Electrochemical impedance spectroscopy at single-walled carbon nanotube network ultramicroelectrodes

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Abstract

Electrochemical impedance spectroscopy (EIS), coupled with chemical vapour deposition (CVD) grown single-walled carbon nanotube (SWNT) network disk-shaped ultramicroelectrodes (UMEs), gives stable, very well-defined and highly reproducible EIS responses for electrolysis of a simple outer sphere redox couple (FcTMA+/2+). The resulting EIS data can be fitted accurately using a simple electrical circuit model, enabling information on double-layer capacitance, diffusion coefficient of the electroactive species and the rate constant of ET (k0) to be extracted in a single EIS experiment. These values are replicated for a range of mediator concentrations and UME sizes (in the range 25–100 μm diameter) demonstrating the robustness of the method. These initial studies bode well for impedance based electroanalysis using SWNT network UMEs.

Introduction

Two-dimensional (2D), planar and electrically connected, single-walled carbon nanotube (SWNT) networks, grown using chemical vapour deposition (CVD) on an inert substrate, are emerging as powerful electrode materials for many applications [1], [2]. The advantages of CVD grown 2D SWNT networks are multi-fold: the SWNTs do not require further purification; metal catalyst content is limited typically to one nanoparticle per SWNT, located at the end of the SWNT; the substrate is insulating and thus the electrochemical response is due to the SWNTs only, providing impressive signal to background ratios; and the SWNT network format is well-defined, of high uniformity and known surface coverage [3].

It has been shown that disk-shaped ultramicroelectrodes (UMEs) fabricated from SWNT networks of low surface coverage (<1%) offer superior characteristics over conventional metal UMEs of the same size and dimensions [1]. SWNT disk UMEs yield a voltammetric response governed by the area of the support, rather than the area of the SWNTs themselves, due to overlap of neighboring diffusion fields, on typical voltammetric timescales [1]. The low intrinsic capacitance of the cCVD grown SWNTs [4] and much reduced surface area lead to much faster response times and unprecedented low background currents compared to conventional solid metal UMEs [1], [2] enabling trace level cyclic voltammetric (CV) detection.

Electrochemical impedance spectroscopy (EIS) is a powerful technique for studying interfacial charge-transfer [5]. The entire electrochemical response, including the rate constant of the electrochemical reaction, double-layer capacitance as well as the diffusion coefficient of the redox species in solution may be measured simultaneously in one experiment [6]. In recent work, EIS has been used as a technique complementing CV for the characterisation of the electrocatalytic activity of modified carbon nanotube electrodes on conducting supports [7], [8], [9], [10], [11]. However, studies have been limited to qualitative interpretation of EIS spectra to assess electron transfer (ET) behaviour at these electrodes [7], [8], [9], [10], [11], [12].

By combining EIS with the SWNT network UMEs it should be possible to quantitatively characterise both Faradaic and non-Faradaic processes. Using SWNT disk UMEs of different sizes and different mediator concentrations, we show that parameters such as double-layer capacitance, standard rate constant of ET (k0) and diffusion coefficient of the redox species can be readily calculated.

Section snippets

SWNT growth and UME fabrication

SWNT networks of <1% surface coverage and density ∼3 μm μm−2, were grown directly onto Si/SiO2 substrates by CVD, using a procedure described in detail elsewhere [3]. SWNT disk-shaped UMEs were fabricated from these networks using a lithographic procedure described in [1]. Briefly, a gold contact is evaporated onto the SWNT network and the surface insulated with photoresist. A disk of the desired diameter is defined using photolithography, exposing SWNT networks to the solution containing the

Results and discussion

The analysis of the diffusion process to a disk UME subjected to an AC flux perturbation has been described theoretically by Baranski [5], who proposed an equivalent circuit to analyze experimental results. A similar circuit, represented in Fig. 1c, has been used to fit EIS data at the SWNT UMEs, with the only modification being the addition of a contact resistance, Rc, to describe the resistance between the gold contact and the SWNT network [5]. Rct is the charge-transfer resistance; Ru is the

Conclusions

By combining EIS with cCVD grown SWNT network disk-shaped UMEs a powerful electrochemical tool is created for investigating charge-transfer processes at SWNTs. We show that important electrochemical parameters such as Cdl, D and k0 can be obtained simultaneously and accurately by fitting an equivalent electrical circuit to the EIS data. Importantly, the SWNT network UMEs give high quality EIS data and demonstrate stable and reproducible responses for the electrolysis of FcTMA+. This bodes well

Acknowledgements

We thank the EPSRC for funding (EP/C518268/1) and ID thanks the University of Warwick for a Postgraduate Fellowship Award and the Overseas Research Students Awards Scheme. Part of the equipment used in this research was obtained through Birmingham Science City with support from Advantage West Midlands and the European Regional Development Fund.

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