Elsevier

Synthetic Metals

Volume 139, Issue 2, 5 September 2003, Pages 521-527
Synthetic Metals

Selective thiolation of single-walled carbon nanotubes

https://doi.org/10.1016/S0379-6779(03)00337-0Get rights and content

Abstract

Single-walled carbon nanotubes (SWCNTs) were derivatized with thiol groups at the ends of the nanotubes. The carbon nanotubes (CNTs) were treated with acid mixtures and modified through a series of chemical reactions. Fourier transform infrared (FT-IR) spectroscopy and nuclear magnetic resonance (NMR) spectroscopy were used to verify the intermediate products of the oxidation and reduction reactions and the final products. The thiolated CNTs were adsorbed on micron-sized silver and gold particles as well as gold surfaces to study the interaction between the thiol groups of the nanotube and the noble metals. The thiol–metal adhesion was studied by scanning electron microscopy (SEM), atomic force microscopy (AFM), wavelength dispersive electron spectroscopy, and Raman spectroscopy. A new type of bonding between the CNT and a noble metal surface was proposed that involves a bow-type single-walled nanotube (SWNT) with its two ends strongly attached to the metal surface.

Introduction

Since its first discovery [1] following the mass production of fullerenes [2], the carbon nanotube (CNT) has been a subject of intense research in chemistry, physics, and materials science [3]. Both single-walled nanotube (SWNT) and multi-walled nanotube (MWNT) come in various sizes and chiralities, thereby exhibiting remarkably diverse physical properties. The CNT is expected to be potentially useful in electronic devices as a quantum wire [4], [5], [6], [7], [8], gas sensor industry [9], nanotweezer technology [10], chemical force microscopy [11], [12], high resolution atomic force microscopy (AFM), and field emission display [13], [14].

A widespread application of the CNT is hindered, however, because there is no proper way to control the production and manipulation of its physical and chemical characteristics. In particular, from a chemical point of view, the CNT is a molecule of little practical interest because of the absence of a functional group, despite the fact that it has a seemingly useful one-dimensional structural motif. Functionalization of the CNT has been pursued in a limited number of cases [15], [16], [17], mostly through linkage bonds. In this paper, we report our success in functionalization of the CNT by directly attaching thiol groups to the CNT through a series of chemical reactions. Such functionalization is selective in the sense that the thiol groups are attached only to the most reactive sites of the CNT, i.e. the ends of the nanotube. Because organic thiol derivatives are generally well known to interact strongly with noble metal surfaces [18], selective thiolation may be used to make a good electrical junction between a CNT and a metal electrode, or to position the CNT relative to a metal surface by taking advantage of the strong thiol–metal interaction.

Although the functionalization of the CNT could bring about numerous potential applications, it is difficult to carry out such a reaction in the first place because of the chemical inertness and low solubility of the CNT in any solvent. Despite such difficulties, however, it has been known that chemical reactions can take place at the defect sites of the CNT in a colloidal state [15], [16], [17]. Although we used basically the same thiolation schemes employed in these earlier studies, our CNT has notable differences from those of previous studies. First, it does not have a long alkyl chain that can be found in compounds such as CNT–(CH2)11–SH synthesized by Smalley and coworkers [15]. Because of the long and flexible alkyl chain, the latter compound does not anchor on metal surface in a specific orientation. Furthermore, the long alkyl chain will give rise to a large contact resistance between the metal and the CNT when the CNT is used as a nanowire in an electronic device. To overcome these problems, Liu et al. synthesized another type of compound, CNT–CONH–(CH2)2–SH, which has one amide bond and only two methylene groups between the CNT and the thiol group [16]. This compound, however, contains an amide bond that tends to react easily in an acidic or basic environment.

In this paper, we report the synthesis of a new form of thiolated CNT, generically represented as CNT–CH2–SH, which contains thiol groups almost directly linked to the main π-conjugated body of the CNT. This compound has a shorter linkage than any other thiolated CNTs reported to date, which means the contact resistance can be minimized for practical applications of the CNT as a nanowire. In addition, this form of thiolated CNT should be chemically much more inert because of the absence of a reactive functional group. Thiol groups were attached, via successive carboxylation, reduction, chlorination and thiolation, to the open ends of the CNT, which were formed by breaking the CNT by sonochemical activation. The intermediate and final products were characterized by various microscopic and spectroscopic methods.

Section snippets

Experiment

The CNTs we used were SWNTs (purchased from Carbon Nanotechnology, Inc.) in aqueous colloidal suspension with surfactant (Triton X-100) and NaOH. After filtration by using a Teflon membrane filter with a pore size of 200 nm, the CNTs were washed with distilled water and methanol to remove the surfactant and NaOH. The CNTs were treated with a H2SO4/HNO3 (3:1) mixture, and sonicated for 24 h at temperatures between 35 and 40 °C. The resulting suspension was filtered with a polycarbonante membrane

Results and discussion

In order to identify the functional groups at the end of the CNT after the acid mixture treatment, we took Fourier transform infrared (FT-IR) spectra. The spectrum (a) in Fig. 1 shows the FT-IR spectrum taken after the acid mixture treatment. The characteristic vibrational modes of carbonyl group (ca. 1700 cm−1) and hydroxyl group (ca. 3400 cm−1) are apparent. The peaks at ca. 2350 and 2900 cm−1 are due to the ambient CO2 and the parylene coating of the IR optics in the spectrometer, respectively.

Acknowledgements

This work was supported by the National R&D Project for Nano Science and Technology for S.K.K. and the Center for Molecular Catalysis Grant for K.K. Personnel support by the BK-21 Program is gratefully acknowledged.

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