focused on single-walled carbon nanotubes of various chiralities, but the researchers feel the results would be as great, and probably even better, with a batch of pristine armchairs.

The key was finding the right balance of temperatures, pressures, reaction times and catalyst ratios to promote growth and retard etching, Barron said. While initial growth took place at 1,000 degrees Celsius, the researchers found the amplification step required lowering the temperature by 200 degrees, in addition to adjusting the chemistry to maximize the yield.

"What we're getting to is that sweet spot where most of the nanotubes grow and none of them etch," Barron said.

Wade Adams, director of Rice's Richard E. Smalley Institute for Nanoscale Science and Technology and principal investigator on the AQW project, compared the technique to making sourdough bread. "You make a little batch of pure metallics and then amplify that tremendously to make a large amount. This is an important increment in developing the science to make AQW.

Adams noted eight Rice professors and dozens of their students are working on aspects of AQW. "We know how to spin nanotubes into fibers, and their properties are improving rapidly too," he said. "All this now has to come together in a grand program to turn quantum wires into a product that will carry vast amounts of electricity around the world."

Barron and his team are continuing to fine-tune their process and hope that by summer's end they can begin amplifying armchair nanotubes with the goal of making large quantities of pure metallics. "We're always learning more about the mechanisms by which nanotubes grow," said Orbaek, who sees the end game as the development of a single furnace to grow nanotubes from scratch, cap them with new catalyst, amplify them and put out a steady stream of fiber for cables.

"What we've done is a baby step," he said. "But it verifies that, in the big picture, armchair quantum wire is technically feasible."

Orbaek said he is thrilled to play a role in achieving amplification, which Smalley recognized as necessary to his dream of an efficient energy grid that would catalyze solutions to many of the world's problems.

"I'd love to meet him now to say, 'Hey, man, you were right,'" he said.

The Robert A. Welch Foundation and the Air Force Office of Scientific Research funded the research.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by Rice University.

Journal Reference:

Alvin W. Orbaek, Andrew C. Owens, Andrew R. Barron. Increasing the Efficiency of Single Walled Carbon Nanotube Amplification by Fe–Co Catalysts Through the Optimization of CH4/H2Partial Pressures. Nano Letters, 2011; 11 (7): 2871 DOI: 10.1021/nl201315j

Narrowest bridges of gold are also the strongest, study finds

At an atomic scale, the tiniest bridge of gold -- that made of a single atom -- is actually the strongest, according to new research by engineers at the University at Buffalo's Laboratory for Quantum Devices.

The counterintuitive finding is the result of experiments probing the characteristics of atomic-scale necks of gold that formed when the pointed, gold tip of a cantilever was pushed into a flat, gold surface. An examination of these tiny, gold bridges revealed that they were stiffest when they comprised just a single atom.

The study was published in June in Physical Review B by a trio of UB researchers: postdoctoral fellow Jason Armstrong and professors Susan Hua and Harsh Deep Chopra, all in UB's Department of Mechanical and Aerospace Engineering.

As engineers look to build devices such as computer circuits with ever-smaller parts, it is critical to learn more about how tiny components comprising a single atom or a few atoms might behave. The physical properties of atomic-scale gadgets differ from those of larger, "bulk" counterparts.

"Everyday intuition would suggest that devices made of just a few atoms would be highly susceptible to mechanical forces," the team said. "This study finds, however, that the ability of the material to resist elastic deformation actually increases with decreasing size."

Another observation the team made while studying the tiny gold necks: abrupt atomic displacements that occur as the gold tip and surface are drawn apart are not arbitrary, but follow well-defined rules of crystallography.

UB's Laboratory for Quantum Devices, led by Chopra and Hua, works on mapping the evolution of various physical properties of materials -- including mechanical, magnetic and magneto-transport behavior -- as sample sizes grow from a single atom to bulk.

This complicated task requires technology capable of capturing a single or few atoms between probes, and further pushing and pulling on the atoms to study their response.

The sophisticated technology that Armstrong, Hua and Chopra invented and built to accomplish the research was recently licensed to Precision Scientific Instruments Inc., a Western New York start-up company founded by the leaders of Murak & Associates LLC, a management consulting practice; SoPark Corporation, an electronics service manufacturer (ESM); and The PCA Group, Inc., a consulting firm that offers total technology solutions.

"The instruments and methods are incredibly precise and capable of deforming the sample at the picometer scale (about 100 times smaller than an atom), which means literally stretching the bond lengths, and simultaneously measuring the forces at the piconewton level, as well as various other properties. As a very broad perspective, by enabling researchers to probe the very small, the technology could speed advances in fields ranging from satellite communications to health care," said Gerry Murak, president and cofounder of Precision Scientific Instruments, Inc.

"Small is exciting, and atomic scale devices are the new frontier of technology. Metrology systems capable of probing the behavior of atomic-scale devices are sorely needed, and this technology gives us a unique platform," Murak said.

Support for the work came from National Science Foundation grants No. DMR-0706074 and No. DMR-0964830.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by University at Buffalo.

Journal Reference:

G. Rubio, N. Agrait, S. Vieira. Atomic-Sized Metallic Contacts: Mechanical Properties and Electronic Transport. Physical Review Letters, 1996; 76 (13): 2302 DOI: 10.1103/PhysRevLett.76.2302

New way to store sun's heat: Modified carbon nanotubes can store solar energy indefinitely, then be recharged by exposure to the sun

 A novel application of carbon nanotubes, developed by MIT researchers, shows promise as an innovative approach to storing solar energy for use whenever it's needed.

Storing the sun's heat in chemical form -- rather than converting it to electricity or storing the heat itself in a heavily insulated container -- has significant advantages, since in principle the chemical material can be stored for long periods of time without losing any of its stored energy. The problem with that approach has been that until now the chemicals needed to perform this conversion and storage either degraded within a few cycles, or included the element ruthenium, which is rare and expensive.

Last year, MIT associate professor Jeffrey Grossman and four co-authors figured out exactly how fulvalene diruthenium -- known to scientists as the best chemical for reversibly storing solar energy, since it did not degrade -- was able to accomplish this feat. Grossman said at the time that better understanding this process could make it easier to search for other compounds, made of abundant and inexpensive materials, which could be used in the same way.

Now, he and postdoc Alexie Kolpak have succeeded n doing just that. A paper describing their new findings has just been published online in the journal Nano Letters, and will appear in print in a forthcoming issue.

The new material found by Grossman and Kolpak is made using carbon nanotubes, tiny tubular structures of pure carbon, in combination with a compound called azobenzene. The resulting molecules, produced using nanoscale templates to shape and constrain their physical structure, gain "new properties that aren't available" in the separate materials, says Grossman, the Carl Richard Soderberg Associate Professor of Power Engineering.

Not only is this new chemical system less expensive than the earlier ruthenium-containing compound, but it also is vastly more efficient at storing energy in a given amount of space -- about 10,000 times higher in volumetric energy density, Kolpak says -- making its energy density comparable to lithium-ion batteries. By using nanofabrication methods, "you can control [the molecules'] interactions, increasing the amount of energy they can store and the length of time for which they can store it -- and most importantly, you can control both independently," she says.

Thermo-chemical storage of solar energy uses a molecule whose structure changes when exposed to sunlight, and can remain stable in that form indefinitely. Then, when nudged by a stimulus -- a catalyst, a small temperature change, a flash of light -- it can quickly snap back to its other form, releasing its stored energy in a burst of heat. Grossman describes it as creating a rechargeable heat battery with a long shelf life, like a conventional battery.

One of the great advantages of the new approach to harnessing solar energy, Grossman says, is that it simplifies the process by combining energy harvesting and storage into a single step. "You've got a material that both converts and stores energy," he says. "It's robust, it doesn't degrade, and it's cheap." One limitation, however, is that while this process is useful for heating applications, to produce electricity would require another conversion step, using thermoelectric devices or producing steam to run a generator.

While the new work shows the energy-storage capability of a specific type of molecule -- azobenzene-functionalized carbon nanotubes -- Grossman says the way the material was designed involves "a general concept that can be applied to many new materials." Many of these have already been synthesized by other researchers for different applications, and would simply need to have their properties fine-tuned for solar thermal storage.

The key to controlling solar thermal storage is an energy barrier separating the two stable states the molecule can adopt; the detailed understanding of that barrier was central to Grossman's earlier research on fulvalene dirunthenium, accounting for its long-term stability. Too low a barrier, and the molecule would return too easily to its "uncharged" state, failing to store energy for long periods; if the barrier were too high, it would not be able to easily release its energy when needed. "The barrier has to be optimized," Grossman says.

Already, the team is "very actively looking at a range of new materials," he says. While they have already identified the one very promising material described in this paper, he says, "I see this as the tip of the iceberg. We're pretty jazzed up about it."

Yosuke Kanai, assistant professor of chemistry at the University of North Carolina at Chapel Hill, says "the idea of reversibly storing solar energy in chemical bonds is gaining a lot of attention these days. The novelty of this work is how these authors have shown that the energy density can be significantly increased by using carbon nanotubes as nanoscale templates. This innovative idea also opens up an interesting avenue for tailoring already-known photoactive molecules for solar thermal fuels and storage in general."

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Massachusetts Institute of Technology. The original article was written by David L. Chandler, MIT News Office.

Journal Reference:

Alexie M. Kolpak, Jeffrey C. Grossman. Azobenzene-Functionalized Carbon Nanotubes As High-Energy Density Solar Thermal Fuels. Nano Letters, 2011; : 110705085331088 DOI: 10.1021/nl201357n

BASF intensifies research and development of innovative products for sustainable electromobility

 Batteries are the key technology for the electromobility of the future. Over the next five years, BASF will be investing a three-digit million euro sum in researching, developing and the production of battery materials. Part of the investment is being channeled into the construction of a production plant for advanced cathode materials in Elyria, Ohio. This new facility with an investment volume of more than $50 million is scheduled to supply the market with cathode materials for the production of high-performance lithium-ion batteries from mid-2012.

In addition to innovative materials for cathodes, BASF has recently also entered the field of electrolyte development. High-quality tailored electrolytes are essential for battery performance. “By entering into the electrolyte business we are expanding our portfolio of innovative solutions for high-performance lithium-ion batteries and as a future system supplier, we will be able to support our customers' competitiveness in the electromobility field,” said Dr. Andreas Kreimeyer, member of the Board of Executive Directors and Research Executive Director of BASF SE. As well as developing materials for lithium-ion batteries, which include solutions for anodes and separators, BASF is also researching future battery concepts such as lithium-sulfur or lithium-air.

“With our research activities we are substantially contributing to making electric cars affordable, environment friendly and sustainable. For this we need batteries and further innovative components that provide a greater driving range with less weight and lower costs,” explained Kreimeyer.

To compensate for the additional battery weight of about 200 kg and allow for an acceptable driving range, the weight of electric vehicles must be reduced through lightweight construction components. This naturally places new demands on the materials, including completely new properties in terms of temperature stability, electromagnetic screening and fire resistance. Although plastics already contribute greatly to vehicular weight savings when incorporated in the chassis, interior and engine compartment, further multifunctional lightweight construction concepts are needed. For example, BASF is working on fast-curing epoxy, polyurethane and polyamide resins for fiber reinforced composites to be used in the manufacture of lightweight vehicle bodies. These materials can provide further weight savings of up to 100 to 150 kilograms in structural components and chassis.

BASF also offers solutions for improving heat management in electric cars. “When the temperatures rise in summer, the car's air conditioner consumes additional energy reducing the vehicles driving range,” explained Kreimeyer. When incorporated in interiors and automotive coatings, pigments that reflect the heat-generating infrared rays of sunlight prevent the temperature from getting too high inside the car. And while the combustion engine provides exhaust heat in winter, an electric vehicle consumes electricity to heat the interior. To keep energy consumption low under these conditions, it is necessary to insulate electrical vehicles against the cold with high performance foams. This also increases the car’s driving range.

Innovations from chemical research and the right energy mix will be key factors in helping electromobility to make its breakthrough – while remaining sustainable. “We take a holistic view of this topic. Electromobility will only significantly contribute to environmental and climate protection when the electricity from the batteries has been generated high efficiently and with less CO2. Therefore we are investing in research to find ways of generating electricity from renewable energy technologies such as wind and solar energy. We are also developing innovative storage technologies because in our latitudes these forms of energy are not available 24/7,” said Kreimeyer.

Policy makers are also called upon to create the appropriate general conditions to ensure that electromobility remains competitive in the global market. This will include government sponsored research and development programs to ensure that Germany retains its technological lead and can take further strides forward in electromobility. The creation of added value and jobs in Germany is another positive outcome.

“If industry together with politics, science and society as a whole all pull in the same direction, electromobility will be successful and become an affordable and sustainable alternative to the classical internal combustion technology,” added Kreimeyer.