Comparison of double-walled with single-walled carbon nanotube electrodes by electrochemistry
Introduction
Carbon nanotubes (CNTs) have emerged as a promising new class of electronic materials due to their nanoscale dimensions and outstanding properties, which include the ability to conduct a current density three orders of magnitude higher than typical conductors, such as copper and aluminium [1], and the ability to conduct electrons ballistically [2], [3], [4]. Such unique properties are already seeing CNTs incorporated into many nanoscale electronic devices such as transistors [5], [6], logic gates [7], [8] and electrodes [9], [10], [11]. Their incorporation into these devices relies upon a high degree of control and manipulation, a process which is currently achieved through methods such as functionalisation [12], [13], polymer wrapping [14], [15] or the use of surfactants [16], [17]. Whilst these methods are effective at attaining control over the nanotubes, modification of the unique physical properties occurs in each case, leading to an undesirable loss of conductivity [18].
Traditionally, single-walled carbon nanotubes (SWCNTs) have been used for device fabrication. However it has recently been suggested that double-walled carbon nanotubes (DWCNTs) may offer a superior alternative [19], [20]. It has been proposed that upon chemical modification, the outer tube will act as a protective sheath, hence preserving the electronic properties of the inner tube. Selective functionalisation has been observed experimentally by Ellis and Bubendorfer [21] and also by Brozena et al. [22] who confirmed selective outer-wall functionalisation of DWCNTs with Raman spectroscopy. Several groups are now taking advantage of this approach. For example, Hayashi and co-workers [19], [23] have utilised selectively functionalised DWCNTs for the purpose of self-assembling CdSe quantum dots onto the nanotubes for use in electronic and biomedical applications.
The two concentric graphene-like tubes making up the coaxial arrangement of DWCNTs are each capable of exhibiting metallic or semi-conducting character and have been found to possess a range of interesting electrical [24], thermal [25] and mechanical [26] properties. For example, Saito et al. [27] have suggested that upon arrangement of a semi-conducting and metallic nanotube, it would be possible to create molecular wires covered by an insulator or molecular capacitors for use in memory devices.
Furthermore, the high electrochemically accessible surface area of CNT arrays, combined with their high electronic conductivity and useful mechanical properties, make CNTs an attractive material for use in electrochemical sensors [28]. Recently, there have been many examples of new electrode platforms, which take advantage of the superior electrical properties of CNTs compared to typical macroscale materials. For example, Gooding et al. [29] have developed a CNT-based enzymatic electrochemical biosensor by covalently attaching protein decorated SWCNTs to a cysteamine modified gold electrode. Bissett and Shapter [10] have reported the production of vertically aligned SWCNT arrays covalently bonded to FTO glass for use in solar cells.
While CNTs can be attached to many surfaces [29], [30], [31], attachment to silicon based electrode devices has attracted much attention as it facilitates incorporation into current silicon-based technology. Pioneering this field was Yu et al. [9] who developed a silicon based electrode surface consisting of SWCNTs covalently bonded to silicon via an ester linkage. The electron transfer rate for a ferrocene redox solution was determined to be 4.54 × 10−3 cm s−1 [32], which is comparable to platinum, glassy carbon and polypropylene composite graphite (CPP) [9], indicating that such a surface would further applications in nano-electronic, opto-electronic and biosensor devices. Yu et al. [33] then went onto develop light harvesting antenna and multi-bit information storage by chemically modifying the immobilised CNTs with ruthenium porphyrin and ferrocene methanol.
While the electrodes of Yu et al. [9], [33] show great potential in the field of nanoscale electronic devices, their long term stability and application to ‘real world’ conditions is limited by the attachment chemistry. Ester linkages are easily made and reasonably stable for a short period of time, however the bond can easily be cleaved in water. Therefore, only electrode platforms with applications in organic solvents are possible. Clearly this means that many desirable applications such as biosensor systems and water quality sensing are not feasible. To solve this problem Flavel et al. [11] have utilised aryl-diazonium chemistry to fabricate an aminophenyl tether layer on a silicon substrate which is capable of forming a stable bond with CNTs and allows for use in aqueous environments.
In this work we demonstrate that DWCNTs are a superior choice of material for incorporation into electronic devices, compared to their single-walled counterparts. Using the previously reported oxidative acid method for functionalising SWCNTs [12], DWCNTs were functionalised and incorporated into two types of CNT-based electrochemical electrodes; a fluorinated tin oxide glass (FTO)- and a silicon–aminophenyl-CNT electrode previously reported for SWCNTs by Bissett and Shapter [10] and Flavel et al. [11], respectively. Electrochemical kinetic measurements were performed to obtain the electron transfer rate, which allowed a comparison with electrodes fabricated with SWCNTs.
Section snippets
CNT functionalisation
Y–Ni catalyst assisted DC arc discharge synthesised SWCNTs (Carbon Solutions Inc., CA, USA, P2-SWCNT) and CVD produced DWCNTs (ShenZhen Nanotech Port Co., Shenzhen, China) were purchased. The CNTs were then functionalised with carboxylic acid groups by ultrasonication (Elma S30 H Ultrasonic) in a 3:1 v/v solution of 98% H2SO4 and 70% HNO3 (Sigma–Aldrich) at a nanotube concentration of 1 mg mL−1 for 8 h and 2.5 h at 0 °C for SWCNTs and DWCNTs, respectively. Acid oxidation was then quenched by
Results and discussion
Valuable insights into the physical and electronic properties of SWCNTs and DWCNTs can be obtained by measurement of their radial breathing modes (RBMs) with Raman spectroscopy. In the case of multi-walled or large diameter CNTs, the Raman cross section is small and the detection of RBMs is quite difficult [34]. However due to the relatively small diameter (<2 nm) of the DWCNTs used here, resonance RBM vibrations can be observed. Fig. 2(a) shows the Raman spectrum between 100 and 500 cm−1 of
Conclusion
This work has demonstrated that DWCNTs are a superior conducting material for use in electrochemical electrodes. DWCNTs were selectively functionalised and incorporated into two types of pre-existing CNT electrodes. Using electrochemical methods the DWCNT electrodes were found to exhibit superior electron transfer kinetics compared to that of SWCNTs. Due to their more complex structure, they provide an improved electron pathway after chemical modification, enabling faster electron kinetics with
Acknowledgements
K.M. wishes to thank the Sir Ross and Keith Smith Honours Scholarship Fund. This work is supported by the Australian Microscopy and Microanalysis Research Facility (AMMRF).
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