J. Phys. Chem. C 2008, 112, 14131–14138
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Effect of Dialysis on the Electrochemical Properties of Acid-Oxidized Single-Walled Carbon Nanotubes Alison Chou, Till Bo¨cking, Rongmei Liu, Nagindar K. Singh,† Grainne Moran, and J. Justin Gooding* School of Chemistry, The UniVersity of New South Wales, Sydney, New South Wales 2052, Australia ReceiVed: December 2, 2007; ReVised Manuscript ReceiVed: May 31, 2008
The impact of residual acid moieties adsorbed on single-walled carbon nanotubes (SWNTs), as a result of the purification and shortening of the SWNTs in a concentrated nitric and sulfuric acid mixture (1:3 ratio, v/v), on the electrochemical properties of carbon-nanotube-modified electrodes was assessed. Two different nanotube samples were prepared: acid-cut SWNTs that were rinsed with water until the washings had a neutral pH to remove excess acid (nondialyzed SWNTs) and acid-cut SWNTs that were dialyzed against a solution of Triton X-100 for a period of 18 h (dialyzed SWNTs). X-ray photoelectron spectroscopic analysis of dialyzed SWNTs showed a reduced concentration of sulfur-containing ions and nitrogen-containing species when compared with that of the nondialyzed SWNTs. These ions can be ascribed to residual acid moieties adsorbed onto the nanotube surfaces during the purification process using acid mixtures. The impact of these residual acidic species on the conductivities of the carbon nanotubes was assessed via electrochemical measurements using self-assembled ferrocene-functionalized nanotube monolayers on a gold electrode. Our measurements showed that the nondialyzed nanotubes exhibited a slower rate of electron transfer compared with the dialyzed nanotubes. We postulate that the adsorbed nitrogen-containing and, to a lesser extent, the sulfur-containing species in the nondialyzed samples become desorbed by the dialysis process, and the vacant sites thus created are subsequently replaced by more electron-withdrawing species such as the perchlorate used as the counterion in the electrochemical experiments. Hence, the concentration of holes formed by charge transfer from the π-electrons to the adsorbed species will be higher, and an electrode constructed with dialyzed carbon nanotubes will exhibit a higher conductivity than that constructed with nondialyzed nanotubes. 1. Introduction The purification of carbon nanotubes in acid mixtures is one of the most commonly used methods for removing amorphous carbon, catalyst particles, and other impurities1,2 from assupplied samples of single-walled carbon nanotubes (SWNTs). An additional effect of the purification process is that the carbon nanotubes are also reduced in length,3 and this process is generally referred to as “cutting” of the nanotubes. Associated with this reduction in length is the breaking of carbon-carbon bonds via oxidation that introduces carboxylic acid groups at the ends and at defect sites on the sidewalls of the nanotubes.4,5 These carboxylic acid groups are useful moieties for further functionalization of the nanotubes by the formation of amide bonds between the carboxylic acids and amine groups, abundant in biological molecules such as proteins.6,7 Thus, this cutting method is usually adopted for devising carbon nanotube electrochemical biosensors. The acid-cutting procedure involves sonicating the nanotubes in a mixture of concentrated nitric and sulfuric acids. Previous studies have shown that the length of the nanotube decreases with increasing sonication time.3,5 Typically, a 4 h sonication generates nanotube fragments with a mean length of about 500 nm.5 Filtration and rinsing with water is usually the only method employed to discard excess acid. The removal of excess acid is verified by testing the pH of the filtrate in the washing step. * To whom correspondence should be addressed. E-mail: Justin.gooding@ unsw.edu.au. † Current address: Department of Chemistry, Materials and Forensics, University of Technology Sydney, Broadway, NSW 2007, Australia.
Such a strategy does not determine whether there are any acids or ionic species adsorbed on the nanotube surfaces. In the case of SWNTs, the presence of adsorbed ionic species is particularly important as it has been shown that acid purification selectively removes small-diameter and metallic SWNTs over semiconducting tubes,8 thus increasing the ratio of semiconducting SWNTs to metallic SWNTs from a ratio of 3:1 obtained during the SWNT synthesis. A consequence of concentrating the semiconducting phase in the acid-purified mixture is that the conductivity of this ensemble will become more sensitive to doping effects from species adsorbed onto the nanotube surface during the purification process. Semiconducting SWNTs are usually found to be p-doped9-11 in both vacuum and aqueous solution. There is substantial evidence that doping with both electron donors and electron acceptors can have a profound impact on the conductivity of SWNTs.12-14 Doping semiconducting nanotube ropes with bromine, for example, increases the conductivity by increasing the number of hole carriers, while atomic potassium, an electron donor,15 significantly increases the conductivity of the nanotube by changing the carriers from holes to electrons.16 Large conductivity changes of SWNTs induced by charge transfer between adsorbed gas molecules such as NO2, NH3,17,18 and O218 adsorbed onto nanotube surfaces have also been observed. In general, doping causes charge transfer from, or to, the nanotube and hence creates electrons and holes as charge carriers.19 The sensitivity of nanotube conductivity to doping effects is sufficiently high such that carbon nanotube sensors
10.1021/jp7113785 CCC: $40.75 2008 American Chemical Society Published on Web 08/15/2008
14132 J. Phys. Chem. C, Vol. 112, No. 36, 2008 have been devised for gas-phase monitoring of species17 and for monitoring a single molecule reacting with a nanotube.20 These studies have prompted us to investigate the effect (if any) that residual products of the acid purification of carbon nanotubes may have on the conductivity of SWNTs used for electrode modification and hence the electrochemical properties of these electrodes. The purpose of the present study was to explore this issue by attempting to remove any residual acids from the acid-purified carbon nanotubes via dialysis.21 Nanotubes that were oxidized in a 1:3 ratio (v/v) of concentrated acids HNO3 and H2SO4 were treated for 18 h with dialysis in 0.1 wt % Triton X-100. The nanotubes, before and after dialysis, were characterized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Changes in the electrochemistry of modified SWNTs were evaluated using such dialyzed tubes and compared with those of SWNTs that were not dialyzed, using ferrocenemethylamine covalently attached to the ends of SWNTs aligned by selfassembly. This strategy was chosen as previously we have shown that electrodes fabricated in this way exhibit a length dependence on the rate of electron transfer between the underlying electrode and the ferrocene.22 2. Experimental Section 2.1. Chemicals and Reagents. Mercaptoethylamine (MEA), and 1,3-dicyclohexylcarbodiimide (DCC) were purchased from Sigma-Aldrich Chemical Co. (Sydney, Australia), and nitric acid, sulfuric acid, and perchloric acid were from Ajax Chemical (Sydney, Australia). SWNTs were HiPco tubes from Carbon Nanotechnologies Inc. Ferrocenemethylamine was synthesized according to the procedure of Kraatz et al.23 All aqueous solutions were prepared with Milli-Q water. 2.2. Nanotube Shortening Procedure. The SWNTs were shortened or cut according to the procedure developed by Liu et al.3 A 2 mg sample of HiPco SWNT was added to 10 mL of an acid mixture of 3:1 concentrated sulfuric acid and concentrated nitric acid. This mixture was sonicated in a 55 Hz sonicator bath for 2, 4, or 6 h. The cut tubes were collected by vacuum filtration on a 0.22 µm nitrocellulose membrane (Millipore, Sydney, Australia) and washed with water until the pH of the filtrate tested neutral before dispersion in 10 mL of ethanol. 2.3. Dialysis of the Nanotubes. Dialysis was carried out following a literature procedure by Willner et al.21 SWNTs of 2, 4, and 6 h cutting time were dispersed in Milli-Q water and dialyzed against a 0.1% Triton X-100 solution for a period of 24 h using Membra-cell MD25-14 dialysis tubing with a 14 000 molecular weight cutoff. The dialyzed tubes were collected by filtration and then rinsed with Milli-Q water before redispersion in ethanol. TEM imaging and XPS surface analysis were performed on the dialyzed tubes. 2.4. Modification of Gold Electrodes. Gold electrodes were polished using 1.0, 0.3, and 0.05 µm Buehler micropolish alumina slurries on a Buehler polishing microcloth (Buehler, Ltd.) until a mirror finish was achieved followed by sonication in a 55 Hz sonicator for 10 min. The electrodes were cleaned by cycling in 0.05 M H2SO4 between -0.3 and +1.5 V versus Ag/AgCl (3 M NaCl) at 150 mV s-1. From the area under the peak due to the reduction of gold oxide in the cleaning cyclic voltammogram, the charge passed was determined, and the surface area of a gold electrode was calculated using the conversion factor of 482 µC cm-2.24 The electrode area determined in this way was typically 0.020 ( 0.002 cm-2, which equates to a surface roughness of approximately 2.5. The clean
Chou et al. gold electrodes were immersed in 10 mM MEA solution in water for 3 h at room temperature followed by rinsing with Milli-Q water and finally drying with nitrogen. To produce an electrode modified with SWNTs aligned normal to the electrode surface, an MEA/Au electrode was placed in a suspension of 0.2 mg of shortened SWNTs in 1 mL of ethanol solution containing 0.5 mg of DCC for 8 h. This modified electrode is denoted as SWNT-VA/MEA/Au. Ferrocenemethylamine was then attached to the modified electrode by immersing DCCactivated MPA/Au or SWNT-VA/MEA/Au in a 10 mM ferrocenemethylamine ethanolic solution. The electrode was incubated in the ferrocenemethylamine solution for 8 h. 2.5. TEM Measurements. Specimens for TEM were prepared by dispersing pristine and cut SWNTs in absolute ethanol with a 10 min sonication prior to placing a drop of the suspension on the standard TEM sample copper grid coated with a carbon film. Images were recorded by a Philips CM 200 field emission transmission electron microscope with a resolution of 0.24 nm. A large number (>100) of lengths of the shortened tubes were measured directly from the images obtained and the distributions fitted using Matlab software (Matlab, v6.0, The Mathworks Inc.). 2.6. XPS Measurements. X-ray photoelectron spectra were collected on a VG ESCALAB 220-iXL spectrometer with a monochromated Al KR source (1486.6 eV). The spectra were accumulated at a takeoff angle of 90°, and an analyzer pass energy of 100 eV was used for the survey scans, while a pass energy of 20 eV was used for the high-resolution spectra acquired over the C 1s, N 1s, O 1s, and S 2p regions. The spectra were analyzed using XPSPEAK41 software. 2.7. Raman Spectroscopy. Samples for Raman spectroscopy were prepared as described in section 2.2, with 6 h of sonication. Washed tubes were dried in an oven overnight at 60 °C and then further heat-treated in a vacuum oven at 320 °C for 5 h to remove adsorbed species. Raman spectra were measured using a Renishaw 2000 Raman microscope with CCD detection at 4 cm-1 resolution. A 50× objective lens was used, and excitation was done using an argon ion laser at 514.5 nm. The Si 521 cm-1 peak was used to check the calibration of the instrument prior to data acquisition. 2.8. Cyclic Voltammetry. Voltammetric measurements were conducted using a BAS 100B potentiostat. The auxiliary electrode was a homemade 1 × 1 cm2 platinum flag electrode, and the working electrodes were constructed using polycrystalline gold wire with a diameter of about 1 mm as described previously.25 All potentials were recorded relative to a Ag/AgCl reference electrode (BAS, Lafayette Inc.). Electrochemical measurements were performed at room temperature in 0.1 M HClO4. Cyclic voltammograms over a range of scan rates were analyzed, and determined ∆Ep values smaller than 200 mV were used as a requirement to extract the apparent electron transfer rate constants using the Laviron method.26 3. Results 3.1. TEM Images of the Dialyzed SWNTs. TEM images of the shortened nanotubes are shown in Figure 1 for the dialyzed samples purified with 2, 4, and 6 h of sonication in the acid mixture. These images were used to determine the lengths of the nanotubes. The average nanotube lengths are given in Table 1 and are consistent with those of nondialyzed acidtreated SWNTs examined by TEM in our previous study.22 3.2. XPS Analysis of the Dialysed 6 h Cut SWNTs. The XP spectra acquired over the C 1s region (Figures S1 and S2 in the Supporting Information) for the dialyzed and nondialyzed 6 h sonicated nanotubes were similar and consistent with
Effect of Dialysis on Acid-Oxidized SWNTs
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Figure 1. TEM images of (a) 2, (b) 4, and (c) 6 h cut SWNTs after 18 h of dialysis in 1 wt % Triton X-100 solution.
TABLE 1: Average Length of SWNTs after Different Cutting Times before and after Dialysis of the Tubesa length/nm before dialysis after dialysis
2h
4h
6h
1175 (633, 411) 1140 (905, 731)
507 (291, 185) 470 (392, 295)
257 (137, 89) 290 (406, 146)
a Numbers in parentheses are the positive and negative standard deviations of the log-normal distribution.
observations of other researchers4,5 and our own observations.5 Both samples show a strong signal for the graphitic C 1s peak at 284.7 eV. Associated with this main feature were less intense chemically shifted peaks extending to ∼289 eV binding energies. These chemically shifted peaks, in conjunction with infrared data, confirmed the presence of carboxylic acid, phenol, and quinone functionalities on the purified nanotubes.27 In both dialyzed and nondialyzed samples the O 1s spectra (Figure S3 in the Supporting Information) showed a dominant peak with the peak maximum at 532.5 eV binding energy which, when deconvoluted, yielded two peaks at 532.0 and 533.2 eV with a 1.6:1 intensity ratio, corresponding to oxygen atoms in carboxylic acid and quinone/phenol groups, respectively. On the higher binding energy side of the main O 1s peak a small and broad peak was discernible in the nondialyzed tubes, which could be fitted to two peaks at 536.0 and 537.7 eV consistent with oxygen atoms attached to sulfur and nitrogen species, respectively. This broad low-intensity peak could not be detected in the dialyzed sample (see below). Significant changes were observed in the N 1s and S 2p peaks after dialysis, which can be easily discerned in the spectra presented in Figure 2. The N 1s region before dialysis contained two prominent peaks at ∼400 and ∼402 eV and a broad peak centered at ∼407 eV. Upon dialysis, the peak at 407 eV could no longer be detected. The fact that this species could be removed from the nanotubes implies that it is an ionic species and is weakly adsorbed most likely at defect sites on the nanotube surface. The peak at 407 eV is assigned to the nitrate ion (NO3-), which remains in the nanotube sample after oxidation in the HNO3/H2SO4 acid mixture. Figure 2 also shows that the intensity of the 401.5 eV peak is greatly reduced following dialysis, being less than 20% of the initial intensity in the nondialyzed sample. Assignment of the nitrogen species giving rise to peaks at ∼400 and ∼402 eV is not straightforward. Peaks in this energy range are usually associated with C-N and N-O bonds. C-N bonds in carbon nanotubes themselves can exhibit two peaks at 398.5 and 400.5 eV, attributable to 3-fold-coordinated nitrogen atoms attached to sp3 carbons and nitrogen atoms substituting sp2 carbons in the graphene struc-
ture.28 This type of incorporation of the nitrogen atoms most likely occurs during the acid-cutting procedure and during the synthesis process. These covalently bound atoms cannot be removed by dialysis, and hence, the observation that the peak at ∼400 eV remains relatively unaffected by dialysis is consistent with its assignment to these species. The binding energy value of 401.5 eV suggests that this species is a nitrogencontaining species with one associated oxygen atom, and we believe it is the NO species. Our justification for this assignment is as follows. Previous studies of nitrogen dioxide29 and nitronium (NO2+) ion30 adsorption on carbon nanotubes have shown that these species can adsorb easily by interactions with the surface π-electrons of the carbon nanotubes and generate adsorbed (NO)n+ species by further reactions at defect sites, resulting in a photoemission feature at 401.5 eV. It is known that nitronium ions (NO2+) generated in the nitric and sulfuric acid mixture3 can react readily with carbon nanotubes during the purification procedure and subsequently liberate adsorbed (NO)n+ species. While XPS is not able to confirm which (NO)n+ ion is formed, we observe that, on dialysis, there is a reduction in the 401.5 eV peak by 80%, suggesting that the majority of the species giving rise to this peak are either ionic (e.g., the NO+ species) and/or weakly adsorbing NO species. However, since NO2+ is a strong oxidizing agent and its salts are routinely used to replace CO ligands with NO ligands in organometallic compounds, we expect it to undergo reduction completely on the carbon nanotube surface to form NO, which subsequently adsorbs on the surface.31 Hence, it is the adsorbed NO species which gives rise to the 401.5 eV binding energy in the XP spectrum. A previous surface study of NO adsorption on SWNTs32 has shown that indeed this species adsorbs weakly inside the nanotubes. We note that, while the NO species is able to diffuse inside the nanotubes (diameter ∼1 nm), the nitrate and hydrosulfate species are not able to do that due to their larger sizes. The diameter of the SWNTs is approximately 1 nm; however, a previous molecular dynamics simulation study indicates that due to the hydrophobic nature of carbon nanotubes, water molecules inside them exhibit a continuous radial distribution only up to 70% of the inside diameter,33 which equates to less than 0.7 nm in our case. NO is a linear molecule with a 0.21 nm length, and at the larger nitrogen end it is approximately 0.15 nm in diameter.34 Nitrate and hydrogen sulfate ions however have sizes of 0.47 and 0.48 nm, respectively, and in the solution phase will be significantly larger because of solvation, thus retricting their entry inside the nanotubes. The broad peak centered at 168 eV in Figure 2b is the spin-orbit split S 2p feature and is consistent with adsorbed
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Figure 2. (a) XP spectra monitoring the N 1s feature of the 6 h cut SWNTs before and after dialysis in 1% Triton X-100 solution. (b) XP spectra monitoring the S 2p spectra of the 6 h cut SWNTs before and after dialysis in 1 wt % Triton X-100 solution.
Figure 3. Raman spectrum of the SWNTs sonicated in a 1:3 ratio of concentrated HNO3 and H2SO4 for 6 h followed by washing and heating to 350 °C to remove adsorbates. The inset shows the RBM mode of a pristine sample (top trace) and that of a nondialyzed purified (6 h of sonication in the 1:3 concentrated HNO3/H2SO4 (v/v) acid mixture and heated to 350 °C) sample (lower trace).
sulfate or hydrogen sulfate species. This species has been previously shown to be detected after treatment of SWNTs with a mixture of nitric and sulfuric acids.3 The observation that the peaks decrease but are not completely removed after dialysis suggests that the species are covalently bound to the nanotubes, as proposed by Yu et al.,3 and slowly hydrolyze in an acidic environment, losing HSO4- to the aqueous solution. The small reduction in intensity we observe in the S 2p peak (∼20%) upon dialysis suggests the hydrolysis process is very slow, and this observation is consistent with that proposed by Yu et al.3 3.3. Raman Spectra of Carbon Nanotubes. Figure 3 shows Raman spectra acquired for 6 h cut undialyzed nanotubes over the 1900-100 cm-1 range, and the inset shows the expanded radial breathing mode (RBM) region (100-300 cm-1). Spectra over the RBM region for the untreated pristine nanotubes are also shown in the inset for comparison purposes. Prior to
purification the RBM region showed features at 180, 204, 225, 246, and 283 cm-1, which correspond to calculated nanotube diameter distribution in the range 0.79-1.24 nm. Following purification and heat treatment, only two features remained in the RBM region, corresponding to diameters of 1.2 and 1.1 nm, which suggests that the lower diameter nanotubes (
