Scanning Electrochemical Microscopy of Individual Single-Walled Carbon Nanotubes

Abstract

Here we report on novel application of scanning electrochemical microscopy (SECM) to enable spatially resolved electrochemical characterization of individual single-walled carbon nanotube (SWNT). The feedback imaging mode of SECM was employed to detect a pristine SWNT (~1.6 nm in diameter and ~2 mm in length) grown horizontally on a SiO 2 surface by chemical vapor deposition. The resulting image demonstrates that the individual nanotube under an unbiased condition is highly active for the redox reaction of ferrocenylmethyltrimethylammonium used as a mediator. Micrometer-scale resolution of the image is determined by the diameter of a disk-shaped SECM probe rather than by the nanotube diameter as assessed using 1.5 and 10 μm-diameter probes. Interestingly, the long SWNT is readily detectable using the larger probe although the active SWNT covers only ~0.05 % of the insulating surface just under the tip. This high sensitivity of the SECM feedback method is ascribed to efficient mass transport and facile electron transfer at the individual SWNT.Electrochemistry of carbon nanotubes is a large and growing field that is both fundamentally and practically important. 1, 2 Unique physical and chemical properties of carbon nanotubes render them attractive electrode materials for electroanalysis and electrocatalysis. Most electrochemical studies of carbon nanotubes, however, have been carried out with an ensemble of nanotubes with various structural and electronic properties, thereby limiting our knowledge about fundamental electrochemistry of carbon nanotubes. To overcome this limitation, electrodes based on individual single-walled carbon nanotubes (SWNTs) were fabricated to voltammetrically reveal their high activity for redox reaction. 3 Moreover, ex-situ SEM and AFM observation of metallic nanoparticles electrodeposited on a SWNT demonstrated preferential nucleation sites (or defects). 4,5 Single molecule fluorescence microscopy was also employed for in-situ monitoring of adsorption and electrolysis of fluorescent redox molecules at discrete, nanometer-sized reactive sites on a SWNT. 6 These non-electrochemical microscopes, however, are applicable only to limited types of redox reactions although their superb spatial resolution is highly attractive.Recently, we proposed a principle of scanning electrochemical microscopy (SECM) 7,8 for spatially resolved electrochemical characterization of individual SWNTs (Figure 1). 9 In this SECM feedback method, a disk-shaped ultramicroelectrode is employed as a probe to electrolyze redox mediators at a diffusion-limited rate. When the tip is held less than a tip diameter away from the SWNT, the tip-generated species effectively diffuses to and reacts at the local surface of the SWNT just under the tip. SWNT are detected at the tip with enhancing its amperometric current response. Consequently, the tip current enhancement based on the feedback effect serves as a measure of the rate of electron transfer at the nanotube surface. Importantly, significant feedback effect is exp...

Intrinsic electrochemical activity of single walled carbon nanotube–Nafion assemblies

“…Notably, a format comprising an individual SWNT on an insulating support as the electrode, where the ends were not exposed to solution, displayed fast ET, 55 as did alternative studies on individual SWNTs. 67 The same conclusion has been reached through studies of individual SWNTs on an insulating support addressed by a microcapillary, 68 and, more recently, scanning electrochemical cell microscopy (SECCM) studies of SWNT networks of the type used herein, demonstrated that high rates of ET were observed from the majority of sidewalls of SWNTs. 69 Here, we report on a composite electrode comprising of a drop-cast Nafion film on a network of SWNTs synthesised by cCVD on an insulating Si/SiO 2 support.…”

Investigation of the Electron Transfer at Si Electrodes: Impact and Removal of the Native SiO2Layer

“…Undoped Si/SiO 2 has been used as insulating substrate for the characterization of single-walled carbon nanotubes 30,31 and gapped Au nanobands 32 by SECM. For these examples the insulating property of undoped Si/SiO 2 substrates is beneficial.…”

“…Although only Li + electrolyte is used in this study, the lithiation is not investigated, since the potential of all Si electrodes was kept at open circuit (E S = 3.2 V) and the first lithiation requires E S ≤ 0.2 V. 26 In contrast to pristine graphite electrodes, pristine Si electrodes are covered by a native SiO 2 surface layer, which is at least 1-3 nm in thickness and possess passivating properties for many charge transfer reactions. [27][28][29] Undoped Si/SiO 2 has been used as insulating substrate for the characterization of single-walled carbon nanotubes 30,31 and gapped Au nanobands 32 by SECM. For these examples the insulating property of undoped Si/SiO 2 substrates is beneficial.…”

Quantitative nanoscale visualization of heterogeneous electron transfer rates in 2D carbon nanotube networks

“…A separate approach has been to study CNTs on an inert substrate to ensure that the electrochemical (EC) signal measured can be uniquely attributed to the CNT material used. Both 2D networks of SWNTs (12,(26)(27)(28) and individual SWNT devices (29,30) have been employed, and facile HET has been reported for several redox complexes at a majority of SWNTs assessed. Such studies suggest highly active sidewalls, but the measurements are typically averaged over many SWNTs and SWNTcontacts (in the 2D networks) or over a length of μm to mm for individual SWNTs.…”