Non-carbon nanotubes

Carbon nanotubes rose to prominence on the back of the buckyball chemistry revolution in the 1990s and are now emerging from prototype applications across academic and industrial laboratories. They have potential in microelectronic circuits, novel sensor devices, special light conductors, and light-emitting nanotubes for display technology.

Indeed, applications as diverse as medical technology, for fibres with ultrahigh tensile strength, in hydrogen storage, for rechargeable batteries, in catalysis, and in nanotechnology are being developed. There are even applications for antifouling coatings for ships.

With this in mind, chemists in Germany who work with inorganic materials have now developed an approach to synthesising tin sulfide nanotubes which could expand the nanotube concept much further still and open up yet more avenues for applications. After all, carbon does not have a monopoly on nanotubes. Early in the development of tubular fullerene structures, inorganic chemists opted to make their nanotubes from metals and non-carbon atoms: tungsten sulfide, nickel chloride, vanadium sulfide, titanium sulfide, and indium sulfide. Many others have been produced.

Wolfgang Tremel and colleagues, Aswani Yella, Martin Panthoefer, Helen Annal Therese, Enrico Mugnaioli, Ute Kolb, of the Johannes Gutenberg-Universitaet in Mainz, Germany, have now developed a new process for the production of tin sulfide nanotubes, which they report in the Wiley journal Angewandte Chemie, the researchers found they could “grow” tin sulfide nanotubes from a drop of metal using a bismuth catalyst.

The team were faced with one of the fundamental problems of synthesising sulfidic nanotubes in that they require a high temperature to force the planar layers of material to bend and fuse into tubular structures. For tin sulfide, the situation is complicated still further by an unstable intermediate that is almost impossible to trap because it decomposes at a lower temperature.

The researchers used a different approach. First, they employed a vapour-liquid-solid (VLS) process, a technique borrowed from semiconductor scientists for producing nanowires as opposed to nanotubes. The process involved mixing bismuth metal powder with minute flakes of tin sulfide and heating this mixture in a tube furnace under a stream of the relatively unreactive noble gas argon. The product of the reaction forms a deposit at the cooler end of the tube.

The team explains that tiny droplets on the nanometre scale are form within the oven. These nanodroplets act as local points of contact for the tin so that the reactants become concentrated within the metal droplet and nanotubes can then grow from these seeds.

“In this process, the metal drop is obtained as a sphere at the end of the tube, and the nanotubes grow out of the sphere like a hair out of a follicle,” explains Tremel. “Catalysis by the metal droplet makes growth possible at low temperatures.”

The team has successfully grown nanotubes comprising multiple layers of tin sulfide with few defects. The nanotubes have diameters of between 30 and 40 nanometres and are 100 to 500 nm in length.

Tin sulfide nanotubes grow from droplets (Credit: Tremel et al/Wiley-VCH)

Angewandte Chemie, in press

Group of Prof. Dr. Wolfgang Tremel

Setting store by microporous polymers

UK chemists have devised a new approach to the storage of hydrogen gas that could power fuel-cell cars and vehicles without the need to carry hazardous cylinders of compressed gas. The approach is base on a highly porous polymer that can trap huge numbers of gas molecules allowing hydrogen gas to be stored in a compact container in a safe form.

Cardiff University’s Neil McKeown and collaborators Peter Budd of The University of Manchester, and David Book of the University of Birmingham are keen to solve the problem of hydrogen storage. The potential is enormous. As the use of fossil fuels comes under increasing environmental, political and practical pressures, alternative energy sources for use in vehicles and elsewhere will be needed in order to sustain current lifestyles. Fuel cells are considered one of the most promising alternatives to the internal combustion engine as they displace the pollution from city streets. The obstacle to their widespread use, however, lies in the need for a large energy-rich gas supply.

Polymer (Credit: Wiley/VCH)

Polymer (Credit: Wiley/VCH)

Microporous materials can have a vast internal surface area relative to their overall volume and ones that can adsorb hydrogen molecules on to the inner surfaces of their pores offer a way to store the gas in a small space. Zeolites and activated carbons have been the focus of much of this research but cost and weight remain obstacles to their implementation in commercially viable systems. McKeown and his colleagues have developed an organic alternative based on polymers, which should be lighter as well as less expensive to mass produce.

The molecular chains in most organic polymers are flexible and can pack efficiently in tight structures. So, to make a porous polymer requires monomer building blocks that would result in inflexible chains. With this in mind, the team constructed polymers from molecules containing fused chemical rings that form a stiff strip. By introducing the occasional twist in the strip, the team could produce a contorted but stiff polymer structure. The contorted molecules cannot pack together efficiently and the results are tiny gaps between the chains. These polymers of intrinsic microporosity (PIMs) have a vast internal surface area. A gram of polymer has a pore surface area equivalent to three tennis courts, or 800 square metres, to which hydrogen molecules can stick.

Neil McKeown

Neil McKeown

The idea is to reversibly adsorb the hydrogen on to the high surface area offered by the polymer due to its microporous structure so that it can be stored at more reasonable pressures and temperature than is the case for gaseous or liquid hydrogen, McKeown told Spotlight, The polymer (or any other microporous material envisaged) would be expected to act like the petrol tank of a car and be able to adsorb and release the hydrogen many times, hopefully, over the whole lifetime of the car and perhaps beyond.

Cartoon used with permission of Peter Welleman, www.cartooncreator.nl

Cartoon used with permission of Peter Welleman, www.cartooncreator.nl

The researchers suggest that by 2010 they will have succeeded in optimizing their polymers to store 6% by weight of hydrogen. There is plenty of work to do before we get something practical, concedes McKeown, but the concept of PIMs gives chemists another route to investigate.

Further reading

Angewandte Chemie International Edition
http://dx.doi.org/10.1002/anie.200504241

Neil B. McKeown
http://www.cardiff.ac.uk/chemy/contactsandpeople/academicstaff/mckeown.html

Peter Budd
http://www.chemistry.manchester.ac.uk/aboutus/staff/showprofile.php?id=50

Dr David Book
http://www.eng.bham.ac.uk/metallurgy/staff/book.shtml

Cartoon fuel cell
http://www.cartooncreator.nl/images/fuelcell.jpg

Suggested searches

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polymers

Hydrogen trapped

New porous materials could make the transport, storage and delivery of hydrogen much easier and open the door to what has been described as the hydrogen-based economy. The hydrogen economy could go some way to side-stepping certain problems associated with dwindling petroleum reserves, pollution, and climate change. Hydrogen-based fuel cells, for instance, could reduce greatly the impact of vehicle pollution in urban environments. The trouble with hydrogen though is finding a safe and effective transport and storage medium.

Many types of material have been investigated as possible storage media from high-surface-area carbon materials to single-walled nanotubes. Now, the storage of hydrogen at medium to low temperatures has been demonstrated by US researchers. They have developed a new method of storing large quantities of hydrogen gas in crystalline compounds. The results may lead to the development of new technologies that exploit the efficiency and low-pollution properties of hydrogen.

Dave Mao

Dave Mao

Currently, most hydrogen power systems use liquid hydrogen, a very cold (20 K) liquid that requires a lot of energy to produce. One alternative might be to trap molecular hydrogen (H2) in crystalline solids where a framework of water molecules encloses molecules of the gas. Until now, hydrogen was thought to be too small a molecule to be contained in a clathrate hydrate in this way. Ho-kwang Dave Mao and Wendy Mao of the University of Chicago and the Geophysical Laboratory at the Carnegie Institution of Washington thought differently and have recently reported that hydrogen clathrate hydrate, a crystal made up of units containing one hydrogen molecule and two water molecules, H2(H2O)2, can be prepared and stored comparatively easily.

To create these crystals, the team compressed hydrogen gas and ice with a pressure of 2,000 to 3,000 atmospheres. Using liquid nitrogen to cool the crystals to 77 K, the researchers showed that the hydrogen was retained even after the pressure was reduced to ambient levels. When the researchers raised the temperature to 140 K, the stored hydrogen was released.

The father and daughter team developing hydrogen storage materials

The father and daughter team developing hydrogen storage materials

Theoretically, the hydrogen gas that can be released from this medium could then be channelled to a generator. At 50 grams of hydrogen per litre of crystal, the clathrate does not contain as much hydrogen by weight as liquid hydrogen, and may be difficult to produce in large quantities. However, the researchers say that related clathrate crystals that use hydrocarbons, such as methane and octane, rather than water should be able to hold more hydrogen.

This result could be a first step toward an alternative way of storing environmentally friendly hydrogen gas, says Dave Mao, although the researchers admit that the field is still in its infancy and that much work is still to be done before the cash cow of the hydrogen economy becomes comes home.

Further reading

Read article
http://intl.pnas.org/cgi/content/abstract/0307449100v1

Ho-kwang “Dave” Mao
http://www.gl.ciw.edu/people/hmao

Intute hydrogen resources
http://www.intute.ac.uk/sciences/cgi-bin/browse.pl?id=78

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