SERS spectroscopy studies on the electrochemical oxidation of single-walled carbon nanotubes in sulfuric acid solutions

Abstract

Surface-enhanced Raman scattering (SERS) and cyclic voltammetry (CV) were used to investigate oxidation–reduction processes of single-wall carbon nanotube (SWNT) films deposited on Au supports in 0.5 M H₂SO₄ solutions. In the potential range (0; +1000) and (0; +1500) mV versus saturated calomel electrode (SCE), the oxidation–reduction reactions of SWNT films are quasi-reversible and irreversible, respectively. Anodic polarization of SWNT films until +1000 mV versus SCE produced compounds similar to the bisulfate intercalated graphite. Regardless of excitation wavelength, i.e. 1064 or 676.4 nm, variation in the Raman spectra exhibited a decrease in the intensity of the bands associated with the radial breathing mode (RBM) situated in the 120–240 cm⁻¹ spectral range. Also an increase in the intensity of the D band is accompanied an up-shift of this band. A gradual decrease of the Breit–Wigner–Fano component was observed at λ_exc=676.4 nm. Partial restoration of the Raman spectra was achieved by a subsequent alkaline solution treatment. Potentials higher than +1000 mV versus SCE resulted in SWNTs breakage and fragments of different length were formed such as closed-shell fullerene. This was observed in the SERS spectrum by: (i) the disappearance of the RBM band, (ii) the increased D-band shifted to ca. 1330 cm⁻¹ and (iii) the appearance of a new band at 1494 cm⁻¹, frequently observed also in the Raman spectrum of fullerenes on the type C₇₀, C₈₄, C₁₁₉, as well as in its derivative compounds (e.g. C₆₀O, clathrates, etc.). Appearance and increase in the intensity of the Raman band at 1494 cm⁻¹ as result of an anodic polarization of the SWNT film in solution of H₂SO₄ 0.5 M in 1-butanol is a further evidence of the nanotubes breakage.

Introduction

Carbon nanotubes (CNTs) are molecular structures of nanometric size, which together with fullerenes, are a key component in the synthesis of novel nanostructured materials. The physical and chemical properties of carbon nanotubes are therefore an inciting subject for both basic research and technological applications. These are two types of nanotubes, multi-walled (MWNTs) and single-wall carbon nanotubes (SWNTs), the latter has received the most interest due to their potential incorporation in nanoscale electronic devices. Regardless of synthesis method, microscopic studies have revealed that samples consist of bundles of 20–100 individual nanotubes aligned in a two-dimensional crystal packing arrangement over essentially their entire length [1], [2]. The bundles, also known as nanoropes, contain both metallic and semiconducting tubes.

Experimental studies of the physical and chemical properties of these materials have revealed that SWNTs can take part in chemical interactions either as bundles or as free standing tubules. SWNT bundles immersed in a HNO₃ solution for a long time leads to increased disorder and partial exfoliation of the nanotubes [3]. Higher disorder is also observed at the electrochemical intercalation of Li [4] and after chemical doping of SWNT bundles with various chemical compounds acting as electron acceptors or donors [5]. Oxidation has decisively helped to identify higher reactivity areas of SWNTs. Chemical treatments of the SWNTs with O₂ [6], CO₂ [7] and concentrated HCl solution [8], [9] revealed a process for opening the carbon tubes. It was determined that reactivity is prevailingly increased at the SWNT end caps [9], [10], [11]. This arises from the curvature of the carbon atom layers which reduces the spatial atomic overlap turning the sp²-type hybridization of the carbon atoms, specific of graphite, into an intermediate one between sp² and sp³ [12], [13]. The chemical interaction of SWNTs with dichlorocarbene and Birch reduction reaction have been used as methods for illustrating the chemical reactivity of carbon nanotube walls [12], [14].

Previously, it was shown that exposure to air and oxygen reduces the CNTs electrical resistance and increases their thermoelectrical power [15], [16], but no detailed structural information is yet available concerning nanotubes changes upon oxidation. The changes in electrical conductivity when SWNTs are exposed to air were first associated to chemical doping, namely charge-transfer interaction between oxygen molecules and CNTs [17], [18], but later studies showed that it should rather be attributed to the modification of the energy barriers at the nanotube/electrode contacts [19]. It remains unclear whether charge transfer is associated to chemisorbed or physisorbed oxygen [18]. In order to elucidate this point, we used Raman spectroscopy that has become a valuable tool to probe structural features of CNTs [20], [21], [22]. Concerning the electrochemical oxidation of SWNTs in sulfuric acid aqueous solutions, few in situ Raman [23], [24] and cyclic voltammetry (CV) [25], [26] studies have been reported so far. The presence of both H₂SO₄ molecules and HSO₄⁻ ions, in the interstitial channels between the tubes, has been proven by an up-shift of the tangential TM-band of 320 cm⁻¹ per hole per C-atom for semiconducting SWNTs [23]. Recently, Corio et al. [24] carried out Raman spectroscopy studies on SWNTs submitted to an electrochemical treatment in aqueous H₂SO₄ solutions, focusing on the change in the occupation of electronic states for metallic and semiconducting tubes. A high specific capacitance (115.7 F g⁻¹) of SWNTs was also reported [26].

In this paper, we show that in the scanning ranges (0; +1000) and (0; +1500) mV versus SCE, modifications induced by cyclic voltammetry in single-wall carbon nanotube (SWNT) films immersed in aqueous solutions of 0.5 M H₂SO₄ have a quasi-reversible and irreversible character, respectively. The presence in the SERS spectrum of a band at 1495 cm⁻¹, observed at oxidation potentials higher +1000 mV versus SCE, is due to the breaking of SWNTs in fragments of shorter length like closed-shell fullerenes. Chemical reaction of the bisulfate intercalated SWNTs with an alkaline solution resulted in partial recovery of the nanotubes. A semiquantitative analysis of the variation of the Raman band associated to radial breathing modes (RBM) revealed that the restoration degree is dependant on the oxidation potential applied to the SWNT film.