- Email: [email protected]
SERS spectroscopy studies on the electrochemical oxidation of single-walled carbon nanotubes in sulfuric acid solutions
Synthetic Metals 144 (2004) 133–142
SERS spectroscopy studies on the electrochemical oxidation of single-walled carbon nanotubes in sulfuric acid solutions S. Lefrant a,∗ , M. Baibarac a,b , I. Baltog b , J.Y. Mevellec a , L. Mihut b , O. Chauvet a a
Institut des Matériaux de Nantes, Lab. de Physique Cristalline, 2 rue de la Houssinière, B.P.32229, 44322 Nantes, France b National Institute of Materials Physics, Lab. 160, Bucharest, P.O. Box MG-7, R-76900, Romania Received 6 November 2003; received in revised form 17 February 2004; accepted 19 February 2004 Available online 15 April 2004
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 H2 SO4 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 λexc = 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. C60 O, 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 H2 SO4 0.5 M in 1-butanol is a further evidence of the nanotubes breakage. © 2004 Elsevier B.V. All rights reserved. Keywords: Carbon nanotubes; Surface-enhanced Raman scattering; Electrochemistry
1. 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
∗ Corresponding author. Tel.: +33-240-373-910; fax: +33-240-373-991. E-mail address: [email protected] (S. Lefrant).
0379-6779/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2004.02.010
[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 [9–11]. 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
134
S. Lefrant et al. / Synthetic Metals 144 (2004) 133–142
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 [20–22]. 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 H2 SO4 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 H2 SO4 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 H2 SO4 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.
solvent. To study the electrochemical properties of SWNT films, we applied both potentiostatic method and cyclic voltammetry. The two electrochemical methods were performed in a conventional three-electrode one-compartment cell, having as working electrode a film of SWNTs deposited on a 25 mm2 Au plate. The auxiliary electrode consisted of a spiral Pt wire. The potential of the working electrode was measured by reference to a saturated calomel electrode (SCE). Prior to SWNT film deposition, the Au plate was chemically treated to clean its surface until no detection of any parasitic Raman signal. The H2 SO4 solutions were argon purged prior to cyclic voltammetry studies on the SWNT films. For the potentiostatic study, we used both aqueous and non-aqueous (1-butanol) H2 SO4 solutions. H2 SO4 , 1-butanol (C4 H9 OH) and fullerene (C60 ) were of synthetic grade from Merck. Electrochemical measurements were carried out using a potentiostat–galvanostat type Princeton Applied Research (PAR), model 173, equipped with a digital coulometer, a PAR pulse generator, model 175 and a Philips-type X–Y recorder. Raman spectra were recorded at room temperature and in ambient air in a backscattering geometry under excitation wavelengths of 676.4 and 1064 nm. For the former excitation wavelength, we used a Raman spectrophotometer Jobin-Yvon T64000 equipped with a microprobe allowing to focus the laser spot on the sample within a micrometer scale. For the 1064 nm excitation light, we used an FT Raman Bruker RFS 100 spectrophotometer, which allows us to investigate sample areas of approx. 1 mm2 . SERS measurements on C60 films were carried out using Au supports with a rough microstructure in the range 10–100 nm, which were prepared according to the previously reported procedure [28].
3. Results and discussion Fig. 1a and b shows the 6th cyclic voltammetry curve recorded on an Au electrode alone and coated with a SWNT film, respectively. Cyclic voltammetry studies were performed in aqueous solution of 0.5 M H2 SO4 , in the potential range of (0; +1500) mV versus SCE at a sweep rate of 100 mV s−1 . Two oxidation peaks located at +1055 and +1170 mV versus SCE and two reduction peaks at +690 and +870 mV versus SCE are recorded for the Au electrode alone (Fig. 1a). According to Eq. (1) [29] the oxidation peaks indicate the formation of an oxide monolayer on the Au electrode:
2. Experimental
2Au + 3H2 O − 6e− → Au2 O3 + 6H+
We used single-walled carbon nanotubes produced by the electric arc technique [3,27]. SWNT (ca. 0.02 g) were dispersed in toluene (10 ml) and homogenized by ultrasonic treatment. The nanotube films, of 200 nm thickness, were deposited on gold supports by vacuum evaporation of the
The difference between the two peaks of oxidation and reduction potentials indicates that irreversible processes take place on the Au support, the gold oxide not entirely reduced at the returning sweep, i.e. from +1500 to 0 mV versus SCE. The voltammogram of the SWNT film deposited
(1)
S. Lefrant et al. / Synthetic Metals 144 (2004) 133–142
135
Fig. 1. Cyclic voltammograms obtained on an Au support alone (a) and coated with a SWNT film (b), carried out in 0.5 M H2 SO4 aqueous solution in the potential range (0; +1500) mV versus SCE (cycle 6) with a sweep rate of 100 mV s−1 . (c) shows the cyclic voltammogram of SWNT film deposited on Au support in 0.5 M H2 SO4 aqueous solution in the potential range (0; +1000) mV vs. SCE using same sweep rate as above.
on the Au support (Fig. 1b) shows an oxidation peak at +335 mV versus SCE and two reduction peaks at +310 and +860 mV versus SCE. In the scanning range (0; +1000) mV versus SCE (Fig. 1c) for the same number of cycles carried out on the SWNT film, one observes a well defined oxidation peak situated at +345 mV with a shoulder at +455 mV whose reduction replica are at ca. +330 and +450 mV, respectively. The quasi-reversible or irreversible character of the oxidation–reduction reactions which take place by sweeping of SWNT films in the potential ranges: (0; +1000) and (0; +1500) mV versus ECS in an aqueous solution of H2 SO4 0.5 M was established on the basis of the reversibility criteria of the CV technique and variations induced in SERS spectra. It is well known that for a reversible system, the potential of separation (E) of the anodic and cathodic peaks and the width peak are related by the Eqs. (2) and (3): E = Ep,a − Ep,c =
0.58 n
(2)
Ep − Ep/2 =
0.59 n
(3)
where Ep ,a and Ep ,c correspond to anodic and cathodic peak potential, respectively and Ep/2 is the half-wave potential. In the case of a reversible charge transfer involving one-electron and no coupled chemical reaction, the value of E and peak width are equal to about 60 mV and the ratio of cathodic to anodic peak currents ip,c /ip,a is equal to unity, respectively. At first sight, a reversible character of the process can be invoked for the potential range (0; +1000) mV versus SCE (Fig. 1c). However, SERS spectra show that in the potential range (0; +1000) mV, the electrochemical process has a quasi-reversible character. In Fig. 2, we display the two characteristic groups of Raman bands for SWNTs (curve 1). In the interval from 1100 to 1700 cm−1 (Fig. 2a), the two usual bands are found, i.e. one broad in the range 1500–1600 cm−1 associated to the tangential stretching modes (TM) and another called “D”, well known for being assigned to disorder state in graphite com-
136
S. Lefrant et al. / Synthetic Metals 144 (2004) 133–142
Fig. 2. Ex situ SERS spectra, at λexc = 1064 nm, of SWNTs film deposited on Au support, submitted to an electrochemical transformation carried out by cyclic voltammetry in 0.5 M H2 SO4 aqueous solution. Raman spectra 1, 2, 3 and 4 correspond to initial state of SWNT film and after 6, 12 and 24 cycles, respectively carried out in the potential range (0; +1000) mV vs. SCE with a sweep rate of 100 mV s−1 . Spectrum 5 exhibits the interaction of the SWNT film swept in the same potential range during of 24 cycles with an 0.5 M KOH aqueous solution.
pounds and/or defects in nanotubes [2,30]. The TM band consists of four Raman lines at ca. 1555, 1573, 1595 and 1610 cm−1 (insert in Fig. 2a), which are structured in this way as a consequence of the splitting of the graphite mode E2g2 resulting from a bending of the graphene layer [31]. In the other spectral range 125–225 cm−1 (Fig. 2b), one observes radial breathing modes (RBM), whose frequencies are related to the tube diameter according with the relation ν(cm−1 ) = 223.75/d (nm) [5]. As the successive CV scans increase on the SWNT film, from 6 to 24, additional changes in the Raman spectrum of the CNTs are observed: (i) a gradual decrease in intensity of the Raman bands in the spectral range 1500–1700 cm−1 ; (ii) a stronger decrease in intensity of the Raman peak at 164 cm−1 , inducing a ratio modification between the relative intensity of the 164 and 178 cm−1 bands (I164 /I178 ); and (iii) an enhanced intensity for the D band shifted from 1275 at 1284 cm−1 , with a profile changed from Lorentzian to Gaussian. This reveals a transition towards a disorder state or the formation of defects on nanotubes. These variations are also observed in the SERS
spectra of the SWNT films when an anodic polarization was applied to +500 and +1000 mV versus ECS in aqueous 0.5 M H2 SO4 solution (curves 1–3 in Fig. 3) through the potentiostatic technique. At a resonant excitation wavelength for metallic nanotubes, i.e. λexc = 676.4 nm, Raman spectra of a SWNT film submitted to a potentiostatic polarization of +500 and +1000 mV versus SCE (curves 1–3 in Fig. 4) exhibit the main modifications that can be described as it follows: (i) a decrease in the intensity of the RBM band simultaneously with a gradual shift from 176 to 182 cm−1 ; (ii) a decrease of ca. 45% at the high energy side of the TM band peaked at about 1591 cm−1 ; (iii) a decrease until disappearance of the component peaking at ∼ 1542 cm−1 . We note that for λexc = 676.4 nm, the analysis of the group of bands associated to TM vibrations, shows that the best fit reveals also four components [32]: three are of Lorentzian type at ca. 1606, 1588 and 1560 cm−1 , and width at half maximum of ca. 32, 26 and 26 cm−1 and a fourth one at ca. 1542 cm−1 of Breit–Wigner–Fano (BWF) type that exhibits an asymmetric profile towards lower energies, which indicates electron–phonon interaction [20]. Concerning the up-shift of the RBM Raman bands as well as the change of the I164 /I178 ratio a short commentary is necessary. For these bands, which are very sensitive to inter-tube interaction [33], a significant shift was observed upon intercalation of various doping species inside the SWNT bundles. Depending on the doping agent used, it was reported a down-shift of the RBM band in the case of alkali-intercalation SWNTs [34] and an up-shift of RBM band for both electrochemical p-doping of SWNTs [35] and SWNT/nitric acid chemical treatment [36]. At present, a complete explanation of the RBM band shifts is still under debate. So far, the variations have been explained on the base the change in an empirical intertube interaction fraction [36] and an electrochemical-induced stress [37]. We think that an explanation on the up-shift of RBM band, can be given taking in account the oxidation reactions which take place during anodic polarization of SWNT electrode in H2 SO4 solution. By analogy with the electrochemical studies carried out on a graphite–H2 SO4 system and according to [23] until +1000 mV versus SCE, the following two oxidation reactions are expected to take place: [SWNT] + (1 + y)H2 SO4 → [SWNT]+ (HSO4 )− (H2 SO4 )y + H+ + e−
(4)
[SWNT] + (HSO4 )− (H2 SO4 )y → [SWNT](1+x)+ (HSO4 )(1+x) − (H2 SO4 )(y−x) + xe− + xH+
(5)
The electrochemical de-insertion of H2 SO4 molecules and HSO4 − ions at the returning sweep of the SWNT film, i.e. from +1000 to 0 mV versus SCE, cannot take place as a consequence of the internal conversion of neutral sulfuric
S. Lefrant et al. / Synthetic Metals 144 (2004) 133–142
137
Fig. 3. Ex situ SERS spectra, at λexc = 1064 nm, of SWNT films deposited on Au support, electrochemically oxidized in 0.5M H2 SO4 aqueous solution at +500 mV (2), +1000 mV (3), +1500 mV (4) and +2000 mV (5) vs. SCE. The Raman spectrum (1) corresponds to the initial state of the SWNT film. The Raman spectrum (6) illustrates the recovery process achieved by a subsequence reaction with 0.5 M KOH, time of 5 min. of the SWNT film previously oxidized at +2000 mV vs. SCE. The Raman spectrum (5c) corresponds to the SWNT film deposited on Ag support, electrochemically oxidized in 0.5 M H2 SO4 aqueous solution at +2000 mV vs. SCE.
acid molecules to bisulfate anions in the intercalate tubes, according to reaction (6) [23]: H2 SO4 + e− → (HSO4 )− + 21 (H2 )
(6)
In our opinion for acid-intercalated SWNTs, the up-shift of the RBM band is induced by stress due to intercalated chemical compounds which in the present work is assimilated to a hydrostatic pressure effect on CNTs [38]. The compression effect originates in the dipole–dipole attraction forces between adjacent H2 SO4 molecules situated inside
the SWNT bundles. As described above, the radial breathing mode consist in two Raman lines at 164 and 178 cm−1 associated with isolated and bundled tubes, respectively [39,40]. We note that the gradual diminution of the ratio I164 /I178 as result of the decrease of the relative intensity of the 164 cm−1 band, observed in Figs. 2 and 3, on spectra 1–4, indicates that in the beginning the electrochemical oxidation of the isolated tubes takes place and afterwards that of bundled tubes. The de-insertion of H2 SO4 molecules and HSO4 − ions can be achieved chemically if the oxidized SWNT film
138
S. Lefrant et al. / Synthetic Metals 144 (2004) 133–142
Fig. 4. Ex situ SERS spectra, at λexc = 676.4 nm, of SWNT films deposited on Au support, electrochemically oxidized in 0.5M H2 SO4 aqueous solution at +500 mV (2), +1000 mV (3), +1500 mV (4) and +2000 mV (5) vs. SCE. The Raman spectrum (1) corresponds to the initial state of the SWNT film. The Raman spectrum (6) illustrates the recovery process achieved by a subsequence reaction with 0.5 M KOH, time of 5 min. of the SWNT film previously oxidized at +2000 mV vs. SCE.
is immersed in aqueous solution of 0.5 M KOH. Partial recovery of the nanotubes is illustrated in spectrum 5 of Fig. 2 by: (i) the increase in intensity of the Raman bands in the spectral ranges 125–225 and 1500–1700 cm−1 ; (ii) a change of the ratio between the intensities of the bands at 164 and 178 cm−1 , which shifts to 170 and 183 cm−1 , respectively; and (iii) the down-shift of the D-band from 1284 to 1280 cm−1 . We note that a similar up-shift of the RBM band was observed on recovered SWNTs with KOH after an oxidizing treatment in H2 SO4 with addition of K2 Cr2 O7 [22]. We think that the above results can be understood in terms of the chemical transformation of the oxidized SWNT film into KOH solution:
2[SWNT] + (HSO4 )− (H2 SO4 )y + 2(2y + 1)KOH → [SWNT] + [SWNT]O + 2yK2 SO4 + 2KHSO4 + (4y + 1)H2 O
(7)
Reaction (7) suggests only a partial recovery nanotubes, another part being converted in an oxidized form with oxygen covalently bound on the nanotube surface. Spectra 4 and 5 from Figs. 3 and 4 indicate a drastic transformation of both metallic and semiconducting SWNTs, when the oxidation potential reaches a value higher than +1000 mV versus SCE. This might be attributed to the use of water as solvent for H2 SO4 . It is well known that for intercalated graphite, above a cell potential of 1 V, “overcharging”
S. Lefrant et al. / Synthetic Metals 144 (2004) 133–142
in a first stage is found to stop, and “overoxidation” regime starts. The presence of H2 O in H2 SO4 allows the electrochemical formation of covalent C–O bonds and thus the graphitic network in the graphene sheet is irreversibly altered [23]. Consequently, it has been supposed that a so called “graphite oxide” is formed electrochemically. In our case, above +1000 mV, the oxidation reaction involves the breaking of the nanotubes and formation of fragments of different length, some of them like closed-shell fullerenes, which exists in two different forms. One has oxygen covalently bound on the surface of carbon nanotube fragments as in “graphite oxide” [23] or like closed-shell C60 O [41] and the other is similar to C60 @{H2 O}n complexes [42]. To illustrate these pictures, Raman spectra of SWNTs obtained, by anodic polarization to +1500 and +2000 mV versus SCE, at the two excitation wavelengths 676.4 and 1064 nm (spectra 4 and 5 from Figs. 3 and 4) are characterized by: (i) the decrease in intensity of the RBM band until its complete disappearance, (ii) the increase in intensity of the D-band which further shifts to ca. 1330 cm−1 and (iii) the appearance of a new band at ca. 1494 cm−1 , equally observed with the two excitation wavelengths. A similar band was observed in vibrational spectra of both fullerenes like C70 , C84 , C119 and its derivative compounds of the epoxide type (C60 O) and clathrates (C60 ·4C6 H6 , C60 ·CCl4 , C60 ·C2 HCl3 ) [41,43–45]. It appears as a result of a lowering of the high icosahedral (Ih ) C60 symmetry which allows the activation of silent and higher order vibrational modes, observable both in IR absorption and Raman spectra. The band at 1495 cm−1 becomes indicative of the breaking of the SWNTs and the formation of fragments of different size, including precursors of closed-shell fullerenes. We notice that this band peaks at 1475 cm−1 , when the electrochemical oxidation process is carried out on a SWNT film deposited on Ag support (spectrum 5c from Fig. 3). Variation of the SERS spectra with the metal type used as support is not surprising, it was also reported for conducting polymers as poly 3-hexyl thiophene and polyaniline [28]. It is well-known that the SERS spectrum originates in two basic enhancement mechanism: (i) electromagnetic, achieved by the resonant excitation of the surface plasmons and (ii) chemically, mainly due to charge transfer processes between the metallic substrates and adsorbed molecules. By using Ag and Au supports manipulated in air, we think that their different behavior is due to the interposition of an intermediate compound layer between CNT fragments and the metal substrates. Silver has a strong oxidation tendency to form a stable compound Ag2 O which, as a surface layer, prevents direct interaction between CNT fragments and the metal substrates. In this case, the SERS spectrum is similar to the regular Raman spectrum. For Au, which is the only metal that shows no direct reaction with oxygen even at high temperature, the lack of a covering oxide layer permits direct interaction with the adsorbed CNT fragments. Thus, an electron of the metal, excited by the incident photon, is transferred by tunnelling into an excited state of the adsorbed CNT fragments. In this way, the
139
Fig. 5. Ex situ SERS spectra, at λexc = 1064 nm, of SWNT films deposited on Au support, electrochemically oxidized in solution of 0.5 M H2 SO4 in 1-butanol at +500 mV (2), +1000 mV (3), +1500 mV (4), +2000 mV (5), +3000 mV (6) and +4000 mV (7) vs. SCE. The Raman spectrum (1) corresponds to the initial state of the SWNT film.
charge-transfer process induces another equilibrium geometry in the excited molecule. The return of the electron to the metal leaves CNT fragments into another vibrational excited state than the neutral molecule leading to the emission of a Raman-shifted phonon. At first sight, the invocation of the band at 1494 cm−1 can be seen as being a speculative experimental detail. An additional fact (see Fig. 5) is that this band is much more clearly observed when an alcohol is used as solvent for H2 SO4 . We note that, as the length of the hydrocarbonate chain of the alcohol increases (ROH, R = CH3 , C2 H5 , C3 H7 , C4 H9 ), the growth in intensity of the band at 1495 cm−1 is more and more pronunced. In Fig. 6a and b, one sees that the SERS spectra of C60 deposited on Au support, using the procedure of the solvent evaporation, are different when the films were prepared from solutions of C60 in toluene (1 mg ml−1 ) and C60 (5 mg) in toluene (2.5 ml) mixed with 1-butanol (2.5 ml), respectively. Raman spectra reveal all vibration active modes of C60 , i.e. two Ag and eight Hg modes [30]. Previous studies have shown that the SERS spectra of C60 depend on the type of the metal used as support, i.e. Au or Ag [32]. Influence of the metal substrate is noticed by the different magnitude of the measured Raman signal and the appearance of new bands. Variation of the Raman intensity with the support roughness relates a more efficiency of surface plasmons excitation. The chemical component to the SERS process is illustrated by different SERS spectra recorded on
140
S. Lefrant et al. / Synthetic Metals 144 (2004) 133–142
alcohols, etc. [47,48] forms donor–acceptor complexes. Recently, for “C60 –H2 O” system was reported the formation of the C60 @{H2 O}n complex, that corresponds to isolated C60 molecules in hydrated state, its origin being explained by the weak donor–acceptor interaction between the two constituents (the role of the electron donor having the water oxygen) [40]. Taking these results into account, we believe that two reactions take place on the SWNT electrode at an oxidation potential higher than +1000 mV versus SCE in the 0.5 M H2 SO4 solution. The first corresponds to the breaking of carbon nanotubes (CNTs) in fragments of different size having a fullerene behavior. The second reaction leads to the formation of some complexes of the type CNTs fragments@{H2 O}n and CNTs fragments @{C4 H9 OH}n where n is the number of H2 O or C4 H9 OH molecules. In the case of the aqueous H2 SO4 solution at oxidation potential higher than +1000 mV versus SCE, we expect also covalent bonding of the oxygen on the nanotube fragments surface. Coming back to Figs. 3 and 4, curves 6a and 6b show that the percentage of tubes which are restored is approximately the same for semiconducting and metallic SWNTs. The subsequent reaction with KOH leads to a partial recovery of the RBM band of about 6% from the initial intensity. This means that the difference, representing ca. 94% of the nanotubes, was transformed in other carbon nanoparticles. This deduction is in agreement with the presence of the additional Raman bands at about 258 and 769 cm−1 , not shown in the Figs. 3 and 4, which does not belong to CNTs and also is unrelated to the metal substrate. These bands are associate with Hg (1) and Hg (4) breathing vibration mode occurring in fullerenes-like particles [30]. The fact that after the reaction with KOH (Fig. 4), the intensity of the RBM, D and TM bands (λexc = 1064 nm) are ca. 6.5, 3.5 and 1.75 times weaker, respectively than the intensity of the corresponding bands of the samples previously oxidized at +1000 mV versus SCE (Fig. 2), indicates that the restoration degree is depending on the applied oxidation potential of the SWNT film. Fig. 6. SERS spectra (λexc = 1064 nm) of C60 film, deposited on Au support, smooth (a) and rough (b), by evaporation of toluene and of a mixture toluene : 1-butanol having the volumetric ratio equal with 1. (c) corresponds to the Raman spectrum (λexc = 1064 nm) of 1-butanol.
Au and Ag substrates. We note in this sense the appearance of a band at 341 cm−1 , when Au is used as metallic support for C60 film, which was associated to C60 –Au vibration [30,32,46]. In spite of the changes observed in the SERS spectra with the substrate roughness, the curves from Fig. 6a and b show that for the mixture of solvents, 1-butanol and toluene, the band at 1495 cm−1 is always present. On the other hand, Fig. 6c proves that it is not associated with 1-butanol. It is known that fullerenes reacting with donors of electrons, e.g. amines, poly(vinylpyrrolidinone),
4. Conclusions This paper reports new results concerning the electrooxidation of single-wall carbon nanotube (SWNT) films in 0.5 M H2 SO4 solution. Films of SWNTs of 200 nm thickness were deposited on Au supports to enhance the Raman signals through the excitation of the surface plasmons. The quasi-reversible and irreversible character of the oxidation–reduction reactions which take place at SWNT films/electrolyte interface was investigated by SERS spectroscopy and cyclic voltammetry (CV) for two scanning ranges: (0; +1000) and (0; +1500) mV versus SCE. The following results were obtained: (i) the anodic polarization of SWNT film until +1000 mV versus SCE leads to the formation of compounds similar to those resulted
S. Lefrant et al. / Synthetic Metals 144 (2004) 133–142
from bisulfate intercalated graphite. The electrochemical transformation of SWNTs was monitored by changes of the Raman bands, in the spectral ranges 120–240 and 1500–1700 cm−1 , associated to radial breathing modes (RBM) and tangential stretching modes (TM) vibrations, respectively; (ii) when SWNTs were submitted to an anodic polarization in a solution of H2 SO4 in water or 1-butanol, at the values of the oxidation potential higher than +1000 mV versus SCE, SERS spectra show a new Raman band peaked at 1495 cm−1 , which is also observed for C60 dissolved in toluene–1-butanol mixture. Its appearance in the SERS spectrum indicates the breaking of SWNTs in fragments of different size, some of them having a fullerene behavior; (iii) the semiquantitative analysis of the variation of Raman band associated with RBM vibrations, after a subsequent alkaline solution treatment of the oxidized SWNTs, has revealed that the restoration degree is depending on the applied oxidation potential of the carbon nanotube film. This fact is a consequence of the formation of compounds having the oxygen covalently bound on nanotube fragments surface and of the complexes of the type CNTs fragments@{H2 O}n and CNTs fragments @{C4 H9 OH}n . Acknowledgements Samples of SWNTs have been provided by the “Groupe de Dynamique des Phases Condensées” of the University of Montpellier II. Part of 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 Institute of Materials Physics, Bucharest and other was supported in the frame of a European program COMELCAN (HRPN-CT-2000-00128). The “Institut des Matériaux Jean Rouxel” is Unité Mixte de recherche CNRS-Université de Nantes No.6502.
References [1] A. Thess, R. Lee, P. Nikolaev, H. Dai, P. Petit, J. Robert, C. Xu, Y.H. Lee, S.G. Kim, A.G. Rinzler, D.T. Colbert, G.E. Scuseria, D. Tomanek, J.E. Fisher, R.E. Smalley, Science 273 (1996) 483. [2] C. Journet, W.K. Maser, P. Bernier, A. Loiseau, M. Lamy de la Chapelle, S. Lefrant, R. Lee, J.E. Fisher, Nature 388 (1997) 756; P.C. Eklund, R.E. Smalley, Appl. Phys. A. 67 (1998) 29. [3] C. Bower, A. Kleinhammes, Y. Wu, O. Zhou, Chem. Phys. Lett. 288 (1998) 481. [4] B. Gao, A. Kleinhammes, X.P. Tang, C. Bower, L. Fleming, Y. Wu, O. Zhou, Chem. Phys. Lett. 307 (1999) 153. [5] A.M. Rao, S. Bandow, E. Richter, P.C. Eklund, Thin Solid Films 331 (1998) 141. [6] M. Cinke, J. Li, C.W. Bauschlicher Jr., A. Ricca, M. Meyyappan, Chem. Phys. Lett. 376 (2003) 761. [7] L. Vallentini, L. Lozzi, C. Cantalini, I. Armentano, J.M. Kenny, L. Ottaviano, S. Santucci, Thin Solid Films 436 (2003) 95. [8] C.N.R. Rao, A. Govindaraj, R. Sen, B.C. Satishkumar, Mat. Res. Innovat. 2 (1998) 128.
141
[9] J. Sloan, J. Hammer, M.Z. Sibley, M.L.H. Green, Chem. Commun. 1 (1998) 347. [10] B.C. Satishkumar, A. Govindaraj, J. Mofoking, G.N. Subbanne, C.N.R. Rao, J. Phys. B.; At. Mol. Opt. Phys. 29 (1996) 4925. [11] B.W. Clare, D.L. Kepert, Inorgan. Chim. Acta 343 (2003) 1. [12] J. Chen, R.C. Haddon, S. Fang, A.M. Rao, W.H. Lee, E.C. Dickey, E.A. Greelke, J.C. Pendergrass, A. Chavan, B.E. Haley, R.E. Smalley, J. Mater. Res. 13 (1998) 2423. [13] R.C. Haddon, K. Raghavachari, Tetrahedron 52 (1996) 5207. [14] J. Chen, M.A. Hamon, H. Hu, Y. Chen, A.M. Rao, P.C. Eklund, C.R. Haddon, Science 282 (1998) 95. [15] P.G. Collins, K. Bradley, M. Ishigami, A. Zettl, Science 287 (2000) 1801. [16] J. Kong, N.R. Franklin, C. Zhou, M.G. Chapline, S. Peng, K. Cha, H. Dai, Science 287 (2000) 622. [17] Z. Yu, L.E. Brus, J. Phys. Chem. A 104 (2000) 10995. [18] S.H. Jhi, S.G. Louie, M.L. Cohen, Phys. Rev. Lett. 85 (2000) 1710. [19] V. Derycke, R. Martel, J. Appenzeller, Ph. Avouris, Appl. Phys. Lett. 80 (2002) 2773. [20] A.M. Rao, E. Richter, S. Bandow, B. Chase, P.C. Eklund, K.A. Williams, S. Fang, K.R. Subbaswamy, M. Menon, A. Thess, R.E. Smalley, G. Dresselhaus, M.S. Dresselhaus, Science 275 (1997) 187. [21] A. Kasuya, M. Sugano, T. Maeda, Y. Saito, K. Tohji, H. Takahashi, Y. Sasaki, M. Fukushima, N. Nishina, C. Horie, Phys. Rev. B 57 (1998) 4999. [22] S. Lefrant, I. Baltog, M. Baibarac, J.Y. Mevellec, O. Chauvet, Carbon 40 (2002) 2201. [23] G.U. Sumanasekera, J.L. Allen, S.L. Fang, A.L. Loper, A.M. Rao, P.C. Eklund, J. Phys. Chem. B 103 (1999) 4292. [24] P. Corio, P.S. Santos, V.W. Brar, G.G. Samsonidze, S.G. Chou, M.S. Dresselhaus, Chem. Phys. Lett. 370 (2003) 675. [25] J.N. Barisci, G.G. Wallace, R.H. Baughman, Electrochim. Acta 46 (2000) 509. [26] J.H. Chen, W.Z. Li, D.Z. Wang, S.X. Yang, J.G. Wen, Z.F. Ren, Carbon 40 (2002) 1193. [27] L. Vaccarini, C. Goze, R. Aznar, V. Micholet, C. Journet, P. Bernier, Synth. Met. 103 (1999) 2492. [28] M. Baibarac, M. Lapkowski, A. Pron, S. Lefrant, I. Baltog, J. Raman Spectrosc. 29 (1998) 825; M. Baibarac, M. Cochet, M. Lapkowski, L. Mihut, S. Lefrant, I. Baltog, Synth. Met. 96 (1998) 63. [29] L.D. Burke, P.F. Nugent, J. Electroanal. Chem. 444 (1998) 18. [30] M.S. Dresselhaus, G. Dresselhaus, P.C. Eklund, Science of Fullerenes and Carbon Nanotubes, New York, Academic Press, 1996. [31] N. Bendiab, R. Almairac, M. Paillet, J.L. Sauvajol, Chem. Phys. Lett. 372 (2003) 210. [32] S. Lefrant, I. Baltog, M. Baibarac, J. Schreiber, O. Chauvet, Phys. Rev. B 65 (2002) 2354011. [33] S. Rols, A. Righi, L. Alvarez, R. Almairac, C. Journet, P. Bernier, J.L. Sauvajol, A.M. Benito, W.K. Maser, E. Munoz, M.T. Martinez, G.F. de la Fuente, A. Girard, J.C. Ameline, Eur. Phys. J. B18 (2000) 201. [34] N. Bendiab, E. Anlaret, J.L. Bandgnies, A. Zahab, J.L. Sauvajol, P. Petit, C. Mathias, S. Lefrant, Phys. Rev. B 64 (2001) 245424. [35] M. Stoll, P.M. Rafailov, W. Frenzel, C. Thomsen, Chem. Phys. Lett. 375 (2003) 625. [36] A. Kukovecz, Ch. Kramberger, M. Holzinger, H. Kuzmany, J. Schalko, M. Mannsberger, A. Hirsch, J. Phys. Chem. B 106 (2002) 6374. [37] S. Gupta, M. Hughes, A.H. Windle, J. Robertson, Diamond and Related Materials, 2004, in press. [38] C. Thomsen, S. Reich, A.R. Goni, H. Jantoljak, P.M. Rafailov, I. Loa, et al., Phys. State Sol. B 215 (1999) 435; J. Sandler, M.S.P. Shaffer, A.H. Windle, M.P. Halsall, M.A. Montes-Morán, C.A. Cooper, R.J. Young, Phys. Rev. B 67 (2003) 35417.
142
S. Lefrant et al. / Synthetic Metals 144 (2004) 133–142
[39] P.C. Marcoux, J. Schreiber, P. Batail, S. Lefrant, J. Renouard, G. Jacob, D. Albertini, J.Y. Mevellec, Phys. Chem. Chem. Phys. 4 (2002) 2278. [40] J.L. Sauvajol, E. Anglaret, S. Rols, L. Alvarez, Carbon 40 (2002) 1697. [41] S. Lebedkin, H. Rietschel, G.B. Adams, J.B. Page, W.E. Hull, F.H. Hennrich, H.J. Eisler, M.M. Kappes, W. Krätschmer, J. Chem. Phys. 110 (1999) 11768. [42] G.V. Andrievsky, V.K. Klochkov, A.B. Boryuh, G.I. Dovbeshko, Chem. Phys. Lett. 364 (2002) 8.
[43] V. Schettino, M. Pagliai, L. Ciabini, G. Cardini, J. Phys. Chem. A 105 (2001) 11192. [44] N.H. Li, S.G. Sun, J. Electroanal. Chem. 436 (1997) 65. [45] S. Nath, H. Pal, D.K. Palit, A.V. Sapre, J.P. Mittal, J. Phys. Chem. B 102 (1998) 10158. [46] C. Giusca, M. Baibarac, S. Lefrant, O. Chauvet, I. Baltog, A. Devenyi, R. Manaila, Carbon 40 (2002) 1565. [47] M. Ichida, T. Sohda, A. Nakamura, Chem. Phys. Lett. 310 (1999) 373. [48] C. Ungurenasu, A. Airinei, J. Med. Chem. 43 (2000) 3186.
SERS spectroscopy studies on the electrochemical oxidation of single-walled carbon nanotubes in sulfuric acid solutions S. Lefrant a,∗ , M. Baibarac a,b , I. Baltog b , J.Y. Mevellec a , L. Mihut b , O. Chauvet a a
Institut des Matériaux de Nantes, Lab. de Physique Cristalline, 2 rue de la Houssinière, B.P.32229, 44322 Nantes, France b National Institute of Materials Physics, Lab. 160, Bucharest, P.O. Box MG-7, R-76900, Romania Received 6 November 2003; received in revised form 17 February 2004; accepted 19 February 2004 Available online 15 April 2004
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 H2 SO4 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 λexc = 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. C60 O, 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 H2 SO4 0.5 M in 1-butanol is a further evidence of the nanotubes breakage. © 2004 Elsevier B.V. All rights reserved. Keywords: Carbon nanotubes; Surface-enhanced Raman scattering; Electrochemistry
1. 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
∗ Corresponding author. Tel.: +33-240-373-910; fax: +33-240-373-991. E-mail address: [email protected] (S. Lefrant).
0379-6779/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2004.02.010
[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 [9–11]. 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
134
S. Lefrant et al. / Synthetic Metals 144 (2004) 133–142
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 [20–22]. 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 H2 SO4 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 H2 SO4 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 H2 SO4 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.
solvent. To study the electrochemical properties of SWNT films, we applied both potentiostatic method and cyclic voltammetry. The two electrochemical methods were performed in a conventional three-electrode one-compartment cell, having as working electrode a film of SWNTs deposited on a 25 mm2 Au plate. The auxiliary electrode consisted of a spiral Pt wire. The potential of the working electrode was measured by reference to a saturated calomel electrode (SCE). Prior to SWNT film deposition, the Au plate was chemically treated to clean its surface until no detection of any parasitic Raman signal. The H2 SO4 solutions were argon purged prior to cyclic voltammetry studies on the SWNT films. For the potentiostatic study, we used both aqueous and non-aqueous (1-butanol) H2 SO4 solutions. H2 SO4 , 1-butanol (C4 H9 OH) and fullerene (C60 ) were of synthetic grade from Merck. Electrochemical measurements were carried out using a potentiostat–galvanostat type Princeton Applied Research (PAR), model 173, equipped with a digital coulometer, a PAR pulse generator, model 175 and a Philips-type X–Y recorder. Raman spectra were recorded at room temperature and in ambient air in a backscattering geometry under excitation wavelengths of 676.4 and 1064 nm. For the former excitation wavelength, we used a Raman spectrophotometer Jobin-Yvon T64000 equipped with a microprobe allowing to focus the laser spot on the sample within a micrometer scale. For the 1064 nm excitation light, we used an FT Raman Bruker RFS 100 spectrophotometer, which allows us to investigate sample areas of approx. 1 mm2 . SERS measurements on C60 films were carried out using Au supports with a rough microstructure in the range 10–100 nm, which were prepared according to the previously reported procedure [28].
3. Results and discussion Fig. 1a and b shows the 6th cyclic voltammetry curve recorded on an Au electrode alone and coated with a SWNT film, respectively. Cyclic voltammetry studies were performed in aqueous solution of 0.5 M H2 SO4 , in the potential range of (0; +1500) mV versus SCE at a sweep rate of 100 mV s−1 . Two oxidation peaks located at +1055 and +1170 mV versus SCE and two reduction peaks at +690 and +870 mV versus SCE are recorded for the Au electrode alone (Fig. 1a). According to Eq. (1) [29] the oxidation peaks indicate the formation of an oxide monolayer on the Au electrode:
2. Experimental
2Au + 3H2 O − 6e− → Au2 O3 + 6H+
We used single-walled carbon nanotubes produced by the electric arc technique [3,27]. SWNT (ca. 0.02 g) were dispersed in toluene (10 ml) and homogenized by ultrasonic treatment. The nanotube films, of 200 nm thickness, were deposited on gold supports by vacuum evaporation of the
The difference between the two peaks of oxidation and reduction potentials indicates that irreversible processes take place on the Au support, the gold oxide not entirely reduced at the returning sweep, i.e. from +1500 to 0 mV versus SCE. The voltammogram of the SWNT film deposited
(1)
S. Lefrant et al. / Synthetic Metals 144 (2004) 133–142
135
Fig. 1. Cyclic voltammograms obtained on an Au support alone (a) and coated with a SWNT film (b), carried out in 0.5 M H2 SO4 aqueous solution in the potential range (0; +1500) mV versus SCE (cycle 6) with a sweep rate of 100 mV s−1 . (c) shows the cyclic voltammogram of SWNT film deposited on Au support in 0.5 M H2 SO4 aqueous solution in the potential range (0; +1000) mV vs. SCE using same sweep rate as above.
on the Au support (Fig. 1b) shows an oxidation peak at +335 mV versus SCE and two reduction peaks at +310 and +860 mV versus SCE. In the scanning range (0; +1000) mV versus SCE (Fig. 1c) for the same number of cycles carried out on the SWNT film, one observes a well defined oxidation peak situated at +345 mV with a shoulder at +455 mV whose reduction replica are at ca. +330 and +450 mV, respectively. The quasi-reversible or irreversible character of the oxidation–reduction reactions which take place by sweeping of SWNT films in the potential ranges: (0; +1000) and (0; +1500) mV versus ECS in an aqueous solution of H2 SO4 0.5 M was established on the basis of the reversibility criteria of the CV technique and variations induced in SERS spectra. It is well known that for a reversible system, the potential of separation (E) of the anodic and cathodic peaks and the width peak are related by the Eqs. (2) and (3): E = Ep,a − Ep,c =
0.58 n
(2)
Ep − Ep/2 =
0.59 n
(3)
where Ep ,a and Ep ,c correspond to anodic and cathodic peak potential, respectively and Ep/2 is the half-wave potential. In the case of a reversible charge transfer involving one-electron and no coupled chemical reaction, the value of E and peak width are equal to about 60 mV and the ratio of cathodic to anodic peak currents ip,c /ip,a is equal to unity, respectively. At first sight, a reversible character of the process can be invoked for the potential range (0; +1000) mV versus SCE (Fig. 1c). However, SERS spectra show that in the potential range (0; +1000) mV, the electrochemical process has a quasi-reversible character. In Fig. 2, we display the two characteristic groups of Raman bands for SWNTs (curve 1). In the interval from 1100 to 1700 cm−1 (Fig. 2a), the two usual bands are found, i.e. one broad in the range 1500–1600 cm−1 associated to the tangential stretching modes (TM) and another called “D”, well known for being assigned to disorder state in graphite com-
136
S. Lefrant et al. / Synthetic Metals 144 (2004) 133–142
Fig. 2. Ex situ SERS spectra, at λexc = 1064 nm, of SWNTs film deposited on Au support, submitted to an electrochemical transformation carried out by cyclic voltammetry in 0.5 M H2 SO4 aqueous solution. Raman spectra 1, 2, 3 and 4 correspond to initial state of SWNT film and after 6, 12 and 24 cycles, respectively carried out in the potential range (0; +1000) mV vs. SCE with a sweep rate of 100 mV s−1 . Spectrum 5 exhibits the interaction of the SWNT film swept in the same potential range during of 24 cycles with an 0.5 M KOH aqueous solution.
pounds and/or defects in nanotubes [2,30]. The TM band consists of four Raman lines at ca. 1555, 1573, 1595 and 1610 cm−1 (insert in Fig. 2a), which are structured in this way as a consequence of the splitting of the graphite mode E2g2 resulting from a bending of the graphene layer [31]. In the other spectral range 125–225 cm−1 (Fig. 2b), one observes radial breathing modes (RBM), whose frequencies are related to the tube diameter according with the relation ν(cm−1 ) = 223.75/d (nm) [5]. As the successive CV scans increase on the SWNT film, from 6 to 24, additional changes in the Raman spectrum of the CNTs are observed: (i) a gradual decrease in intensity of the Raman bands in the spectral range 1500–1700 cm−1 ; (ii) a stronger decrease in intensity of the Raman peak at 164 cm−1 , inducing a ratio modification between the relative intensity of the 164 and 178 cm−1 bands (I164 /I178 ); and (iii) an enhanced intensity for the D band shifted from 1275 at 1284 cm−1 , with a profile changed from Lorentzian to Gaussian. This reveals a transition towards a disorder state or the formation of defects on nanotubes. These variations are also observed in the SERS
spectra of the SWNT films when an anodic polarization was applied to +500 and +1000 mV versus ECS in aqueous 0.5 M H2 SO4 solution (curves 1–3 in Fig. 3) through the potentiostatic technique. At a resonant excitation wavelength for metallic nanotubes, i.e. λexc = 676.4 nm, Raman spectra of a SWNT film submitted to a potentiostatic polarization of +500 and +1000 mV versus SCE (curves 1–3 in Fig. 4) exhibit the main modifications that can be described as it follows: (i) a decrease in the intensity of the RBM band simultaneously with a gradual shift from 176 to 182 cm−1 ; (ii) a decrease of ca. 45% at the high energy side of the TM band peaked at about 1591 cm−1 ; (iii) a decrease until disappearance of the component peaking at ∼ 1542 cm−1 . We note that for λexc = 676.4 nm, the analysis of the group of bands associated to TM vibrations, shows that the best fit reveals also four components [32]: three are of Lorentzian type at ca. 1606, 1588 and 1560 cm−1 , and width at half maximum of ca. 32, 26 and 26 cm−1 and a fourth one at ca. 1542 cm−1 of Breit–Wigner–Fano (BWF) type that exhibits an asymmetric profile towards lower energies, which indicates electron–phonon interaction [20]. Concerning the up-shift of the RBM Raman bands as well as the change of the I164 /I178 ratio a short commentary is necessary. For these bands, which are very sensitive to inter-tube interaction [33], a significant shift was observed upon intercalation of various doping species inside the SWNT bundles. Depending on the doping agent used, it was reported a down-shift of the RBM band in the case of alkali-intercalation SWNTs [34] and an up-shift of RBM band for both electrochemical p-doping of SWNTs [35] and SWNT/nitric acid chemical treatment [36]. At present, a complete explanation of the RBM band shifts is still under debate. So far, the variations have been explained on the base the change in an empirical intertube interaction fraction [36] and an electrochemical-induced stress [37]. We think that an explanation on the up-shift of RBM band, can be given taking in account the oxidation reactions which take place during anodic polarization of SWNT electrode in H2 SO4 solution. By analogy with the electrochemical studies carried out on a graphite–H2 SO4 system and according to [23] until +1000 mV versus SCE, the following two oxidation reactions are expected to take place: [SWNT] + (1 + y)H2 SO4 → [SWNT]+ (HSO4 )− (H2 SO4 )y + H+ + e−
(4)
[SWNT] + (HSO4 )− (H2 SO4 )y → [SWNT](1+x)+ (HSO4 )(1+x) − (H2 SO4 )(y−x) + xe− + xH+
(5)
The electrochemical de-insertion of H2 SO4 molecules and HSO4 − ions at the returning sweep of the SWNT film, i.e. from +1000 to 0 mV versus SCE, cannot take place as a consequence of the internal conversion of neutral sulfuric
S. Lefrant et al. / Synthetic Metals 144 (2004) 133–142
137
Fig. 3. Ex situ SERS spectra, at λexc = 1064 nm, of SWNT films deposited on Au support, electrochemically oxidized in 0.5M H2 SO4 aqueous solution at +500 mV (2), +1000 mV (3), +1500 mV (4) and +2000 mV (5) vs. SCE. The Raman spectrum (1) corresponds to the initial state of the SWNT film. The Raman spectrum (6) illustrates the recovery process achieved by a subsequence reaction with 0.5 M KOH, time of 5 min. of the SWNT film previously oxidized at +2000 mV vs. SCE. The Raman spectrum (5c) corresponds to the SWNT film deposited on Ag support, electrochemically oxidized in 0.5 M H2 SO4 aqueous solution at +2000 mV vs. SCE.
acid molecules to bisulfate anions in the intercalate tubes, according to reaction (6) [23]: H2 SO4 + e− → (HSO4 )− + 21 (H2 )
(6)
In our opinion for acid-intercalated SWNTs, the up-shift of the RBM band is induced by stress due to intercalated chemical compounds which in the present work is assimilated to a hydrostatic pressure effect on CNTs [38]. The compression effect originates in the dipole–dipole attraction forces between adjacent H2 SO4 molecules situated inside
the SWNT bundles. As described above, the radial breathing mode consist in two Raman lines at 164 and 178 cm−1 associated with isolated and bundled tubes, respectively [39,40]. We note that the gradual diminution of the ratio I164 /I178 as result of the decrease of the relative intensity of the 164 cm−1 band, observed in Figs. 2 and 3, on spectra 1–4, indicates that in the beginning the electrochemical oxidation of the isolated tubes takes place and afterwards that of bundled tubes. The de-insertion of H2 SO4 molecules and HSO4 − ions can be achieved chemically if the oxidized SWNT film
138
S. Lefrant et al. / Synthetic Metals 144 (2004) 133–142
Fig. 4. Ex situ SERS spectra, at λexc = 676.4 nm, of SWNT films deposited on Au support, electrochemically oxidized in 0.5M H2 SO4 aqueous solution at +500 mV (2), +1000 mV (3), +1500 mV (4) and +2000 mV (5) vs. SCE. The Raman spectrum (1) corresponds to the initial state of the SWNT film. The Raman spectrum (6) illustrates the recovery process achieved by a subsequence reaction with 0.5 M KOH, time of 5 min. of the SWNT film previously oxidized at +2000 mV vs. SCE.
is immersed in aqueous solution of 0.5 M KOH. Partial recovery of the nanotubes is illustrated in spectrum 5 of Fig. 2 by: (i) the increase in intensity of the Raman bands in the spectral ranges 125–225 and 1500–1700 cm−1 ; (ii) a change of the ratio between the intensities of the bands at 164 and 178 cm−1 , which shifts to 170 and 183 cm−1 , respectively; and (iii) the down-shift of the D-band from 1284 to 1280 cm−1 . We note that a similar up-shift of the RBM band was observed on recovered SWNTs with KOH after an oxidizing treatment in H2 SO4 with addition of K2 Cr2 O7 [22]. We think that the above results can be understood in terms of the chemical transformation of the oxidized SWNT film into KOH solution:
2[SWNT] + (HSO4 )− (H2 SO4 )y + 2(2y + 1)KOH → [SWNT] + [SWNT]O + 2yK2 SO4 + 2KHSO4 + (4y + 1)H2 O
(7)
Reaction (7) suggests only a partial recovery nanotubes, another part being converted in an oxidized form with oxygen covalently bound on the nanotube surface. Spectra 4 and 5 from Figs. 3 and 4 indicate a drastic transformation of both metallic and semiconducting SWNTs, when the oxidation potential reaches a value higher than +1000 mV versus SCE. This might be attributed to the use of water as solvent for H2 SO4 . It is well known that for intercalated graphite, above a cell potential of 1 V, “overcharging”
S. Lefrant et al. / Synthetic Metals 144 (2004) 133–142
in a first stage is found to stop, and “overoxidation” regime starts. The presence of H2 O in H2 SO4 allows the electrochemical formation of covalent C–O bonds and thus the graphitic network in the graphene sheet is irreversibly altered [23]. Consequently, it has been supposed that a so called “graphite oxide” is formed electrochemically. In our case, above +1000 mV, the oxidation reaction involves the breaking of the nanotubes and formation of fragments of different length, some of them like closed-shell fullerenes, which exists in two different forms. One has oxygen covalently bound on the surface of carbon nanotube fragments as in “graphite oxide” [23] or like closed-shell C60 O [41] and the other is similar to C60 @{H2 O}n complexes [42]. To illustrate these pictures, Raman spectra of SWNTs obtained, by anodic polarization to +1500 and +2000 mV versus SCE, at the two excitation wavelengths 676.4 and 1064 nm (spectra 4 and 5 from Figs. 3 and 4) are characterized by: (i) the decrease in intensity of the RBM band until its complete disappearance, (ii) the increase in intensity of the D-band which further shifts to ca. 1330 cm−1 and (iii) the appearance of a new band at ca. 1494 cm−1 , equally observed with the two excitation wavelengths. A similar band was observed in vibrational spectra of both fullerenes like C70 , C84 , C119 and its derivative compounds of the epoxide type (C60 O) and clathrates (C60 ·4C6 H6 , C60 ·CCl4 , C60 ·C2 HCl3 ) [41,43–45]. It appears as a result of a lowering of the high icosahedral (Ih ) C60 symmetry which allows the activation of silent and higher order vibrational modes, observable both in IR absorption and Raman spectra. The band at 1495 cm−1 becomes indicative of the breaking of the SWNTs and the formation of fragments of different size, including precursors of closed-shell fullerenes. We notice that this band peaks at 1475 cm−1 , when the electrochemical oxidation process is carried out on a SWNT film deposited on Ag support (spectrum 5c from Fig. 3). Variation of the SERS spectra with the metal type used as support is not surprising, it was also reported for conducting polymers as poly 3-hexyl thiophene and polyaniline [28]. It is well-known that the SERS spectrum originates in two basic enhancement mechanism: (i) electromagnetic, achieved by the resonant excitation of the surface plasmons and (ii) chemically, mainly due to charge transfer processes between the metallic substrates and adsorbed molecules. By using Ag and Au supports manipulated in air, we think that their different behavior is due to the interposition of an intermediate compound layer between CNT fragments and the metal substrates. Silver has a strong oxidation tendency to form a stable compound Ag2 O which, as a surface layer, prevents direct interaction between CNT fragments and the metal substrates. In this case, the SERS spectrum is similar to the regular Raman spectrum. For Au, which is the only metal that shows no direct reaction with oxygen even at high temperature, the lack of a covering oxide layer permits direct interaction with the adsorbed CNT fragments. Thus, an electron of the metal, excited by the incident photon, is transferred by tunnelling into an excited state of the adsorbed CNT fragments. In this way, the
139
Fig. 5. Ex situ SERS spectra, at λexc = 1064 nm, of SWNT films deposited on Au support, electrochemically oxidized in solution of 0.5 M H2 SO4 in 1-butanol at +500 mV (2), +1000 mV (3), +1500 mV (4), +2000 mV (5), +3000 mV (6) and +4000 mV (7) vs. SCE. The Raman spectrum (1) corresponds to the initial state of the SWNT film.
charge-transfer process induces another equilibrium geometry in the excited molecule. The return of the electron to the metal leaves CNT fragments into another vibrational excited state than the neutral molecule leading to the emission of a Raman-shifted phonon. At first sight, the invocation of the band at 1494 cm−1 can be seen as being a speculative experimental detail. An additional fact (see Fig. 5) is that this band is much more clearly observed when an alcohol is used as solvent for H2 SO4 . We note that, as the length of the hydrocarbonate chain of the alcohol increases (ROH, R = CH3 , C2 H5 , C3 H7 , C4 H9 ), the growth in intensity of the band at 1495 cm−1 is more and more pronunced. In Fig. 6a and b, one sees that the SERS spectra of C60 deposited on Au support, using the procedure of the solvent evaporation, are different when the films were prepared from solutions of C60 in toluene (1 mg ml−1 ) and C60 (5 mg) in toluene (2.5 ml) mixed with 1-butanol (2.5 ml), respectively. Raman spectra reveal all vibration active modes of C60 , i.e. two Ag and eight Hg modes [30]. Previous studies have shown that the SERS spectra of C60 depend on the type of the metal used as support, i.e. Au or Ag [32]. Influence of the metal substrate is noticed by the different magnitude of the measured Raman signal and the appearance of new bands. Variation of the Raman intensity with the support roughness relates a more efficiency of surface plasmons excitation. The chemical component to the SERS process is illustrated by different SERS spectra recorded on
140
S. Lefrant et al. / Synthetic Metals 144 (2004) 133–142
alcohols, etc. [47,48] forms donor–acceptor complexes. Recently, for “C60 –H2 O” system was reported the formation of the C60 @{H2 O}n complex, that corresponds to isolated C60 molecules in hydrated state, its origin being explained by the weak donor–acceptor interaction between the two constituents (the role of the electron donor having the water oxygen) [40]. Taking these results into account, we believe that two reactions take place on the SWNT electrode at an oxidation potential higher than +1000 mV versus SCE in the 0.5 M H2 SO4 solution. The first corresponds to the breaking of carbon nanotubes (CNTs) in fragments of different size having a fullerene behavior. The second reaction leads to the formation of some complexes of the type CNTs fragments@{H2 O}n and CNTs fragments @{C4 H9 OH}n where n is the number of H2 O or C4 H9 OH molecules. In the case of the aqueous H2 SO4 solution at oxidation potential higher than +1000 mV versus SCE, we expect also covalent bonding of the oxygen on the nanotube fragments surface. Coming back to Figs. 3 and 4, curves 6a and 6b show that the percentage of tubes which are restored is approximately the same for semiconducting and metallic SWNTs. The subsequent reaction with KOH leads to a partial recovery of the RBM band of about 6% from the initial intensity. This means that the difference, representing ca. 94% of the nanotubes, was transformed in other carbon nanoparticles. This deduction is in agreement with the presence of the additional Raman bands at about 258 and 769 cm−1 , not shown in the Figs. 3 and 4, which does not belong to CNTs and also is unrelated to the metal substrate. These bands are associate with Hg (1) and Hg (4) breathing vibration mode occurring in fullerenes-like particles [30]. The fact that after the reaction with KOH (Fig. 4), the intensity of the RBM, D and TM bands (λexc = 1064 nm) are ca. 6.5, 3.5 and 1.75 times weaker, respectively than the intensity of the corresponding bands of the samples previously oxidized at +1000 mV versus SCE (Fig. 2), indicates that the restoration degree is depending on the applied oxidation potential of the SWNT film. Fig. 6. SERS spectra (λexc = 1064 nm) of C60 film, deposited on Au support, smooth (a) and rough (b), by evaporation of toluene and of a mixture toluene : 1-butanol having the volumetric ratio equal with 1. (c) corresponds to the Raman spectrum (λexc = 1064 nm) of 1-butanol.
Au and Ag substrates. We note in this sense the appearance of a band at 341 cm−1 , when Au is used as metallic support for C60 film, which was associated to C60 –Au vibration [30,32,46]. In spite of the changes observed in the SERS spectra with the substrate roughness, the curves from Fig. 6a and b show that for the mixture of solvents, 1-butanol and toluene, the band at 1495 cm−1 is always present. On the other hand, Fig. 6c proves that it is not associated with 1-butanol. It is known that fullerenes reacting with donors of electrons, e.g. amines, poly(vinylpyrrolidinone),
4. Conclusions This paper reports new results concerning the electrooxidation of single-wall carbon nanotube (SWNT) films in 0.5 M H2 SO4 solution. Films of SWNTs of 200 nm thickness were deposited on Au supports to enhance the Raman signals through the excitation of the surface plasmons. The quasi-reversible and irreversible character of the oxidation–reduction reactions which take place at SWNT films/electrolyte interface was investigated by SERS spectroscopy and cyclic voltammetry (CV) for two scanning ranges: (0; +1000) and (0; +1500) mV versus SCE. The following results were obtained: (i) the anodic polarization of SWNT film until +1000 mV versus SCE leads to the formation of compounds similar to those resulted
S. Lefrant et al. / Synthetic Metals 144 (2004) 133–142
from bisulfate intercalated graphite. The electrochemical transformation of SWNTs was monitored by changes of the Raman bands, in the spectral ranges 120–240 and 1500–1700 cm−1 , associated to radial breathing modes (RBM) and tangential stretching modes (TM) vibrations, respectively; (ii) when SWNTs were submitted to an anodic polarization in a solution of H2 SO4 in water or 1-butanol, at the values of the oxidation potential higher than +1000 mV versus SCE, SERS spectra show a new Raman band peaked at 1495 cm−1 , which is also observed for C60 dissolved in toluene–1-butanol mixture. Its appearance in the SERS spectrum indicates the breaking of SWNTs in fragments of different size, some of them having a fullerene behavior; (iii) the semiquantitative analysis of the variation of Raman band associated with RBM vibrations, after a subsequent alkaline solution treatment of the oxidized SWNTs, has revealed that the restoration degree is depending on the applied oxidation potential of the carbon nanotube film. This fact is a consequence of the formation of compounds having the oxygen covalently bound on nanotube fragments surface and of the complexes of the type CNTs fragments@{H2 O}n and CNTs fragments @{C4 H9 OH}n . Acknowledgements Samples of SWNTs have been provided by the “Groupe de Dynamique des Phases Condensées” of the University of Montpellier II. Part of 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 Institute of Materials Physics, Bucharest and other was supported in the frame of a European program COMELCAN (HRPN-CT-2000-00128). The “Institut des Matériaux Jean Rouxel” is Unité Mixte de recherche CNRS-Université de Nantes No.6502.
References [1] A. Thess, R. Lee, P. Nikolaev, H. Dai, P. Petit, J. Robert, C. Xu, Y.H. Lee, S.G. Kim, A.G. Rinzler, D.T. Colbert, G.E. Scuseria, D. Tomanek, J.E. Fisher, R.E. Smalley, Science 273 (1996) 483. [2] C. Journet, W.K. Maser, P. Bernier, A. Loiseau, M. Lamy de la Chapelle, S. Lefrant, R. Lee, J.E. Fisher, Nature 388 (1997) 756; P.C. Eklund, R.E. Smalley, Appl. Phys. A. 67 (1998) 29. [3] C. Bower, A. Kleinhammes, Y. Wu, O. Zhou, Chem. Phys. Lett. 288 (1998) 481. [4] B. Gao, A. Kleinhammes, X.P. Tang, C. Bower, L. Fleming, Y. Wu, O. Zhou, Chem. Phys. Lett. 307 (1999) 153. [5] A.M. Rao, S. Bandow, E. Richter, P.C. Eklund, Thin Solid Films 331 (1998) 141. [6] M. Cinke, J. Li, C.W. Bauschlicher Jr., A. Ricca, M. Meyyappan, Chem. Phys. Lett. 376 (2003) 761. [7] L. Vallentini, L. Lozzi, C. Cantalini, I. Armentano, J.M. Kenny, L. Ottaviano, S. Santucci, Thin Solid Films 436 (2003) 95. [8] C.N.R. Rao, A. Govindaraj, R. Sen, B.C. Satishkumar, Mat. Res. Innovat. 2 (1998) 128.
141
[9] J. Sloan, J. Hammer, M.Z. Sibley, M.L.H. Green, Chem. Commun. 1 (1998) 347. [10] B.C. Satishkumar, A. Govindaraj, J. Mofoking, G.N. Subbanne, C.N.R. Rao, J. Phys. B.; At. Mol. Opt. Phys. 29 (1996) 4925. [11] B.W. Clare, D.L. Kepert, Inorgan. Chim. Acta 343 (2003) 1. [12] J. Chen, R.C. Haddon, S. Fang, A.M. Rao, W.H. Lee, E.C. Dickey, E.A. Greelke, J.C. Pendergrass, A. Chavan, B.E. Haley, R.E. Smalley, J. Mater. Res. 13 (1998) 2423. [13] R.C. Haddon, K. Raghavachari, Tetrahedron 52 (1996) 5207. [14] J. Chen, M.A. Hamon, H. Hu, Y. Chen, A.M. Rao, P.C. Eklund, C.R. Haddon, Science 282 (1998) 95. [15] P.G. Collins, K. Bradley, M. Ishigami, A. Zettl, Science 287 (2000) 1801. [16] J. Kong, N.R. Franklin, C. Zhou, M.G. Chapline, S. Peng, K. Cha, H. Dai, Science 287 (2000) 622. [17] Z. Yu, L.E. Brus, J. Phys. Chem. A 104 (2000) 10995. [18] S.H. Jhi, S.G. Louie, M.L. Cohen, Phys. Rev. Lett. 85 (2000) 1710. [19] V. Derycke, R. Martel, J. Appenzeller, Ph. Avouris, Appl. Phys. Lett. 80 (2002) 2773. [20] A.M. Rao, E. Richter, S. Bandow, B. Chase, P.C. Eklund, K.A. Williams, S. Fang, K.R. Subbaswamy, M. Menon, A. Thess, R.E. Smalley, G. Dresselhaus, M.S. Dresselhaus, Science 275 (1997) 187. [21] A. Kasuya, M. Sugano, T. Maeda, Y. Saito, K. Tohji, H. Takahashi, Y. Sasaki, M. Fukushima, N. Nishina, C. Horie, Phys. Rev. B 57 (1998) 4999. [22] S. Lefrant, I. Baltog, M. Baibarac, J.Y. Mevellec, O. Chauvet, Carbon 40 (2002) 2201. [23] G.U. Sumanasekera, J.L. Allen, S.L. Fang, A.L. Loper, A.M. Rao, P.C. Eklund, J. Phys. Chem. B 103 (1999) 4292. [24] P. Corio, P.S. Santos, V.W. Brar, G.G. Samsonidze, S.G. Chou, M.S. Dresselhaus, Chem. Phys. Lett. 370 (2003) 675. [25] J.N. Barisci, G.G. Wallace, R.H. Baughman, Electrochim. Acta 46 (2000) 509. [26] J.H. Chen, W.Z. Li, D.Z. Wang, S.X. Yang, J.G. Wen, Z.F. Ren, Carbon 40 (2002) 1193. [27] L. Vaccarini, C. Goze, R. Aznar, V. Micholet, C. Journet, P. Bernier, Synth. Met. 103 (1999) 2492. [28] M. Baibarac, M. Lapkowski, A. Pron, S. Lefrant, I. Baltog, J. Raman Spectrosc. 29 (1998) 825; M. Baibarac, M. Cochet, M. Lapkowski, L. Mihut, S. Lefrant, I. Baltog, Synth. Met. 96 (1998) 63. [29] L.D. Burke, P.F. Nugent, J. Electroanal. Chem. 444 (1998) 18. [30] M.S. Dresselhaus, G. Dresselhaus, P.C. Eklund, Science of Fullerenes and Carbon Nanotubes, New York, Academic Press, 1996. [31] N. Bendiab, R. Almairac, M. Paillet, J.L. Sauvajol, Chem. Phys. Lett. 372 (2003) 210. [32] S. Lefrant, I. Baltog, M. Baibarac, J. Schreiber, O. Chauvet, Phys. Rev. B 65 (2002) 2354011. [33] S. Rols, A. Righi, L. Alvarez, R. Almairac, C. Journet, P. Bernier, J.L. Sauvajol, A.M. Benito, W.K. Maser, E. Munoz, M.T. Martinez, G.F. de la Fuente, A. Girard, J.C. Ameline, Eur. Phys. J. B18 (2000) 201. [34] N. Bendiab, E. Anlaret, J.L. Bandgnies, A. Zahab, J.L. Sauvajol, P. Petit, C. Mathias, S. Lefrant, Phys. Rev. B 64 (2001) 245424. [35] M. Stoll, P.M. Rafailov, W. Frenzel, C. Thomsen, Chem. Phys. Lett. 375 (2003) 625. [36] A. Kukovecz, Ch. Kramberger, M. Holzinger, H. Kuzmany, J. Schalko, M. Mannsberger, A. Hirsch, J. Phys. Chem. B 106 (2002) 6374. [37] S. Gupta, M. Hughes, A.H. Windle, J. Robertson, Diamond and Related Materials, 2004, in press. [38] C. Thomsen, S. Reich, A.R. Goni, H. Jantoljak, P.M. Rafailov, I. Loa, et al., Phys. State Sol. B 215 (1999) 435; J. Sandler, M.S.P. Shaffer, A.H. Windle, M.P. Halsall, M.A. Montes-Morán, C.A. Cooper, R.J. Young, Phys. Rev. B 67 (2003) 35417.
142
S. Lefrant et al. / Synthetic Metals 144 (2004) 133–142
[39] P.C. Marcoux, J. Schreiber, P. Batail, S. Lefrant, J. Renouard, G. Jacob, D. Albertini, J.Y. Mevellec, Phys. Chem. Chem. Phys. 4 (2002) 2278. [40] J.L. Sauvajol, E. Anglaret, S. Rols, L. Alvarez, Carbon 40 (2002) 1697. [41] S. Lebedkin, H. Rietschel, G.B. Adams, J.B. Page, W.E. Hull, F.H. Hennrich, H.J. Eisler, M.M. Kappes, W. Krätschmer, J. Chem. Phys. 110 (1999) 11768. [42] G.V. Andrievsky, V.K. Klochkov, A.B. Boryuh, G.I. Dovbeshko, Chem. Phys. Lett. 364 (2002) 8.
[43] V. Schettino, M. Pagliai, L. Ciabini, G. Cardini, J. Phys. Chem. A 105 (2001) 11192. [44] N.H. Li, S.G. Sun, J. Electroanal. Chem. 436 (1997) 65. [45] S. Nath, H. Pal, D.K. Palit, A.V. Sapre, J.P. Mittal, J. Phys. Chem. B 102 (1998) 10158. [46] C. Giusca, M. Baibarac, S. Lefrant, O. Chauvet, I. Baltog, A. Devenyi, R. Manaila, Carbon 40 (2002) 1565. [47] M. Ichida, T. Sohda, A. Nakamura, Chem. Phys. Lett. 310 (1999) 373. [48] C. Ungurenasu, A. Airinei, J. Med. Chem. 43 (2000) 3186.