IS THERE anything carbon nanotubes can鈥檛 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. 快猫短视频s 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 Kr谩l and Moshe Shapiro at the Weizmann Institute of Science in Rehovot, Israel, worked out that if you ran a 鈥減olar鈥� liquid such as water past a conducting nanotube, electrons should flow through the walls of the tubes in the same direction.
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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鈥檚 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.
Kr谩l鈥檚 theory doesn鈥檛 explain the Bangalore team鈥檚 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鈥檚 findings. At least two of the groups 快猫短视频 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.