Point of Zero Charge of Single-Walled Carbon Nanotubes

It has been shown through theoretical models that the point of zero charge (PZC) of single- walled carbon nanotubes (SWNT) changes with the size of the tubes. By modifying post-growth treatments, we show that different chiralities/diameters have different PZC and could allow us to separate bulk SWNT samples. In our lab, the SWNT are synthesized with a Co catalyst incorporated in an MCM41 matrix [1]. In the earlier work, highly dispersed size controllable Co clusters were produced by reduction treatment of Co-MCM41. Using the Co-MCM41 catalysts and performing the reaction at the different temperatures allows tuning over a range of different diameter/chiral tubes with small diameter distributions. The product SWNT has to be treated with harsh acids, a traditional method of functionalizing the surface of the SWNT for potential applications. The problem of this method is that the small diameter (<1nm) SWNTs are easy to be consumed in such a harsh conditions, therefore, this kind of post-treatment will not be favorable because it changes the distribution of the tubes we are trying to separate. We refine the method to functionalize the SWNT by using a more mild treatment that utilizes ozone concentrated water, according to fluorescence measurement, the small tubes survives and the distribution remains unchanged.

 We have shown that the functionalization of SWNT using ozone gives comparable results as with nitric acid treatment. By modifying the functionalization treatment method, we can still maintain the same surface charge characteristics but maintain PZC characteristics. As shown in the figure below, the PZC of ozone treated tubes is actually lower than that of nitric acid treated and could be modified by annealing. This information allows for a less hazardous treatment and can be used in functionalizing SWNT surfaces. 

[1] Loebick, C. Podila, R., Reppert, J., Chudow, J. Ren, F., Haller, G., Rao, A., Pfefferle, L. J. Am. Chem. Soc., 7, 11127 (2010).