12944
J. Phys. Chem. C 2007, 111, 12944-12953
Functionalizing Single-Walled Carbon Nanotube Networks: Effect on Electrical and Electrochemical Properties Ioana Dumitrescu,† Neil R. Wilson,‡ and Julie V. Macpherson*,† Department of Chemistry, and Department of Physics, UniVersity of Warwick, CoVentry, CV4 7AL, U.K. ReceiVed: NoVember 3, 2006; In Final Form: June 6, 2007
Chemical functionalization is an important aspect of single-walled carbon nanotube (SWNT) research, of interest to many proposed applications of SWNTs, including electrical and electrochemical sensing. In this study, the effects of two common in situ treatments on the electrochemical and solution conductance properties of SWNTs are assessed. The first is acid reflux, used for the purification of SWNTs and a common first step toward chemical functionalization of SWNTs. The second is an air plasma treatment, compatible with microfabrication processing. Rather than studying bulk quantities and using bulk analysis techniques, we investigate two-dimensional networks of individual SWNTs grown on an insulating substrate, enabling the effects of the treatments to be investigated at the level of individual SWNTs, as well as ensemble average behavior. The SWNTs are grown using catalyzed chemical vapor deposition, and electrical, electrochemical, atomic force microscopy, field emission scanning electron microscopy, and micro-Raman analysis are performed before and after applying the treatments. It is found that the major effect of the acid treatment is cutting of the SWNTs followed by gradual etching at the cut ends. Micro-Raman spectroscopy indicates preferential oxidative attack at the metallic SWNTs and minimal damage to the sidewalls. In contrast, plasma treatment does not affect the morphology of the SWNTs. Raman microscopy indicates a dramatic change in SWNT electronic structure, with a possible increase in sp3-hybridized carbon. Both treatments have a negligible effect on the voltammetric response of a simple outer-sphere electron-transfer redox process, Ru(NH3)63+/2+. However, both acid reflux and air plasma treatment enhance the electron-transfer kinetics for the oxidation of inner-sphere dopamine. In both cases this is likely due to the creation of defect sites. A key result of these studies is the strong correlation between increasing functionalization (with a view to increasing chemical sensitivity) and decreasing conductivity, which is an important consideration for electrical and electrochemical applications. It is clear that a balance must be struck between the two to enhance the performance of a SWNT device.
1. Introduction Chemical functionalization of single-walled carbon nanotubes (SWNTs) has developed into an important area of research.1-3 Both covalent and noncovalent chemical functionalization strategies have been employed, for example, in the solubilization of SWNTs,1,4 dispersal of SWNTs in polymers, separation of metallic and semiconducting SWNTs, and increasing the selectivity and sensitivity of SWNT-based sensors, among many others. Even the simple process of purifying SWNTs results in chemical functionalization.5 With many of the popular approaches to growing SWNTs, e.g., HiPCO, laser ablation, and arc discharge,6 purification is an important first step for removing amorphous carbon and catalytic nanoparticles, prior to use. One of the most popular purification treatments is the use of strong acid.7-9 Acid treatment is thought to result in SWNT cutting, the opening of SWNT ends, and oxidative damage of the sidewall,7 resulting in the introduction of electronic scattering centers. Tube opening and sidewall damage result in the production of defects in the SWNT sp2 structure which are functionalized with oxygencontaining groups such as carboxylic acids (-COOH), ketones, and alcohols.10-13 These groups, especially -COOH, can also * Corresponding author. E-mail:
[email protected]. † Department of Chemistry. ‡ Department of Physics.
serve as useful anchors for further chemical functionalization; hence, acid treatment is a crucial step in SWNT processing. The covalent addition of molecules directly to the sidewall also leads to the introduction of electronic scattering centers due to a change in hybridization of the carbon atom from sp2 to sp3.14 This process causes drastic changes in the electronic states near the Fermi level and results in an increase in resistivity of the SWNT.15 Catalyzed chemical vapor deposition (cCVD) can produce very high-quality SWNTs, i.e., free from amorphous carbon, with a very low proportion of catalyst and with very few defects present (as low as ∼1 µm-1 of SWNT).16 The growth defects present on these “pristine” SWNTs are likely to be found on the sidewall and include stone-walls defects, sp3-hybridized sites, and lattice vacancies.5 Two-dimensional (2D) networks of pristine SWNTs, grown by cCVD onto insulating surfaces, are proving very useful for electrical and electrochemical applications, such as transparent conductors, sensors, field effect transistors, and electrodes for electrochemistry.17-21 The networks form once the density of SWNTs exceeds a critical value, the percolation threshold, so that percolating pathways of SWNTs extend over macroscopic distances. Due to their high aspect ratio, SWNTs can form interlinking 2D networks on surfaces even at low surface coverages (∼1%). The networks
10.1021/jp067256x CCC: $37.00 © 2007 American Chemical Society Published on Web 08/15/2007
Functionalizing SWNT Networks are composed of individual SWNTs connected only to other SWNTs at a few points. For a random distribution of SWNTs, both metallic (mSWNTs) and semiconducting (sSWNTs) nanotubes are present;22 thus, increasing the density also changes the nature of the electrical response. For the network to form at all, its density must be above the percolation threshold. However, if the density is low enough that the mSWNTs do not form a continuous network, i.e., below the mSWNT percolation threshold, current must flow through sSWNTs and the overall network response is semiconducting. If the network density is above the metallic percolation threshold, the SWNT network will be semimetallic in nature.23 A metallic film is preferred for transparent conductors and electrochemical applications,20 whereas a semiconducting film is often more suited to field effect transistor and sensing applications.21 For most sensing applications employing “semiconducting” SWNT networks, interactions between the analyte of interest, be it gases, chemical vapors,24,25 or biomolecules,17,26 and the SWNT modulate the conductance properties of the network. Detection sensitivity can be increased by coating with chemoselective materials27 or through the addition of sidewall defects. Defects are considered to act as preferential molecular adsorption sites.24,28 Dynamic electrochemistry represents an alternative detection strategy for SWNT-based sensing, provided the analyte of interest is electroactive.19,20 There is some evidence that the presence of carboxylic acid groups on the SWNT can increase the electrocatalytic activity or electrochemical reversibility of the SWNT-based electrode toward specific analytes.29,30 However, in these studies the SWNTs were not electrochemically isolated from the electroactive contact. In this paper, we investigate the effects of two differing chemical functionalization procedures on the electrical and electrochemical performance of high-density SWNT networks. The first is an acid treatment (common first step in the purification of non-cCVD-grown SWNTs) and the second is air plasma oxidation; both have previously been shown to result in the introduction of oxygen functionalities into SWNTs.13,31 However, the mechanism of functionalization, and hence the effect of functionalization, is different. The use of SWNT networks grown directly by cCVD onto an insulating surface also enables the effect of the treatment to be studied both at the individual SWNT level, through atomic force microscopy (AFM) and field emission scanning electron microscopy (FE-SEM), and at the ensemble averaged level, via electrical and electrochemical measurements. Moreover, each SWNT on the surface is equally exposed to the treatment, which contrasts with conventional procedures where typically large bundles of SWNTs are present and the inner SWNTs are protected from the treatment by the outer ones. 2. Experimental Section 2.1. Preparation of SWNT Networks. SWNTs were grown on Si/SiO2 substrates (300 nm thermally grown SiO2) using cCVD under a H2/CH4 atmosphere and employing Fe nanoparticles as the catalyst.23 Fe was deposited either by sputtering, using a sputter coater (SC7640, Quorum Technologies; 11 mA working current, 10 s sputtering time) equipped with a homemade Fe target, or by soaking the substrates for 1 h in a 1:200 diluted ferritin solution (Sigma-Aldrich). The ferritin protein shells were removed by a 1 min exposure to a 100 W oxygen plasma (Emitech K1050X plasma asher). SWNT densities (F), defined as total length of SWNT/µm2 substrate, were calculated from either high-contrast FE-SEM or AFM
J. Phys. Chem. C, Vol. 111, No. 35, 2007 12945 images.20 All work was carried out using highly connected networks, above the metallic percolation threshold, with densities ca. 3.5 ( 1 µm SWNT/µm2 substrate. SWNT diameters were determined using AFM and found to be in the range of 1-3 nm. The shape of the G-band and the absence of the D-band in the Raman spectra confirmed the presence of highpurity SWNTs.32 2.2. Treatment of SWNT Networks. 2.2.1. Reflux in 3 M HNO3. SWNT sample plates were placed at the bottom of a flask (with an attached condenser) which contained 100 mL of ∼3 M HNO3, prepared by a 5-fold dilution of concentrated HNO3 (70%, Fischer Scientific). The system was refluxed at 110 °C for various times. This acid treatment is milder than that routinely employed for the purification of SWNTs grown by other techniques7 but has been shown to oxidatively attack SWNTs.33 We deliberately chose this milder treatment to enable us to investigate the time-dependent effects of acid attack on the SWNTs on a reasonable time scale and to avoid any damage to the insulating silicon oxide film. 2.2.2. Air Plasma Treatment. A 13.56 MHz rf inductively coupled plasma source (Emitech K-1050X) was used with an operating air pressure of 0.6 mbar which corresponded to a constant air flow rate of 10 sccm. 2.3. SWNT Network Characterization. The morphology of the SWNT networks (and electrodeposited silver) was examined using FE-SEM (Zeiss SUPRA 55 VP FE-SEM). FE-SEM allowed characterization of the SWNT network over large areas. Image contrast is due to the local potential differences between the SWNTs and the insulating SiO2 substrate caused by differential charging.34 SWNTs connected to one another in the network show significantly greater contrast than isolated ones, which are barely visible. FE-SEM images are thus very useful for obtaining SWNT network densities, as described elsewhere.20 For all figures, the color of the FE-SEM images has been reversed using image analysis software, to allow easier visualization of the SWNT network and silver nanoparticles (vide infra). AFM analysis was performed using either a Veeco EnviroScope AFM with Nanoscope IV controller or a Veeco Multimode AFM with Nanoscope IIIa controller. Scanned conductance microscopy (SCM), a form of electric force microscopy (EFM), was used to probe the conductance of the SWNTs.35 SCM requires a substrate which has an insulating, dielectric surface and an underlying conductive bulk, such as SiO2 on Si, as used here. If a conducting object lies on such a substrate, it will be capacitatively coupled to both the conductive bulk and to the AFM tip placed above it. If the substrate is grounded and a voltage is applied to the AFM tip, an electric field is created between the tip and object, which can be detected as in conventional EFM. Hence, SCM allows the conductivity of the nanotubes to be probed without the need for contact electrodes. SCM imaging was performed in “lift-mode” with the tip set to scan each line in tapping mode and then to retrace at a set distance of 30 nm, with a bias voltage of 12 V applied to the tip. Raman spectra of SWNT networks were recorded using a Renishaw inVia Raman microscope with incorporated Leica microscope and CCD detector. SWNT peaks were calibrated against a Si peak at 521 cm-1. A 514.5 nm (2.41 eV) excitation wavelength of an Ar laser, at 10 mW power, focused in a ∼2.5 µm spot, was used in the experiments. 2.4. Device Fabrication. Three types of electrochemical device were used to probe the electrochemical response and conductance properties in solution (i.e., wet gate) of SWNT networks. The first two are described in detail elsewhere.19,36
12946 J. Phys. Chem. C, Vol. 111, No. 35, 2007
Figure 1. (a) Schematic and (b) FE-SEM image of the type 3 experimental droplet cell setup used for the wet gate measurements. Scale bar for (b) is 10 µm. Gold electrodes are separated by a 90 µm by 1 mm SWNT network area. A photoresist layer protects the gold contacts, and a 325 µm by 60 µm area of SWNT network is exposed to solution. Source-drain current (Isd) is recorded as function of potential at the Ag/AgCl RE (Vwg) immersed in the solution drop. A constant bias of 25 mV (Vsd) is applied between source and drain.
Briefly, type 1, used for SWNT network templated silver deposition, comprised a gold contact (100 nm Au with 10 nm Cr adhesive layer) thermally evaporated across one edge of the SWNT network substrate. The gold contact was clipped to the working electrode terminal of a potentiostat, in a three-electrode setup, for chronoamperometry. Type 2, used for cyclic voltammetry (CV) measurements, was fabricated by thermally evaporating gold (10 nm Cr, 100 nm Au) through a shadow mask to form band electrodes, separated by an area of 90 µm by 1 mm. A 1.5 µm thick photoresist layer (Microposit S1818, Rohm and Haas Electronic Materials) was subsequently spun over the sample. An area 400 µm by 20 µm of SWNT network in between the gold band contact electrodes was then exposed using the 458 nm He/Ne laser wavelength of a confocal microscope (Zeiss LSM 510) and removed by MF319 developer (Shipley Microposit). For comparison, a gold band electrode was also fabricated in the same way. However, here the photoresist was exposed on one of the contact electrodes rather than in the area between them. For the type 3 devices (shown in Figure 1) used for wet gate measurements, the contact electrodes were deposited in the same way as for the type 2 device. T35ES image reversal photoresist (Microchemical GMBH) was spin-coated onto the device and photolithographically patterned to protect a defined region of the nanotube network, 350 µm wide, in the gap between the two electrodes. A harsh oxygen plasma treatment (100 W for 1 min) was used to remove the unprotected SWNT network. The remaining T35ES photoresist was removed with acetone. A further photolithography step (S1818, with an underlayer of Omnicoat) was then employed to insulate the contact electrodes, with a section of the gap (60 µm by 325 µm) between the electrodes exposed. 2.5. Solutions. All chemicals were used as received, without any further purification. Aqueous solutions were prepared using Milli-Q reagent water (Millipore Corp.). Silver deposition solutions consisted of 1 mM AgNO3 (99+%, BDH AnalR) in 0.2 M KNO3 (Analytical Grade, Fisher Scientific) supporting electrolyte. Solutions for CV and wet gate measurements consisted of 1 mM hexaamineruthenium(III) chloride (Ru(NH3)63+; 99%, Strem Chemicals) in 0.1 M KNO3 and 1 mM dopamine (Sigma Chemicals) in 0.1 M citric acid (99.5%, Aldrich Chemicals), pH 7.2 phosphate buffer aqueous solution (Sigma-Aldrich). 2.6. Procedures. Chronoamperometric measurements were carried out using a commercial potentiostat (CH750A electro-
Dumitrescu et al. chemical workstation, CH Instruments). Type 1 device was electrodeposited with silver by stepping the applied potential from 0.0 to -0.4 V (well into the wave for Ag+ reduction) versus a Ag/AgCl (saturated AgCl) reference electrode (RE), for a time period of 30 s and then back to open circuit. The device was rinsed with ultrapure water before imaging with FE-SEM. CV and wet gate experiments employed a two-electrode droplet cell setup.20 In both cases, electrical connection to the gold electrodes was made with a sharp tip probe (xyz 300TR Quarter Research). A drop of electrolyte solution (∼50 µL) containing the species of interest was placed over the exposed SWNT area, and the Ag/AgCl RE was positioned inside the drop. Current-voltage curves were recorded using a DAQ card (DT9800, Data Translations) for both analog output and input, controlled by purpose-written LabVIEW software. For wet gate measurements, a constant bias of 25 mV was applied at the source electrode (Vsd) while sweeping the gate potential (Vwg) applied to the RE, in the range of -0.7 to 0.7 V. Source-drain current (Isd) as function of Vwg was converted to a voltage by an Ithaco 1211 virtual earth current preamplifier connected to the drain electrode and recorded using the DAQ card. 3. Results and Discussion 3.1. Acid Reflux Treatment. Figure 2 shows 25 µm × 25 µm FE-SEM images of the same SWNT network sample after (Figure 2a) growth, and after (Figure 2b) 2 h, (Figure 2c) 6 h, and (Figure 2d) 14 h of acid reflux treatment. Here, the dark lines are the SWNTs, and the white background is the insulating SiO2 surface. After growth the SWNTs form a multiply interconnected, random 2D network on the SiO2. After (Figure 2b) 2 h of reflux, the density appears decreased and the SWNTs shorter. This trend is more pronounced after (Figure 2c) 6 h and (Figure 2d) 14 h. For 2 h reflux times the resulting density was close to the percolation threshold; thus, small variations across the sample can result in areas where the network breaks down (see Supporting Information 1). The image in Figure 2b is taken in an area where the network is continuous. The FE-SEM images in Figure 2 were analyzed to extract the SWNT network density, F, and the average length of the SWNTs, L. Post growth the density was measured to be 3.4 µmSWNT/µm2; with the high interconnectivity it is difficult to measure the lengths of individual SWNTs. However, for samples grown under similar cCVD conditions and having lower densities, lengths of SWNTs were found to vary in the range of 1-100 µm, averaging at 5-10 µm.36 A typical value of L ) 7 µm is used here for the pristine sample. Lengths of SWNTs post reflux could readily be measured from the FE-SEM images. Knowing F, an average value for L and the average diameter, d, of SWNTs, the following parameters were estimated: percentage surface coverage, %S ) (Fd) × 100; average separation, s ) 1/F; the average number of intersections per SWNT, i ) L/s. The densities, average lengths, and computed values, using d ) 2 nm, are given in Table 1. As can be seen from Figure 2 and Table 1, as the acid reflux treatment time is increased, the average density decreases from 3.4 to 2 µmSWNT/µm2, and the average length decreases from ∼7 to 1.2 µm. The behavior was reproducible. Only SWNTs connected to one another give good contrast in the FE-SEM images; thus, it is the density of the connected network that decreases with reflux time. AFM images of a pristine sample and 14 h acid-refluxed sample are shown in Figure 3, parts a and b, respectively. AFM of the pristine sample shows a well-connected network of
Functionalizing SWNT Networks
J. Phys. Chem. C, Vol. 111, No. 35, 2007 12947
Figure 2. FE-SEM images of a SWNT network (a) pre and post (b) 2 h, (c) 6 h and, (d) 14 h of 3 M HNO3 reflux treatment. Scale bar is 10 µm.
TABLE 1: Measured Values of the Network Density and Average Length of SWNT for Pristine and Acid-Treated Samples a sample
F/µm-1
L/µm
%S
s/µm
i
Fth
pristine 2 h refluxed 6 h refluxed 14 h refluxed
3.4 3 2.4 2
7 3 1.5 1.2
0.68 0.60 0.48 0.40
0.29 0.33 0.42 0.50
23.8 9 3.6 2.4
0.8 1.9 3.8 4.8
a The percent surface coverage, average separation, and number of SWNT-SWNT intersections are estimated from the measured density. The threshold density, Fth, for the formation of a percolating network of nanotubes, given the measured average length, is given in the final column as predicted by eq 1.
SWNTs lying flat on the SiO2 substrate. After acid treatment the AFM topography shows SWNTs of much shorter length, clear evidence that the SWNTs have been cut. The magnified region of Figure 3b shows what appears to be the remaining fragments of what was once a single SWNT, suggesting that the SWNT was first cut and then the cut ends gradually etched away. The many unconnected fragments of SWNTs indicate that the total mass of SWNT present did not decrease as significantly as indicated by the decrease in density calculated from the FE-SEM images. It is also important to note that not all SWNTs are shortened to the same extent; a few long (∼5-7 µm) nanotubes are evident in the FE-SEM images even after 14 h of reflux. AFM analysis showed that these longer nanotubes had larger diameters (typically 5-8 nm), possibly indicating that they were either small multiwalled carbon nanotubes (MWNTs) or bundles of several SWNTs. Figure 4 shows Raman spectra obtained on the same sample as in Figure 2, after growth, and 2, 6, and 14 h of acid reflux treatment. The spectra cover the 1300-1700 cm-1 region where both the tangential modes derived from the in-plane Raman vibrations in graphite (G-band, 1500-1600 cm-1) and the disorder modes (D-band in the region 1300-1400 cm-1) are seen.32 Raman spectra are normalized with respect to the Si peak at 950 cm-1. Given that only about 10% of the SWNTs on the sample are resonant with the excitation laser, and with a laser spot size of 2.5 µm diameter, each spectra recorded corresponded to the signal from only a few SWNTs.32 All the spectra presented here were averaged over 10 points on the sample. With the 2.41 eV laser excitation energy used, lowfrequency vibrations (radial breathing mode) were rarely present due to the resonance conditions32 and when averaged over the 10 points were barely discernible from the background. However the G- and D-bands were observable and give useful information about the sample. In particular, the shape and position of the G-band positively identifies SWNTs and the barely discernible D-band indicates they are of high quality.32 Note that the intensity of the G-band peak decreases only by 9% even after 14 h of acid reflux, indicating only a marginal decrease in the total mass of SWNTs present. (Only SWNTs of certain diameter, in resonance with the 2.41 eV laser excitation energy, appear in the Raman spectrum.) For all spectra the intensity of the D-band is virtually unchanged and
