ARTICLE pubs.acs.org/JPCC
Nanographite Impurities within Carbon Nanotubes are responsible for their Stable and Sensitive Response Toward Electrochemical Oxidation of Phenols Emma J. E. Stuart† and Martin Pumera* Division of Chemistry & Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371
bS Supporting Information ABSTRACT: Electrochemists employ carbon nanotube surfaces to achieve highly stable and sensitive electrochemical detection of phenols. There is a large amount of interest in the electrochemical oxidation of phenolic compounds, especially for environmental detection. Herein, we investigate the reasons behind the observed stable and sensitive response on carbon nanotube surfaces. We unambiguously demonstrate that the reported stability and sensitivity exhibited by CNTs is not actually ‘inherent’ to CNT materials but is instead caused by the nanographite impurities contained within them. These findings will have profound impact on the way electrochemical sensors are designed.
’ INTRODUCTION Carbon nanotubes are at the forefront of chemistry, physics, and materials science research.1 These unique carbon nanomaterials have also been employed in electrochemical devices, such as energy storage devices and sensors.24 There are many important properties in electrochemistry that carbon nanotubes have been assigned for such as the electrocatalytic effect, stability toward passivation and high sensitivity. For industrial and practical applications, the possibility of achieving high levels of stability and the avoidance of passivation provided through the use of CNTs is especially compelling. However, despite countless reports exclaiming the excellent electrochemical properties of carbon nanotubes, it has become apparent that there is controversy surrounding the interpretation of results obtained on CNT electrodes. In general, it has been found that there are two types of impurities in CNTs which can have an influence upon their electrochemistry, these are (i) residual catalyst metallic impurities and (ii) nanographite impurities. Compton et al. were the first to report that the electrocatalytic effect exhibited by carbon nanotubes is, in the case of several analytes, caused by metallic impurities within them, this finding was first demonstrated with example of hydrazine5 and then later with hydrogen peroxide,6 glucose,7 and halothane.8 These discoveries were confirmed by many other groups and have been extended to encompass the “electrocatalysis” of organic peroxides,9 amino acids10 and peptides.11 Here should r 2011 American Chemical Society
be noted that of course not all compounds are prone to elecrocatalysis on metallic impurities. Compton et al. also documented that CNTs do not pose any inherent electrocatalytic properties and that their electrochemistry is similar to that of graphite.12 It was suggested by the same group that carbon onions (a form of nanographitic impurities) might actually be responsible for the fast heterogeneous electron transfer observed on CNT surfaces for various compounds.13 We extended this view with our finding that nanographite impurities contained within CNTs govern their electrochemical activity toward the standard electrochemical probe ferro/ferricyanide allowing us to conclude that the reported resemblance between the electrochemistry of CNTs and graphite is only due to the large quantities of nanographite impurities in CNT materials.14 This discovery was only strengthened when we explored the reduction of the azo group in methyl orange at CNT surfaces, we found that nanographite impurities dominate the electrochemistry of carbon nanotubes and are responsible for the observed “electrocatalysis” of CNTs toward the reduction of azo groups,15 redox properties of hydroquinone,16 amino acids and enzyme cofactors.17 The topic of CNT “electrocatalysis” is complex, as
Received: December 16, 2010 Revised: February 22, 2011 Published: March 17, 2011 5530
dx.doi.org/10.1021/jp111941s | J. Phys. Chem. C 2011, 115, 5530–5534
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it was also found that oxygen containing groups are electrocatalytic toward the oxidation of endiols.18 There is a large amount of interest in the electrochemical oxidation of phenols, especially for environmental detection. There have been reports of responses with excellent stability and sensitivity at CNT surfaces toward the oxidation of various phenols.19,20 In view of these previous findings, we started to investigate the reasons for such stability and sensitivity. Herein, we unambiguously demonstrate that the reported stability and sensitivity exhibited by CNTs is due to nanographite impurities contained within them, rather than the CNTs themselves.
’ EXPERIMENTAL SECTION Apparatus. All electrochemical measurements were carried out using an electrochemical analyzer μAutolabIII (Ecochemie, Utrecht, The Netherlands). Voltammetric and amperometric experiments were performed in a 5 mL electrochemical cell at room temperature (25 °C) and a three-electrode configuration was employed. Amperometric experiments were carried out in a stirred solution (rpm ∼ 500). An Ag/AgCl electrode acted as a reference electrode while a platinum electrode served as an auxiliary electrode. Materials. GC, Pt, and Ag/AgCl electrodes were acquired from CHInstruments, TX, USA. p-Cresol was purchased from Alfa Aesar. Phosphate buffer (pH 7.4), phenol, DWCNTs (i.d. 1.32.0 nm; o.d. 5 nm; length of 50 μm; Sigma-Aldrich product code 637351 (product codes in the following text are related to the Sigma-Aldrich catalogue if not stated otherwise) we characterized this sample by cyclic voltammetry,21 TEM,22 SEM,21 Raman spectroscopy,21,22 XPS22 and ICP22 in our respective previous publications. They contained the following amounts of impurities (8.78 wt % according to TGA) which according to TEM/EDX analysis consisted of Co-, Mo-, and Fe-based compounds; the amount of nanographitic impurities in the DWCNT sample was 8.5 wt %),22 pure CNTs (o.d. 90110 nm, length 59 μm; these were previously characterized by TEM,23 XPS,23 TGA,14,23 EPR,23 and magnetic susceptibility measurements;23 we found that the amount of Fe impurities contained within them is ∼3 ppm (0.0003 wt %) and no detectable amount of nanographitic impurities; p.c. 659258),14,23 metal and metal oxide particles (Fe3O4 (product code (p.c.) 637106), MoO2 (p.c. 234761), MoO3 (p.c. 203815), Co (p.c. 266639), and Co3O4 (p.c. 637025)) and graphite nanofibers (p.c. 698830)24 were obtained from Sigma-Aldrich. All CNT and graphite nanofiber materials had been characterized in detail previously. Procedures. No functionalization was performed for any of the nanomaterials tested. CNT suspensions were prepared by dispersion of nanotubes in DMF at a total concentration of 0.5 mg mL1. The dispersed CNTs were placed in an ultrasonic bath for a period of 5 min before 1 uL of the suspended CNT material was removed and deposited onto a glassy carbon electrode surface. The CNT suspension was left to dry in air at room temperature; the DMF solution evaporated leaving a randomly distributed CNT film across the electrode surface. A similar procedure was carried out with the other nanomaterials tested.
’ RESULTS AND DISCUSSION First, we explored the reasons behind the highly sensitive voltammetric response of CNT materials, using two different phenolic compounds, phenol and p-cresol. The voltammetric
Figure 1. (A) Cyclic voltammograms resulting from the electrochemical oxidation of phenol (10 mM) at GC electrodes modified with DWCNTs (red line), pure CNTs (black line), Fe3O4 (pink line), Co3O4 (blue line), and MoO3 (green line). (B) Cyclic voltammograms for the electrochemical oxidation of phenol (10 mM) at glassy carbon electrodes modified with DWCNTs (red line), pure CNTs (black line) and nanographitic impurities (blue line). The voltammetric profile gained at the bare GC electrode surface (dashed black line) is also shown. C) Amperometric response for 10 mM phenol at DWNCTs (red line), nanographitic impurities (blue line), pure CNTs (black line), and bare GC (dashed black line). Conditions for A and B: scan rate, 100 mV s1; background electrolyte, 50 mM phosphate buffer (pH 7.4); reference electrode, Ag/ AgCl. Conditions for C: applied potential, þ0.8 V; background electrolyte, 50 mM phosphate buffer (pH 7.4); stirring rate, 500 rpm. 5531
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The Journal of Physical Chemistry C response for the oxidation at phenol at a double-walled carbon nanotube (DWCNT)-modified electrode surface is displayed in Figure 1, A (red line). Oxidation of phenol at DWCNTs originates at þ406 mV, exhibits a maximum at þ664 and a small shoulder at þ851 mV while the reversed scan shows small reduction peaks at þ265 and þ28 mV. It can be seen in Figure 1, A that the voltammetric profile gained for the electrochemical oxidation of phenol at DWCNTs is significantly different to the profile gained for oxidation of the same compound at the bare GC surface. When compared to the voltammetric response at DWCNTs, the electrochemical oxidation of phenol at the bare GC electrode originates at an increased potential of þ448 mV, has a maximum at a similar potential (þ598 mV) to that observed in the DWCNT profile and exhibits no reduction signals in the reversed scan. The response at DWCNTs gives rise to a much larger oxidation peak (þ109 μA) while detection of the oxidation of phenol at the bare GC electrode is much poorer (þ19 μA). To determine the reason for the increased sensitivity exhibited by DWCNTs toward the redox behavior of phenol we subjected phenol to electrochemical oxidation at a pure CNT-modified electrode (pure CNTs are CNTs containing no metallic or carbon-based impurities within them). The voltammetric response at pure CNTs is analogous to the response observed for the oxidation of phenol at the bare GC electrode, therefore DWCNTs also exhibit enhanced phenol detection abilities when compared to pure CNTs. As pure CNTs contain no metallic or carbon-based impurities the dramatic difference between the response observed at the pure CNT-modified electrode and that at the DWCNT-modified electrode indicates that it is the impurities within DWCNTs that are responsible for the observed improvement in phenol sensing. First, we performed an experiment to determine if metallic impurities within DWCNTs govern their electrochemical behavior. DWCNTs contain Fe-, Mo-, and Co-based impurities,21 therefore we modified GC electrodes with Fe3O4, MoO2, MoO3, Co and Co3O4 nanoparticles (see Figure 1A, for clarity only the response at Fe3O4, MoO3 and Co3O4 metal nanoparticles are shown as voltammetric profiles for the corresponding metals and lower metal oxides tested were very similar). We studied the oxidation of phenol at each of these surfaces and found that the voltammetric profiles gained for all of the metallic nanoparticles are very similar to the profile gained for the oxidation of phenol at the bare GC electrode. These findings allow us to conclude that metallic impurities present in DWCNTs are therefore not responsible for the sensitive and stable phenol detection observed at DWCNTs. The next step was to test phenol oxidation at a GC electrode modified with nanographite impurities. We used graphite nanofibers as they are almost identical in structure to the nanographite impurities found within CNTs, therefore studying the oxidation of phenol at an nanographite (NG) surface allows us to determine whether nanographite impurities are the cause of the observed enhanced phenol sensing at DWCNTs. Oxidation of phenol at NGs (Figure 1B, blue line) begins at þ374 mV, exhibits a maximum at þ644 mV, it also gives rise to two reduction waves in the reversed scan (þ265 mV and þ15 mV). The voltammetric profile is strikingly similar to that observed for the oxidation of phenol at DWCNTs with the oxidation of phenol at the NG surface also being at a greatly enhanced current signal (þ144 μA) when compared to the response at the bare GC electrode (þ19 μA). These
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observations demonstrate that it is the nanographite impurities contained within DWCNTs that are responsible for the improved electrochemical detection of phenol seen at DWCNT surfaces. Having discovered that nanographite impurities are the reason for the enhanced phenol detection observed at DWCNTs, we wondered whether the same was true for other phenolic compounds. To find out, we subjected 10 mM p-cresol to oxidation at DWCNTs (Figure 2A, red line). Electrochemical oxidation of p-cresol at a DWCNT-modified GC electrode surface begins at þ312 mV, exhibiting a peak at þ502 mV with a current of þ158 μA. When we compare this voltammetric signal with the response observed for the oxidation of p-cresol at a bare GC electrode (Figure 2A, dashed black line) it is clear that DWCNTs are also capable of enhanced p-cresol sensing. At the bare GC electrode, oxidation of p-cresol originates at þ358 mV and reaches a maximum at þ595 mV with a current signal (þ45 μA) that is much lower than that observed at the DWCNT-modified surface. As with phenol, the response at pure CNTs was found to be identical to the voltammetric signal seen for the oxidation of p-cresol at the bare GC electrode surface. As before, we first tested whether metallic impurities are influencing the electrocatalytic properties of DWCNTs toward the oxidation of p-cresol. We tested the oxidation of p-cresol at GC electrode surfaces that had been modified with Fe3O4, MoO2, MoO3, Co and Co3O4 nanoparticles (see Figure 2A, for clarity only the response at Fe3O4, MoO3 and Co3O4 metal nanoparticles are shown as voltammetric profiles for the corresponding metals and lower metal oxides tested were very similar). The findings were very similar to the response observed for the oxidation of phenol at metal nanoparticle surfaces. All responses at metal nanoparticle-modified GC electrode surfaces are comparable to the voltammetric signal observed at the bare GC electrode, allowing us to conclude that residual metallic impurities present within DWCNTs are in no way responsible for the enhanced detection capabilities of DWCNTs toward p-cresol. Suggesting that, as with phenol, the improved p-cresol sensing at CNTs is being caused by the alternate impurities contained within CNTs: nanographite impurities. When the oxidation of p-cresol at an nanographite-modified electrode surface was explored, the observed voltammetric signal (Figure 2B, blue line) was very similar to the response observed for the electrochemical oxidation of p-cresol at DWCNTs. Oxidation of p-cresol at nanographite impurities starts at þ290 mV, reaches a maximum at þ500 mV and also exhibits an enhanced current of þ198 μA. The resemblance between the electrochemical response for the oxidation of p-cresol at DWCNTs and at nanographite impurities demonstrates that it is the nanographite impurities present in DWCNTs that are responsible for the stability improvements observed for the electrochemical detection of p-cresol at DWCNT-modified electrodes. Here it should be mentioned that the typical RSD of the peak currents was between 3 and 5% which is analytically acceptable figure for instrumental methods. With the discovery that nanographite impurities cause the sensitive and stable voltammetric measurements of two phenolic compounds at DWCNT surfaces, we decided to explore the possibility that nanographite impurities are also responsible for the sensitive and stable amperometric measurements of the same phenolic compounds at DWCNTs. We monitored the current time response for a stirred 10 mM phenol solution when holding a DWCNT-modified GC electrode at a potential of þ0.8 V 5532
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over a time period of 1000 s (Figure 1C, red line), and compared this against the response of the same compound at a bare GC electrode under the same conditions (Figure 1C, dashed black line). While the amperometric response at the bare GC electrode shows immediate current diminution (t1/2 = 0.6 s; RSD of decay measurements was
