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IS THERE
anything carbon nanotubes can't do? They have been hailed as a
strong, semiconducting wonder ingredient that will make
materials stronger and help miniaturise electronics systems.
And their ability to act as filters might one day be exploited
to build artificial livers.
Now an even
bolder claim has been added to the list. Scientists this week
report that simply flushing water past a bundle of nanotubes
makes them generate a current. These miniature power sources,
say the researchers, might one day lead to a new breed of
implants, such as heart pacemakers that are powered by the
flow of body fluids alone (Science, DOI:
10.1126/science.1079080).
The idea that
it is possible to make nanotubes generate electricity by doing
little more than sticking them under a running tap was mooted
in 2001. Petr Král and Moshe Shapiro at the Weizmann Institute
of Science in Rehovot, Israel, worked out that if you ran a
"polar" liquid such as water past a conducting nanotube,
electrons should flow through the walls of the tubes in the
same direction.
In the
molecules of a polar liquid, some atoms are slightly
positively charged while others carry a balancing negative
charge. When the positive part of the liquid's molecules are
close to the surface of a single-walled nanotube, they attract
electrons (see
Graphic), which are carried along with the liquid as it
flows past. Because electrons can only flow lengthwise along
the tubes, the flow of polar molecules will produce a small
but potentially useful current.
Ajay Sood, a
physicist at the Indian Institute of Science in Bangalore, his
student Shankar Ghosh and colleagues at the nearby Raman
Research Institute have now tested the idea. They attached
electrodes to the top and bottom of a clump of randomly
oriented nanotubes and suspended them in a metre-long glass
tube. They then pumped water through the tube and measured the
voltage across the nanotubes, some of which would by chance be
aligned to the water flow.
Sure enough,
the flow produced a voltage across the clumps of nanotubes
that increased as the flow speeded up. At 2 millimetres per
second, the flow produced 2.7 millivolts across the clump, and
when the water flow was reversed the voltage between the
electrodes flipped too. Sood found that when he added
hydrochloric acid to the water, which increased the number of
positive hydrogen ions in the liquid, the voltage was
boosted.
Král's theory
doesn't explain the Bangalore team's results entirely, as the
induced voltage did not increase linearly with flow speed.
Sood suspects that the way the layered, laminar flow of the
water around the nanotubes distributes charge may hold an
answer.
But some
experts in nanotube chemistry are unconvinced by Sood's
findings. At least two of the groups New Scientist
contacted for comment were unconvinced by the work. Both
groups wish to remain anonymous. One team say they failed to
find any evidence that nanotubes could be turned into
generators when they tried a similar experiment. But Sood has
filed for a patent on the technology and hopes to prove the
critics wrong.
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