Amorphous Carbon Impurities Play an Active Role in Redox

Nov 21, 2011 - Lu Wang , Adriano Ambrosi , and Martin Pumera. Analytical Chemistry .... Martin Pumera. Chemistry - An Asian Journal 2013 8 (6), 1200-1...
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Amorphous Carbon Impurities Play an Active Role in Redox Processes of Carbon Nanotubes Adriano Ambrosi and Martin Pumera* Division of Chemistry & Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371, Singapore

bS Supporting Information ABSTRACT: Carbon nanotubes (CNTs) produced via chemical vapor deposition (CVD) generally contain significant amounts of impurities ranging from residual metallic catalysts to nanographitic and amorphous carbon debris. Here, we show that in addition to the graphitic portion of impurities, also the amorphous carbon impurities significantly alter the electrochemical behavior of CNTs. We use a particular type of amorphous carbon black to specifically simulate amorphous carbon impurities and cyclic voltammetry with a ferro/ferricyanide redox probe to measure the heterogeneous electron transfer (HET) rate. The presence of about 30 wt % of amorphous carbon impurities resulted in an HET rate increased about 16 times compared to that of impurity-free CNTs. Electrochemical investigations of CNTs should, therefore, be performed only after a careful and complete purification from residual metal catalysts, nanographitic, and also amorphous carbon impurities. Our findings have direct implications for the construction of CNT-based electrochemical devices, such as batteries, supercapacitors, and sensors.

1. INTRODUCTION Carbon nanotubes (CNTs) with unique structural and physical properties represent promising material for fundamental research and new technological applications.1 Development and progress in the fabrication methods opened the way to large-scale production and offered the opportunity to use CNTs in several applications ranging from material building-blocks in construction industry2 and nanocomposites in energy storage devices3,4 to electronic nanodevices.5 Despite the fact that research on synthesis of carbon nanotubes has been carried out for two decades,6 commonly used synthetic procedures result in significant amounts of metallic and carbonaceous impurities in the CNT samples. These impurities consist of (i) residual metal catalyst impurities; (ii) nanographitic impurities; and (iii) amorphous carbon impurities.7 These impurities represent a major obstacle for the determination of CNT properties and also hinder their use in advanced technological applications.8 The use of vigorous purification procedures not only raise production costs but also is challenging since it is difficult to purify the samples without damaging the carbon nanotubes themselves.9 Several methods have been proposed to purify CNT samples,10,11 but also, procedures to synthesize CNTs without amorphous carbon have been shown.12 Very recently, an interesting study has been published on the effect of amorphous carbon on the catalyst particle.13 It is of interest to mention that washing with nitric acid, as is a method typically used for removal of metallic impurities from CNT samples, destroys walls of CNTs and produces amorphous carbon instead.14 r 2011 American Chemical Society

Electrochemical properties of CNTs are among the most researched and exploited, especially in the fabrication of batteries, capacitors, and sensing devices.15 It has been claimed in several publications that CNT-based materials decrease overpotential for the oxidation/reduction of many important compounds, amplify current signals, exhibit an electrocatalytic effect, and show negligible electrode surface fouling.15 This was until the discovery that the impurities present in most commercial CNT samples play an extremely active role in such properties with some cases where the impurities were exclusively responsible for such special behaviors. A pioneer work in such direction was done by Compton et al. who first demonstrated that the reported electrocatalytic effect on the oxidation of hydrazine16 and glucose17 and on the reduction of hydrogen peroxide18 is caused by the residual metallic impurities within CNTs. Following those works, we have shown that metallic impurities within CNTs participate in the redox behavior of amino acids,19 regulatory peptides,20 organic peroxides,21 sulphides,22 and even at concentrations in the order of hundreds of ppm,23,24 and also, we elucidated the role of multicomponent metallic impurities.25 Compton et al. demonstrated also that formation of nanographitic impurities in CNT samples, so-called graphitic onions, dominate the electrochemistry of CNTs toward a wide range of biologically important compounds.26 We recently investigated the influence of carbon-based impurities and showed that Received: October 10, 2011 Revised: November 20, 2011 Published: November 21, 2011 25281

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nanographitic particles actually dominate the electrochemistry of CNTs, altering significantly the heterogeneous electron transfer (HET) rate toward the redox probe ferro/ferricyanide.27 It has been shown also that such nanographitic impurities govern the electrochemistry of many other industrially and biologically important compounds containing various redox active groups.28 30 However, the influence of amourphous carbon impurities present in CNT samples on their electrochemical properties has not been studied. Here, we wish to investigate for the first time the specific influence of amorphous carbon impurities on the electrochemistry of CNTs. We wish to show that when amorphous impurities are present, they play an important active role and dominate the electrochemical properties of carbon nanotubes.

2. EXPERIMENTAL SECTION 2.1. Materials. N,N-dimethyl formamide (DMF), potassium chloride, potassium phosphate disodium salt, potassium ferrocyanide, and potassium ferricyanide were purchased from SigmaAldrich (Singapore). Multiwalled carbon nanotube samples, labeled as MWCNT-A and MWCNT-B were purchased from Sigma Aldrich; the sample MWCNT-C was purchased from Bucky, TX, USA. Transmission electron micrographs were obtained with a JEM 1400 transmission electron microscope (JEOL, Japan) operating at 120 kV. Samples for TEM were prepared by depositing 1 μL of the material suspension (1 mg/mL) onto a copper TEM grid and allowed to dry in air. All MWCNTs were openended (representative TEM images can be found also in our previous publications, see refs 31 and 32). MWCNT-A had a diameter of 90 110 nm and a length of 5 9 μm; MWCNT-B had a diameter of 10 30 nm and a length of 1 10 μm. MWCNT-C had a diameter of 8 15 nm and a length of 5 10 μm. Carbon black (a-CB) (particle size 29 nm) was obtained from Asbury Carbons, NJ. Glassy carbon electrodes (diameter = 3 mm) were obtained from CH Instruments, TX, USA. 2.2. Apparatus. Raman spectroscopy analysis was performed using a microRaman LabRam HR instrument from Horiba Scientific in backscattering geometry with a CCD detector, a 514.5 nm Ar laser, and a 100 objective mounted on an Olympus optical microscope. The calibration is initially made using an internal silicon reference at 520 cm 1 and gives a peak position resolution of less than 1 cm 1. The spectra are measured from 1000 to 3250 cm 1. All voltammetric experiments were performed on a μAutolab type III electrochemical analyzer (Eco Chemie, The Netherlands) connected to a personal computer and controlled by General Purpose Electrochemical Systems Version 4.9 software (Eco Chemie). Electrochemical experiments were performed in a 5 mL voltammetric cell at room temperature by using a three-electrode configuration. A platinum electrode (Autolab) served as an auxiliary electrode, while an Ag/ AgCl electrode (CH instruments) served as a reference electrode. All electrochemical potentials in this article are stated versus the Ag/AgCl reference electrode. 2.3. Procedures. Carbon materials were used as received without further purification. For the electrochemistry measurements, the CNTs and amorphous carbon impurity solutions were prepared at the required ratios by dispersion in DMF at a total concentration of 5 mg mL 1. The suspension was then placed into an ultrasonic bath for 30 min after which 1 μL of the suspension was pipetted onto the surface of the glassy carbon (GC) electrode, which was previously polished with 0.05 mm alumina on a polishing cloth. The suspension was allowed to

Figure 1. Transmission electron micrographs and Raman spectra of MWCNT-A, MWCNT-B, and a-CB.

evaporate at room temperature, creating a randomly distributed CNT film on the surface of the GC electrode. Cyclic voltammetric experiments were performed at a scan rate of 100 mV s 1 using 0.1 M potassium chloride as the supporting electrolyte and 1 mM potassium ferro/ferricyanide as the electrochemical probe. The k0 values were determined using the method developed by Nicholson.33 The roughness factor was not taken into account. The diffusion coefficient D = 7.26  10 6 cm2 s 1 for [Fe(CN)6]3/4 in 0.1 M KCl was used.34

3. RESULTS AND DISCUSSION Before starting our discussion, it is important to highlight again that carbon-based impurities in CNT samples consist in part of (a) graphitic/nanographitic particles and in part of (b) carbonaceous materials with a very low grade of crystallinity and high percentage of sp3 hybridized carbon. Such materials are generally defined as amorphous carbon.35 To investigate the influence of amorphous carbon on the electrochemistry of carbon nanotubes, we employed three types of carbon nanotubes, labeled in following text as MWCNT-A (pure carbon nanotubes without metallic, nanographitic, and amorphous impurities), MWCNT-B (carbon nanotubes containing large amounts of metallic and carbonaceous impurities), and MWCNT-C (carbon nanotubes free of carbonaceous impurities but containing metallic impurities). As a simulant of pure amorphous carbon, we used a particular type of amorphous carbon black (a-CB) that was poorly graphitized and with a low grade of crystallinity. The established fine structure of CB is comprised of coexisting crystalline carbon (sp2) and amorphous carbon (sp3) at different ratios depending on the synthetic procedure adopted.36 Different CBs are in fact classified according to such ratio ranging from 25282

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Figure 3. \Cyclic voltammograms of MWCNT-B and a-CB using 1 mM [Fe(CN)6]3/4 in a 0.1 M KCl solution. Carbon-impurity-rich MWCNT-B resembles the electrochemical behavior of a-CB.

Figure 2. (a) Cyclic voltammograms of MWCNT-A with different ratios of a-CB impurities added. (b) Effect of the presence of a-CB impurities in the CNT sample. Peak-to-peak separation of oxidation and reduction of [Fe(CN)6]3/4 decreases significantly with only 15 wt % of impurities present in the sample. The corresponding k0 is also shown for a-CB additions between 0 wt % and 30 wt % (blue graph). Solution: 1 mM [Fe(CN)6]3/4 in 0.1 M KCl.

amorphous-CB when it possesses a low grade of crystallinity (sp2 carbon) and high percentage of sp3 carbon; and graphitized-CB when is rich in sp2 carbon.37 We precisely introduced different amounts of such amorphous carbon material (a-CB) to pure (carbon-impurity-free) CNT samples and investigated the HET rate using ferro/ferricyanide as a redox probe, as it will be described in the following text. First, we used transmission electron microscopy (TEM) and Raman spectroscopy to characterize the materials used in this work. Figure 1 shows the Raman spectra and TEM images of MWCNT-A used as impurity-free CNTs, compared to the spectrum of MWCNT-B used in this work as the carbonimpurity-rich sample and to the spectrum of a-CB. The Raman spectrum of MWCNT-A is typical of highly graphitized CNTs, with an intense G band at 1590 cm 1 and a very low D band at 1360 cm 1 indicating a highly ordered structure. Also, the 2D band at ∼2700 cm 1 is intense and typical of ordered structures. The Raman spectrum of a-CB is typical of a poorly graphitized carbon black, with a high ID/IG ratio and also the low grade of crystallinity is indicated by the large full width at half maximum (fwhm) of the D band.37 MWCNT-B also presents a Raman spectrum with a high ID/IG ratio and poorly defined second-order bands (∼2700 cm 1). This indicates a material with an abundance of defects in the structure as well as

the presence of poorly graphitized carbon impurities. The similarity between the Raman spectrum of a-CB with that of MWCNT-B is apparent. TEM images in Figure 1 also confirms the characteristics resulting from Raman spectroscopy analysis. MWCNT-A presents a clean tubular structure of about 100 nm diameter with no visible deposition of amorphous carbon on the nanotube wall. MWCNT-B has a smaller diameter (10 30 nm) and some carbonaceous amorphous material is visible together with a nanotube. Typical aggregate of particles with no defined crystallinic structure can be seen in the TEM image of a-CB. In order to demonstrate the effect of amorphous carbon impurities to the electrochemistry of CNTs, we prepared dispersions of MWCNT-A with an increasing amount of a-CB added, between 0 wt % up to 30 wt %. These dispersions have been used to modify GC electrodes, and then the HET rate was measured by means of cyclic voltammetry (CV) with a ferro/ ferricyanide redox probe. Figure 2a shows selected cyclic voltammograms of pure MWCNT-A, a-CB, and MWCNT-A with added 5, 15, and 25 wt % a-CB. It can be seen that the peak-topeak separation decreases with the increasing addition of a-CB to the pure MWCNT-A. It is also surprising that a-CB alone allows an extremely fast HET rate toward the ferro/ferricyanide complex with a ΔE of about 75 mV, much lower than ΔE of pure MWCNT-A (315 mV). This clearly indicates a strong influence of possible amorphous carbon impurities to the global electrochemical behavior of CNT unpurified samples. Figure 2b shows the quantitative results of the peak-to-peak separation vs the increasing amount of a-CB added and compared to that of pure MWCNT-A and a-CB samples. It can be seen that for an increasing amount of a-CB impurities, the peak-to-peak separation decreases starting from an average value of 315 mV for pure MWCNT-A and reaches a value of 116 mV with added 30 wt % aCB impurities. The corresponding variation of the observed HET rate constant (k0) is also illustrated in Figure 2b. The influence of the a-CB impurities is evident since k0 varies from a value of 2.2  10 4 cm s 1 for the pure MWCNT-A to the value of 3.5  10 3 cm s 1 in the presence of 30 wt % impurities. The HET rate is, therefore, significantly increased about 16 times. It is important to know that unpurified CNT samples could contain up to 40 wt % of carbon-based impurities, and therefore, their influence is significant.38 To further verify such influence, 25283

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The Journal of Physical Chemistry C we decided to also test a carbon-impurity-rich CNT sample. Figure 3 shows representative cyclic voltammograms of MWCNT-B and a-CB alone. The similarity between the two signals is striking, which further confirms the conclusion illustrated in the previous paragraph. To prove that this is a general case, we employed another carbon-impurity-free CNT sample (MWCNT-C) obtained from a different supplier and observed very similar trends upon introduction of a-CB impurities, as shown in Figure S1 of the Supporting Information.

4. CONCLUSIONS In summary, we demonstrate here that in addition to the graphiticnanographitic impurities, the electrochemistry of CNTs is also strongly influenced by the amorphous portion of carbon impurities. We used in this work a poorly graphitized CB material (a-CB) to simulate the amorphous carbon impurities. Such material was introduced in a controlled manner into a carbonimpurity-free CNT sample. By means of cyclic voltammetry, we showed that such amorphous carbon impurities strongly alter the HET rate measured toward the ferro/ferricyanide redox probe. These findings further confirm that before a correct and reliable evaluation of the HET rate of CNT samples can be made, a prior careful purification is always necessary to remove not only residual metallic impurities but also graphitic and amorphous carbon impurities, which play an active role on the electrochemical behavior of CNTs even at very low concentrations. Our finding has a profound implication to several applications of CNTs: (i) CNTs are typically treated with nitric acid to remove metallic impurities. Such treatment partly destroys CNT walls and transforms them to amorphous carbon. Therefore, electrochemical properties of such CNTs might be dominated by amorphous carbon after such washing procedure. (ii) There is an important implication for the construction of electrochemical devices, such as batteries, supercapacitors, or sensors based on CNTs. (iii) From a broader point of view, it can be more beneficial to use amorphous carbon than CNTs in some electrochemical applications. ’ ASSOCIATED CONTENT

bS

Supporting Information. Raman spectroscopy and CV analysis of MWCNT-C with different added amounts of a-CB. This material is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was partially supported by a NAP start-up fund grant (no. M58110066) provided by NTU.

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