Embedded rods: Chitin-silicon dioxide nanocomposite made by self-organization and sol-gel chemistry

October 11, 2010

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(PhysOrg.com) -- Self-organization processes involving chemical building blocks are the basis for many biological processes and are increasingly of interest in the field of materials synthesis, for example in the production of highly ordered nanocomposites or high-porosity materials with special properties.

In the journal Angewandte Chemie, Bruno Alonso and Emmanuel Belamie from the Charles Gerhardt Institute in Montpellier (France) have introduced a novel, highly versatile approach to the large-scale synthesis of a new family of bioorganic–inorganic nanocomposites -- with a previously unattainable degree of control over the composition and structure of the materials produced.

Nanocomposites are solid materials made of different substances, one of which is in the form of nanoparticles. The properties of the composites differ significantly from those of the pure individual components. Nanocomposites can also serve as “molds” for the production of porous substances. These have potential application in the areas of gas storage, catalysis, or materials separation.

For their synthesis, the researchers chose to use a sol–gel process, a popular technique for the production of inorganic network structures. In the first step they needed to generate a sol: a suspension of finely divided nanoscopic particles in a solvent. Their challenge was to obtain co-suspension of the two different components, silicon dioxide precursors (siloxane oligomers) and chitin nanorods from shrimp shells (a renewable resource). However, these two components require different conditions to remain in stable suspensions without uncontrolled precipitation. The researchers produced an alcohol suspension by slowly replacing water with ethanol. Through slow removal of the solvent, a gel formed. Gels are gelatinous substances; they contain solid but loose, cross-linked, three-dimensional polymer structures.

The sol can be “poured” into a desired mold and dried or it can be spray-dried into spherical particles. This process results in a nanocomposite made of chitin rods that are fully embedded in a silicon dioxide matrix. The mechanism by which this occurs is based on a self-organized aggregation of the chitin molecules and weak attractive forces between chitin and siloxane oligomers.

The stability of the alcohol suspensions opens up a wide range of possibilities for the production of materials with controllable volume ratios, spatial arrangements, and morphologies. If a magnetic field is applied during preparation of the material, the chitin rods line up in parallel. If the nanocomposite is heated, the chitin rods can be burned off to leave behind cavities. This forms a highly porous material with interesting properties.

More information: Emmanuel Belamie, Chitin–Silica Nanocomposites by Self-Assembly, Angewandte Chemie International Edition, http://dx.doi.org/ … ie.201002104

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Extreme darkness: Carbon nanotube forest covers NIST's ultra-dark detector

August 18, 2010

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This is a colorized micrograph of the world's darkest material -- a sparse "forest" of fine carbon nanotubes -- coating a NIST laser power detector. Image shows a region approximately 25 micrometers across. Credit: Aric Sanders, NIST

Harnessing darkness for practical use, researchers at the National Institute of Standards and Technology have developed a laser power detector coated with the world's darkest material -- a forest of carbon nanotubes that reflects almost no light across the visible and part of the infrared spectrum.

NIST will use the new ultra-dark detector, described in a new paper in Nano Letters,* to make precision laser power measurements for advanced technologies such as optical communications, laser-based manufacturing, solar energy conversion, and industrial and satellite-borne sensors.

Inspired by a 2008 paper by Rensselaer Polytechnic Institute (RPI) on "the darkest man-made material ever,"** the NIST team used a sparse array of fine nanotubes as a coating for a thermal detector, a device used to measure laser power. A co-author at Stony Brook University in New York grew the nanotube coating. The coating absorbs laser light and converts it to heat, which is registered in pyroelectric material (lithium tantalate in this case). The rise in temperature generates a current, which is measured to determine the power of the laser. The blacker the coating, the more efficiently it absorbs light instead of reflecting it, and the more accurate the measurements.

The new NIST detector uniformly reflects less than 0.1 percent of light at wavelengths from deep violet at 400 nanometers (nm) to near infrared at 4 micrometers (µm) and less than 1 percent of light in the infrared spectrum from 4 to 14 µm. The results are similar to those reported for the RPI material and in a 2009 paper by a Japanese group. The NIST work is unique in that the nanotubes were grown on pyroelectric material, whereas the other groups grew them on silicon. NIST researchers plan to extend the calibrated operating range of their device to 50 or even 100 micrometer wavelengths, to perhaps provide a standard for terahertz radiation power.

NIST previously made detector coatings from a variety of materials, including flat nanotube mats. The new coating is a vertical forest of multiwalled nanotubes, each less than 10 nanometers in diameter and about 160 micrometers long. The deep hollows may help trap light, and the random pattern diffuses any reflected light in various directions. Measuring how much light was reflected across a broad spectrum was technically demanding; the NIST team spent hundreds of hours using five different methods to measure the vanishingly low reflectance with adequate precision. Three of the five methods involved comparisons of the nanotube-coated detector to a calibrated standard.

Carbon nanotubes offer ideal properties for thermal detector coatings, in part because they are efficient heat conductors. Nickel phosphorous, for example, reflects less light at some wavelengths, but does not conduct heat as well. The new carbon nanotube materials also are darker than NIST's various Standard Reference Materials for black color developed years ago to calibrate instruments.

New super strong alloy discovered

September 8, 2010

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Dr Peter Liddicoat next to the atom probe.

(PhysOrg.com) -- International team of researchers has discovered a new super-strength light alloy and had their key findings published in Nature Communications.

A North Carolina State University researcher and colleagues have figured out a way to make an aluminum alloy, or a mixture of aluminum and other elements, just as strong as steel.

That's important, says Dr. Yuntian Zhu, professor of materials science and the NC State researcher involved in the project, because the search for ever lighter - yet stronger - materials is crucial to devising everything from more fuel-efficient cars to safer airplanes.

In a paper published in the journal Nature Communications, Zhu and his colleagues describe the new nanoscale architecture within aluminum alloys that have unprecedented strength but also reasonable plasticity to stretch and not break under stress. Perhaps even more importantly, the technique of creating these nanostructures can be used on many different types of metals.

Zhu says the aluminum alloys have unique structural elements that, when combined to form a hierarchical structure at several nanoscale levels, make them super-strong and ductile.

The aluminum alloys have small building blocks, called "grains," that are thousands of times smaller than the width of a human hair. Each grain is a tiny crystal less than 100 nanometers in size. Bigger is not better in materials, Zhu says, as smaller grains result in stronger materials.

Zhu also says the aluminum alloys have a number of different types of crystal "defects." Nanocrystals with defects are stronger than perfect crystals.

The unexpectedly high level of strengthening appears to be due to two factors. Firstly, the way that the alloying elements are arranged within the grains is thought to increase the dislocation-storage capacity of the alloy. Secondly, the clustering of elements between the grains could limit nanocrystal growth, increase the cohesion of the grains, and resist embrittlement and defect generation.

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Imaging of nano-sized grains inside an aluminum alloy. Colored blobs show the grains: colored dots show clusters of zinc and magnesium atoms that strengthen the material. (Yonghao Zhao/UC Davis graphic)

Now, Zhu plans on working on strengthening magnesium, a metal that is even lighter than aluminum. He's collaborating with the Department of Defense on a project to make magnesium alloys strong enough to be used as body armor for soldiers.

Zhu's colleagues on the Nature Communications paper are affiliated with the University of Sydney in Australia; the University of California, Davis; and Ufa State Aviation Technical University in Russia.

Radically simple technique developed to grow conducting polymer thin films

November 1, 2010 By Mike Rodewald

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Sequence of images illustrating growth of polymer film in tubes over 35 seconds

(PhysOrg.com) -- Oil and water don't mix, but add in some nanofibers and all bets are off.

A team of UCLA chemists and engineers has developed a new method for coating large surfaces with nanofiber thin films that are both transparent and electrically conductive. Their method involves the vigorous agitation of water, dense oil and polymer nanofibers. After this solution is sufficiently agitated it spreads over virtually any surface, creating a film.

"The beauty of this method lies in its simplicity and versatility," said California NanoSystems Institute (CNSI) researcher Richard B. Kaner, a professor of chemistry and biochemistry and a professor of materials science and engineering at the UCLA Henry Samueli School of Engineering and Applied Science. "The materials used are inexpensive and recyclable, the process works on virtually any substrate, it produces a uniform thin film which grows in seconds and the entire thing can be done at room temperature."

Conducting polymers combine the flexibility and toughness of plastics with electrical properties. They have been proposed for applications ranging from printed electronic circuits to supercapacitors but have failed to gain widespread use because of difficulties processing them into films.

"Conducting polymers have enormous potential in electronics, and because this technique works with so many substrates, it can be used in a broad spectrum of applications, including organic solar cells, light-emitting diodes, smart glass and sensors," said Yang Yang, a professor of materials science and engineering at the Samueli School of Engineering and Applied Science and faculty director of the Nano Renewable Energy Center at the CNSI.

One of the potential applications is smart, or switchable, glass that can change between states when an electric current is applied — for example, switching between see-through and opaque states to let light in or block it. The UCLA research group is applying the technique to other nanomaterials in addition to polymer nanofibers in the hopes of expanding the number of available applications.

The team's solution-based technique, published in the peer-reviewed journal Proceedings of National Academy of Sciences, was discovered serendipitously when a transparent film of polymer spread up the walls of a container while nanofibers in water were being purified with chloroform.

"What drew me in immediately was the eerie phenomenon of what appeared to be self-propelled fluid flow," said Julio M. D'Arcy, lead author on the PNAS paper and a senior graduate student in the Kaner's UCLA lab.

"Now I can tell people that I make films in L.A.," he joked.

When water and oil are mixed, a blend of droplets is formed, creating a water–oil interface that serves as an entry point for trapping polymer nanofibers at liquid–liquid interfaces. As droplets unite, a change in the concentration of blended solids at the water–oil interface leads to a difference in surface tension. Spreading up a glass wall occurs as result of an attempt to reduce the surface-tension difference. Directional fluid flow leads to a continuously conductive thin film comprised of a single monolayer of polymer nanofibers. The uniformity of the film surface is due to the particles being drawn out of the water–oil interface, sandwiched between two fluids of opposing surface tensions.

Development of the technology is occurring in collaboration with Fibron Technologies Inc., with support from the National Science Foundation through a Small Business Technology Transfer grant. Fibron is a small company that has licensed the technology from UCLA. It was founded by Kaner, who serves as chief scientific adviser, and two of his former Ph.D. students — Christina Baker and Henry Tran, who have gone on to take leadership roles in the company.

Fibron's CEO, Christian Behrenbruch, said "working with UCLA to develop this technology has been a win-win. It enables us to access incredibly innovative people, but also, the NSF has helped enable the establishment of a formal and transparent IP releationship with the university. The good news is that this technology is moving rapidly into commercial development."

Other techniques exist for creating thin films of conducting polymers, but each technique tends to work only a limited number of applications, or they are not feasible for scaling up. A method has long been sought which would overcome the limitations of each of the previous methods. The water and oil technique, with a bit of nanotechnology thrown in, might provide just that — a scalable universal method for creating large thin films of conducting polymers.

Organic solvent system may improve catalyst recycling and create new nanomedicine uses

October 28, 2010

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Wei Lin holds a gold/organic aqua regia solution while Rongwei Zhang holds a silicon substrate coated with 200-nanometer gold. The image on the monitor shows gold recovered from the solution using calcinations. Credit: Credit: Gary Meek

Noble metals such as platinum and palladium are becoming increasingly important because of growth in environmentally friendly applications such as fuel cells and pollution control catalysts. But the world has limited quantities of these materials, meaning manufacturers will have to rely on efficient recycling processes to help meet the demand.

Existing recycling processes use a combination of two inorganic acids known as "aqua regia" to dissolve noble metals, a class of materials that includes platinum, palladium, gold and silver. But because the metals are often dissolved together, impurities introduced in the recycling process may harm the efficiency of catalysts produced from the recycled materials. Now, researchers at the Georgia Institute of Technology have developed a new organic solvent process that may help address the problem – and open up new possibilities for using these metals in cancer therapeutics, microelectronics and other applications.

The new Georgia Tech solvent system uses a combination of two chemicals – thionyl chloride and a variety of organic reagents such as pyridine, N,N-dimethylformamide (DMF), pyrimidine or imidazole. The concentrations can be adjusted to preferentially dissolve gold or palladium, and more importantly, no combination of the organic chemicals dissolves platinum. This ability to preferentially dissolve noble metals creates a customized system that provides a high level of control over the process.

"We need to be able to selectively dissolve these noble metals to ensure their purity in a variety of important applications," said C.P. Wong, a Regents professor in the Georgia Tech School of Materials Science and Engineering. "Though we don't fully understand how it works yet, we believe this system opens a lot of new possibilities for using these metals."

A paper describing the research was published recently in the journal Angewandte Chemie.

Catalyst systems that make use of more than one metal, such as palladium with a gold core, are becoming more widely used in industrial processes. To recycle those, the new solvent system – dubbed "organic aqua regia" – could first use a combination of thionyl chloride and DMF to dissolve out the gold, leaving hollow palladium spheres. Then the palladium spheres could be dissolved using a different combination.

So far, the researchers have demonstrated that the solvent system can selectively dissolve gold and palladium from a mixture of gold, palladium and platinum. They have also used it to remove gold from a mixture of gold and palladium.

Beyond recycling, the new solvent system could also provide new ways of producing nanometer-scale cancer chemotherapy agents that involve these metals. And the new solvent approach could have important implications for the electronics industry, which uses noble metals that must often be removed after specific processing steps. Beyond selectivity, the new approach also offers other advantages for electronics manufacturing – no potentially harmful contamination is left behind and processing is done under mild conditions.

"In semiconductor production, people want to avoid having a metal catalyst remaining in devices, but in many cases, they cannot use existing water-based processes because these can damage the semiconductor oxides and introduce contamination with free ions in the aqueous solution," explained Wei Lin, a graduate research assistant in Wong's laboratory. "Use of this organic system avoids the problem of moisture."

Use of the selective process could also facilitate recycling of noble metals used in electronics manufacturing. Wire-bonding, metallization and interconnect processes currently use noble metals.

Noble metals are also the foundation for widely-used chemotherapy agents, but the chemistry of synthesizing them involves a complex process of surfactants and precursors. Wong believes the new Georgia Tech solvent process may allow creation of novel compounds that could offer improved therapeutic effects.

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Wei Lin holds a gold/organic aqua regia solution. The image on the monitor shows gold recovered from the solution using calcinations. Credit: Credit: Gary Meek

"We hope this will open up some new ways of making these important pharmaceutical compounds as well as novel gold and palladium catalytic systems," he said.

Lin discovered the new solvent system by accident in 2007 while using thionyl chloride in an unrelated project that involved bonding carbon nanotubes to a gold substrate. "I left my sample in the solution and went to lunch," he recalled. "Then I received a couple of phone calls and the sample stayed in the solution for too long. When I got it out, the gold was gone."

The researchers were intrigued by the discovery and pursued an explanation as they had time over the past three years. They tested other reagents mixed with the thionyl chloride, and learned the proportions necessary for selective dissolution of palladium and gold. They worked with other researchers at Georgia Tech, including nanotechnology pioneer Zhong Lin Wang, to develop a fundamental understanding of the process – research that is continuing.

The chemicals used by the Georgia Tech research team are well known in organic chemistry, and are used today in polymer synthesis. Beyond their selectivity, the new solvent system is more environmentally friendly than traditional aqua regia – which is a combination of concentrated nitric and hydrochloric acids – and can operate at mild conditions. Potential disadvantages compared to traditional aqua regia include higher costs and slower dissolution rates.

"We have opened up a new approach to noble metals using organic chemistry," Wong added. "We don't yet thoroughly understand the mechanism by which this works, but we hope to develop a more complete understanding that may lead to additional applications."

Pivoting hooks of graphene's chemical cousin could revolutionize work of electron microscopes

November 1, 2010

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This is a sample seen attached to the graphene oxide. Credit: University of Warwick/ Nano Letters

The single layer material Graphene was the subject of a Nobel prize this year but research led by a team of researchers at the University of Warwick has found molecular hooks on the surface of its close chemical cousin, Graphene Oxide, that will potentially provide massive benefits to researchers using transmission electron microscopes. They could even be used in building molecular scale mechanisms.

The research team, which includes Drs. Jeremy Sloan, Neil Wilson and PhD student Priyanka Pandey from the Department of Physics and Dr. Jon Rourke from the Department of Chemistry together with the groups of Drs. Kazu Suenaga and Zheng Liu from AIST in Japan and Drs. Ian Shannon and Laura Perkins in Birmingham were looking at the possibility of using Graphene as a base to mount single molecules for imaging by transmission electron microscopy. As Graphene forms an electron transparent sheet just one atom thick it would enable high precision, high contrast imaging of the molecules being studied as well as the study of any interactions they have with the supporting graphene.

While this idea is great in theory, Graphene is actually very difficult to create and manipulate in practice. The researchers therefore turned to Graphene's easier to handle cousin, Graphene Oxide. This choice turned out to be a spectacularly better material as they found extremely useful properties, in the form of ready-made molecular hooks that could make Graphene Oxide the support material of choice for future transmission electron microscopy of any molecule with oxygen on its surface.

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This is a graphic of sample binding to a graphene oxide "hook". Credit: University of Warwick / Nano Letters

Graphene Oxide's name obscures the fact that it is actually a combination of carbon, oxygen and hydrogen. For the most part it still resembles the one atom thin sheet of pure Graphene, but it also has "functional groups" consisting of hydrogen paired with oxygen. These functional groups can bind strongly to molecules with external oxygens making them ideal tethers for researchers wishing to study them by transmission electron microscoscopy.

This feature alone will probably be enough to persuade many researchers to turn to Graphene Oxide as a support for the analysis of a range of molecules by transmission electron microscopy, but the researchers found yet another intriguing property of these handy hooks – the molecules attached to them move and pivot around them.

Dr Jeremy Sloan said: "Under the right conditions the functional groups not only provide molecular tethers that hold molecules in an exact spot they also allow the molecule to be spun in that position. This opens up a range of new opportunities for the analysis of such molecules but could also be a useful mechanism for anyone seeking to create molecular sized "machinery"."

Intricate, curving 3-D nanostructures created using capillary action forces

October 19, 2010

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Twisting spires are one of the 3-D shapes researchers at the University of Michigan were able to develop using a new manufacturing process. Credit: A. John Hart

ANN ARBOR, Mich.---Twisting spires, concentric rings, and gracefully bending petals are a few of the new three-dimensional shapes that University of Michigan engineers can make from carbon nanotubes using a new manufacturing process.

The process is called "capillary forming," and it takes advantage of capillary action, the phenomenon at work when liquids seem to defy gravity and travel up a drinking straw of their own accord.

The new miniature shapes, which are difficult if not impossible to build using any material, have the potential to harness the exceptional mechanical, thermal, electrical, and chemical properties of carbon nanotubes in a scalable fashion, said A. John Hart, an assistant professor in the Department of Mechanical Engineering and in the School of Art & Design.

They could lead to probes that can interface with individual cells and tissues, novel microfluidic devices, and new materials with a custom patchwork of surface textures and properties.

A paper on the research is published in the October edition of Advanced Materials, and is featured on the cover.

"It's easy to make carbon nanotubes straight and vertical like buildings," Hart said. "It hasn't been possible to make them into more complex shapes. Assembling nanostructures into three-dimensional shapes is one of the major goals of nanotechnology. The method of capillary forming could be applied to many types of nanotubes and nanowires, and its scalability is very attractive for manufacturing."

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By using unique two-dimensional templates, researchers at the University of Michigan could coax carbon nanotubes to grow in intricate, curving three-dimensional structures. Credit: A. John Hart

Hart's method starts by stamping patterns on a silicon wafer. His ink in this case is the iron catalyst that facilitates the vertical growth of the carbon nanotubes in the patterned shapes. Rather than stamp a traditional, uniform grid of circles, Hart stamp hollow circles, half circles and circles with smaller ones cut from their centers. The shapes are arranged in different orientations and groupings. One such grouping is a pentagon of half circles with their flat sides facing outward.

He uses the traditional "chemical vapor deposition" process to grow the nanotubes in the prescribed patterns. Then he suspends the silicon wafer with its nanotube forest over a beaker of a boiling solvent, such as acetone. He lets the acetone condense on the nanotubes, and then lets the acetone evaporate.

As the liquid condenses, capillary action forces kick in and transform the vertical nanotubes into the intricate three-dimensional structures. For example, tall half-cylinders of nanotubes bend backwards to form a shape resembling a three-dimensional flower.

"We program the formation of 3D shapes with these 2D patterns," Hart said. "We've discovered that the starting shape influences how the capillary forces change the structures' geometry. Some bend, others twist, and we can combine them any way we want."

The capillary forming process allows the researchers to create large batches of 3D microstructures---all much smaller than a cubic millimeter---over essentially limitless areas, Hart said. In addition, the researchers show that their 3D structures are up to 10 times stiffer than typical polymers used in microfabrication. Thus, they can be used as molds for manufacturing of the same 3D shapes in other materials.

"We'd like to think this opens up the idea of creating custom nanostructured surfaces and materials with locally varying geometries and properties, " Hart said. "Now, we think of materials as having the same properties everywhere, but with this new technique we can dream of designing the structure and properties of a material together."