Archive for September 2010
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Serendipity: Nano LEDs accidentally produced during NIST nanowire research
Graphic illustrates a single row of nanowires (cylinders with red tops) with fin-shaped
nanowalls extending outward. Credit: NIST.
Speaking of ZnO nanowires, chemists at NIST who were perfecting new methods of creating these nanowire report that they have stumbled upon a way to create light-emitting nanowires that operate like very tiny LEDs.
TEM of four rows of the nanowires and nanowalls. Credit: NIST.
The chemists, Babak Nikoobakht and Andrew Herzing, had already found a way to grow ZnO nanowires horizontally across a substrate that was seeded with gold nanoparticles. These nanoparticles serve as growth sites and medium for the crystallization of zinc oxide molecules: As the zinc oxide nanocrystal grows, it pushes the gold nanoparticle along the surface of the substrate (“surface-directed” fabrication).
While working with a gallium nitride substrate, Nikoobakht and Herzing, unexpectedly found that when they increased the thickness of the gold nanoparticle from less than 8 nm to approximately 20 nm, the nanowires sprouted a secondary structure. Visually, the structure is said to look like a “dorsal fin.” Technically, it is a nanowall that forms where the ZnO portion is electron-rich and the gallium nitride portion is electron-poor.
At this location, the nanowall becomes a p-n heterojunction where electrons can flow across it when voltage is applied to the nanowire-nanowall combination. This flow of electrons produces light, which led the researchers to dub it a “nano LED.”
According to NIST
“Unlike previous techniques for producing heterojunctions, the NIST “surface-directed” fabrication method makes it easy to locate individual heterojunctions on the surface. This feature is especially useful when a large number of heterojunctions must be grouped in an array so that they can be electrically charged as a light-emitting unit.”
The duo says the simple design of the nanowires means that is scalable to literally any platform size. For example, the nano LEDs, with further improvements, could be used as light sources and detectors in photonic devices or lab-on-a-chip platforms.
TEM of nano LEDs emitting light. Credit: NIST.
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Inexpensive radial ZnO nanowire brushes ’scrub’ toxic compounds from water
Low-magnification SEM image of the hierarchical nanostructure consisting of radially aligned ZnO nanowires grown on electrospun poly-l-lactide nanofibers. Inset: schematic representation of the hierarchical nanostructure. Credit: JACerS.
In a story just published in one of ACerS’ technical journals, a group of researchers from Sweden’s Royal Institute of Technology (KTH) report they have discovered what appears to be a relatively easy and inexpensive way to make a mat of photocatalytic water purification fibers that are coated with radial arrays of ZnO nanowires. The KTH group says that a prototype of a continuous-flow water purification system teamed with a UV light source worked well at decomposing three organic pollutants: methylene blue, monocrotophos and diphenylamine. Their paper is available in the “Early View” (online only) section of the Journal of the American Ceramic Society.
There is much interest in any purification system that combines UV and ZnO. UV irradiation is a great sterilization technique. Moreover, ZnO in the presence of UV light provides an enhanced antibacterial effect.
The ability of metal oxides, such as TiO2 and ZnO, to act as a photocatalytic agent isn’t new, and several other groups have previously demonstrated this also using nanoparticles of the oxides. These efforts have mainly been fairly simple demonstrations where a colloidal suspension of nanoparticles are mixed with water and an organic contaminant. Nanoparticles are an obvious choice because they offer the potential of a high collective surface area. While proving that the contaminants are decomposed, these demonstrations, however, haven’t addressed a practical way of removing the nanoparticles from the water other than batch centrufugation.
In effect, the KTH group is trying to tackle this problem by coming up with a fast, cost-efficient way to achieve the same water purification effects through other methods. There solution is to grow metal oxide nanowire arrays (NWA) on an anchored substrate. ZnO NWAs can be grown on a substrate by various techniques like gas phase deposition or chemical bath deposition. The KTH group also knew that researchers already had shown that ZnO NWAs could be grown on nonwoven polyethylene nanofibers using the above-mentioned chemical bath deposition technique. The nanofibers, themselves, could be formed via standard electrospinning techniques.
Looking for durable fibers that could withstand substantial water flow and turbulence, the group opted for nanofibers of poly-l-lactide (PLLA), a highly flexible material.
The KTH group’s procedure is to electrospin and aggregate an initial mat of PLLA fibers. The PLLA fibers then go through a two-step process that, first, “seeds” the fibers with ZnO nanoparticles to provide the initial nucleation sites for the NWAs. In the second step, the NWAs are grown by putting the seeded fibers in a mixed aqueous solution of of Zn(NO3)2 and hexamine, and heated. After several hours in the solution, the fibers are removed and finally dried.
To create a functioning proof-of-concept water purification system, the researchers filled a glass column with the NWA-coated fibers. The column was connected to a pump-and-reservoir continuous-flow loop system (see column sample and schematic diagram). To provide a UV light source, the glass column was illuminated by a mercury vapor lamp. Part of the system passed through a UV-vis spectrophotometer to monitor the changes in absorption spectra in real time.
The first sign of success for the system came when the group tested its ability to handle a 5 ppm solution of methylene blue. After 80 minutes, about 90% of the methylene blue had decomposed. Repeated tests using the same PLLA–ZnO fibers gave identical results, indicating the structural and chemical stability of the nanostructure.
The researchers then tested the systems ability to handle two widely used and toxic organic compounds, monocrotophos (MCP), an organophosphate insecticide, and diphenylamine (DPA) an antioxidant for fruit storage. For both MCP and DPA, the purification system work, but not nearly as well as with methylene blue. After 125 minutes, the system reached what appeared to be its maximum decomposition ceiling of 50%. They weren’t surprised by the lower decomposition levels of MCP and DPA and noted that other researchers found similar results in the absence of additives to enhance the photocatalytic decomposition. They say they are currently investigating methods to improve the decomposition percentage by testing the effects of various additives.
They are also testing a system that uses only solar radiation instead of a dedicated UV source.
Regardless, it looks like their work could be an important step in developing an efficient, low-cost system to remove agricultural and industrial pollutants. Scaling up the production of the PLLA-ZnO NWAs is fairly straightforward, so the main hurdles appear to be finding the right additives to surpass the decomposition ceiling.
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ACerS new Career Center jobsite launched
The American Ceramic Society has launched its new Career Center. Best of all, it’s free.
Job seekers will find a broad range of opportunities spanning academia, government labs and private sector companies of all sizes. Job posters will find an audience of both established and emerging leaders in traditional and advanced ceramic segments that include areas such as energy, biomedical, environmental issues, national security, aerospace, transportation, nanotechnology and more.
Job seekers can create a profile, upload resumes and apply for posted jobs. Companies with open jobs in the materials industry can post online to their target audience.
As I mentioned, posting a position opening or posting a résumé is free. All you need to do it register. Check it out. www.ceramics.org/careers
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Intriguing scientists and engineers among 2010 MacArthur ‘genius’ Fellows
The John D. and Catherine T. MacArthur Foundation named 23 new MacArthur Fellows for 2010. Included in this list is John Dabiri whose work with fluid dynamics we’ve featured several times on this blog.
Dabiri is a biophysicist who studies animal locomotion, specifically the movement of jellyfish. Dabiri has shown that explaining the mechanisms of locomotion depends on detailed mathematical analysis of the fluid vortex rings that jellyfish form in the surrounding water by contracting their bell.
Dabiri’s research has applications in fluid dynamics, including the design of wind power generators, and bioinspired robotic locomotion.
While studying the vortices left behind by fish swimming in a school, Dabiri noticed that some rotated clockwise, while others rotated counter-clockwise. Dabiri, therefore, wants to examine whether alternating the rotation of vertical-axis turbines in close proximity will help improve efficiency.
The second observation he made was that the vortices formed a “staircase” pattern, which contrasts with current wind farms that place turbines neatly in rows. With optimal placement, Dabiri thinks ten times more energy could be harvested out of the same wind farm using vertical instead of horizontal turbines.
Dabiri is testing his ideas in partnership with Windspire Energy.
Some of our older posts featuring Dabiri and others’ work on jellyfish can be seen here:
John Blottman on bioinspiration and jellyfish
Video of the week: Smart materials and ‘bioinspiration’
School of fish offers new school of thought for turbine design
MacArthur Fellows each receive $500,000 in “no strings attached” support over the next five years. A full list of 2010 recipients can be seen here. While all of this year’s 20 MacArthur winners are worth checking out, materials science folks and technophiles might want to read about three other winners:
Nergis Mavalvala (MIT) research focuses on minimizing, if not circumventing, barriers imposed by quantum physics on the precision of standard optical interferometers, such as cooling the large components of the device into a coherent quantum state. She is also exploring squeezed coherent states and optical springs and other topics that are found “at the intersection of optics, condensed matter, and quantum mechanics.”
Michal Lipson (Cornell University) is a leader in silicon-based photonics. She is designing optical and hybrid opto-electronic devices, creating, for example, waveguides by etching silicon. Her approaches to fabrication engineering is opening new frontiers for using light for optical information-processing devices and computers.
Drew Barry (Walter and Eliza Hall Institute of Medical Research) is primarily a biomedical illustrator, but his visualization work illustrates what could and should be done in chemistry, physics and engineering. The results are animations that show, as he puts it, “What’s happening down at the microscopic level. It is an accurate representation of, ‘If you could see it, it would look something like this.’
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Dutch report advances with low-cost, low-temp Pd-film-on-ceramic hydrogen production membrane
The nonprofit Energy Research Center of the Netherlands reports that it is making significant advances with hydrogen separation membrane technology at an experimental plant in Italy. An ECN 0.4m2 Hysep 108 module composed of thin-film Pd layers on ceramic supports is being tested in partnership with an Italy-based Tecnimont KT, at the company’s Chieti test site. Tecnimont KT, a subsidiary of the larger Maire Tecnimont S.p.A., is a process engineering firm that specializes in applications for the chemical, petrochemical and refining industries.
ECN says the hydrogen plant employs a reformer-and-membrane-modules configuration that integrates membrane separation and reaction modules. ECN says that data so far confirms the feasibility of running these modules at a lower operating temperature. Instead of working in the costlier 850-900°C range, the RMM runs at less than 650°C, yielding 20 normal cubic meters per hour.
ECN says that two performance indicators are being closely monitored: the flux (or hydrogen yield per m2 of membrane) and the purity of the product. It says these indicators have remained through 500 hours of operation and more than 50 thermal cycles.
Besides operating at lower temperatures, an big advantage of ECN’s membranes is that they require low-cost starting materials and standard fabrication technologies. It says that it is pleased that, as the work shifted from the lab to field production, good results continues to be found. It says it will continue with the long-term stability testing under field conditions to “demonstrate the membrane technology as a market-ready technology.”
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