pubs.acs.org/Langmuir © 2009 American Chemical Society
Robust Forests of Vertically Aligned Carbon Nanotubes Chemically Assembled on Carbon Substrates David J. Garrett,† Benjamin S. Flavel,‡ Joseph G. Shapter,‡ Keith H. R. Baronian,§ and Alison J. Downard*,† †
MacDiarmid Institute for Advanced Materials and Nanotechnology, Department of Chemistry, University of Canterbury, Private Bag 4800, Christchurch, New Zealand, ‡School of Chemistry, Physics and Earth Sciences, Flinders University, Bedford Park, SA 5042, Australia, and §Christchurch Polytechnic Institute of Technology, P.O. Box 540, Christchurch, New Zealand Received July 14, 2009. Revised Manuscript Received August 3, 2009 Forests of vertically aligned carbon nanotubes (VACNTs) have been chemically assembled on carbon surfaces. The structures show excellent stability over a wide potential range and are resistant to degradation from sonication in acid, base, and organic solvent. Acid-treated single-walled carbon nanotubes (SWCNTs) were assembled on amineterminated tether layers covalently attached to pyrolyzed photoresist films. Tether layers were electrografted to the carbon substrate by reduction of the p-aminobenzenediazonium cation and oxidation of ethylenediamine. The aminemodified surfaces were incubated with cut SWCNTs in the presence of N,N0 -dicyclohexylcarbodiimide (DCC), giving forests of vertically aligned carbon nanotubes (VACNTs). The SWCNT assemblies were characterized by scanning electron microscopy, atomic force microscopy, and electrochemistry. Under conditions where the tether layers slow electron transfer between solution-based redox probes and the underlying electrode, the assembly of VACNTs on the tether layer dramatically increases the electron-transfer rate at the surface. The grafting procedure, and hence the preparation of VACNTs, is applicable to a wide range of materials including metals and semiconductors.
Introduction The vertical alignment of carbon nanotubes (CNTs) on surfaces provides new materials with interesting properties for future applications in electronic, optoelectronic, and sensing devices. For example, CNTs have been shown to have particular advantages in bioelectrochemistry, where they can be used as highly conducting nanowires to connect redox enzymes to macrosized electrodes.1-5 The retention of enzyme activity and the promotion of direct electron transfer between the enzyme and the electrode lead to the possibility of nanoscale biosensors with very high selectivity and sensitivity. Vertically aligned CNT (VACNT) forests also show fast electron transfer for solution-based redox probes; on comparable surfaces, the electron-transfer rate is faster than for randomly dispersed CNTs.6,7 This is the result of the oxygen functionalities at the ends of acid-treated CNTs promoting fast electron transfer.7-9 Clearly, vertical alignment maximizes the number of CNT ends exposed at the electrode surface. *To whom correspondence should be addressed. Tel: 64-3-3642501. Fax: 64-3-3642110. E-mail:
[email protected]. (1) Gooding, J. J.; Wibowo, R.; Liu, J. Q.; Yang, W. R.; Losic, D.; Orbons, S.; Mearns, F. J.; Shapter, J. G.; Hibbert, D. B. J. Am. Chem. Soc. 2003, 125, 9006– 9007. (2) Guiseppi-Elie, A.; Lei, C. H.; Baughman, R. H. Nanotechnology 2002, 13, 559–564. (3) Patolsky, F.; Weizmann, Y.; Willner, I. Angew. Chem., Int. Ed. 2004, 43, 2113–2117. (4) Sotiropoulou, S.; Chaniotakis, N. A. Anal. Bioanal. Chem. 2003, 375, 103– 105. (5) Yu, X.; Chattopadhyay, D.; Galeska, I.; Papadimitrakopoulos, F.; Rusling, J. F. Electrochem. Commun. 2003, 5, 408–411. (6) Gooding, J. J.; Chou, A.; Liu, J. Q.; Losic, D.; Shapter, J. G.; Hibbert, D. B. Electrochem. Commun. 2007, 9, 1677–1683. (7) Chou, A.; Bocking, T.; Singh, N. K.; Gooding, J. J. Chem. Commun. 2005, 842–844. (8) Banks, C. E.; Davies, T. J.; Wildgoose, G. G.; Compton, R. G. Chem. Commun. 2005, 829–841. (9) Banks, C. E.; Moore, R. R.; Davies, T. J.; Compton, R. G. Chem. Commun. 2004, 1804–1805.
1848 DOI: 10.1021/la902575w
Chemical assembly is a simple approach to preparing VACNT forests and is an extension of long-established methods for the preparation of chemically modified electrodes.10 This strategy involves the immobilization of presynthesized CNTs on suitably prepared surfaces, relying on interactions between the carboxylic acid groups on the ends of acid-cut CNTs and appropriate functionalities on the substrate surface. Because of the presence of multiple carboxylic acid groups on each cut CNT end, there is a possibility of multiple attachment points to the surface, ensuring a largely upright orientation of CNTs. A number of different methods have been used to prepare the substrate and to induce VACNT assembly. For example, amine-terminated alkanethiols have been self-assembled on gold surfaces to fabricate VACNT forests using conditions that promote an amide linkage with the CNT.1,11,12 Amine-terminated alkanethiols on gold have also been used for the electrostatic assembly of CNTs utilizing an electric-field-assisted process.13 To assemble CNTs on a silicon (100) surface, an ethyl undecylenate tether layer was grafted to the hydride-terminated silicon surface, converted to the corresponding alcohol, and then reacted with CNTs, giving an ester link.14 More recently, VACNTs were directly assembled on a hydroxylated silicon (100) surface without an intervening tether layer.15 A characteristic of each of these self-assembly approaches is its limitation to a small number of substrate materials because of the specificity of the method used to prepare the substrate for reaction (10) Gooding, J. J. Electrochim. Acta 2005, 50, 3049–3060. (11) Diao, P.; Liu, Z. F.; Wu, B.; Nan, X. L.; Zhang, J.; Wei, Z. ChemPhysChem 2002, 3, 898–901. (12) Cai, L.; Bahr, J. L.; Yao, Y.; Tour, J. M. Chem. Mater. 2002, 14, 4235–4241. (13) Chen, Z.; Yang, Y.; Wu, Z.; Luo, G.; Xie, L.; Liu, Z.; Ma, S.; Guo, W. J. Phys. Chem. B 2005, 109, 5473–5477. (14) Yu, J. X.; Losic, D.; Marshall, M.; Bocking, T.; Gooding, J. J.; Shapter, J. G. Soft Matter 2006, 2, 1081–1088. (15) Yu, J. X.; Shapter, J. G.; Quinton, J. S.; Johnston, M. R.; Beattie, D. A. Phys. Chem. Chem. Phys. 2007, 9, 510–520.
Published on Web 09/29/2009
Langmuir 2010, 26(3), 1848–1854
Garrett et al.
with CNTs. In contrast, Papadimitrakopoulos and co-workers have developed a more general chemical assembly method utilizing a physisorbed Nafion/Fe(OH)x(3 - x)þ layer.16 Electrostatic interactions between the carboxyl-terminated CNTs and the cationic iron hydroxides are used to assemble the CNTs. In this work, we describe a simple chemical assembly method that can be used to assemble VACNTs on a very wide range of substrates. The method is based on the well-known surface grafting of nanoscale organic layers via the reduction of aryldiazonium salts.17-19 Grafting has been demonstrated on a large number of substrates, including graphitic carbons, metals, semiconductors, and even insulating materials (such as Teflon and glass).19,20 The process can be effected by electrochemical21,22 or chemical reduction20,23,24 or, for many substrates, by spontaneous reaction with the aryldiazonium salt.19,23,25,26 In addition to its broad substrate compatibility, grafting from aryldiazonium salts has the advantage of yielding tether layers that are attached to the substrate via covalent bonds.19 The strength of the bond between the tether layer and surface depends on the substrate; for carbon, the formation of a C-C bond gives very stable surface layers.22 Patterning methods applicable to this grafting approach have recently been demonstrated,27-31 allowing the method to be extended to the formation of patterned VACNT assemblies. There have been several previous reports describing the use of diazonium salts with SWCNTs; however, those approaches aimed to achieve alignment of the nanotubes parallel to the surface (silicon32 and silver33) by reacting diazonium salts with nanotube sidewalls. In this work, we demonstrate our new approach to the preparation of VACNT assemblies using electrochemically grafted aminophenyl layers on carbon surfaces. However, the results can be equally well applied to a wide range of other substrates and to layers formed by spontaneous or chemical reduction of the diazonium salt. We also describe preliminary results for the preparation of VACNT forests on an amineterminated tether layer grafted by electrochemical oxidation of (16) Chattopadhyay, D.; Galeska, I.; Papadimitrakopoulos, F. J. Am. Chem. Soc. 2001, 123, 9451–9452. (17) Bourdillon, C.; Delamar, M.; Demaille, C.; Hitmi, R.; Moiroux, J.; Pinson, J. J. Electroanal. Chem. 1992, 336, 113–123. (18) Downard, A. J. Electroanalysis 2000, 12, 1085–1096. (19) Pinson, J.; Podvorica, F. Chem. Soc. Rev. 2005, 34, 429–439. (20) Mevellec, V.; Roussel, S.; Tessier, L.; Chancolon, J.; Mayne-L’Hermite, M.; Deniau, G.; Viel, P.; Palacin, S. Chem. Mater. 2007, 19, 6323–6330. (21) Delamar, M.; Hitmi, R.; Pinson, J.; Saveant, J. M. J. Am. Chem. Soc. 1992, 114, 5883–5884. (22) Allongue, P.; Delamar, M.; Desbat, B.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Saveant, J. M. J. Am. Chem. Soc. 1997, 119, 201–207. (23) Barriere, F.; Downard, A. J. J. Solid State Electrochem. 2008, 12, 1231– 1244. (24) Pandurangappa, M.; Lawrence, N. S.; Compton, R. G. Analyst 2002, 127, 1568–1571. (25) Stewart, M. P.; Maya, F.; Kosynkin, D. V.; Dirk, S. M.; Stapleton, J. J.; McGuiness, C. L.; Allara, D. L.; Tour, J. M. J. Am. Chem. Soc. 2004, 126, 370–378. (26) Adenier, A.; Barre, N.; Cabet-Deliry, E.; Chausse, A.; Griveau, S.; Mercier, F.; Pinson, J.; Vautrin-Ul, C. Surf. Sci. 2006, 600, 4801–4812. (27) Downard, A. J.; Garrett, D. J.; Tan, E. S. Q. Langmuir 2006, 22, 10739– 10746. (28) Garrett, D. J.; Lehr, J.; Miskelly, G. M.; Downard, A. J. J. Am. Chem. Soc. 2007, 129, 15456–15457. (29) Charlier, J.; Palacin, S.; Leroy, J.; Del Frari, D.; Zagonel, L.; Barrett, N.; Renault, O.; Bailly, A.; Mariolle, D. J. Mater. Chem. 2008, 18, 3136–3142. (30) Ghorbal, A.; Grisotto, F.; Charlier, J.; Palacin, S.; Goyer, C.; Demaille, C. ChemPhysChem 2009, 10, 1053–1057. (31) Cougnon, C.; Gohier, F.; Belanger, D.; Mauzeroll, J. Angew. Chem., Int. Ed. 2009, 48, 4006–4008. (32) Flatt, A. K.; Chen, B.; Tour, J. M. J. Am. Chem. Soc. 2005, 127, 8918–8919. (33) Yoo, B. K.; Myung, S.; Lee, M.; Hong, S.; Chun, K.; Paik, H.-j.; Kim, J.; Lim, J. K.; Joo, S.-W. Mater. Lett. 2006, 60, 3224–3226. (34) Barbier, B.; Pinson, J.; Desarmot, G.; Sanchez, M. J. Electrochem. Soc. 1990, 137, 1757–1764. (35) Deinhammer, R. S.; Ho, M.; Anderegg, J. W.; Porter, M. D. Langmuir 1994, 10, 1306–1313.
Langmuir 2010, 26(3), 1848–1854
Article
a primary aliphatic diamine.34,35 Although the grafting of aliphatic amines is at present more limited in terms of substrates,36,37 it is another very simple approach to covalently attached tether layers.
Experimental Section Materials and Reagents. Pyrolyzed photoresist film (PPF) samples were prepared as described previously.38 The average surface roughness of the samples was e0.6 nm, and the surface resistivity was 18-25 Ω/sq. PPF surfaces were cleaned by sonication (2 min) in isopropanol (IPA) before use. The preparation and drying of [Bu4N]BF4 and acetonitrile (ACN) for electrochemistry have been described previously.38 SWCNTs prepared by the HiPco process were purchased from Carbon Nanotechnologies Incorporated. Uncut SWCNTs were acid-treated by adding 25 mg to 27 mL of mixed acid (3:1 concentrated H2SO4/HNO3) and sonicating for 10 h;39 ice was regularly added to the ultrasonicator bath to maintain the temperature close to 20 °C. Following sonication, the solution was poured into 500 mL of distilled water. After standing overnight, the solution was filtered under suction through Millipore 0.22 μm hydrophilic polyvinylidene fluoride filter membranes and washed with copious amounts of water. The dried SWCNT cakes were easily peeled from the filters and resuspended in DMSO to give a 1 mg mL-1 stock solution. Imaging by scanning electron microscopy (SEM) revealed that cut SWCNTs had a wide range of lengths and bundle diameters (Figure S1, Supporting Information). Electrochemistry. All electrochemical experiments were conducted using an Eco Chemie Autolab potentiostat. PPF samples were mounted in a glass cell that exposed a circular area of the surface to the cell solution.38 A Viton O-ring defined the geometric area of the working electrode (0.26 cm2 for grafting amine films and 0.13 cm2 for subsequent analysis). In all experiments, the secondary electrode was a Pt wire and the reference was either SCE (for aqueous solutions) or Ag/Agþ (0.01 M AgNO3 in 0.1 M [Bu4N]BF4-ACN) (for ACN solutions). All voltammetric measurements were obtained at a scan rate of ν = 100 mV s-1. Solutions for voltammetry were prepared using Mill-Q water (>18 MΩ cm). The pH was controlled using phosphate buffers (10 mM) with added NaClO4 to give an ionic strength of 0.1 M. Preparation of Chemically Assembled VACNT Electrodes. Aminophenyl (AP) films were grafted to PPF surfaces from
the corresponding diazonium salt, prepared in situ.40 One molar equivalent of NaNO2 dissolved in the minimum amount of water was added to 10 mM p-phenylenediamine in 0.5 M HCl at room temperature; this reaction yields the p-aminobenzenediazonium cation. AP films were grafted to PPF by cycling once between 0 and -0.6 V vs SCE (ν = 100 mV s-1), followed by holding the potential at -0.6 V for 2 min. A final cyclic voltammetric scan confirmed that the electrode was covered with a film that blocked the further reduction of the diazonium salt. Ethylenediamine (en) films were grafted to PPF from ACN solutions containing 0.1 M [Bu4N]BF4 and 1 mM en. Six potential cycles were recorded between 0.3 and 1.3 V vs Ag/Agþ (ν = 100 mV s-1). The oxidation peak for the amine disappeared after the first scan, and the background current decreased further on each subsequent scan, consistent with the grafting of a surface film. All film-modified surfaces were sonicated in acetone for 10 s and then in IPA for 10 s prior to further analysis. SWCNTs were assembled on amine-terminated films by submerging the amine-modified electrode in a DMSO solution (36) Adenier, A.; Chehimi, M. M.; Gallardo, I.; Pinson, J.; Vila, N. Langmuir 2004, 20, 8243–8253. (37) Gallardo, I.; Pinson, J.; Vila, N. J. Phys. Chem. B 2006, 110, 19521–19529. (38) Brooksby, P. A.; Downard, A. J. Langmuir 2004, 20, 5038–5045. (39) Liu, J.; Rinzler, A. G.; Dai, H. J.; Hafner, J. H.; Bradley, R. K.; Boul, P. J.; Lu, A.; Iverson, T.; Shelimov, K.; Huffman, C. B.; Rodriguez-Macias, F.; Shon, Y. S.; Lee, T. R.; Colbert, D. T.; Smalley, R. E. Science 1998, 280, 1253–1256. (40) Lyskawa, J.; Belanger, D. Chem. Mater. 2006, 18, 4755–4763.
DOI: 10.1021/la902575w
1849
Article
Garrett et al. Scheme 1. Strategy for Assembling VACNTs on Covalently Attached Amine-Terminated Films
(2 mL) of cut SWCNTs (0.2 mg mL-1). N,N0 -dicyclohexylcarbodiimide (DCC, 1 mg mL-1) was added to the reaction solution immediately prior to the introduction of the amine-modified electrode. The reaction mixture was maintained at 65 °C for the required time. VACNT electrodes were sonicated in acetone for 10 s and then in IPA for 10 s prior to further analysis. Surface Characterization. SEM images were obtained using a JEOL 7000 HRSEM with an acceleration voltage of 5 kV. The diameters of VACNT bundles were determined from SEM images (250 nm 250 nm) by fitting circles to all bundles in an image. The circle diameters were measured using Image J v 1.37 image analysis software. AFM images were recorded on a Digital Instruments Dimension 3100 instrument using a Nanoscope IV controller. The images were obtained with a NSC11 B cantilever (MikroMasch) operating in tapping mode at a frequency of 315 kHz; 5 μm 5 μm scans were recorded at 5 μm s-1, and 1 μm 1 μm scans were recorded at 1 μm s-1 with gain parameters optimized for each sample. Post-processing of AFM images was performed on Nanoscope Version 5.31R1 offline software and included digital leveling of some images and roughness calculations.
Results and Discussion Preparation of Chemically Assembled VACNTs. Scheme 1 outlines the two procedures used to assemble acid-treated SWCNTs on covalently attached, amine-terminated tether layers. In pathway a, an aminophenyl (AP) film was grafted to the PPF surface by the reduction of p-aminobenzenediazonium salt, prepared in situ in the electrochemical cell, and in pathway b, an aliphatic amine-terminated film was grafted by the oxidation of ethylenediamine (en). On the basis of our earlier work and the grafting conditions used, both pathways are expected to result in multilayer films.38,41 An alternative strategy for preparing an AP layer was also investigated in initial experiments. Nitrophenyl films were grafted to PPF by the reduction of the corresponding aryldiazonium salt in 0.1 M [Bu4N]BF4-ACN. The resulting films were electroreduced in 0.1 M H2SO4, which converts the majority of nitrophenyl groups to AP groups.42 All results obtained after the assembly of SWCNTs on these films were similar to those obtained on AP films prepared as shown in Scheme 1a, and hence only the results obtained with the latter films are included here. The assembly of SWCNTs on the tether layers followed the same procedure for each type of film (Scheme 1). The film-coated substrate and cut SWCNTs were reacted for 4-24 h under conditions suitable for the formation of amide bonds between the amines of the film and carboxylic acid groups at the ends and sidewall defects of nanotubes (DMSO with DCC at 65 °C). Bond formation involving the multiple carboxylic acid groups at the end (41) Cruickshank, A. C.; Tan, E. S. Q.; Brooksby, P. A.; Downard, A. J. Electrochem. Commun. 2007, 9, 1456–1462. (42) Yu, S. S. C.; Tan, E. S. Q.; Jane, R. T.; Downard, A. J. Langmuir 2007, 23, 11074–11082.
1850 DOI: 10.1021/la902575w
of each SWCNT is expected to result in vertical alignment of SWCNTs, as found in studies using amine-terminated tether layers on other substrates.1,12,43 The resulting films are referred to as AP-SWCNT and en-SWCNT, respectively. We note that in addition to amide bond formation a contribution from the electrostatic assembly of SWCNTs is also expected under these conditions. VACNTs have been assembled on amine-terminated SAMs of alkanethiols on gold electrodes solely via electrostatic interactions,13 and the possibility that both amide linkages and electrostatic interactions may play a part in SWCNT assembly on amine-terminated tether layers (in the presence of coupling agents) has been highlighted by Tour and co-workers.12 Characterization of VACNT Surfaces by Microscopy. PPF surfaces grafted with AP films were examined by AFM and SEM before and after reaction with SWCNTs for 4, 8, 12, and 24 h. The images, shown in Figure 1 , provide convincing evidence for the assembly of a layer of VACNTs. AFM images b-e show that the density of the SWCNT assembly increases with reaction time. The rms surface roughness increases for up to 8 h and then decreases as the layer of SWCNTs “fills in”. The apparent height of the layer decreases for long reaction time because the AFM tip tracks across the surface of the SWCNT layer and cannot approach the underlying substrate. This observation illustrates why the true height of VACNT forests is not expected to be revealed by this mode of imaging, a phenomenon that we and others have previously noted.1,3,14,15 However, AFM images of surfaces early in the assembly process (Figure 1 b,c) suggest that the assembled SWCNTs have maximum lengths of ∼60 nm. We assume that the assembly process “selects” short nanotubes because they diffuse to the surface more quickly than do longer ones. The proposed vertical alignment of SWCNTs is strongly supported by AFM images f-j and SEM micrographs k-o. Spherical features correspond to the tops of bundles of VACNTs; a few bundles are seen lying horizontally on the surface (indicated by arrows in image l). A close inspection of SEM images l-o reveals average VACNT bundle diameters of approximately 14.6 (standard deviation 1.3), 16.4 (2.2), 16.7 (1.9), and 18.1 nm (2.7 nm) after 4, 8, 12, and 24 h of reaction, respectively, consistent with the chemical assembly mechanism proposed previously.15 van der Waal’s interactions between the hydrophobic sidewalls of the SWCNTs are proposed to account for the increasing bundle diameter with assembly time. Although the SWCNT sidewalls are not expected to be devoid of carboxylic acid groups after acid pretreatment, the horizontal alignment of SWCNTs through the interaction of sidewall acid groups with the tether layer does not appear to be significant. Images of surfaces bearing horizontally aligned or randomly dispersed CNTs are markedly different from those obtained in this work; such assemblies have a tangled (43) Diao, P.; Liu, Z. F. J. Phys. Chem. B 2005, 109, 20906–20913.
Langmuir 2010, 26(3), 1848–1854
Garrett et al.
Article
Figure 1 . AFM (a-j) and (SEM k-o) images of PPF surfaces modified with AP films (a, f, k) and AP films after reaction with SWCNTs for 4 (b, g, l), 8 (c, h, m), 12 (d, i, n), and 24 h (e, j, o). The rms surface roughness of each sample was calculated from AFM data over a surface area of 25 μm2. Arrows indicate bundles of SWCNTs lying horizontally on the surface. Table 1. Cyclic Voltammetric ΔEp Values (ν = 100 mV s-1) Obtained at Bare PPF, Film-Grafted PPF, AP-SWCNT, and en-SWCNT Surfaces in 1 mM Fe(CN)64-/3-/1 M KCl and 1 mM FcOH/0.1 M [Bu4N]BF4-ACN ΔEp (mV) SWCNT assembly time (h) film/probe
bare PPF
amine film only
AP/ Fe(CN)64-/3en/ Fe(CN)64-/3en/FcOH
100 100 127
>800 85 219
4
8
24
254 104 144
100 105 136
97 130 137
“rat’s nest” appearance (Figure S2, Supporting Information, and references therein). Characterization of VACNT Electrodes by Cyclic Voltammetry. The performance of VACNT assemblies as electrodes was assessed by cyclic voltammetry of the Fe(CN)63-/4- couple in 1 M KCl. Table 1 lists ΔEp data obtained from cyclic voltammetric scans, and Figure 2 shows representative voltammograms. Figure 2a, scan i, is the voltammogram obtained at a bare PPF surface in a solution containing 1 mM of both Fe(CN)63- and Fe(CN)64-. The response is chemically reversible but electrochemically quasi-reversible with ΔEp = 100 mV. After an AP film was grafted and soaked in the reaction solution in the absence of SWCNTs (i.e., 1 mg mL-1 DCC in DMSO at 65 °C) for 24 h, scan ii was obtained in the redox probe solution. No peaks appear at the potentials seen at the bare PPF surface or within the scan limits, indicating that ΔEp > 800 mV. As frequently reported for multilayer films grafted by aryldiazonium cation grafting, the film acts as an insulating barrier, slowing the rate of electron transfer (44) D’Amours, M.; Belanger, D. J. Phys. Chem. B 2003, 107, 4811–4817. (45) Downard, A. J.; Prince, M. J. Langmuir 2001, 17, 5581–5586.
Langmuir 2010, 26(3), 1848–1854
Figure 2. Cyclic voltammograms of (a, b) 1 mM Fe(CN)64-/3-/1 M KCl and (c) 1 mM FcOH/0.1 M [Bu4N]BF4-ACN at (a) APmodified PPF and (b, c) en-modified PPF. Scans: (i) bare PPF, (ii) amine-modified PPF after soaking in DMSO with 1 mg mL-1 DCC for 24 h at 65 °C, (iii) amine-modified PPF after 4 h of reaction with SWCNTs, and (iv) amine-modified PPF after 24 h of reaction with SWCNTs.
between the underlying electrode and solution species.40,44-46 When the scan is repeated after the incubation of the film for 4 h in the SWCNT reaction solution, significant currents are seen and ΔEp = 254 mV (scan iii). Clearly the blocking properties of the film are now decreased, and the rate of electron transfer is higher. Soaking the AP film in the SWCNT reaction solution for 8 and 24 h (scan iv) leads to almost identical cyclic voltammograms (ΔEp = 100 and 97 mV, respectively). These scans are very similar to those obtained at bare PPF and show first that the VACNT assembly restores the electron-transfer rate to that observed prior to film grafting and second that the effective surface area of the (46) Ortiz, B.; Saby, C.; Champagne, G. Y.; Belanger, D. J. Electroanal. Chem. 1998, 455, 75–81.
DOI: 10.1021/la902575w
1851
Article
electrode is not increased by nanotube assembly. This latter finding is consistent with the presence of closely packed, short (compared with the diffusion-layer thickness) nanotubes, as evidenced by the rms roughness values shown in Figure 1. When the experiments were repeated beginning with an en film, the cyclic voltammograms shown in Figure 2b were obtained. In contrast to the results described earlier, grafting the en film to PPF leads to a small decrease in ΔEp for the Fe(CN)63-/4- couple (compare scans i and ii, recorded at bare PPF and the en film, respectively), indicating a small increase in the electron-transfer rate. The reaction of the en film with SWCNTs leads to a small increase in ΔEp and a decrease in peak currents in voltammograms of the Fe(CN)63-/4- couple; these changes are greatest for the longest reaction time as demonstrated by scan iv, which was obtained after 24 h of reaction with SWCNTs. Unlike the situation for the AP film, these small changes indicate a decreased rate of electron transfer between the Fe(CN)63-/4- couple and the electrode in the presence of the en-VACNT layer. We have previously demonstrated that en films show significant blocking behavior toward hydroxymethylferrocene (FcOH) voltammetry in 0.1 M [Bu4N]BF4-ACN solution.41 Hence we used this probe solution to examine the effects of SWCNT assembly on en films under conditions where the tether layer alone causes a pronounced decrease in the electron-transfer rate. Figure 2c shows cyclic voltammograms of 1 mM FcOH obtained at bare PPF (scan i), at an en film after soaking in the blank reaction solution for 24 h (scan ii), and after the reaction of the film with SWCNTs for 4 and 24 h (scans iii and iv, respectively). As previously observed, grafting the en film to PPF increases ΔEp for the FcOH0/þ couple. In contrast to the experiments conducted in aqueous solution (described above), reaction with SWCNTs decreases the blocking properties of the layer in ACN and the effect is most marked at the longest reaction time. After 24 h of reaction (scan iv), the voltammogram is very similar to that recorded at a bare PPF surface (scan i). To summarize the results described above, for two filmsolvent combinations (AP in aqueous solution and en in ACN), the reaction of the film with SWCNTs leads to a significant increase in the rate of electron transfer between the redox probe and the electrode, compared with the rate at the film-coated electrode. In contrast, for the en film in an aqueous medium, there is a small decrease in the apparent electron-transfer rate. To investigate the latter system in more detail, cyclic voltammograms of the Fe(CN)63-/4- couple were recorded at the en film and at the en-SWCNT assembly in phosphate-buffered solutions over the pH range of 3.5-12. Figure 3a shows a plot of ΔEp for the Fe(CN)63-/4- voltammograms versus pH, obtained at an enmodified PPF surface. (Note that the ΔEp values shown in Figure 3 are not the same as those in Table 1 because the medium is different.) The curve has the expected shape for an acid-base titration and gives a protonation constant of approximately 9. The similarity of this value to the second protonation constant of en in homogeneous solution (9.8) suggests that the film environment is solutionlike; that is, the en layer is loosely packed and highly permeable to water molecules and ions. The pH-dependent changes in ΔEp shown in Figure 3a appear to be controlled by electrostatics. Thus, at low pH, the rate of electron transfer for the Fe(CN)63-/4- couple is fast because of favorable electrostatic interactions between the anionic redox probe and a cationic (protonated) porous en film. As the pH increases, the film becomes increasingly deprotonated and the loss of electrostatic attraction between the probe and the surface of the film decreases the rate of electron transfer. Note, however, that even at pH 12, 1852 DOI: 10.1021/la902575w
Garrett et al.
Figure 3. Plots of ΔEp for the Fe(CN)63-/4- couple vs solution pH
obtained from cyclic voltammograms (ν = 100 mV s-1) recorded at (a) en-modified PPF and (b) en-SWCNT assembly on PPF.
where the film is neutral, ΔEp ≈ 520 mV, demonstrating that in aqueous solution the en film is inherently less blocking toward Fe(CN)63-/4- than is the AP film, for which ΔEp is greater than 800 mV in 0.1 M KCl (Figure 2a, scan ii). This is consistent with the proposed loosely packed and porous structure of the en film and the known insulating properties of multilayer aryl films grafted from diazonium salts.40,44-46 After the reaction of the en film with SWCNTs for 24 h, the cyclic voltammetry was repeated, giving the plot shown in Figure 3b. A pKa value of approximately 7 can be determined from this curve and is consistent with the presence of carboxylic acid groups at the ends of the cut SWCNTs. Comparing the titration curves in Figure 3a,b shows that the change in ΔEp versus pH is less abrupt for the en-SWCNT surface than for the en film. This may be due to a high concentration of carboxylic acid groups in the en-SWCNT assembly, leading to interactions between neighboring carboxylic acid groups and hence a range of pKa values. A contribution from unreacted -NH3þ groups in the underlying layer would also contribute to this effect. Again, the voltammetry of the Fe(CN)63-/4- couple appears to be controlled by electrostatic interactions with the surface. At pH 3.5, the surface will be close to neutral, allowing relatively fast electron transfer with the redox probe. As the pH increases, the surface becomes increasingly negatively charged and electrostatic repulsions with the anionic probe slow the rate of electron transfer. Interestingly, a consideration of the data in Figure 3a,b strongly suggests that the assembly of SWCNTs on the en film has an inherently accelerating effect on the rate of electron transfer for the Fe(CN)63-/4- probe. When the surfaces are neutral, ΔEp ≈ 520 mV at the en film (high pH), whereas ΔEp ≈ 150 mV at the en-SWCNT surface (pH 3.5), which is consistent with faster electron transfer after reaction with SWCNTs. Hence the en-SWCNT surfaces exhibit similar behavior in aqueous solution to that of AP-SWCNT films under aqueous conditions and to en-SWCNT films in ACN. Langmuir 2010, 26(3), 1848–1854
Garrett et al.
An increase in the rate of electron transfer to solution redox probes after the assembly of CNTs on insulating tether layers has been noted a number of times.15,43,47,48 Similar effects are also seen after the assembly of metallic nanoparticles on tether layers.49,50 Mechanisms for electron transfer in both covalently linked and electrostatically assembled systems are under consideration, but a clear understanding has not yet emerged.43,49,50 The present results provide another example of a system where the assembly of conducting nanomaterials on insulating tether layers strongly accelerates the rate of electron transfer across the interface. Stability of VACNT Assemblies. The mechanical and chemical stabilities of AP-SWCNT films (prepared with 24 h of reaction with SWCNTs) were examined by sonication of the surfaces in acid, base, and pentane. Before and after each sonication treatment, cyclic voltammograms were recorded in 0.1 M KCl solutions containing 1 mM Fe(CN)63- and Fe(CN)64-, 1 mM FcOH or 1 mM Ru(NH3)63þ. The potential ranges for voltammograms were -0.1 to 0.6 V for Fe(CN)63-/4- and FcOH, and 0.1 to -0.5 V for Ru(NH3)63þ. At AP-modifed PPF, cyclic voltammograms of FcOH and Ru(NH3)63þ revealed only a sigmoidal response with very low currents consistent with a small amount of pinhole diffusion. A solution of Fe(CN)63-/4- gave no response attributed to the redox couple. Prior to sonication, ΔEp values obtained at the AP-SWCNT assemblies were 95, 105, and 125 mV for FcOH, Ru(NH3)63þ, and Fe(CN)63-/4-, respectively. The sonication of AP-SWCNT assemblies in 0.1 M H2SO4, 0.1 M NaOH, and pentane for 5 min had no effect on the cyclic voltammograms of FcOH and Ru(NH3)63þ. Sonication in acid and pentane for 5 min did not change the response of Fe(CN)63-/4-, but after sonication of the AP-SWCNT assembly in 0.1 M NaOH, ΔEp for the cyclic voltammogram of Fe(CN)63-/4- increased to 244 mV. This large increase in ΔEp for Fe(CN)63-/4- under conditions that do not affect the voltammograms of the other redox probes suggests that the AP-SWCNT assemblies are stable to sonication and that the change in the voltammetry of Fe(CN)63-/4- is due to the increased deprotonation of carboxylate groups on the SWCNT ends in basic solution and hence greater electrostatic repulsion with the anionic redox probe, which slows the rate of electron transfer. The stability range of the SWCNT assemblies was established by applying increasingly positive potentials from 0.7 to 1.5 V and increasingly negative potentials from -0.3 to -1.5 V (both in 0.2 V increments) to AP-SWCNT films in 0.1 M KCl/1 mM Fe(CN)63-/4- solution. After applying each potential for 60 s, cyclic voltammograms were recorded in the probe solutions, as described above. There were no changes in the cyclic voltammograms of FcOH or Ru(NH3)63þ after polarization at potentials down to -1.5 V or up to 1.3 V, but polarization at 1.5 V resulted in a distortion of the FcOH cyclic voltammograms and a small decrease in the peak currents for the Ru(NH3)63þ/2þ couple. For the Fe(CN)63-/4- couple, ΔEp increased and the peak currents decreased as increasingly high negative and positive potentials were applied to the AP-SWCNT assemblies. After polarization at -1.5 V, ΔEp was approximately 270 mV, whereas after polarization at 1.3 V, ΔEp was close to 290 mV. Application of 1.5 V led to poorly defined voltammograms. The surfaces exposed to potentials of -1.5 and 1.5 V were subsequently soaked for (47) Liu, J. Q.; Chou, A.; Rahmat, W.; Paddon-Row, M. N.; Gooding, J. J. Electroanalysis 2005, 17, 38–46. (48) Hauquier, F.; Pastorin, G.; Hapiot, P.; Prato, M.; Bianco, A.; Fabre, B. Chem. Commun. 2006, 4536–4538. (49) Bethell, D.; Brust, M.; Schiffrin, D. J.; Kiely, C. J. Electroanal. Chem. 1996, 409, 137–143. (50) Zhao, J.; Bradbury, C. R.; Fermin, D. J. J. Phys. Chem. C 2008, 112, 6832– 6841.
Langmuir 2010, 26(3), 1848–1854
Article
30 min in 0.1 M KCl, cycled 20 times in Ru(NH3)63þ solution, and returned to Fe(CN)63-/4- solution. In both cases, the deterioration of the electrochemical properties of the surface appeared to be substantially reversed: cyclic voltammograms were welldefined with ΔEp values of
