Nearly every issue of major science journals contains new developments in nanotechnology which may eventually help us develop nanoscale medical implants, sensors, pollution scavengers, and other devices. In designing nanomachines, one has to think about how to power them. Batteries or other external power sources would add to the cost and size of the devices, so it would be preferable if they could be self-powered, having their own power cell or some power-harvesting mechanism.

Several years ago, scientists found that they could create an electric current by pushing water through a single-walled carbon nanotube (SWCNT)—the direction of the electric potential along the tube could even be flipped by changing the course of the water flow. Last year, Chinese scientists led by Lianfeng Sun managed to make hydroelectric power converters based on this phenomenon, which led them to suggest that "SWNTs can be exploited as unique, tunable molecular channels for water and might find potential application in nanoscale energy conversion."

However, before it’s sensible to look into further applications, it’s necessary to figure out how water and SWCNTs generate hydroelectric voltage. Without a basic understanding of the mechanisms involved, it would be difficult to design an efficient power-harvesting technique.

Quanzi Yuan and Ya-Pu Zhao from the Chinese Academy of Sciences investigated how water interacted with SWCNTs at the atomic level, and their work appeared today in an early edition of the Journal of the American Chemical Society. They computationally simulated a system where osmotic pressure was pushing water molecules through a SWCNT that was 12.3 � long and 8.14 � in diameter. As a frame of reference, a molecule of water has a diameter of about 2.75 �. They then varied different parameters to determine the source of the voltage and its properties.

One of the first things Yuan and Zhao noticed was that water moved through the nanotube in a perfect single-file formation. Water molecules normally form ordered hydrogen bonds with one another, and the ability to hydrogen bond was responsible for the formation of this single-file chain.

Each water molecule in that chain has a dipole moment and is polar, as the oxygen atom is more electronegative than the hydrogen atoms. Thus, when hydrogen bonded in a single-file fashion, all the water molecules contribute to give the collective chain a dipole moment as well.

The dipole moment of the chain creates a polarity difference through the SWCNT, resulting in a charge of 0.134 e at one end of the tube and a charge of -0.005 e at the other end. Yuan and Zhao calculated that the voltage difference between the two ends was 17.2 mV, the electric current was 1.72 �A, and the electric field of the tube was 107 V/m.

Based on these results, Yuan and Zhao conclude that "the structure of a water-filled SWCNT" makes it "a promising candidate for a synthetic nanoscale power cell, as well as a practical nanopower harvesting device." While the authors succeeded in figuring out the fundamental reason behind the voltage generation and identified the process as useful for real-world applications, further computational work would be helpful, as it is still unclear how factors like changing the diameter of the nanotube or the velocity of the moving water molecules will influence the voltage generation.

Journal of the American Chemical Society, 2009. DOI: 10.1021/ja8093372