Charging Nanowalls: Adjusting the Carbon Nanotube Isoelectric Point via Surface Functionalization

Controlling the point of zero charge (PZC) of carbon nanotubes is important for depositing finely dispersed metal nanoparticles from precursors in solution for fabricating chemical and biological sensor and catalyst surfaces. HiPco single-walled carbon nanotubes (p-SWNTs) were functionalized with carboxylic acid (COOH-SWNT), nitroso (NO-SWNT), and maleic anhydride (MA-SWNT) groups. The presence of attached moieties on the carbon nanotube surface was verified using X-ray photoelectron (XPS) and attenuated total reflection infrared (ATR-IR) spectroscopies. PZC measurements (in parentheses) were in the descending order: NO-SWNTs (7.5) > p-SWNTs (3.5) > MA-SWNTs (2.0) > COOH-SWNTs (1.2). The trend in measured PZC values correlated with the electron withdrawing character of the attached moieties, consistent with Hammett a constants. Variations in the electron withdrawing character of the moieties led to SWNTs with differing semiconducting character, as observed in the UV-vis-NIR E-11 semiconducting region and Raman D-to-G band ratios. These results suggest a tunability of the SWNT PZC via sidewall functionalization, a factor to consider for practical SWNT nanomaterial fabrication.