Assessment of the Electrochemical Behavior of Two-Dimensional

Anal. Chem. 2006, 78, 7006-7015

Assessment of the Electrochemical Behavior of Two-Dimensional Networks of Single-Walled Carbon Nanotubes Neil R. Wilson,† Manon Guille,‡ Ioana Dumitrescu,‡ Virginia R. Fernandez,‡,§ Nicola C. Rudd,‡ Cara G. Williams,‡ Patrick R. Unwin,*,‡ and Julie V. Macpherson*,‡

Departments of Chemistry and Physics, University of Warwick, Coventry, CV4 7AL UK

Scanning electrochemical microscopy (SECM) has been employed in the feedback mode to assess the electrochemical behavior of two-dimensional networks of singlewalled carbon nanotubes (SWNTs). It is shown that, even though the network comprises both metallic and semiconducting SWNTs, at high density (well above the percolation threshold for metallic SWNTs) and with approximately millimolar concentrations of redox species the network behaves as a thin metallic film, irrespective of the formal potential of the redox couple. This result is particularly striking since the fractional surface coverage of SWNTs is only ∼1% and SECM delivers high mass transport rates to the network. Finite element simulations demonstrate that under these conditions diffusional overlap between neighboring SWNTs is significant so that planar diffusion prevails in the gap between the SECM tip and the underlying SWNT substrate. The SECM feedback response diminishes at higher concentrations of the redox species. However, wet gate measurements show that at the solution potentials of interest the conductivity is sufficiently high that lateral conductivity is not expected to be limiting. This suggests that reaction kinetics may be a limiting factor, especially since the low surface coverage of the SWNT network results in large fluxes to the SWNTs, which are characterized by a low density of electronic states. For electroanalytical purposes, significantly, two-dimensional SWNT networks can be considered as metallic films for typical millimolar concentrations employed in amperometry and voltammetry. Moreover, SWNT networks can be inexpensively and easily formed over large scales, opening up the possibility of further electroanalytical applications. Single-walled carbon nanotubes (SWNTs) are attracting much attention as components in electrical and electrochemical devices, with applications ranging from field emitters and field effect transistors to new electrode materials.1 The development of carbon * To whom correspondence should be addressed. E-mail: j.macpherson@ warwick.ac.uk. † Department of Physics. ‡ Department of Chemistry. § Present address: Department of Analytical Chemistry, University of Burgos, Burgos, Spain 09001.

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nanotube networks as “macroelectronic materials”2 has been fueled in the main by applications as transparent flexible conductors,3 environmentally sensitive field effect transistors for use as chemical sensors,4,5 field activated optical modulators,6 and diodes.7 For applications reliant on the high conductivity of SWNTs, such as use as an electrode material, the presence of a high density of metallic nanotubes (mSWNTs) is important, while for field effect transistor applications, the behavior of the semiconducting nanotubes (sSWNTs) needs to dominate. Two-dimensional random SWNT networks (one layer of SWNTs lying horizontally on a surface) can be produced on a surface by direct growth using chemical vapor deposition (CVD) techniques.8-10 Alternatively, SWNTs that have been pregrown and purified (often by acid treatment) can be spin-coated,11,12 spraycoated,13 solution-casted and transfer printed,11 or vacuum filtered14 onto substrates, again resulting in the formation of random networks. These types of deposition and growth methods can be (1) (a) Rinzler, A. G.; Hafner, J. H.; Nikolaev, P.; Lou, L.; Kim, S. G.; Tomanek, D.; Norlander, P.; Colbert, D. T.; Smalley, R. E. Science 2005, 269, 1550. (b) Collins, P. G.; Bradley, K.; Ishigami, M.; Zettl, A. Science 2001, 287, 1801. (c) Tans, S. J.; Verschueren, A. R. M.; Deeker, C. Nature (London) 1998, 393, 49. (2) Snow, E. S.; Novak, J. P.; Lay, M. D.; Houser, E. H.; Perkins, F. K.; Campbell, P. M. J. Vac. Sci. B 2004, 22, 1990. (3) Bradley, K.; Gabriel, J.-C. P.; Gru ¨ ner, G. Nano Lett. 2003, 3, 1353. (4) (a) Novak, J. P.; Snow, E. S.; Houser, E. J.; Park, D.; Stepnowski, J. L.; McGill, R. A. Appl. Phys. Lett. 2003, 83, 4026. (b) Abraham, J.; Philip, B.; Witchurch, A.; Varadam, V.; Reddy, C. Smart Mater. Struct. 2004, 13, 1045. (5) Snow, E. S.; Perkins, F. K. Nano Lett. 2005, 5, 2414. (6) Wu, Z.; Chen, Z.; Du, X.; Logan, J. M.; Sippel, J.; Nikolou, M.; Kamaras, K.; Reynolds, J. R.; Tanner, D. B.; Hebard, A. F.; Rinzler, A. G. Science 2004, 305, 1273. (7) Zhou, Y.; Gaur, A.; Hur, S.; Kocabas, C.; Meitl, M.; Shim, M.; Rogers, J. Nano Lett. 2004, 4, 2031. (8) Snow, E. S.; Novak, J. P.; Campbell, P. M.; Park, D. Appl. Phys. Lett. 2003, 82, 2145. (9) Day, T. M.; Wilson, N. R.; Macpherson, J. V. J. Am. Chem. Soc. 2004, 126, 16724. (10) (a) Kocabas, C.; Hur, S.-H.; Gaur, A.; Meitl, M. A.; Shim, M.; Rogers, J. A. Small 2005, 1, 1110. (b) Ismach, A.; Kantorovich, D.; Joselevich, E. J. Am. Chem. Soc. 2005, 127, 11554. (c) Huang, L. M.; Cui, X. D.; White, B.; O’Brien, S. P. J. Phys. Chem. B 2004, 108, 16451. (11) Meitl, M.; Zhou, Y.; Gaur, A.; Jeon, S.; Usrey, M.; Strano, M.; Rogers, J. Nano Lett. 2004, 4, 1643. (12) Stadermann, M.; Papadakis, S. J.; Falvo, M. R.; Novak, J.; Snow, E.; Fu, Q.; Liu, J.; Fridman, Y.; Boland, J. J.; Superfine, R.; Washburn, S. Phys. Rev. B 2004, 69, 201402. (13) Artukovic, A.; Kaempgen, M.; Hecht, D. S. Roth, S.; Gru ¨ ner, G. Nano Lett. 2005, 5, 757. (14) Hu, L.; Hecht, D. S.; Gru ¨ ner, G. Nano Lett. 2004, 4, 2513. 10.1021/ac0610661 CCC: $33.50

© 2006 American Chemical Society Published on Web 08/30/2006

used in conjunction with a variety of different surfaces, ranging from mechanically robust silicon oxide to flexible polymers. Consequently, SWNT networks are increasing in popularity because they are very easy to fabricate, are inexpensive to produce in large quantities, and can be synthesized on a variety of substrates. Recent work has begun to explore the characteristics of twodimensional SWNT networks as electrode materials for voltammetric and amperometric studies.9,15 Random, highly interconnected SWNT networks grown by CVD on an insulating support were investigated by voltammetry and conductivity measurements under solution,9,15 using a “wet gate” arrangement.16 It appeared that the activity of the SWNT network electrode depended strongly on the electrode potential applied to detect the solution species. At sufficiently negative electrode potentials, the sSWNTs (p-type through ambient oxygen doping17) appeared to “switch off” since the current signal was smaller than that observed at positive potentials. A recent theory paper suggests that the rate of charge transfer does not necessarily fall to zero in “switched off” sSWNTs and that sSWNTs in this state should instead be considered highly resistive.18 Thus, the decrease in faradaic current observed at negative potentials in voltammetric experiments could be due to a decreased conductivity of the network or to a decrease in the rate of charge transfer at the SWNT/solution interface. The spacing between SWNTs and the experimental time scale are also likely to have important effects on the amperometric response, as they determine the degree to which there is overlap of neighboring diffusion fields, associated with SWNTs in the network. If the time scale is such that the diffusion fields overlap significantly, the amperometric response would resemble that of a macroelectrode19 rather than a network of individual “nanoband” electrodes. Thus, questions for the use of SWNT networks as electrode materials are as follows: When can the electrode be treated as a homogeneous surface (in terms of diffusion)? When is the conductivity of the network limiting? How does the transition from individual to collective behavior depend on factors such as the time scale of the measurement, SWNT spacing, electrode potential, and redox mediator concentration? In this paper, we use scanning electrochemical microscopy (SECM) operating in the feedback mode, to probe the redox activity of high-density networks of SWNTs, with the aim of addressing these questions. SECM has proven powerful for the measurement of the conductivity of ultrathin films and local electron-transfer kinetics.20,21 Particularly advantageous is the fact that for SECM feedback the substrate does not need to be (15) Day, T. M.; Unwin, P. R.; Wilson, N. R.; Macpherson, J. V. J. Am. Chem. Soc. 2005, 127, 10639. (16) Rosenblatt, S.; Yaish, Y.; Park, J.; Gore, J.; Sazonova, V.; McEuen P. L. Nano Lett. 2002, 2, 869. (17) Avouris, P. Acc. Chem. Res. 2002, 35, 1026. (18) Heller, I.; Kong, J.; Williams, K. A.; Deeker, C.; Lemay, S. G. J. Am. Chem. Soc. 2006, 128, 7353. (19) (a) Scharifker, B. R. J. Electroanal. Chem. 1988, 240, 61. (b) Amatore, C.; Saveant, J. M.; Tessier, D. J. Electroanal. Chem. 1983, 147, 39. (20) (a) Slevin, C. J.; Unwin, P. R. J. Am. Chem. Soc. 2000, 122, 2597. (b) Zhang, J.; Barker, A. L.; Mandler, D.; Unwin, P. R. J. Am. Chem. Soc. 2003, 125, 9312. (c) O’Mullane, A. P.; Macpherson, J. V.; Unwin, P. R.; CerveraMontesinos, J.; Manzanares, J. A.; Frehill, F.; Vos, J. G. J. Phys. Chem. B 2004, 108, 7219. (d) Whitworth, A. L.; Mandler, D.; Unwin, P. R. Phys. Chem. Chem. Phys. 2005, 7, 356. (e) Ruiz, V.; Nicholson, P. G.; Jollands, S.; Thomas, P. A.; Macpherson, J. V.; Unwin, P. R. J. Phys. Chem. B 2005, 109, 19335.

electrically connected.22 For studies of SWNTs, this means that the sample can be investigated “as grown”, i.e., in pristine condition, without any of the extra processing steps that are required for the formation of contacts in electrical transport measurements8 or for direct electrochemical measurements under solution.9 This is an important point as work on individual SWNTs subjected to processing indicated that in some cases contamination related to sample preparation may be an issue.23 The SECM studies reported herein allow us to identify conditions where highdensity SWNT networks achieve positive feedback in SECM measurements and can thus be treated as a metallic film. EXPERIMENTAL SECTION Materials and Solutions. All chemicals were used as received. Aqueous solutions were prepared using Milli-Q reagent water (Millipore Corp., resistivity >18 MΩ cm) and contained either Ru(NH3)63+ (obtained as the chloride salt; Strem Chemicals, Newbury Port, MA), both Ru(NH3)63+ and Fe(phen)32+ (as the sulfate salt; BDH), or IrCl63- (as the potassium salt; Aldrich) at varying concentrations in the range 0.3-20 mM. In all cases, KNO3 (Fisher Scientific), at a concentration of 0.1-0.2 M served as a supporting electrolyte. SWNT Network Synthesis. SWNT networks were grown on Si/SiO2 substrates (300 nm thermally grown SiO2) by catalyzed CVD, using iron as a catalyst either in the form of solution-based nanoparticles (ferric nitrate nonahydrate; Aldrich Chemicals)24,25 or sputter-deposited at submonolayer thickness and then hightemperature annealed to form nanoparticles.26 In general, we note that Fe nanoparticles, formed by the latter approach, resulted in more uniform growth across large-scale substrates.27 The samples were placed in a 1-in. tube furnace and annealed under a flow of H2; CH4 was then introduced as the carbon-containing gas. Typical experimental conditions were 1500 sccm CH4 with 500 sccm H2, at 875 °C. Patterned SWNT samples were obtained by sputtering Fe through a shadow mask (a copper TEM grid was used for simplicity) onto the Si/SiO2 substrate, for subsequent SWNT growth. The density of the network was qualitatively varied by altering the growth conditions.27 The heights of SWNTs formed by CVD were determined by atomic force microscopy (AFM) and were typically in the range 1-3 nm. Quantitative information on SWNT densities on Si/SiO2 surfaces was obtained using a commercial software package (WsXM, freeware) from images taken using a field emissionscanning electron microscope (FE-SEM; SUPRA 55 VP, Zeiss). Throughout, densities are expressed as the length of SWNT per unit area of substrate (µmSWNT µm-2). Details on how information (21) (a) Liljeroth, P.; Vanmaekelbergh, D.; Ruiz, V.; Kontturi, K.; Jiang, H.; Kauppinen, E.; Quinn, B. M. J. Am. Chem. Soc. 2004, 126, 22, 7126. (b) Ruiz, V.; Liljeroth, P.; Quinn, B. M.; Kontturi, K. Nano Lett. 2003, 3, 1459. (22) Xiong, H.; Gross, D. A.; Guo, J.; Amemiya, S. Anal. Chem. 2006, 78, 1946. (23) Heller, I.; Kong, J.; Heerng, H. A.; Williams, K. A.; Lemay, S. G.; Dekker: C. Nano Lett. 2005, 5, 137. (24) Hafner, J. H.; Cheung, C. L.; Oosterkamp, T. H.; Lieber, C. M. J. Phys. Chem. B 2001, 105, 743. (25) Wilson, N. R.; Cobden, D. H.; Macpherson, J. V. J. Phys. Chem. B 2002, 106, 13102. (26) Peng, H. B.; Ristroph, T. G.; Schurmann, G. M.; King, G. M.; Yoon, J.; Narayanamurti, V.; Golovchenko, J. A. Appl. Phys. Lett. 2003, 83, 4238. (27) Wilson, N. R.; Edgeworth, J. E. Unpublished results.

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Figure 1. (a) Schematic of the steps involved in fabricating a SWNT network electrode for wet gate measurements and (b) FE-SEM image of a finished device.

such as fractional surface coverage, average SWNT-SWNT spacing, and number of SWNTs per unit area were extracted from density measurements are given in the Supporting Information. SECM Measurements. A two-electrode arrangement was employed with a 25-µm-diameter Pt disk ultramicroelectrode (UME) serving as the working electrode and Ag/AgCl (saturated AgCl) or Ag wire as a quasi-reference electrode (QRE). The RG values of the UMEs used were measured from optical micrographs and confirmed by SECM approach curve experiments to inert substrates.28 A small known volume (in the range 20-70 µL) of solution containing Ru(NH3)63+, IrCl63-, or both Ru(NH3)63+ and Fe(phen)32+, in KNO3 supporting electrolyte was placed onto the SWNT network substrate, and the Pt UME and the reference electrode were immersed in the droplet. Within the experimental time scales employed, solution evaporation was not problematic; i.e., there was no noticeable change in the steady-state UME current for diffusion-controlled electrolysis of the mediator in bulk solution during the course of the various experiments. For SECM approach and imaging measurements, the UME potential was controlled with a purpose-built triangular wave/pulse generator (Colburn Electronics, Coventry, UK) and the current measured with a home-built current follower (gains of 10-5-10-9 A/V). Current-potential characteristics for the UME working electrode were recorded using a PC equipped with a data acquisition card (Lab PC card, National Instruments, Austin, TX). Fine control of the UME position was achieved with a set of stages, in an x, y, and z arrangement, driven by inchworm motors (incorporating optical encoders) and a controller from Burleigh Instruments (models TSE-75 and 6200-3-3, respectively; Fischer, NJ). The electrochemical cell was shielded from electrical noise by a Faraday cage.

Prior to SECM measurements, the substrates were immobilized at the base of an electrochemical cell, which has been described previously.29 If necessary, solutions were deaerated with nitrogen prior to use. The lid of the cell contained several holes through which solutions could be changed and humidified nitrogen passed in order to maintain an oxygen-free atmosphere throughout the experiments. Approach curves were run by translating the UME toward the substrate typically at a speed of 1 µm s-1. The tip was held at a potential to electrolyze the redox mediator of choice at a diffusion-controlled rate: Fe(phen)32+ (oneelectron oxidation; +0.90 V); Ru(NH3)63+ (one-electron reduction; -0.45 V); IrCl63- (one-electron oxidation; +0.90 V). Imaging experiments were carried out in solutions containing 2 mM Ru(NH3)63+ in 0.2 M KNO3, with a 5-µm-diameter Pt disk UME (RG ) 35). With the tip held at a potential for the diffusioncontrolled reduction of Ru(NH3)63+, the tip was translated toward the surface until a change in the tip current from bulk was noted. Images, obtained over an area of 100 µm × 100 µm (scan rate, 5 µm s-1), were then recorded at different tip heights. A quantitative value for the tip-substrate separation was obtained by recording approach curves in known locations from the scanned image, where essentially positive feedback was observed (vide infra). Contact Electrode Fabrication. For complementary wet gate measurements, a four-step fabrication process was employed, as depicted schematically in Figure 1a. (1) Cr/Au (10 nm/60 nm) contact electrodes were thermally evaporated onto the SWNT substrates (used previously for SECM approach curve measurements) through a copper mask, to produce two Au electrodes, 1 mm in length, separated by an 80-µm gap. (2) TI35ES image reversal photoresist from MicroChemicals GMBH was photolitho-

(28) Amphlett, J. L.; Denuault, G. J. Phys. Chem. B 1998, 102, 9946-9951.

(29) Macpherson, J. V.; Unwin, P. R. J. Phys. Chem. 1995, 99, 3338.

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graphically patterned to protect a defined region of the nanotube network, 350 µm wide, in the gap between the two electrodes. (3) A 1-min oxygen plasma treatment (100 W, 0.6 mbar in an Emitech K-1050X plasma asher) was used to remove all the SWNTs except those protected by the photoresist, which was subsequently removed using acetone. (4) A further photolithography step (S1818 positive photoresist, with an underlayer of Omnicoat) was used to insulate the contact electrodes, with a section of the gap (60 µm by 450 µm) between the electrodes exposed. The resulting electrode device is shown in the FE-SEM image in Figure 1b. For wet gate measurements, the voltage was controlled using a PC card (Data Translation DT9800 DAQ card) with LabVIEW software. The current was measured with the same card using a current follower fabricated in-house. Finite Element Modeling. A model to treat diffusion to SWNTs was formulated (vide infra), and modeling was carried out using a commercial finite element method modeling package (COMSOL Multiphysics, version 3.2a), in conjunction with MATLAB (version 7.0, release 14). This was run on a Dell PC under Windows XP with an Intel Pentium 4 processor (2.50 GHz) and 1.5 GB of RAM. RESULTS AND DISCUSSION Simulation of Diffusional Feedback at Arrays of SWNTs. As highlighted in the introduction, the extent to which the diffusion fields associated with neighboring SWNTs overlap is an important consideration in defining the response of SWNT networks. To deduce when a SWNT network can be treated as homogeneous (as far as diffusion to the SWNTs is concerned) Fick’s second law was solved under steady-state conditions for an array of infinitely long parallel cylinders (representing the SWNTs) on an inert surface. To simplify the simulations, the cylinders were considered to be evenly spaced so that the problem reduced to 2D diffusion:

(

0)D

)

∂2c ∂2c + ∂x2 ∂z2

(1)

where D is the diffusion coefficient and x and z are the horizontal and vertical coordinates, respectively, with respect to the tube; see Figure 2. The finite element simulation domain was created such that there were axes of symmetry through the center of a cylinder and midway between two adjacent cylinders, defined by the half-spacing, h, as shown in Figure 2a. A conducting surface placed opposite the cylinder array surface represents the SECM tip. The latter imposes a boundary condition that allows a steady state to be achieved. This configuration obviously simplifies the SWNT network, which consists of randomly oriented and randomly spaced SWNTs, rather than aligned regularly spaced cylinders. However, the use of a simple unit cell allows the key features influencing diffusion to be defined and reduces the memory and computation time requirements considerably compared to a full 3D simulation. The SWNTs were assumed to be uniformly active and at a uniform potential; this simplifying assumption removes the effect of the conductivity of the network from the simulations, allowing the effect of diffusion to the SWNT array to be studied independent of any potential gradient on the surface.

Figure 2. (a) Schematic of the system of interest for the simulation of diffusional feedback at arrays of SWNTs. (b) Simulation domain (not to scale).

The boundary conditions employed (eqs 2-6) assumed that

x ) x r2 - (r - z)2

z ) r ( xr2 - x2, 2r < z < d, z ) 0,

05 µmSWNT µm-2 SWNT network.

probe edge, a large distance from the region of interest, will always make first contact with the surface. With the tip in proximity to the substrate, the tip-generated species diffuse to the sample surface and can be turned over electrochemically due to electron transfer at the substrate/solution interface (see Figure 5a). The extent to which this occurs on an unbiased sample depends on the lateral conductivity of the sample20,21 and the electron-transfer kinetics. For spatially heterogeneous samples, it may also be necessary to consider heterogeneities in diffusion (although this is not an issue here, vide supra). Note that the tip currents in Figure 6 have been normalized by the steady-state, diffusion-limited current for the particular mediator when the tip is placed far from the surface, i(∞), while the tip-substrate separation, d, has been normalized by the UME radius, a. In all of the families of approach curves shown, there is a monotonic decrease in the normalized tip current as the concentration of the redox mediator increases (concentration values are given in the figure captions), although the effect is slightly more pronounced with the Ru(NH3)63+/2+ mediator. Under steady-state conditions, the tip response has a contribution from mediator diffusion to the UME through the volume of solution trapped in the tip-substrate gap. When the substrate is inert, this results in a decrease in the tip current with decreasing tip-substrate separation (negative feedback35). Regeneration of the electroactive species at the substrate surface enhances the tip current, and for the case of a highly active surface, the current increases with decreasing tip-substrate separation (positive feedback35). It is well-established that positive feedback can occur at highly conductive macroscopic substrates, even when the substrate is not connected to an external potential source, as the substrate is poised at a potential established by the species in bulk solution (see Figure 5a).36 For poorly conducting samples, however, the contribution of mediator regeneration to the tip (35) Kwak, J.; Bard. A. J. Anal. Chem. 1989, 61, 1221. (36) Wipf, D. O.; Bard, A. J. J. Electrochem. Soc. 1991, 138, 469.


Figure 6. Experimental approach curves recorded on two highdensity SWNT samples with a 25-µm-diameter Pt UME (RG ) 10). Tip biased at (a) Etip ) -0.45 V for Ru(NH3)63+ reduction and (b) Etip ) 0.90 V for IrCl63- oxidation. In each case, the concentrations of mediator were (bottom to top) 20, 10, and 1 mM.

Figure 7. (a) FE-SEM image of a patterned SWNT surface. (b) 100 µm × 100 µm SECM feedback image of the same sample taken with the tip biased at a potential to reduce Ru(NH3)63+ (2 mM in 0.1 M KNO3 supporting electrolyte) at a diffusion-controlled rate at a tip-substrate separation of ∼1 µm. The tip current has been normalized with respect to that which flowed at a steady state with the tip positioned in the bulk of the solution.

current may only be apparent in the limit of relatively low concentrations of electroactive species.21 At higher concentrations of redox species, the lateral flux of electrons through the resistive sample, needed to sustain the feedback process, results in significant ohmic drop across the sample, diminishing the extent to which the redox couple is regenerated at the substrate.20,21 For both mediators, it is clear (from Figure 6) that essentially pure positive feedback is obtained with approximately millimolar concentrations, but at higher concentrations (10 and 20 mM), there is a limitation from either sample conductivity or electrontransfer kinetics. The lateral conductivity of homogeneous samples can be obtained from the concentration dependence of the feedback curves,20,21 as is evident from studies of Langmuir monolayers, 20b closely packed Au nanoparticle arrays,21a and composite thin films.20e,21b Using the lower values for network conductivity determined from the data in Figure 4b, deviations from positive feedback behavior37 would only be expected when the concentration of redox mediator is >0.5 M.21 This value is at least 1 order of magnitude greater than the concentrations employed to obtain the approach curve data in Figure 6, suggesting that the conductivity of the network is not responsible for the trends

observed in the approach curves. However, it should be noted that this simple analysis assumes that the substrate surface has a uniform conductivity and electroactivity. It follows from the above discussion that electron kinetics is the most likely factor causing the trend in the change in the approach curves with mediator concentration. For conventional metal electrodes, the standard electron-transfer rate constant should be independent of the concentration of the redox species, if Butler-Volmer kinetics apply.38 However, the low density of electronic states in both sSWNTs and mSWNTs (mSWNTs are effectively “semimetallic”-like graphite with a low density of states18) could lead to a concentration-dependent feedback response similar to that found in other two-phase problems.39 Recent theoretical work suggests that the electron-transfer kinetics of an (37) The approach curve increases in sensitivity as d/a decreases; thus, an estimate of the concentration at which conductivity becomes rate limiting was given by comparing i/i(∞) values at a d/a value of 0.1 and observing when the normalized current decreased by more than 10% compared with positive feedback. (38) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, Fundamentals and Applications, 2nd ed.; John Wiley: New York, 2001; p 87. (39) Barker, A. L.; Unwin, P. R.; Amemiya, S.; Zhou, J. F.; Bard, A. J. J. Phys. Chem. B 1999, 103, 7260.


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individual SWNT is expected to depend critically on the potential dependence of the density of states of the SWNT,18 intimately linked to its structure. In networks of the type considered herein, there may thus be significant variation in electron-transfer kinetics at individual tubes, although these effects may be averaged out as the network will tend to be biased by the bulk solution at a potential that drives electron transfer. In earlier experiments with individual SWNTs, significant kinetic effects were noted in some voltammetric measurements,23 consistent with the observations here. It is important to note the extremely high fluxes that individual SWNTs see in networks. Since the fractional surface coverage is only ∼1%, each nanotube supports a flux equivalent to 100 times the flux at a uniformly active surface. Thus, kinetic effects are expected to be significant. Effect of Network Density on SECM Feedback. The results above demonstrate that for high-density unbiased networks the feedback tip current depends on the concentration of the redox mediator. From the discussion above, it is clear that network density should also have a significant effect on the feedback current. The effect of network density on SECM feedback can clearly be visualized by SECM imaging of a patterned SWNT network surface. Figure 7a shows an FE-SEM image of a patterned sample. Defined near-circular zones (∼30 µm in diameter) of highdensity SWNT growth are clearly visible, separated by lower density regions. Figure 7b shows a typical 100 µm × 100 µm SECM image, recorded with a 5-µm-diameter Pt disk UME, at a scan speed of 5 µm s-1, positioned ∼1 µm from the surface of the sample. The solution contained 2 mM Ru(NH3)63+ in 0.2 M KNO3, and the tip was biased at a potential to reduce Ru(NH3)63+ at an apparent diffusion-controlled rate. The tip current has been normalized with respect to that which flowed at steady state with the tip positioned in the bulk of the solution. For the relatively low concentration of mediator solution employed herein, the feedback response over the high-density network areas tends toward positive feedback. Over the lower density areas, the current is significantly diminished but is higher than for negative feedback. The ability to selectively pattern SWNT networks onto substrates opens up the possibility of selective chemical tethering of target molecules (though attachment to the SWNT)40 in defined locations. For molecules that show electrocatalytic activity, SECM could also serve as a useful detection tool, as amply demonstrated in other studies of patterned surfaces by SECM.41 Preliminary SECM studies were also carried out on a lower density SWNT network sample (3.0 ( 0.8 µmSWNT µm-2) with an electrolyte solution containing both Ru(NH3)63+ and Fe(phen)32+ (at various concentrations) in 0.2 M KNO3 supporting electrolyte. For these approach curve measurements, the SECM tip was first biased at a potential to produce Ru(NH3)62+, from the diffusioncontrolled reduction of Ru(NH3)63+ (Etip ) -0.45 V). After obtaining an approach curve, of steady-state tip current versus tip-sample separation, the applied tip bias was changed to generate Fe(phen)33+ from the diffusion-controlled oxidation of Fe(phen)32+ (Etip ) +0.90 V). The tip current was again measured as the probe was translated toward the substrate surface. In this (40) Chen, R. J.; Bangsaruntip, S.; Drouvalakis, K. A.; Kam, N.; Shim, M.; Li, Y.; Kim, W.; Utz, P.; Dai, H. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 4984. (41) Niculescu, M.; Gaspar, S.; Schulte, A.; Csoregi, E.; Schuhmann W. Biosens. Bioelectron. 2004, 19, 1175.


Figure 8. Experimental approach curves (solid lines) for a 25-µmdiameter Pt UME (RG ) 7) approaching a medium-density SWNT network 3.0 ( 0.8 µmSWNT µm-2 . The tip was biased at (a) Etip ) -0.45 V (Ru(NH3)63+ reduction) and (b) 0.9 V (Fe(phen)32+ oxidation). Concentration of Ru(NH3)63+, in 0.2 M KNO3 (from bottom to top): 12, 6.3, 1.3, and 0.3 mM. Concentration of Fe(phen)32+ in 0.2 M KNO3 (from bottom to top): 10, 7.2, and 3.1 mM. All approach curves were recorded in the same spot on the sample.

way, SECM measurements could be made in the same location on the sample. As the results in Figure 8 clearly show, for network densities close to Fthmet, not only is concentration important in determining the degree of positive feedback, but the formal potential of the redox couple now appears to play a more important role. In particular, the normalized tip current for Ru(NH3)63+ reduction does not rise appreciably above unity, even at the lowest mediator concentrations used (0.3 mM; Figure 8a). In contrast, even at high concentrations of Fe(phen)32+ (up to 10 mM), the tip current shows significant positive feedback; see Figure 8b. We are currently exploring this effect in more detail. However, for network densities below Fthmet (i.e., with no continuous metallic pathways), changes in network conductance with solution potential are likely to be much more important. CONCLUSIONS SECM feedback measurements show that, even at surface coverages as low as 1%, two-dimensional networks of SWNTs, comprising both mSWNTs and sSWNTs, behave as thin metallic

films exhibiting positive feedback, irrespective of the formal redox potential of the electroactive species. The importance of diffusion field overlap of neighboring SWNTs has been investigated by simulations of a simplified thinlayer cell arrangement, with a (tip) electrode positioned over a two-dimensional array of parallel SWNTs. The simulations demonstrate that at relatively large tip-substrate separations the diffusion fields for SWNTs overlap and the network acts as a uniformly active surface (as seen by the tip). At small tipsubstrate separations (less than ∼10 times the characteristic SWNT separation), the tip electrode acts to constrain the diffusion fields at the SWNTs, diminishing the degree of overlap and so reducing the tip current. For the high-density networks used herein (surface coverage, ∼1%), the simulations indicate that heterogeneities in diffusion to the substrate are largely insignificant for practical SECM tip-substrate separations. Wet gate measurements on high-density SWNT networks enabled the lateral conductivity of the network to be determined as a function of solution potential. For a typical high-density sample, sheet resistivities in the range 140-500 kΩ square-1 were obtained. These values indicate that network conductivity should not limit the SECM feedback current response. The low surface coverage of SWNTs in the network results in very large fluxes to the SWNTs. Thus, while essentially positive feedback can be achieved with millimolar concentrations of mediator, at higher concentrations, kinetic effects appear to become apparent in the SECM approach curves, which are consistent with the low density of electronic states of SWNTs. We are currently carrying out a range of studies to explore electron-transfer kinetics at SWNT networks.

The results presented in this paper demonstrate that, for electroanalytical purposes, high-density, two-dimensional SWNT networks act as metallic films even at extremely low fractional surface coverages of ∼1%. They can be inexpensively and easily formed and patterned over large areas; various electroanalytical applications of SWNT networks are also underway in our laboratory. ACKNOWLEDGMENT J.V.M. thanks the Royal Society for the award of a University Research Fellowship. N.R.W. thanks the EPSRC for funding (EP/ C518268/1). V.R.F. and M.G. thank the EU Human Potential Programme SUSANA (Supramolecular Self-Assembly of Interfacial Nanostructures, contract HPRN-CT-2002-00185) and Marie Curie Fellowship Fund, respectively. N.C.R. and I.D. thank the University of Warwick for Postgraduate Fellowship Awards. We also acknowledge the assistance of Mr. Tom Day and Mr. Jonathan Edgeworth (Warwick Chemistry) for providing us with SWNT network samples. SUPPORTING INFORMATION AVAILABLE Details of the determination of SWNT network parameters from FE-SEM images. This material is available free of charge via the Internet at http://pubs.acs.org.

Received for review June 11, 2006. Accepted August 1, 2006. AC0610661


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Assessment of the Electrochemical Behavior of Two-Dimensional

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