Elsevier

Carbon

Volume 40, Issue 12, 2002, Pages 2201-2211
Carbon

SERS studies on single-walled carbon nanotubes submitted to chemical transformation with sulfuric acid

https://doi.org/10.1016/S0008-6223(02)00089-1Get rights and content

Abstract

Surface-enhanced Raman scattering (SERS) at 676.44 nm and 1064 nm excitation wavelengths was used to investigate chemical transformation of single-walled carbon nanotubes (SWNTs) deposited on a gold support. Sulfuric acid was used as the chemical reagent. Special attention was paid to the changes in the Raman bands associated to radial and tangential vibration modes. Partial restoration of the Raman spectra by a subsequent alkaline treatment indicates a transformation with a certain degree of reversibility. The recovery reaction achieved with a 0.5 M KOH solution showed that the variations of tangential and radial band groups are not correlated. The intensity changes of the radial bands is a principal indicator for the chemical transformation of the SWNTs. Particular attention was paid to radial bands at 164 and 176 cm−1, observed with 1064 nm and 676.44 nm excitation wavelength, respectively, and their 14 cm−1 up-shifted replicas i.e. the bands at 178 and 190 cm−1. A different behavior of these bands in the anti-Stokes side was observed.

Introduction

The investigation of the physical and chemical properties of carbon nanotubes is an inciting subject for fundamental research as well as for technological applications. A single-walled nanotube (SWNT) is as a graphene sheet rolled up into a seamless cylinder, with both ends capped with hemispheres made of hexagonal and pentagonal carbon rings [1]. Theoretical calculations have predicted that the electronic properties of SWNTs depend on the tube diameter d and on the helicity of the hexagonal carbon ring alignment on the nanotube surface, defined by a chiral angle θ, which in turn depends on the n and m integers, which denote the number of unit vectors na1 and ma2 in the hexagonal lattice of the graphite:d(n,m)=Ch/π=31/2aC–C(m2+mn+n2)1/2/π; θ=tan−1 [31/2m/(m+2n)]Ch is the length of the chiral vector Ch, aC–C=1.42 Å is the nearest-neighbor C–C distance. The single-walled nanotube is metallic if nm=3 k, k=1,2,3…, and semiconducting otherwise [2]. A slight variation in these parameters causes a shift from a metallic to a semiconducting state. No matter what the synthesis method, microscopic studies have revealed that the nanotubes form bundles of 20 to 100 individual tubes aligned in a two-dimensional crystal packing arrangement over essentially their entire lengths [3].

The chemical properties of the nanotubes are related to carbon atom rings as a structural element. Oxidation process studies led to major progress in understanding the chemical behavior of nanotubes. Chemical treatments with oxygen [4], carbon dioxide [5] and HCl solution [6], [7] revealed a process of opening of the carbon tubes from which it was inferred that chemical reactivity is highest at the end caps [4], [5], [7], [8], [9]. This arises from the curvature of the carbon layers, which reduces the spatial atomic overlap turning the sp2-type hybridization of the carbon atoms, typical for graphite, into an intermediate between sp2 and sp3[10], [11], [12]. The addition of functional organic groups such as dichlorocarbene and Birch reduction were used as a method for studying the side-wall reactivity of carbon nanotubes [10], [13]. Another study concerned the intercalation of HNO3 molecules into the nanotube bundles; nanotubes immersed for few hours in a 70% HNO3 solution reveal an increased disorder and partial exfoliation of the bundles [14]. Particular interest was paid to the investigation of the chemical and electrochemical doping of SWNTs with various electron donors and acceptors [15], [16]. As a rule, a chemical transformation of the nanotubes changes the phonon spectrum. To identify the intrinsic properties of the transformed tubes, a Raman study on a small number of nanotubes is needed. An appropriate method is provided by surface-enhanced Raman scattering (SERS), which operates with enhanced Raman signals of 102 to 104 times [17]. The double origin of the enhancement of the Raman signal—electromagnetic and chemical—causes often that the SERS spectra differ considerably from the regular Raman spectra. The electromagnetic enhancement is related to the resonant excitation of surface plasmons (SPs) and the chemical component is mainly due to charge transfer processes taking place between the metallic substrate and adsorbed molecules [17]. The selection of film thickness is an important parameter in SERS studies because a surface chemical reaction involves only few molecular layers and the penetration depth of the surface electromagnetic wave associated to the surface plasmons is much larger, sometimes over 10 nm. Correspondingly, it helps to adjust the weight of the chemical component in SERS generation. Earlier SERS studies on conducting polymers and carbon nanotubes deposited on an Au or Ag substrate of average roughness of ∼50 nm showed that the surface chemical effects are easily observed on films of up to ∼30 nm thickness, that the electromagnetic enhancement prevailed for the thicker films, and that an enhanced Raman signal can still be observed with films of∼150 nm thickness [18], [19], [20].

This work presents new SERS data on the transformation of SWNT films submitted to a chemical attack by sulfuric acid. A semiquantitative analysis of the variations of Raman bands associated to radial and tangential vibration modes revealed an irreversible transformation of the nanotubes of metallic type. In order to avoid some perturbing effects due to surface chemical reactions, we used nanotube films of ca. 150 nm thickness. The different behavior of the metallic and semiconducting tubes is revealed by the use of resonant excitation wavelength at 676.44 and 1064 nm, respectively.

Section snippets

Experimental

We used single-walled nanotubes produced and purified in the Groupe de Dynamique des Phases Condensées of Montpellier University [21]. SWNTs are insoluble in toluene. The films of ca. 150 nm thickness have been obtained by the evaporation of toluene from a known amount of nanotubes dispersed in toluene. The ‘solutions’ of 0.2 wt% nanotubes were made several days in advance and were ultrasonically homogenized for ca. 30 min immediately before film preparation. A suitable roughness of an Au plate

Results and discussion

The SERS spectra on SWNT films at 1064 nm and 676.44 nm excitation wavelengths are presented in Fig. 1. The Raman spectra exhibit the well known three main groups of bands whose relative intensities and peak positions vary with the excitation wavelengths. In the interval from 1100 to 1700 cm−1 two bands are found: A broad one in the range of 1500–1600 cm−1 associated to the tangential stretching modes (TM), and another, frequently referred as ‘D band’ which is not intrinsically related to the

Conclusions

We have investigated the chemical transformation of carbon nanotubes by SERS spectroscopy. Sulfuric acid was used as chemical reagent. The transformation of carbon nanotubes through the action of sulfuric acid is featured by a certain reversibility, a subsequent alkaline treatment applied to the reacted sample restores the Raman spectrum. SERS spectra were measured at two excitation wavelengths, 676.44 nm and 1064 nm, which reveal better the behavior of metallic and semiconducting tubes,

Acknowledgements

The authors thank the Groupe de Dynamique des Phases Condensées of the Montpellier University for providing the SWNT samples. They wish to express their full appreciation to Professor H.P. Boehm and the referees for their useful comments and suggestions to improve this paper. This work was performed in the frame of the Scientific Cooperation between the Laboratory of Crystalline Physics of the Institute of Materials, Nantes, and the Laboratory of Optics and Spectroscopy of the National

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