Carbon Nanotubes Provide a Charge

A recent Report by S. Ghosh and co-workers (“Carbon nanotube flow sensors,” 14 Feb., p. [1042][1]) describes how flowing liquid over a mat of carbon nanotubes induces a voltage parallel to the flow. The authors explain their result in terms of “a direct forcing of the free charge carriers in the nanotubes by the fluctuating Coulombic field of the liquid flowing past the nanotubes.” I suggest a more prosaic explanation: It is well known that most porous materials develop a “streaming potential” in response to a liquid flow because the flow carries along counterions that accumulate in a thin layer near the solid-liquid interface (the Debye layer). Quincke first observed this effect in 1859 in powdered glass, ivory chips, animal bladder, graphite, and iron filings, among other materials ([1][2], [2][3]), and Helmholtz provided a quantitative explanation in 1879 ([3][4]). There is no reason for carbon nanotubes to be immune to it. The purification treatment reported by Ghosh et al. of long exposure to concentrated HCl would leave the surface of the nanotubes negatively charged, so one would expect an excess of positive charges in the Debye layer. This is consistent with the observed sign of the voltage in their experiments. More viscous solutions produce a lower voltage because the flow penetrates a lesser distance into the interior of the mat. The saturation in the observed voltage can be explained by electrode polarization. The fact that graphite did not produce a voltage in their control experiment is not surprising, given that (i) it has vastly smaller surface area and (ii) unlike the nanotubes, it presumably was not treated with acid before the measurement. The results of Ghosh and co-workers are interesting and may lead to useful devices, but the data presented seem consistent with classical electrokinetics. 1. 1.[↵][5] 1. G. Quincke , Ann. Physik 107(2), 1 (1859). [OpenUrl][6] 2. 2.[↵][7] 1. G. Quincke , Ann. Physik 110(2), 38 (1860). [OpenUrl][8] 3. 3.[↵][9] 1. H. L. F. von Helmholtz , Ann. Physik 7(3), 337 (1879) (no.), translated by P. Bocquet, Two Monographs on Electrokinetics (Engineering Research Institute, University of Michigan, Ann Arbor, MI, 1951). [OpenUrl][10] # Response {#article-title-2} Cohen suggests an electrokinetic mechanism for our observation of voltages induced by fluid flow over a mat of single-walled carbon nanotubes (SWNTs). In this purely ionic mechanism, the voltage appears as a streaming potential involving the ions carried by fluid flow in the diffuse Debye layer at the interface, while the mobile charge carriers (electrons and holes) in the solid play no role. Although an electrokinetic mechanism should suffice for the case of a nonconducting solid/liquid interface (e.g., with powdered glass, ivory chips, and so forth as the solid), we believe that it cannot effectively explain the present case of conducting nanotubes (resistivity ∼ 0.02 ohm-m). Assuming, as suggested by Cohen, that the SWNTs are negatively charged at the interface (so as to be consistent with the direction of the observed voltage), the streaming potential at the low flow velocities ( u ) obtained in our experiments (several orders of magnitude smaller than the thermal velocities) is expected to be linear in u , which is in strong disagreement with the observed sublinear dependence. As stated in our Report, the flow-induced voltages at these flow velocities are about 10 times smaller for multiwalled carbon nanotubes. These results again contradict the electrokinetic mechanism as a possible explanation. For a conducting solid/liquid interface (SWNTs in the present case), the charge on the solid surface is also screened by the carriers in the conducting solid. The usual treatment of the electrokinetic mechanism for the insulating solid/liquid interface is then not quite applicable per se. Figure 2 of our Report clearly shows that the induced voltage increases with increasing ionic concentration, in sharp contrast to a electrokinetic mechanism. We believe that the classical, purely electrokinetic mechanism, although very apt in the case of an insulating solid/liquid interface, is not effective in the case of the conducting solid/liquid interface in our study. Our mechanism involves the forcing of charge carriers (electrons and holes) in the SWNT itself by the ionic flow over the interface. [1]: /lookup/doi/10.1126/science.1079080 [2]: #ref-1 [3]: #ref-2 [4]: #ref-3 [5]: #xref-ref-1-1 View reference 1. in text [6]: {openurl}?query=rft.jtitle%253DAnn.%2BPhysik%26rft.volume%253D107%26rft.issue%253D2%26rft.spage%253D1%26rft.atitle%253DANN%2BPHYSIK%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [7]: #xref-ref-2-1 View reference 2. in text [8]: {openurl}?query=rft.jtitle%253DAnn.%2BPhysik%26rft.volume%253D110%26rft.issue%253D2%26rft.spage%253D38%26rft.atitle%253DANN%2BPHYSIK%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [9]: #xref-ref-3-1 View reference 3. in text [10]: {openurl}?query=rft.jtitle%253DAnn.%2BPhysik%26rft.volume%253D7%26rft.issue%253D3%26rft.spage%253D337%26rft.atitle%253DANN%2BPHYSIK%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx