Carbon nanotube powder microelectrodes for nitrite detection

Sensors and Actuators B 84 (2002) 194–199

Carbon nanotube powder microelectrodes for nitrite detection$ Peifang Liu*, Junhu Hu Department of Chemistry, Wuhan University, Wuhan 430072, PR China Accepted 15 January 2002

Abstract Multi-wall carbon nanotubes (MWCNTs) are filled in the cavity at the tip of a microelectrode to form a carbon nanotube powder microelectrode (CNTPME) for testing the nanotubes under various conditions. Anodic treatment is found to cut the nanotubes into shorter species, increase double layer capacity, and make the nanotubes able to strongly adsorb Os(bpy)32þ. The CNTPME modified with anodic pretreatment and pre-adsorbed Os(bpy)32þ (CNTPME-Os(bpy)32þ) shows high catalytic activity for nitrite reduction in acidic solutions. The sensitivity, temperature coefficients of such modified electrode for nitrite detection are 4.9 and 6.8 A cm2 M1, 0.14 and 0.16% (8C)1 for steps 1 and 2 of nitrite reduction, respectively with a lower detection limit 107 M for S=N ¼ 4. These features plus the miniaturizability make the CNTPME-Os(bpy)32þ a very promise candidate for nitrite sensor. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Carbon nanotube; Powder microelectrode; Nitrite reduction; Modified electrode

1. Introduction Nitrite has been an important species in environment protection and live processes. Developing high quality nitrite sensors has been paid much attention [1]. There have been many reports about various nitrite sensors. Among them electrochemical sensors, especially amperometric ones, show quick response, high sensitivity and are readily miniaturized. For amperometric methods, the reduction of nitrite is more favorable than the oxidation. This is because the high overpotential necessary for nitrite oxidation tends to invite interference from coexisting impurities [2]. It was found in our previous work [3,4] that nitrite reduction in acidic solution on glassy carbon and Au electrodes is controlled by a preceding chemical process with very thin reaction zone which has a thickness close to the double layer. It has been shown that this kind of homogeneous preceding reaction can be accelerated by enlarging the electrode/solution interfacial area, a method usually used to speed up the reactions controlled by surface processes. Powder microelectrodes (PMEs)have been proved an effective technology for the purpose. PME, which was originated in our laboratory, is a convenient technique for both characterization of powder materials and electroanalytical applications [5]. In fact, the so-called PME is a combination of porous and microelectrode. Besides, the PME has the $ *

Presented at Eurosensors XIII. Corresponding author. Fax: þ86-27-87874669.

properties of the thin layer cell to some extent. No binder is need for preparing a PME, this not only makes the preparation simple but also can prevent impurities and keep the powder material in its pristine state [5]. Besides the nano-size effects common to nano materials, carbon nanotubes also show unique size distribution and hollow geometry, which bestow them unique electronic, mechanical and chemical properties and potentiality of wide applications. There have been some important works about the electrocatalysis of carbon nanotubes [6–9] and chemical sensors [6,10,11]. In fact, carbon nanotubes have shown considerably complexity on both microstructure and physical or chemical properties depending on preparation and pretreatment and even the construction of the electrodes. In general, single-wall carbon nanotubes (SWCNTs) are shaped into an electrode by filtering suspension of nanotubes on a membrane filter [12a] to form a paper of nanotube. However, for construction of an electrode multi-wall carbon nanotubes (MWCNTs) are usually mixed with bromoform [6], mineral oil [7], or liquid paraffin and then packed into a glass capillary. Microelectrodes prepared from the single carbon nanotubes with large diameter (80–200 nm) were reported [13]. In this work, MWCNTs were filled in the cavity at the tip of a microelectrode to form a carbon nanotube powder microelectrode (CNTPME). The properties of CNTPMEs and the effect of electrochemical (anodic pretreatment) and chemical modification (Os(bpy)32þ preadsorption) on their catalytic activity for electro-reduction and detection of nitrite are reported.

0925-4005/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 ( 0 2 ) 0 0 0 2 4 - 2

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2. Experimental The main experimental methods used in this work were the PME and steady state polarization. 2.1. Preparation of CNTPME A Pt micro disk electrode of 100 mm in diameter was first chemically etched in aqua regia to form a cavity of hundreds micrometers deep at the tip of the microelectrode. Then the cavity was filled with the powder of carbon nanotubes to form a CNTPME. 2.2. Anodic pre-treatment of nanotube and pre-adsorption of Os(bpy)32þ The CNTPME was anodically polarized in 0.05 M H2SO4 at 1.4 V (versus SCE) for 80 min and then socked in 5 mM Os(bpy)32þ solution for 1 h to pre-adsorb Os(bpy)32þ. The CNTPME-Os(bpy)32þ was rinsed with doubly distillation water and dried in air prior to measurements. 2.3. Instruments and chemicals Electrochemical experiments were carried out using a potentiostat (Pine model, USA) and 3036 X–Y recorder (Sichuan 4th Instrumental Factory, China). Scanning electron microscope (SEM, JEM-100CXII, Japan) and transmission electron microscope (TEM, Nicong, Japan) were used to observe the morphologic changes of nanotubes brought about by anodic pretreatment. All potentials in the paper are referred to the saturated calomel electrode. All of chemicals were analytical reagents and were used as received except for Os(bpy)32þ which was prepared according to [14]. All solutions were prepared with doubly distilled water. Acidic NaNO2 solutions were freshly prepared prior to use. MWCNTs were made from catalytic decomposition of CH4 [15] and were kindly donated by Professor Zhang of Xiamen University. The nanotubes were soaked in hot (80 8C ) 4 N HNO3 about 8 h to remove nickel catalyst and then washed with doubly distilled water throughout and dried in air.

3. Results and discussion 3.1. CNTPME without anodic pretreatment There is a pair of CV current peaks near 0.35 V for the CNTPME as shown in Fig. 1a, attributable to the redox reactions of surface groups at MWCNT. The types of surface groups on carbon nanotubes depend on the pretreatment and the media used. As reported in the literature, there are mainly three types of pretreatments in two different kinds of media to form different surface groups on nanotubes. These include airoxidation, plasma-oxidation in gas phase [16] and acid-oxidation [16–20], cyclic voltermetry [12b] in liquid phase to form

Fig. 1. Cyclic voltamograms of CNTPMEs in 0.05 M H2SO4: (a) untreated; (b) anodically pretreated; (c) after anodic treatment and Os(bpy)32þ pre-adsorption.

surface groups containing oxygen, carbon and even sulfur [17]. Gas-treatment preferentially forms hydroxyl and carbonyl groups, while liquid-treatment forms carboxylic acid, phenolic hydroxides [8,20], OH [18] and quinone (cyclic voltermmetry in various pH aqueous) [12b]. Based on the similarity of CV behavior between MWCNT used and glassy carbon, surface quinoidal groups seem probably to be most responsible for the observed redox current peaks. After pre-adsorbing Os(bpy)32þ, the CV showed another pair of current peaks near 0.6 V due to Os(bpy)32þ/3þ redox couple, but they disappeared in less then an hour in background solution. This observation indicates that the adsorbed mediators Os(bpy)32þ tend to leach out easily. When the pristine CNTPME was used to test nitrite reduction in an acid solution containing 5 mM NaNO2, both the first and second steps of nitrite reduction could be recognized on the steady state polarization curve in the potential range studied (Fig. 2a). Their plateau current densities were 0.58 and 0.93 mA cm2 with Tafel slopes about 234 and 147 mV per decade, respectively. Their halfwave potentials were 0.32 and 0.13 V, respectively positively shifted about 72 and 370 mV compared to the glassy carbon electrode. With a glassy carbon electrode, the current of step 2 was not appreciable and the plateau current density for step 1 was about 30 times smaller than that with CNTPME in the same potential range. Above mentioned results show that the pristine CNTPME already exhibits notably higher catalytic activity for nitrite reduction than a glassy carbon electrode. This apparently higher activity may be ascribed to the powder nature of the electrode and properties of nanotube (see the subsequent sections). 3.2. Anodically pretreated CNTPME After anodic pretreatment, the double layer charging current of CNTPME in the background solution was about

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Fig. 2. Steady state polarization curves for NO2 reduction in 0.05 M H2 SO4 þ 5 mM NaNO2 at CNTPMEs: (a) without pretreatment; (b) after anodic treatment; (c) after anodic treatment and Os(bpy)32þ pre-adsorption.

doubled, implying an increase in surface area by a factor about two (Fig. 1b). The activity of the electrode for nitrite reduction in acidic solution was also roughly doubled as shown in Fig. 2b and Table 1. This increase in catalytic activity is explained in the framework of CE mechanism with very thin chemical reaction zone for the preceding chemical step. 3.3. Anodically pretreated CNTPME with adsorbed Os(bpy)32þ (CNTPME-Os(bpy)32þ) The pair of current peaks round 0.6 V in background solution due to Os(bpy)32þ/3þ redox couple (Fig. 1c) did not decrease appreciably overnight, indicating strong adsorption of Os(bpy)32þ on the anodically treated CNTPME. Carbon nanotube is known to be highly hydrophobic [15]. It may be inferred that proper anodic pretreatment may change the hydrophobicity of carbon nanotubes to match the hydrophobicity of Os(bpy)32þ and therefore increases the ability of adsorption of nanotube for Os(bpy)32þ. Barisci et al. [12b] has reported that wetting of carbon nanotube can be effectively facilitated by electrochemical treatment.

According to TEM observation, all the nanotubes used in this work have about the same inner diameter (about 3 nm) while the outer diameters are mainly round three values, namely about 17, 22, 37 nm (Fig. 3). The length of the carbon nanotubes ranges about 200–2000 nm. From these dimensions and the double layer capacitance found from CV, the number of nanotubes in a PME may be estimated to be in the order of magnitude 1010–1011. (there must be a range spanning an order of magnitude, because the length changes in the same range). On the other hand, according to the charge of Os(bpy)32þ redox peak, the number of the adsorbed Os(bpy)32þ is found to be close in the order of magnitude to the number of carbon nanotubes. The result might imply that Os(bpy)32þ species are adsorbed on the openings at the ends of the nanotubes. There are only a few papers concerning the location of foreign species on or in nanotubes [21,22]. Wu et al. studied the filling of Pb, C, Al and S atoms into carbon nanotubes and found that Pb atoms tend to enter the tubules while the additional C atoms rather stay at the mouths of tubules. The behaviors of filling depend on the length and diameter of tubules [21]. Gao et al. studied K doping of SWCNT crystals and found that K atoms may

Table 1 The parameters for nitrite reduction in 0.05 M H2SO4 þ 5 mM NaNO2 on different electrodes iL (mA cm2)a

Electrode

iL CNTPME CNTPME after anodic treatment CNTPME after anodic treatment and Os adsorption Glassy carbon a b

1b

0.58 0.87 1.41

Tafel slope, b (mV per decade)

Half-wave potential j1/2 (V)

iL2

b1

b2

j1/21

j1/22

0.93 1.15 2.41

235 131 59

147 83 73

0.32 0.4 0.55

0.13 0.073 0.07

0.25

0.5

0.013 

iL is limiting current density for NO2 reduction. All superscripts 1 and 2 in the second row represent steps 1 and 2 for NO2 reduction, respectively.

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Fig. 3. TEM of nanotubes: (a) untreated; (b) anodically pretreated. Magnification 72,000 (1 mm represents 13.9 nm).

Fig. 4. SEM of nanotubes: (a) untreated; (b) anodically pretreated.

be located between tubules or inside the tubule, depending on the number of doped K atoms and the crystal types of nanotubes [22]. No report was found about the sites of large cations such as Os(bpy)32þ at carbon nanotubes. In view of the large size, it seems plausible for Os(bpy)32þ to stay at the mouths of tubules. The plateau currents of the two steps of NO2 reduction on the CNTPME-Os(bpy)32þ increased remarkably compared with cases described in previous sections and reached 1.41 and 2.41 mA cm2, respectively (Fig. 2c and Table 1). The Tafel slopes for the first and second steps were 59 and 73 mV per decade, respectively indicating the first step being apparently reversible. The half-wave potentials for the steps 1 and 2 of nitrite reduction positively shifted about 300 and 430 mV compared to those on the bare glassy carbon electrode. The half-wave potential for step 1 was close to the CV peak potential of Os(bpy)32þ/3þ redox, implying a mediating function of Os(bpy)32þ/3þ for nitrite reduction. Although the mechanism of the above mentioned effects of anodic pretreatment remains to be further studied, the microscopic images in Figs. 3 and 4 may provide some clues. The pristine nanotubes appeared very long and intertwined each other closely (Fig. 4a). They became much

shorter and dispersed after anodic treatment (Fig. 4b). The TEM pictures (Fig. 3a and b) clearly show that a part of the nanotubes had been cut into shorter pieces, leaving more openings. The caps of carbon nanotubes are easily opened with various methods, such as heating in air, O2 and CO2 [17,19], or liquid Pb [23], immersion or refluxing in hot HNO3 [7,17,24] or mixture with H2SO4 [18,25] or KMnO4 [26], etched lightly in mixture of H2SO4 and H2O2 [20] and heated in aqueous containing oxidants such as KMnO4, OsO4, RuO4, etc. [26]. However the yields for different method are different [26]. Not all methods, which can open the caps of the nanotubes can cut them into short fragments. For example, for the untreated nanotubes only the caps were opened as shown in Fig. 3a. Similar phenomena have been reported [25]. According to Mazzoni et al. [27] first-principle calculations, the energy needed to open the cap of SWCNT is significantly less than that for cutting the tubule in the middle wall owing to the release of curvature strain of the cap. It seems that the anodic pretreatment has enough energy to open the caps and cut nanotubes into short fragments. For comparison, a variety of carbon powders were also tested, including two KB carbons (specific surface areas of

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Table 2 The parameters for detection of nitrite in 0.05 M H2SO4 on CNTPME-Os(bpy)32þ Electrode

CNTPME CNTPME after anodic treatment CNTPME after anodic treatment and Os adsorption

Temperature coefficient (% (8C)1)

Activation energy (kJ mol1)

Sensitivity (A cm2 M1)

Step 1

Step 2

Step 1

Step 2

Step 1

Step 2

0.67 0.2 0.16

0.3 0.14

11.43 5.1 2.7

3.44 2.3

0.55 4.9

0.65 6.8

50 and 950 m2/g, respectively), RB carbon (1500 m2/g) and XC-72 (160 m2/g). In general, their double layer current increased about 1.5–2 times due to anodic treatment. However, such a pretreatment did not lead to any improvement in their ability of adsorbing Os(bpy)32þ. For all these materials, pre-adsorbed Os(bpy)32þ quickly leached out regardless with or without anodic pretreatment. Only the plateau current of step 1 for NO2 reduction occurred at these electrode in the potential region studied and it was essentially not affected by the pretreatment regardless of the increase in double layer capacity. The independence of nitrite reduction activity on the increase of electrochemical interfacial areas can be explained by the small size of the ultra-fine structures in these carbons. According to our previous analysis, the ultra-fine structures with sizes smaller than the thickness of the preceding chemical reaction zone has little contribution to accelerating the rate of the total reaction. This may be the situation with the carbons tested in this work except the nanotubes. In fact, most high-surface area carbons have a great part of the surface area in microporous structures where the pore sizes may be smaller than the thickness of the double layer. For activated carbons with specific surface area of 1000 m2 g1 only about 33% of surface areas can be used to form double layer [28]. In contrary, the outer and inner surface area of the carbon nanotubes opened is the major part of their electrochemical interface and is also usable for speeding up the CE reaction. The difference between nanotubes and the other carbons can be seen clearly from the comparison of treated nanotubes with KB950: they have similar double layer capacity but the former showed a much better performance in nitrite reduction. Above data further prove that high surface area electrodes can accelerate reduction of nitrite in acid solution and the PMEs is a useful simple version of this kind. Moreover, the carbon nanotube material showed unique properties and is a better choice than other carbon powders for this purpose. 3.4. Application of CNTPME-Os(bpy)32þ to detection of nitrite in acidic solution The performances of CNTPME-Os(bpy)32 for detection of nitrite in 0.05 M H2SO4 are listed in Table 2. Based on the current plateaus of the steps 1 and 2, the sensitivity of nitrite reduction reached 4.9 and 6.8 A cm2 M1, respectively.

Lower detection limit (mM)

0.1 0.1

The lower limit was estimated to be 107 M for S=N ¼ 4. The temperature coefficients were very low, 0.14– 0.16% (8C)1. The estimated activation energies for steps 1 and 2 were 2.3 and 2.7 kJ mol1, respectively. Impurities such as Fe2þ, Fe3þ, Mg2þ, Cl, NO3 and PO43 showed no interference to nitrite detection when their concentrations were 100 times higher than nitrite; but ascorbic acid did show interference to the current of step 2 of nitrite reduction. The high performance, simple preparation and miniaturizability make the CNTPME-Os(bpy)32þ a promising candidate for high quality nitrite sensors.

4. Conclusions Experiments show that the PME is effective method for characterization of carbon nanotubes. The anodic pretreatment increases real surface area of nanotubes and their ability to adsorb Os(bpy)32þ. Nitrite reduction in acidic solution is effectively catalyzed by CNTPME-Os(bpy)32þ. The anodic pretreatment may cut nanotubes shorter and create more openings at the ends to help hold adsorbed Os(bpy)32þ. The CTPME-Os(bpy)32þ made from anodically pretreated CNTPME may be a good candidate for high quality of NO2 sensors in acidic mediate.

Acknowledgements The authors are grateful to the National Natural Science Foundation of China for financial support to this work (code number: 20173041). References [1] M. Bertotti, D. Pletcher, Amperometric determination of nitrite via reduction with iodide using microelectrodes, Anal. Chem. Acta 337 (1997) 49–55. [2] A.P. Doherty, J.G. Vos, Electrocatalytic reduction of nitrite at an [Os(bipy)2(PVP)10Cl]Cl-modified electrode, J. Chem. Soc., Faraday Trans. 88 (1992) 2903–2907. [3] Peifang Liu, Juntao Lu, Jiawei Yan, Nitrite reduction at powder microelectrodes, J. Electroanal. Chem. 469 (1999) 196–200. [4] Peifang Liu, Jiawei Yan, Powder microelectrode modified with Nafion-Os(bpy)32þ and PVP composite and its catalysis for NO2 reduction, Electrochemistry 6 (2000) 146–150. (in Chinese).

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Biographies Peifang Liu received her MS degree in electrochemistry from Wuhan University, China in 1966. Presently, she is a professor in electrochemistry in Wuhan University, China. Her current fields of interest include electrochemical and bioelectrochemical sensors, chemical power sources, chemically modified electrodes and electrochemistry of nano materials, etc. Junhu Hu received his MS degree in Electrochemistry from Wuhan University, China in 2000. Now, he is a lecturer in Yun Yang University in Hubei province China. His current fields of interest are physical chemistry, electrochemical sensors and chemically modified electrodes, etc.