Electrochemistry at carbon nanotube electrodes: is the nanotube tip more active than the sidewall?

The molecular engineering of an electrode surface is of paramount importance for the development of electrochemical devices with region-specific electron-transfer capabilities. As they have a unique one-dimensional molecular geometry and excellent electronic properties, carbon nanotubes (CNTs) have been widely used as functional electrodes in various electrochemical systems. Indeed, carbon nanotubes have been demonstrated to enhance the electrochemical activity of biomolecules and to promote the electron-transfer reactions of redox proteins, such as myoglobin, cyctochrome c, and microperoxidase MP-11. Recent studies have suggested that much of the enhanced electrochemical activity and electron-transfer rate at carbon nanotube electrodes arises from the edge-plane-like nanotube ends and that the nanotube sidewall is comparable to the basal plane of highly orientated pyrolytic graphite (HOPG). However, no convincing experimental evidence has been obtained owing to technical difficulties in distinguishing the electrochemical role of the nanotube tip from its sidewall, or vice versa, for conventional randomly orientated nanotubes or relatively short aligned nanotubes. This situation was further complicated by oxygen-containing groups, often introduced through chemical/electrochemical oxidation of the CNT tips or sidewalls, which could affect the nanotube electrode kinetics. The recent availability of superlong ( 5 mm) vertically aligned carbon nanotubes (SLVACNTs) enabled us to study the electrochemistry of the nanotube tip and sidewall specifically by selectively masking regions of the nanotube with a nonconducting polymer coating (e.g. polystyrene, PS) such that the electrolyte has access the nanotube sidewall or tip only. The effectiveness of the polymer masking was checked by fully coating the nanotube with polystyrene under the same conditions: no electrochemical signal was observed at all. Various electrochemical probes, including K3[Fe(CN)6], b-nicotinamide adenine dinucleotide disodium salt hydrate (NADH, reduced form), hydrogen peroxide (H2O2), oxygen, cysteine, and ascorbic acid (AA) with specific electrochemical sensitivities to various surface states of an electrode, were then used to monitor the electrochemical activities of the nanotube tip and sidewall. Depending on the electrochemical species used, we found that both the nanotube tip and sidewall could play a dominant role in electrochemistry at the carbon nanotube electrode. Furthermore, oxygen-containing surface functionalities induced, for example by electrochemical oxidation, were also demonstrated to regulate electrochemical activities of the carbon nanotube electrode. These new findings reported herein address the longstanding issue concerning the relative roles of the nanotube tip and sidewall to electrochemistry at carbon nanotube electrodes, and should facilitate the design and development of novel CNT-based electrodes of practical significance. In a typical experiment, SLVA-CNTs (5 mm long) were produced on a SiO2/Si wafer by the water-assisted chemical vapor deposition (CVD) of high-purity (99.99%) ethylene in the presence of an Fe catalyst with helium/H2 (2.5:1 v/v) as a carrier gas under 1 atm pressure at 700 8C. Figure 1a shows a digital photograph of the as-synthesized SLVA-CNT array. The corresponding scanning electron microscope (SEM) image is reproduced in Figure 1b, which shows closely packed well-aligned individual nanotubes. Transmission electron microscopic (TEM) observation of the constituent nanotubes individually dispersed in ethanol clearly reveals a double-walled carbon nanotube (DWNT) with an average outer diameter of 4 nm (Figure 1c). Figure 1d shows a schematic representation of the procedure for preparing the nanotube electrode from the assynthesized SLVA-DWNTarray. To start with, a small bundle of the superlong CNTs was taken out from the as-synthesized SLVA-DWNTs and connected to a copper wire (Step 1 in Figure 1d) with silver epoxy (see inset in Figure 1d). The CNT electrode with only the nanotube tip exposed (designated as the CNT-T electrode) was then prepared by thoroughly coating the copper-wire-supported CNTs with a PS solution (15 wt% in toluene) and drying at 50 8C in air (Step 2 in Figure 1d), followed by partially cutting off the free end of the polymer-wrapped CNTs (Step 3 in Figure 1d). The access of aqueous electrolytes to the innerwall of the wt nanotube can be effectively limited by the hydrophobic nature of the small DWNT. On the other hand, the CNT electrode with only the nanotube sidewall exposed (designated as the CNT-S electrode) was prepared by coating the two ends of the copper-wire-supported CNTs with the PS solution and drying at 50 8C in air (Step 4 in Figure 1d). To prepare the corresponding nanotube electrodes with oxygencontaining surface functionalities (designated as O-CNT-T and O-CNT-S), the newly prepared CNT-T and CNT-S electrodes were polarized at 1.8 V in 0.1m phosphate-buffered [*] Dr. K. Gong, Dr. S. Chakrabarti, Prof. Dr. L. Dai Department of Chemical and Materials Engineering and Department of Chemistry and UDRI University of Dayton 300 College Park, Dayton, OH 45469 (USA) Fax: (+1)937-229-3433 E-mail: [email protected]