J. Phys. Chem. C 2010, 114, 2633–2639
2633
Electron Transfer Kinetics at Single-Walled Carbon Nanotube Electrodes using Scanning Electrochemical Microscopy Ioana Dumitrescu, Petr V. Dudin, Jonathan P. Edgeworth, Julie V. Macpherson,* and Patrick R. Unwin* Department of Chemistry, UniVersity of Warwick, CoVentry, CV4 7AL United Kingdom ReceiVed: September 12, 2009; ReVised Manuscript ReceiVed: December 20, 2009
An important open question on the electrochemistry of single-walled carbon nanotube (SWNT) electrodes concerns the sites at which electron transfer (ET) occurs. This issue is addressed herein for the case of a simple outer sphere redox couple, (ferrocenymethyl)trimethylammonium (FcTMA+). Using relatively sparse networks ( 1.0 ( 0.6 cm s-1 is assigned, which is of similar size to other electrode materials and suggests that the sidewall of SWNTs has considerable ET activity. 1. Introduction There is considerable interest in the application of singlewalled carbon nanotubes (SWNTs) as voltammetric sensors in electrochemistry, due to unique properties, such as high-aspect ratio, nanometer-sized dimensions, good electrical conductivity1 and low intrinsic capacitance.2-4 However, there are still significant unresolved issues regarding the fundamental properties of SWNTs as electrode materials. Although several studies have highlighted the advantageous characteristics of SWNT electrodes for voltammetric analysis, such as greatly enhanced detection sensitivity,4-6 electrocatalytic effects,7 and reduced fouling, other work has suggested that the SWNT sidewall plays no role electrochemically and that only open ends, edge-plane defect sites8-10 and, in some cases, the catalytic nanoparticles from which SWNTs are formed11 are responsible for the electron transfer (ET) activity observed. In many of these latter studies, the SWNTs are first grown in bulk, purified, and then randomly dispersed from solution on an underlying support electrode which itself will have some electroactivity. The use of harsh acid purification techniques and solubilization adjuvants, typically required prior to application, can alter the native structural12,13 and electronic properties of SWNTs,14 and affect the behavior of SWNT-based electrodes, particularly toward inner sphere ET processes.15 To circumvent the problems described above, some recent studies have employed SWNTs grown using catalyzed chemical vapor deposition (cCVD) flat onto Si/SiO2 substrates.3,4,15-19 The main advantage to this approach is that SWNTs can be grown cleanly (with no need for further purification)20 and with a low defect density.21 Furthermore, the metal content is typically limited to one nanoparticle per SWNT, encapsulated at one end of the SWNT. Crucially, and in contrast to the electrodes described above, as the underlying substrate is insulating, the electrochemical response is due solely to the SWNTs.3,18 Studies on cCVD grown SWNT networks of low surface coverage * To whom correspondence should be addressed. E-mail: (P.R.U.)
[email protected]; (J.V.M.)
[email protected].
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