Tubes in space

Carbon nanotubes form in space but use a metal-free chemistry until now unavailable to chemists on Earth. The discovery is a surprising outcome of laboratory experiments designed by Joseph Nuth at NASA’s Goddard Space Flight Center, in Greenbelt, Maryland, and his colleagues. They were hoping to understand how carbon atoms are recycled in stellar nurseries, the regions of space where stars and planets are born, but the finding could have applications in nanotechnology, as well as help explain some characteristics of supernovae.

Writing in the journal Astrophys J Lett, Nuth and colleagues explain how astrochemistry makes carbon nanotubes without requiring a metal catalyst. Nanotubes are produced, they say, when graphite dust particles are exposed to a mixture of carbon monoxide and hydrogen gases, conditions that exist in interstellar space.

The finding corroborates the discovery of graphite whiskers, bigger than nano nanotubes, in three meteorites. The meteoric discovery hinted at why some supernovae appear dimmer and farther away than they ought to be based on calculations using current models. Nuth’s approach is a variation of a well-established way to produce gasoline or other liquid fuels from coal. It’s known as Fischer-Tropsch synthesis, and researchers suspect that it could have produced at least some of the simple carbon-based compounds in the early solar system. Nuth proposes that the nanotubes yielded by such reactions could be the key to the recycling of the carbon that gets released when carbon-rich grains are destroyed by supernova explosions.

Stellar Nursery
A stellar nursery could be home to carbon nanotube factories (Credit: NASA, http://apod.nasa.gov/apod/ap021102.html)

The structure of the carbon nanotubes produced by Nuth and colleagues was determined by materials scientist Yuki Kimura, of Tohoku University, Japan, using transmission electron microscopy. He observed particles on which the original smooth graphite gradually morphed into an unstructured region and finally to an area rich in tangled hair-like masses. A closer look with an even more powerful microscope showed that these tendrils were in fact cup-stacked carbon nanotubes, resembling a stack of disposable drinking cups with the bottoms removed. If further testing indicates that the new method is suitable for materials-science applications, it could supplement, or even replace, the familiar way of making nanotubes, explains Kimura.

Researchers might also now evaluate whether graphite whiskers absorb light. A positive result would lend credence to the proposition that the presence of these molecules in space affects the observations of some supernovae.

LINKS

Astrophys J Lett, 2010, 710, L98-L101

http://dx.doi.org/10.1088/2041-8205/710/1/L98

Catalytic troublemaker

Porous solid catalysts are a mainstay of the modern chemical industry, allowing reactions that would otherwise take an age to progress to be run much, much faster. One group of such catalysts are the zeolites and particularly important among them is one known as ZSM-5, an aluminosilicate material with an MFI structure. However, despite its attractions, ZSM-5 can behave badly because its chemical building blocks do not join together perfectly. This leads to chemical starting materials on which the catalyst is to act often becoming stuck before they can get into the reactive pores and be converted into product. Now, Dutch scientist Marianne Kox has discovered the nature of the miniscule deviations that can make ZSM-5 such a troublemaker.

Catalytic ZSM-5 isn't always on its best behaviour (Credit: Nature Materials/Weckhuysen et al)

Catalysts are essential to the production of a vast array of pharmaceutical drugs, agrochemicals, fuels and countless other chemical products that are made from simple starting materials. Kox and colleague Lukasz Karwacki, together with researchers at the Max Planck Institute for Coal Research in Mülheim an der Ruhr, Germany, ExxonMobil Chemical Europe Inc, Machelen, Belgium, the Centre for Nanoporous Materials, at the University of Manchester, UK, UOP LLC, a Honeywell Company, in Des Plaines, Illinois, USA, and Nicholas Copernicus University, Torun, Poland, have used a raft of spectroscopic techniques, on the micro scale to analyse the structure of zeolite ZSM-5 and have obtained spatial and time-resolved data on the three-dimensional interior of these porous materials. The data reveal the deviations from one porous unit to the next that can lead to reduced efficiency, catalytic poisoning, and unwanted chemical by-products.

Catalytic ZSM-5 (Credit: Nature Materials/Weckhuysen et al)

Kox is working as part of the Vici project run by Bert Weckhuysen, Professor of Inorganic Chemistry and Catalysis at Utrecht University in The Netherlands. Details of the research were published in Nature Materials. The team developed a new approach that correlates confocal fluorescence microscopy with focused ion beam–electron back-scatter diffraction, transmission electron microscopy lamelling and diffraction, atomic force microscopy and X-ray photoelectron spectroscopy to study a wide range of coffin-shaped zeolite crystals of differing shapes, sizes, structures, and chemical compositions.

The powerful combination of techniques demonstrates “a unified view on the morphology-dependent MFI-type [zeolite] intergrowth structures and provides evidence for the presence and nature of internal and outer-surface barriers for molecular diffusion,” the team say. “It has been found that internal-surface barriers originate not only from a 90° mismatch in structure and pore alignment but also from small angle differences of 0.5 to 2 degrees for particular crystal morphologies. Furthermore, outer-surface barriers seem to be composed of a silicalite outer crust with a thickness varying from 10 to 200 nanometres.”

LINKS

Nature Mater, 2009, 8, 959-965
http://dx.doi.org/10.1038/nmat2530

Bert Weckhuysen

Microbial power

New insights into the workings of a metal-munching bacteria and how it exploits semiconducting nanominerals could provide a new approach to making biological fuel cells for an almost all-natural power supply for electronic gadgets and medical devices.

Fuel cells are essentially electrical batteries that have a fuel supply rather than relying on built in chemical energy. Researchers have been developing microbial fuel cells that use a positive electrode, anode, coated with a bacterial film. These cells use a fuel comprising a substrate that the bacteria can break down to release electrons. Tapping into this electron release process to draw a current requires transferring the electrons to the anode.

Prof. Dr. Kazuhito Hashimoto

Prof. Dr. Kazuhito Hashimoto

Now, a team led by Kazuhito Hashimoto of the University of Tokyo, Japan, has investigated how this transfer is carried out in the subterranean microbe Shewanella loihica. Instead of breathing air, this microbe respires by reducing the iron(III) ions in iron oxides from the sediments in which it lives to the iron(II) state. This releases electrons which are used to release energy for it to live, grow, and multiply.

The team added microbial cells to a solution containing very finely divided nanoscopic iron(III) oxide particles and poured the solution into a chamber containing electrodes. A layer of bacteria and iron oxide particles was gradually deposited on to the electrodes at the bottom of the chamber. When the cells were “fed” lactate ions, a current was detected. Electrons from the metabolism of the lactate are thus transferred from the bacteria to the electrode.

SEM image of iron oxide and microbial cells on electrode after more than a day of current generation (Credit: Wiley/VCH)

SEM image of iron oxide and microbial cells on electrode after more than a day of current generation (Credit: Wiley/VCH)

The Tokyo team has now demonstrated using scanning electron microscopy how, in the presence of iron(III) oxide nanoparticles, the metal-reducing bacteria form aggregates that can conduct electricity. The SEM images show a thick layer of cells and nanoparticles on the electrode; the surfaces of the cells are completely coated with iron oxide particles. The researchers were able to show that the semiconducting properties of the iron oxide nanoparticles, which are linked to each other by the cells, contribute to the surprisingly high current recorded.

The cells act as an electrical connection between the individual iron oxide particles. Cytochromes, enzymes in the outer cell membrane of these bacteria, transfer electrons between the cells and the iron oxide particles without having to overcome much of an energy barrier. The result is a conducting network that even allows cells located far from the electrode to participate in the generation of current, the researchers explain.

We are now succeeding in the direct real-time observation of the layer formation using a special optical microscope, Hashimoto told Spotlight, This will be reported shortly.

Further reading

Angew. Chem. Int. Edn 2009, 48, 508-511
http://dx.doi.org/10.1002/anie.200804750

Prof. Dr. Kazuhito Hashimoto laboratory
http://www.light.t.u-tokyo.ac.jp/english/contact-us.htm

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