Investigation on electrochemical properties of carbon nanotubes

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Abstract

The electrochemical behavior of carboxyl-modified carbon nanotube (CNT) films was investigated. The structure of the modified CNT was characterized by scanning electron microscopy, Raman spectroscopy and infrared spectroscopy. Experimental results showed that the physical structure of CNTs was not changed, but the ends of CNT were opened, and oxidized into carboxylate groups, which might react with other reagents. Cyclic voltammetry of Fe2+ was conducted in 0.2 M HClO4. A stable, quasi-reversible voltammetric response is seen for Fe3+/2+ at the carboxyl-modified CNT electrode, and the anodic and the cathodic peak potentials were 1.120 and 0.145 V vs. saturated calomel electrode at a scan rate of 0.15 V s−1, respectively. Both anodic and cathodic peak currents depended linearly on the square root of the scan rate over the range of 0.025–0.2 V s−1, which suggested that the process of the electrode reactions was diffusion-controlled. There were significant differences in voltammetry between the non-modified CNT surface and the carboxyl-modified CNT surface for Fe2+. The low level and stable detection of hydrazine was performed in a phosphate buffer pH 6.6. The peak current increased linearly with the hydrazine concentration from 0.01 to 1 mM and the concentration limit of quantitation was 0.01 mM. The results obtained are discussed in detail.

Introduction

Carbon nanotubes (CNTs) have attracted the attention of many scientists worldwide. The small dimensions, strength and the remarkable physical properties of these structures make them a very unique material with a whole range of promising applications [1], [2], [3], [4]. Depending on their atomic structure, CNTs behave electrically as metal or as a semiconductor [5], [6], [7]. The subtle electronic properties suggest that CNTs have the ability to promote electron-transfer reactions when used as an electrode in chemical reactions [8]. But as a nanotube is such an unusual macromolecule, it leads to indissolubility of nanotubes. However, chemical reactions might happen on nanotubes when some changes in the structure of nanotubes take place and some active functional groups appear on it, such as hydroxyl and carboxyl [9], [10], [11], [12]. However, the electrochemical properties of CNTs haven't been much studied so far. Several reports about CNT electrode in electrochemistry have appeared in journals, some of them did not show well-resolved voltammograms in aqueous solutions, such as Liu, about the cast films of SWNTs on Pt and Au electrodes [13], and Barisci, about the sheets of SWNTs [14]. Recently, our electrochemical experiments of CNT electrodes have gained a significant development in carboxyl-modified CNT electrodes.

In the current work, the carboxyl-modified CNT electrodes prepared by soaking in concentrated nitric acids [15] were compared with non-modified CNT electrodes in structure, stability, and electrode kinetics for some systems. Scanning electron microscopy (SEM), Ramam spectroscopy, infrared (IR) spectra and cyclic voltammetry revealed some important effects on carboxyl-modified CNT films, which might have substantial fundamental and electroanalytical value. The application of carboxyl-modified CNT electrodes in the low level and stable detection of hydrazine in phosphate buffer pH 6.6 is discussed.

Section snippets

Fabrication of nanotubes and carboxyl modification

CNTs were synthesized by hot filament chemical vapor deposition. The distance between the filament and the substrate was 21 mm (Mo net was placed between filament and substrate, and negative bias was put between Mo net and substrate) and the temperature of filament is approximately 1950 °C. The substrate Si, ∼1 cm2 and 0.5 mm thick was coated by a catalyst of the 1:1 mixture of Ni and Fe (NiFe) [16]. The film of NiFe was not pretreated with thickness of approximately 20 nm, and the size of

Structure analysis

An SEM image of the carboxyl-modified CNT film is shown in Fig. 1. CNTs diameters were in the range of 10–40 nm and the lengths of the CNT were between several hundreds nanometer and a few micrometer. From Fig. 1, it can be seen that the end of the carboxyl-modified CNTs were opened but the physical structure of the CNTs was not damaged.

The Raman spectrum of the carboxyl-modified film shown in Fig. 2 is qualitatively similar to that of non-modified CNT films, with no apparent additional peaks,

Conclusion

The carboxyl-modified CNTs were successfully obtained by treating raw CNTs in concentrated nitric acid. From SEM micrograph, Raman spectroscopy and IR spectra, we can see that the physical structure of the CNTs were not changed, but the ends of CNT were opened and oxidized into carboxylate groups, which may react with other reagents. The carboxyl-modified CNT film electrodes showed stable cyclic voltammetric behavior in 1 mM Fe2+/0.2 M HClO4 solution involving the reduction of the carboxylic

Acknowledgements

This work has been financially supported by the Applied Research in Basics Foundation (7327) of Scientific Committee of Chongqing City.

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