Electrochemical Opening of Single-Walled Carbon Nanotubes Filled

Jun 21, 2008 - Electrochemical Opening of Single-Walled Carbon Nanotubes Filled with Metal Halides and with Closed Ends ... E-mail: gregory.wildgoose@...
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J. Phys. Chem. C 2008, 112, 10389–10397

10389

Electrochemical Opening of Single-Walled Carbon Nanotubes Filled with Metal Halides and with Closed Ends Andrew F. Holloway,† Kathryn Toghill,† Gregory G. Wildgoose,*,† Richard G. Compton,† Michael A. H. Ward,‡ Gerard Tobias,*,‡ Simon A. Llewellyn,‡ Bele´n Ballesteros,‡ Malcolm L. H. Green,‡ and Alison Crossley§ Physical and Theoretical Chemistry Laboratory, UniVersity of Oxford, South Parks Road, Oxford OX1 3QZ, Inorganic Chemistry Laboratory, UniVersity of Oxford, South Parks Road, Oxford OX1 3QR, and Materials Department, UniVersity of Oxford, Parks Road, Oxford OX1 3PH, United Kingdom ReceiVed: March 11, 2008; ReVised Manuscript ReceiVed: April 19, 2008

The electrochemical opening of closed-ended, pristine single-walled carbon nanotubes (SWCNTs) upon the application of either a sufficiently oxidizing or reducing electrode potential is reported. Hitherto, it has been unclear whether the side walls of SWCNTs are electrochemically active, or whether, like their multiwalled counterparts (MWCNTs), the electroactive sites on SWCNTs also reside at the edge-plane-like defects at the open ends of the tubes. Evidence is presented herein that suggests the latter case is true, i.e., that SWCNTs require edge-plane sites to be electroactive. Comparisons of the voltammetric response of end-closed SWCNTs (EC-SWCNTs), end-open (EO-SWCNTs), and SWCNTs encapsulating a metal halide filling (MX@SWCNTs, where MX represents either NaI or CuI) in aqueous electrolytes indicate that SWCNTs undergo electrochemical opening if the applied electrode potential is greater than +1.2 V vs SCE or less than -1.5 V vs SCE. This was further confirmed using ex situ X-ray photoelectron spectroscopy. The nonaqueous voltammetry of NaCl@SWCNTs, NaI@SWCNTs, CuI@SWCNTs, and ZnCl2@SWCNTs in dimethyl formamide (DMF) containing 0.1 M tetrabutyl ammonium perchlorate (TBAP) all exhibited voltammetric responses identical to that of EC-SWCNTs unless the potential was cycled beyond ca. +1.6 V vs Ag (+2.129 V vs the cobaltocene/ cobaltocenium redox couple) whereupon voltammetry corresponding to the filling material was observed, again indicating that the SWCNTs had become open-ended. Evidence for quantized charging of the ECSWCNTs is presented in terms of the unusual “bow-tie” shape of the background charging current in DMF is also presented. 1. Introduction Carbon nanotubes (CNTs) exist in two principle forms, multiwalled carbon nanotubes (MWCNTs), arguably first discovered in the late 1970s1–3 and then sensationally rediscovered by Iijima in 1991,4 and single-walled carbon nanotubes (SWCNTs), the first conclusive evidence of which was presented by Iijima in 1993.1,5 Since then, CNTs have been the subject of intense research in numerous scientific fields, not least that of electrochemistry with potential applications in, for example, sensors, catalysts, power sources, and energy storage devices. Conceptually, one may consider a SWCNT to be formed by seamlessly “rolling up” a sheet of graphite, and there are several ways that this can be achieved. This depends on the so-called “roll-up vector” [n, m] where n and m are integers, which is often used to describe the chirality of SWCNTs. The manner in which the tube is “rolled up” affects the electronic properties of the tube such that when |n - m| ) 3q where q is an integer the tubes possess metallic conductivity. Hence, on average onethird of SWCNTs are metallic conductors, and the rest are semiconducting.6–10 * Corresponding author. E-mail: [email protected]. Tel: +44 (0)1865 275406. Fax: +44 (0)1865 275410 (G.G.W.). E-mail: [email protected]. Tel: +44 (0)1865 272600. Fax: +44 (0)1866 272690 (G.T.). † Physical and Theoretical Chemistry Laboratory. ‡ Inorganic Chemistry Laboratory. § Materials Department.

The electronic structure of SWCNTs can be described, using the tight binding model, as consisting of a series of Van Hove singularities (VHS) in the density of states; the allowed transitions between the different VHS give rise to the optical spectra observed for SWCNTs. The energy separation between the ith VHS is given by eq 1 and is inversely proportional to the diameter, d/nm, of the nanotubes

∆Eii )

2iγ0aC-C + δE d

(1)

where γ0 is the nearest-neighbor overlap integral (γ0 ) 2.5 eV), aC-C is the C-C bond length (0.142 nm), and δE is a correction for the intertube interaction in a bundle of SWCNTs (=0.2 eV).6–10 The diameter, d, correlates with the [n, m] chiral indices of the SWCNTs. The application of an electrical potential in an electrochemical experiment can change the occupancy of the VHS leading to “bleaching” of the observed optical spectra as demonstrated by spectroelectrochemical experiments, most notably from the Dunsch group.11–15 This change in the occupancy of the VHS within the electronic structure of the SWCNTs upon the application of a certain electrode potential leads to the charging of the tubes, so-called “quantized charging”, which may be considered formally as a Faradaic process separate from the usual double-layer charging of the electrodeelectrolyte interface.11,14–17 The Faradaic charging of the SWCNTs occurs over a broad range of potentials depending on the large range of different [n, m]-SWCNTs within any given sample.

10.1021/jp802127p CCC: $40.75  2008 American Chemical Society Published on Web 06/21/2008

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Figure 1. Overlaid cyclic voltammograms recorded in 1.0 mM potassium ferrocyanide + 0.1 M KCl at scan rates of 10, 25, and 50 mV s-1 at (a) a bare GC electrode, (b) an EC-SWCNT modified GC electrode, (c) an EO-SWCNT modified GC electrode, and (d) an EC-SWCNTs modified GC electrode after conditioning at +1.5 V for 60 s in 0.1 M KCl.

The charge developed on the SWCNTs is balanced by movement of oppositely charged electrolyte ions from bulk solution to the walls of the SWCNTs, thus maintaining charge neutrality.12 As such, no distinct peak corresponding to this charging can usually be observed above the classical background charging caused by the double-layer capacitance. That being said, broad, ill-defined voltammetric features which may possibly be attributed to quantized charging can be observed in nonaqueous electrolytes when the appropriate choice of electrolyte salt is used, usually alkylammonium ions with a suitably hydrophobic anion, such as tetrabutylammonium tetrafluoroborate used by Dunsch et al.12–15,17,18 or the tetrabutylammonium perchlorate salt used herein. Both MWCNTs and SWCNTs can be formed with the ends of the tubes closed (EC-MWCNTs or EC-SWCNTs, respectively) by fullerene-like caps, and this is usual in the cases where the CNTs are formed using the arc-discharge method. Wang et al. have shown that, in the case of MWCNTs, the application of an electrical potential beyond a certain value “activates” the MWCNTs.19 The electroactive sites on MWCNTs have been shown, by analogy to graphite (which possesses both a basalplane crystal face comprising a plane containing all the carbon atoms in one sheet of graphite and an edge-plane crystal face which is perpendicular to the basal plane), to be located at the edge-plane-like tube ends, while in comparison, the more basalplane-like tube walls can be considered to be electrochemically inactive.20–22 Thus, the electrochemical activation demonstrated by Wang et al. can be understood in terms of the oxidative

rupture of the fullerene-like caps to reveal edge-plane-like sites at the tube ends. As these open-ended MWCNTs (EOMWCNTs) have almost indistinguishable electrochemical properties to a well-prepared edge-plane pyrolytic graphite (eppg) electrode,20–22 it is a simple matter to demonstrate that the multiwalled carbon nanotubes have opened by comparing the cyclic voltammetric response of the MWCNTs to a range of standardredoxprobessuchaspotassiumferrocyanide,hexaamineruthenium(III) chloride, epinephrine, or norepinephrine, for example.20–22 In the case of SWCNTs, however, the location of the active sites from which electron transfer can occur is less well understood. The smaller diameter of the SWCNTs induces a greater degree of curvature reducing the overlap between p-orbitals on adjacent carbon atoms involved in π-bonding and increasing the chemical reactivity of the sidewalls. This means that in the case of SWCNTs not only are the tube ends electroactive, but in contrast to the MWCNTs, the walls of the SWCNTs may also be electrochemically active. This question of whether the whole tube is electroactive in a SWCNT or just the tube ends (as is the case in MWCNTs) is one that is actively being investigated in many laboratories including our own. However, this ambiguity means that clearly demonstrating whether SWCNTs can be electrochemically activated by opening the ends of the tubes or not is a more complicated task than in the case of MWCNTs studied previously. In order to address this problem, we present a method whereby we fill SWCNTs with metal halides, MX@SWCNTs,

Electrochemical Opening of SWCNTs

J. Phys. Chem. C, Vol. 112, No. 28, 2008 10391 separate process from the onset of solvent breakdown and therefore separate from any subsequent followup chemical reactions that may be induced by decomposition of the solvent on the SWCNTs. The fact that the SWCNTs become open upon the application of a certain voltage affects the observed electrochemical properties of these materials and provides important insights into our understanding of how best to use such materials in applications such as in sensors, super capacitors, and new battery technologies.

Figure 2. (a)Ten overlaid cyclic voltammograms of NaI@SWCNTs supported on a BDD electrode in 0.1 M NaClO4 scanning initially from 0.0 to +1.5 V vs SCE at 10 mV s-1. Inset: an exploded view of the area of interest showing the oxidation peak at +0.98 V. (b) Three overlaid scans of CuI@SWCNTs supported on a BDD electrode in 0.1 M NaClO4 scanning initially from 0.0 to +1.5 V vs SCE at 10 mV s-1. Inset: an exploded view of the area of interest showing the series of voltammetric peaks (see text).

where MX denotes the filling material, and then close the ends of the SWCNTs to completely encapsulate the metal halide within the nanotubes.23 The electrochemical and spectroelectrochemical properties of filled nanotube have been studied previously, again most notably by Dunsch et al. who studied C60@SWCNTs or “fullerene peapods”,11,12,17,18,24,25 and in no instance has it been demonstrated that electron transfer can occur through the walls of the encapsulating SWCNTs to or from the filling material.12,13,18,24 We report that by applying a potential to the MX@SWCNTs supported on an electrode surface beyond a certain value in either an oxidative or reductive manner we can observe voltammetry corresponding to the various metal halide filling materials used in the MX@SWCNTs. Thus, we can determine the potential limits beyond which the SWCNTs have become opened, and this is confirmed by comparing the voltammetry of unfilled EO-SWCNTs and EC-SWCNTs as well as using ex situ techniques such as X-ray photoelectron spectroscopy. Voltammetric evidence of possible quantized charging of the tubes is also presented. The opening of the MX@SWCNTs was investigated in a range of different aqueous and nonaqueous electrolytes and also on several different electrode substrates, and the process was largely independent of these, demonstrating that the electrochemical opening was a

2. Experimental Section 2.1. Reagents and Equipment. All reagents were purchased from Aldrich (Gillingham, UK) with the exception of tetrabutylammonium perchlorate and tetrabutylammonium hexafluorophosphate (Fluka, Gillingham, UK) and were of the highest commercially available grade and used without further purification. Single-walled carbon nanotube samples used in this study were supplied by Thomas Swan & Co. Ltd. (Consett, UK), except for the sample of EO-SWCNTs that was purchased from Nanolab (Brighton, MA, USA). Aqueous solutions were prepared using ultrahigh-quality (UHQ) deionized water from a Millipore (Vivendi, UK) UHQ-grade water system with a resistivity of not less than 18.2 MΩ cm at 298 K. Nonaqueous solutions were dried over alumina and 5 Å molecular sieves to remove trace water prior to use. Cyclic voltammetry was performed on a µAutolab type III computer-controlled potentiostat (EcoChemie, Utrecht, Netherlands) using a standard three-electrode configuration. A bright platinum wire served as the counter electrode, while either a saturated calomel reference electrode (SCE, Radiometer, Copenhagen, Denmark) or a silver wire quasi-reference electrode was used for aqueous and nonaqueous voltammetry, respectively. The cell assembly was completed using either a glassy carbon electrode (GC, BAS Technicol, USA, diameter 3 mm) or a boron-doped diamond electrode (BDD, Windsor Scientific, Slough, UK, diameter 3 mm) as the working substrate electrode. All electrolyte solutions were degassed with pure argon (BOC gases, Guildford, UK) for 30 min prior to commencing any voltammetric measurements. High-resolution transmission electron microscopy (HRTEM) was performed using a JEOL 4000EX microscope, and energydispersive X-ray (EDX) analysis using a JEOL 2010 microscope on an Oxford Instruments LZ5 windowless detector controlled by INCA software. The samples were ground and dispersed in ethanol and then placed dropwise onto a holey carbon support grid. Scanning transmission electron microscopy (STEM) was performed in a JEOL JEM-3000FX FEGTEM using annular dark-field (Z-contrast) imaging in STEM mode at 300 kV, giving a STEM resolution of ca. 0.15 nm. The images were oversampled and so were reconvolved using a simple low-pass Butterworth filter to reduce noise. X-ray photoelectron spectroscopy (XPS) was performed using a VG Clam 4 MCD analyzer system at the OCMS Begbroke Science Park, University of Oxford, UK, using X-ray radiation from the Mg KR band (hν ) 1253.6 eV). All XPS experiments were recorded using an analyzer energy of 100 eV for survey scans and 20 eV for detailed scans with a takeoff angle of 90°. The base pressure in the analysis chamber was maintained at not more than 2.0 × 10-9 mbar. Each modified electrode sample was mounted on a stub using double-sided adhesive tape and then placed in the ultrahigh-vacuum analysis chamber of the spectrometer. Analysis and peak fitting of the resulting spectra was performed using the XPSPeak v.4.1 software freely avail-

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Figure 3. (a) A wide survey scan (0-1200 eV) of the XPS spectrum of the CuI@SWCNTs. Inset: The ten cumulative detailed spectra over the I 3d (top left) and the Cu 2p regions (bottom right). (b,c) Left: ten cumulative spectra recorded over the Cu 2p region of CuI@SWCNTs immobilized onto an eppg electrode. Right: the corresponding fitted deconvolution of the Cu 2p3/2 peak before and after poising the electrode potential for 30 s at +1.2 V vs SCE, respectively.

able from the UKSAF Web site,26 using the Shirley baseline background correction method. Assignment of the spectral peaks was made using the UKSAF26 and NIST27 databases. 2.2. Formation of End-Closed Filled and End-Closed Empty SWCNTs. To form the end-closed filled single-walled carbon nanotubes (MX@SWCNTs where MX represents the metal halide filling material), the following procedure was adopted:28–31 under a dinitrogen atmosphere, 50 mg of singlewalled carbon nanotubes (SWCNTs, Thomas Swan Co. Ltd.) were ground with 500 mg of the desired filling material (CuI, NaI, NaCl, or ZnCl2) using an agate pestle and mortar, and then dried under vacuum at 100 °C for 12 h. The mixture was placed in a silica quartz ampule (under dinitrogen) and sealed under vacuum. The ampule was heated at 5 °C min-1 to 900 °C for 12 h (700 °C in the case of ZnCl2), cooled to 50 °C below the melting point of the filling material at 1 °C min-1, and then allowed to cool to room temperature. On cooling, the ends of the tubes become closed.23 The sample was removed from the quartz ampule and washed by sonicating the sample either in a saturated aqueous solution of ammonia in the case of CuI@SWCNTs or in pure deionized water in the case of the

other MX@SWCNTs for 15 min to remove any material on the exterior of the filled nanotubes. Next, the samples were filtered on a polycarbonate membrane (Whatman Cyclopore, 1 µm pore size) and rinsed with deionized water. This procedure was repeated for a total of four washes. Finally, each sample of MX@SWCNTs was subjected to one further rigorous washing procedure by stirring them in the appropriate solution at 60 °C for 48 h, before filtering them on a polycarbonate membrane, rinsing with deionized water, and then drying the samples in air at 60 °C. To form the ultrapure, end-closed unfilled SWCNTs (ECSWCNTs), the following purification and end-closure procedure, developed in the Green group, was adopted: 100 mg of the asmade SWCNTs (Thomas Swan Co. Ltd. UK) were first ground in a pestle and mortar before being steam-treated32 at 900 °C for 1 h to remove the more reactive amorphous carbon33 and graphitic fragment impurities. After this, the sample was allowed to cool to room temperature before the procedure was repeated again. During the steam purification, the graphitic shells coating the catalytic metal particles are removed;34 thus, the sample was next thoroughly washed in 6 M HCl to remove the now-exposed

Electrochemical Opening of SWCNTs

J. Phys. Chem. C, Vol. 112, No. 28, 2008 10393 and BDD electrodes were first polished using successive grades of diamond lapping spray (Kemet, UK) from 6.0 to 0.1 µm particle size. After each successive polishing, the electrodes were sonicated briefly and rinsed in ethanol to remove any adhered microparticles. Finally, a 20 µL aliquot of the appropriate casting suspension was placed onto the electrode surface and the solvent allowedtoevaporateatroomtemperatureleavingtheMX@SWCNTs, EO-SWCNTs, or EC-SWCNTs immobilized onto the electrode surface.

Figure 4. Overlaid cyclic voltammograms of EC-SWCNTs immobilized onto a GC electrode in DMF containing 0.1 m TBAP, scan rate 100 mV s-1. The potential was initially scanned from 0.0 to -1.0 V vs Ag, and then up to increasingly positive potentials between +1.2 and +1.6 V vs Ag.

Figure 5. Overlaid cyclic voltammograms of a NaCl@SWCNTs modified GC electrode in DMF containing 0.1 M TBAP, scan rate 100 mV s-1, scanning to increasingly positive potentials from +1.4 to +2.0 V vs Ag in 100 mV increments (first and second scans overlaid for each incremental increase in the oxidative vertex potential). Inset: Three overlaid cyclic voltammograms of the NaCl@SWCNTs recorded over the region 0.0 to +1.4 V vs Ag, i.e., below the onset of tube opening showing the “bow-tie”-shaped background charging current.

residual metal catalyst particles (introduced during the manufacture of the SWCNTs), before being filtered and dried at 80 °C. Finally, the sample was heated in an induction furnace at 1600 °C for 4 h under vacuum at a pressure no greater than 5 × 10-5 Pa. Upon cooling to room temperature, the tube ends become closed.23 After this, the sample was again washed in 6 M HCl, filtered, and allowed to dry. MX@SWCNT samples were characterized by either HRTEM or STEM (Supporting Information). Although HRTEM is the most commonly used technique for the characterization of filled SWCNTs,30 STEM has also proven useful for the atomic detection of molecules both inside35 and outside36 the SWCNTs. 2.4. Immobilization of the SWCNTs onto the Working Electrode Substrate. To immobilize the MX@SWCNTs, EOSWCNTs, or EC-SWCNTs onto the working electrode, a casting suspension (1 mg/mL) of each type of SWCNT material studied was made up in chloroform with 30 min sonication. The GC

3. Results and Discussion 3.1. Voltammetry of EC-SWCNTs and EO-SWCNTs in Aqueous Electrolyte. First, the available range of the potential window in 0.1 M NaClO4 was determined using cyclic voltammetry at a bare GC electrode and also at a bare BDD electrode. Initially, the potential was cycled from 0.0 to +1.0 V and back to -1.0 V vs SCE before the potential range was extended by 100 mV in both the reductive and oxidative directions. The potential window was determined to be in the ranges of ca. -1.5 to +1.5 V and ca. -3.0 to +3.0 vs SCE for the bare GC and BDD electrodes, respectively. A similar set of potential ranges was observed when the electrolyte was changed to 0.1 M KCl, indicating that the onset of large oxidative or reductive currents used to determine the potential window is simply due to solvent breakdown and is not due to any oxidation or reduction of the electrolyte salt components. The potential window for the EC-SWCNT or EO-SWCNT modified GC or BDD electrodes in either electrolyte studied was consistently found to be ca. -1.25 to +1.2 V vs SCE. The fact that the onset of large oxidative or reductive currents at the SWCNT-modified electrode surface is independent of the underlying electrode substrate is interesting in itself and indicates that either the onset of solvent breakdown occurs more readily on the SWCNTs or that oxidation or reduction of the SWCNTs is occurring, or as we will demonstrate below, it is more likely that both processes are occurring at similar potentials. The voltammetric behavior of a bare GC electrode and a GC electrode modified with either the EC-SWCNTs or the EOSWCNTs was compared using a 1.0 mM solution of the standard redox probe potassium ferrocyanide in 0.1 M KCl, shown in Figure 1a-c, respectively. At the bare GC electrode, an almost reversible couple is observed centered at +0.18 V vs SCE with a peak-to-peak separation of 100 mV at 10 mVs-1, which is a typical voltammetric response of a GC electrode using this redox probe.37 The wave shape and linear variation of the peak current with the square root of the voltage scan rate indicate that the voltammetry is under diffusion control.37 In the case of the EC-SWCNTs, the observed voltammetry is remarkably similar to that of the GC electrode, in terms of the lack of any additional background charging capacitance, usually a typical feature of voltammetry at CNT modified electrodes. The peak current is slightly reduced and the peak-to-peak separation has increased slightly to 186 mV. This tentatively suggests that the EC-SWCNTs are not in themselves electroactive and the effect of immobilizing them onto the GC electrode is to form a partially blocked electrode.37 Note that the film of EC-SWCNTs on the electrode surface is still observable to the naked eye after performing this experiment indicating that the EC-SWCNTs have not simply fallen off the electrode surface. A clear difference between the voltammetry at the ECSWCNTs and the EO-SWCNTs is observed. The EO-SWCNTs exhibit a characteristically large background charging current. The voltammetric wave shape has also altered, with the oxidation wave shape becoming less peak-shaped and more

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Figure 6. Ten overlaid cyclic voltammograms recorded in DMF containing 0.1 M TBAP at 100 mV s-1 scanning initially from 0.0 to -1.0 V vs Ag and then up to +2.0 V beyond the threshold of tube opening for (a) ZnCl2@SWCNT, (b) NaI@SWCNT, and (c) CuI@SWCNT modified GC electrodes.

sigmoidal with increasing scan rate, while on the reverse scan, the reduction wave has become more symmetrical and sharper, characteristic of a surface-bound species (confirmed by the linear variation of reductive peak current with the voltage scan rate).37 The large increase in the capacitive background for the EOSWCNTs clearly indicates that these open-ended tubes are now electroactive. This is particularly interesting given that it suggests that SWCNTs also require open tube ends, with edgeplane defect sites, in order for them to be electroactively similar to their MWCNT counterparts; this question has until now remained largely unclear as discussed in the Introduction. The unusual voltammetric wave shapes of the oxidation wave may be understood by considering the ends of the nanotubes to form a random array of nanoelectrodes with overlapping diffusion domains, the theory of which is described elsewhere.37 The surface-bound nature of the reduction wave can be explained by considering that the oxidation product is the low-spin d5 iron(III) ferricyanide species. This is known to be sufficiently labile to be toxic38 and so may undergo some degree of ligand substitution with surface oxygen groups such as carboxyl or quinonyl structures (which are known to decorate the edgeplane-like defects at the tube ends) on the time scale of the experiment. This odd behavior with potassium ferrocyanide has been observed previously at MWCNT-modified electrodes and on well-prepared edge-plane pyrolytic graphite electrodes (eppg).39–41 As such, some authors are beginning to question whether it truly should be considered as a model outer-sphere redox probe with some evidence of possible inner-sphere

electron transfer mechanisms presented in the literature, particularly on graphitic electrodes.42,43 In order to determine whether the EC-SWCNTs can be electrochemically activated in a similar manner to their MWCNT counterparts by the application of a sufficiently oxidative potential, the EC-SWCNTs were conditioned at +1.5 V for 60 s in 0.1 M KCl. Subsequently, the observed voltammetry of the pretreated EC-SWCNTs in 1.0 mM potassium ferrocyanide (Figure 1d) was very similar to that of the EO-SWCNTs in Figure 1c, suggesting that the tubes had been electrochemically oxidized and opened at the applied potential of +1.5 V. The slight distortion of the oxidative peak in Figure 1c arises due to the background capacitive charging, especially at higher scan rates. Control experiments whereby the EO-SWCNTs and the bare GC electrode were also conditioned at +1.5 V for 60 s exhibited identical voltammetry to that shown in Figure 1a,c, respectively, indicating that no electrochemical oxidation/ activation of these materials occurred. A series of experiments where the conditioning potential was increased from 1.0 to 1.2 Vs were then carried out to ascertain the onset potential at which the EC-SWCNTs open. Conditioning at 1.0 V had no effect on the voltammetry. However, conditioning at 1.1 V had some effect and conditioning at 1.2 V gave indistinguishable voltammetry from that of the EO-SWCNTs. One can infer from this that the onset potential of EC-SWCNT opening in aqueous solution is ca. +1.2 V vs SCE. Conditioning EC-SWCNTs at -1.5 V was also found to result in tube opening.

Electrochemical Opening of SWCNTs The same potential limits for the opening and activation of the EC-SWCNTs was observed on a BDD electrode, again confirming that the underlying substrate has no effect on the electrochemical opening of the EC-SWCNTs. Intuitively, the application of a sufficiently oxidizing potential to the ECSWCNTs is thought to induce the opening of the fullerene-like caps at the tube ends by removing electrons from the bonding HOMO. Perhaps less immediately obvious is the opening of the tubes by the application of a sufficiently reducing potential, which may involve the addition of electrons into the antibonding LUMO again resulting in the opening of the endcaps. 3.2. Voltammetry of CuI@SWCNTs and NaI@SWCNTs in Aqueous Electrolyte. To further demonstrate that the ends of the SWCNTs are opened electrochemically, we employed a method similar to that used previously to demonstrate the chemical opening of EC-SWCNTs, for example, using molten salts or steam treatment.23,32,44 SWCNTs were filled with either CuI or NaI as described in section 2, which results in closure of the tube ends encapsulating the metal iodide salts,23 to form CuI@SWCNTs or NaI@SWCNTs, respectively. The CuI@ SWCNTs or NaI@SWCNTs were separately immobilized onto a BDD electrode and their voltammetric response was explored in either 0.1 M KCl or 0.1 M NaClO4. Figure 2a shows the resulting voltammetry for the NaI@ SWCNTs initially scanned in a positive direction from 0.0 to +1.5 V vs SCE, i.e., beyond the threshold potential at which tube opening occurs at 10 mV s-1. On the first voltammetric cycle, only a very small shoulder on the shoulder of the onset of solvent breakdown could be observed at ca 1.05 V. However, on subsequent cycles a broad, ill-defined oxidative peak could be clearly observed at +1.06 V, which subsequently gradually shifted to +0.98 V on subsequent scans. This peak was only observed after the potential had been scanned to sufficiently positive or negative potentials to induce tube opening as confirmed by scanning initially in either a positive or negative direction, with identical voltammetry observed in either case. Changing the electrolyte from KCl to NaClO4 had no discernible effect on the voltammetric response of these materials. Figure 2b shows the voltammetry when the NaI@SWCNTs were replaced with CuI@SWCNTs on a BDD electrode under the same experimental conditions described above. Again, the broad peak at ca. +0.96 V vs SCE is observed only after cycling the potential beyond the threshold limit where tube opening occurs. On closer inspection, smaller voltammetric features could be observed in the region of ca. 0 to +0.5 V in Figure 2b. An oxidative peak is again observed at +0.92 V after cycling beyond +1.2 V but not before (confirmed by increasing the oxidative vertex potential in 100 mV increments from +1.0 to +1.5 V vs SCE), and a new, quasi-reversible process with oxidative and reductive peaks observed at -0.06 and -0.26 V, respectively, in addition to two irreversible oxidative processes at +0.32 and +0.50 V (inset Figure 2b). None of these peaks is observed unless the potentials are scanned beyond the potential threshold at which tube opening is thought to occur, and none of these peaks is observed in the voltammetry of unfilled EC-SWCNTs under the same experimental conditions, which was performed as a control experiment. The voltammetry of CuI microparticles abrasively immobilized onto an edge-plane pyrolytic graphite electrode was recorded for comparison in 0.1 M NaClO4 at 10 mV s-1 in order to fingerprint the voltammetric features observed in the case of CuI@SWCNTs. This allows us to assign the oxidative peak at ca. +0.96 V observed in both the NaI@SWCNTs and the CuI@SWCNTs as being due to the oxidation of the iodide

J. Phys. Chem. C, Vol. 112, No. 28, 2008 10395 anion. The reduction peak at -0.26 V is attributed to the reduction of CuI to form metallic copper and iodide anions, while the oxidative processes observed at -0.06, +0.32, and +0.50 V likely correspond to the oxidation of copper metal to form copper iodide, copper(II) ions, or copper(I) ions. If the MX@SWCNTs remained closed then no electron transfer can take place through the tube walls to the filling material. In effect, the SWCNTs screen the filling material from any external applied voltage, especially if the tubes themselves are undergoing some degree of quantized charging, rather like a nanoscale Faraday cage.17,18 Furthermore, the MX@SWCNTs were thoroughly washed as described above to remove any material from the external surfaces of the SWCNTs. Therefore, in order to observe any voltammetry corresponding to the filling material the tubes must have undergone electrochemically induced opening. Further confirmation that electron transfer had occurred to the CuI within the CuI@SWCNTs after electrochemical opening was sought using ex situ XPS. A sample of the CuI@SWCNTs was abrasively immobilized onto a specially designed edgeplane pyrolytic graphite (eppg) electrode stub and the potential held at +1.2 V for 30 s in 0.1 M NaClO4. After this, the electrode was removed while still under the applied potential, gently rinsed in deionized water, and allowed to dry before being mounted in the XPS spectrometer. A wide survey scan (Figure 3a) and ten cumulative detailed scans were performed over the C 1s, O 1s, Cu 2p, and I 3d regions of interest on the CuI@SWCNT before (Figure 3b) and after the electrolysis was performed (Figure 3c). After electrolysis, no change in the binding energies of the I 3d5/2 or I 3d3/2 peaks was observed from that in the CuI@SWCNT sample before electrolysis was performed, as the XPS technique is relatively insensitive to changes in the oxidation state of iodine vs iodide.45–47 However, significant changes in the Cu 2p region of the spectra recorded after electrolysis was performed could be observed (Figure 3b,c). After poising the electrode potential at +1.2 V, a shoulder on the Cu 2p3/2 peak could be observed at a binding energy ca. 2 eV higher than the principle Cu 2p3/2 peak at 931.9 eV and a “shake-up” structure could be observed between 940 and 945 eV, neither of these features was observed in the spectra of the CuI@SWCNTs prior to electrolysis. Both of these features are indicative of the presence of copper(II),26,45–47 indicating that some oxidation of the copper(I) to copper(II) has occurred and therefore the nanotubes must have been opened. The Cu 2p3/2 peak was deconvoluted using XPSPeak v 4.1 software26 and is shown in Figure 3c. Unfortunately, due to the X-ray beam diameter being slightly larger than the electrode diameter, a quantitative analysis of the amount of copper(I) oxidized to copper(II) in the CuI@SWCNTs could not be performed, as some spectral intensity likely arises from CuI@SWCNTs immobilized onto the insulating PTFE mantle surrounding the electrode which would not have been subjected to electrolysis. As the XPS technique is unable to distinguish between copper(I) and copper(0),45–47 an ex situ XPS investigation of the reduction of CuI or the opening of the tubes at sufficiently negative potentials could not be explored. 3.3. Voltammetry of MX@SWCNTs and EC-SWCNTs in Nonaqueous Electrolyte. The electrochemical opening of the EC-SWCNTs was also examined in dimethyl formamide (DMF) containing 0.1 M TBAP, partly to examine the influence of the solvent on this opening process, and also to examine the effect of changing the solvation properties of the electrolyte, thus allowing us to be able to observe any quantized charging of the SWCNTs. The use of a silver quasi-reference electrode in

10396 J. Phys. Chem. C, Vol. 112, No. 28, 2008 DMF meant that we also added 5.0 mM of cobaltocenium hexafluorophosphate (CCHFP) to act as a redox marker, and all peak potentials are quoted relative to the midpeak potential of the cobaltocene/cobaltocenium couple. Experiments were also performed in the absence of the cobaltocenium redox marker and confirmed, in all cases, that no voltammetric features were observed in the region that would otherwise be obscured by the cobaltocene/cobaltocenium redox couple. The potential window was found using a bare GC electrode to be between ca. -2 and +2 V vs Ag. However, all data were collected between -1.0 and +2.0 V vs Ag. Figure 4 shows the voltammetric response of the ECSWCNTs modified GC electrode scanning initially from 0.0 to -1.0 V vs Ag and back up to increasingly positive potentials between +1.2 and +1.6 V vs Ag. The CoCp2/CoCp2+ couple can clearly be seen at -0.529 V vs Ag. No distinct voltammetric peaks can be observed, even after cycling to +1.6 V in the ECSWCNT. However, the shape of the cyclic voltammograms recorded at the EC-SWCNT modified GC electrode are slightly different from the standard almost rectangular shape seen in aqueous media. The background charging current is “bow-tie”like, with a much smaller background charging current observed at ca. +0.5 V vs Ag than at any other potential. We attribute the unusual shape of the background charging current to the appearance of quantized charging effects, such that either side of ca. +0.5 V the normal capacitive double-layer charging background current has, super imposed on it, both broad, illdefined oxidative and reductive waves resulting from quantized charging of the EC-SWCNTs themselves.24,25,48 The different solvation of the tetraalkyl ammonium and perchlorate ions in the DMF electrolyte compared to the large, well-hydrated solvation spheres of aqueous electrolytes may explain why quantized charging is observable in DMF but not in aqueous electrolytes.48 Put simply, in DMF the electrolyte ions are less well solvated and better able to shed their solvation spheres allowing a closer approach to the CNT surface and thus better balancing any charge generated on the nanotube. The same experiments were repeated using the NaCl@ SWCNTs in place of the EC-SWCNTs used above, the results of which are presented in Figure 5. Again, the vertex potential was gradually increased by 100 mV increments in the oxidative limit up to +2.0 V vs Ag. If the potential is not cycled beyond ca. 1.6 V vs Ag, then the observed voltammetry is indistinguishable from that of the EC-SWCNTs, having a typical “bowtie” shape due to quantized charging (inset Figure 5). However, once the potential is cycled above this threshold potential a quasi-reversible redox process is observed with oxidative and reductive peaks at +1.288 and +1.142 V vs CoCp2+/CoCp2, respectively. The wave shape of these peaks is suggestive of a surface bound species, and is attributable to the oxidation of the chloride ions49,50 from the NaCl filling, and subsequent reduction of the products indicating that the SWCNTs have been oxidatively opened. Similar behavior was observed with ZnCl2@SWCNTs (Figure 6a), whereby ten scans up to +1.8 V vs Ag revealed that in the first scan no redox peaks attributable to the filling material are observed, but that after cycling beyond the threshold potential for oxidative opening redox peaks are again observed again centered at +1.220 V vs CoCp2+/CoCp2. Upon repetitive cycles, these peaks are found to gradually grow as all the SWCNTs become opened and then stabilize, suggesting that the mass transport of either the filling material or its oxidation products out of the tubes and away from the electrode surface into bulk solution is slow on the experimental time scale.

Holloway et al. Similar behavior was observed for both the NaI@SWCNTs and the CuI@SWCNTs (Figure 6b,c, respectively) except that the redox peaks are shifted to slightly more positive potentials than in the cases where the filling material was a metal chloride, centered at 1.250 V vs CoCp2+/CoCp2, which is in agreement with previous studies of iodide oxidation in DMF (after correction between reference electrodes).49,50 The shift in peak potential between chloride and iodide and the invariance of the halide redox process with the concomitant metal ions in the filling material used (e.g., NaCl vs NaI and NaI vs CuI) lends further support to our assignment that these peaks are due to the oxidation/reduction of these halide ions from the filling materials, indicating that the nanotubes have been oxidatively opened beyond ca. 1.6 V vs Ag. Furthermore, no such voltammetric peaks were observed in the voltammetry recorded using EO-SWCNTs under the same conditions, indicating that these peaks are not attributable to either the open ends of the tubes themselves or any surface functional groups which may decorate them. Experiments were also performed scanning in the negative direction to the onset of solvent breakdown with each of the MX@SWCNTs studied (before and after oxidatively opening the MX@SWCNTs; in these experiments the ferrocene/ferrocenium couple was used so as to avoid the CoCp2+/CoCp2 couple obscuring any voltammetric features), but no new voltammetric features were observed that could be attributed to the reduction of the metal ions in the filling material even after opening of the tubes had occurred. Either the metal ions inside the tubes could not be reduced in the potential range studied or their ion mobility/solvation was much lower than that of the halide ions. The similarity in the threshold potential beyond which the nanotubes are oxidatively opened in both aqueous and nonaqueous solvents, containing different electrolyte salts, indicates that this process may be independent of the solvent used and is not simply caused as a result of reactions with the products of solvent or electrolyte breakdown. 4. Conclusions The voltammetric behavior of EC-SWCNTs, EO-SWCNTs, NaI@SWCNTs, and CuI@SWCNTs in aqueous electrolyte suggests that the SWCNTs, which are initially closed-ended in all cases (not the EO-SWCNTs), become opened by the application of a sufficiently oxidizing potential greater than +1.2 V vs SCE, or a sufficiently reducing potential beyond -1.5 V vs SCE. This was demonstrated by comparing the voltammetric response of the EC-SWCNTs and EO-SWCNTs using ferrocyanide as a standard redox probe, and by the observation of an oxidative peak attributable to the oxidation of iodide ions from the filling materials encapsulated in either the NaI@SWCNTs or CuI@SWCNTs. Further voltammetric peaks corresponding to changes in the oxidation state of the copper ions in the CuI@SWCNTs were also observed after oxidative opening of the tubes. Ex situ XPS experiments on the CuI@SWCNTs confirmed that the Cu(I) ions in the CuI@SWCNTs could be oxidized to form Cu(II) ions upon oxidatively opening the CuI@SWCNTs. Cyclic voltammetry was also performed on the EC-SWCNTs, EO-SWCNTs,NaCl@SWCNTs,NaI@SWCNTs,CuI@SWCNTs, and ZnCl2@SWCNTs in DMF containing 0.1 M TBAP as supporting electrolyte. The EC-SWCNTs and EO-SWCNTs were found to exhibit an unusual background charging current, which was described as being “bow-tie”-like in shape and is attributed to evidence of quantized charging of the SWCNTs.

Electrochemical Opening of SWCNTs This is evident in this electrolyte system due to the weaker solvating properties of DMF compared to water and the more hydrophobic nature of the electrolyte ions allowing a closer approach to the hydrophobic SWCNT tube walls and thus stabilizing any charge developed on the SWCNTs as a result of quantized charging. Evidence of tube opening upon the application of an oxidative potential beyond ca. +1.6 V vs Ag was provided by the observation of a new, quasi-reversible process at centered at +1.220 V vs CoCp2+/CoCp2 in the case of the NaCl@SWCNTs and ZnCl2@SWCNTs, and at +1.250 V vs CoCp2+/CoCp2 in the case of NaI@SWCNTs and CuI@SWCNTs. These voltammetric features are consistent with the oxidation of the halide ions. However, no evidence for the oxidation or reduction of the metal ions used was observed. Therefore, we have provided experimental evidence that, like their MWCNT counterparts, SWCNTs can be also be oxidatively opened by the application of a suitable electrode potential. This has important implications for applications ranging from battery devices and power storage devices (e.g., super capacitors) and sensors utilizing SWCNTs, particularly those made using arc-discharge methods which are normally formed as ECSWCNTs. This also has potential applications in new areas such as drug delivery, where the targeted release of drugs encapsulated within SWCNTs may be controlled potentiostatically. Acknowledgment. The authors are grateful to Thomas Swan Co. Ltd. for SWCNT samples and funding. M.A.H.W. thanks the EPSRC for funding. This work was funded in part by a Marie Curie Intra-European Fellowship within the 6th European Community Framework Program MEIF-CT-2006-024542 (GT). G.G.W. thanks St. John’s College, Oxford, for a Junior Research Fellowship. The authors thank Dr. Hugh Bishop, Oxford University, for his assistance in obtaining and analyzing the XPS spectra. The authors are indebted to Prof. J. R. Dilworth, Jason Holland, and Dr. Chris Blanford for their assistance. Supporting Information Available: HRTEM and STEM characterization of the MX@SWCNTs. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Monthioux, M.; Kuznetsov, V. L. Carbon 2006, 44, 1621. (2) Oberlin, A.; Endo, M. J. Cryst. Growth 1976, 32, 335. (3) Wiles, P. G.; Abrahamson, J. Carbon 1978, 16, 341. (4) Iijima, S. Nature 1991, 354, 56. (5) Iijima, S.; Ichihashi, T. Nature 1993, 363, 603. (6) Kavan, L.; Dunsch, L. ChemPhysChem 2007, 8, 974. (7) Kavan, L.; Rapta, P.; Dunsch, L. Chem. Phys. Lett. 2000, 328, 363. (8) Kavan, L.; Rapta, P.; Dunsch, L.; Bronikowski, M. J.; Willis, P.; Smalley, R. E. J. Phys. Chem. B 2001, 105, 10764. (9) Rao, A. M.; Eklund, P. C.; Bandow, S.; Thess, A.; Smalley, R. E. Nature 1997, 388, 257. (10) Sumanasekara, G. U.; Allen, J. L.; Fang, S. L.; Loper, A. L.; Rao, A. M.; Eklund, P. C. J. Phys. Chem. B 1999, 103, 4294. (11) Kavan, L.; Dunsch, L.; Kataura, H. Carbon 2004, 42, 1011. (12) Tarabek, J.; Kavan, L.; Kalbac, M.; Rapta, P.; Zukalova, M.; Dunsch, L. Carbon 2006, 44, 2147. (13) Kavan, L.; Kalbac, M.; Zukalova, M.; Dunsch, L. Carbon 2005, 44, 99. (14) Kavan, L.; Kalbac, M.; Zukalova, M.; Dunsch, L. J. Phys. Chem. B 2005, 109, 19613.

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