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

Carbon 40 (2002) 2201–2211

SERS studies on single-walled carbon nanotubes submitted to chemical transformation with sulfuric acid S. Lefrant a , I. Baltog b , *, M. Baibarac b , J.Y. Mevellec a , O. Chauvet a a

Institut des Materiaux de Nantes, Lab. de Physique Cristalline, 2 Rue de la Houssiniere, B.P. 32229, 44322, Nantes, France b National Institute of Materials Physics, Lab.160, Bucharest, P.O. Box MG-7, R-76900, Romania Received 9 April 2001; accepted 23 February 2002

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 21 , observed with 1064 nm and 676.44 nm excitation wavelength, respectively, and their 14 cm 21 up-shifted replicas i.e. the bands at 178 and 190 cm 21 . A different behavior of these bands in the anti-Stokes side was observed.  2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon nanotubes; B. Chemical treatment; C. Raman spectroscopy; D. Chemical structure

1. 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 u, which in turn depends on the n and m integers, which denote the number of unit vectors na 1 and ma 2 in the hexagonal lattice of the graphite: d(n,m) 5 Ch /p 5 3 1 / 2 a C – C (m 2 1 mn 1 n 2 )1 / 2 /p ; u 5 tan 21 [3 1 / 2 m /(m 1 2n)] *Corresponding author. Tel.: 140-1-493-0195; fax: 140-1493-0267. E-mail address: [email protected] (I. Baltog).

˚ is Ch is the length of the chiral vector C h , a C – C 51.42 A the nearest-neighbor C–C distance. The single-walled nanotube is metallic if n 2 m 5 3 k, k51,2,3 . . . , 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 twodimensional 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 [4,5,7–9]. This arises from the curvature of the carbon layers, which reduces the spatial atomic overlap turning the sp 2 -type hybridization of the carbon atoms, typical for graphite, into an intermediate between sp 2 and sp 3 [10–12]. The addition of functional organic groups such as dichlorocarbene and Birch reduction were used as

0008-6223 / 02 / $ – see front matter  2002 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 02 )00089-1

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a method for studying the side-wall reactivity of carbon nanotubes [10,13]. Another study concerned the intercalation of HNO 3 molecules into the nanotube bundles; nanotubes immersed for few hours in a 70% HNO 3 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 10 2 to 10 4 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 [18–20]. 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.

2. Experimental We used single-walled nanotubes produced and purified ´ of in the Groupe de Dynamique des Phases Condensees 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 as a SERS active support has been created by mechanical scratching with P 2500 abrasive paper leading to a SERS signal of ca. 25 times stronger than a regular Raman spectrum of SWNT powder sample. The sample prepared in such way was reacted with 0.5 M and 10 M aqueous sulfuric acid. For the recovery chemical treatment, the sample was dipped into 0.5 M KOH. Sulfuric acid and potassium hydroxide (Merck) were of synthetic grade. The SERS spectra were recorded at room temperature and in ambient air in a backscattering geometry under excitation at wavelengths of 676.44 and 1064 nm. 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 1064 nm excitation light we used an FT Raman Bruker RFS 100 spectrophotometer.

3. 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 21 two bands are found: A broad one in the range of 1500–1600 cm 21 associated to the tangential stretching modes (TM), and another, frequently referred as ‘D band’ which is not intrinsically related to the nanotube structure; it is also present in the Raman spectrum of graphite materials. The D band is indicative of disorder induced in the graphitic lattice or defects in nanotubes [2,3]. A distinguishing feature of the TM bands, namely the formation of new components with a maximum at |1540 cm 21 is due to the contribution of the metallic nanotubes, and appears at laser excitation energies of 1.7–2.2 eV [22], as seen in spectrum 1, Fig. 1a. This wide band, asymmetric towards lower frequencies, fits a Breit–Wigner–Fano (BWF) profile which indicates electron–phonon type interactions [23,24]. Another group, located in the high frequencies range from 1700 to 3500 cm 21 , corresponds to the second-order Raman spectrum. As a rule, the most intense bands are those detected at approximately twice the frequency of the D and TM bands. Like their first-order counterparts, they behave resonantly when the excitation wavelength is changed. A significant detail is that the relative intensity of these bands is lowest for excitation energies ranging from 1.7 to 2.2 eV, i.e for resonant excitation of the metallic tubes.

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Fig. 1. SERS spectra, at lexcit 51064 nm (1) and lexcit 5676.44 nm (2) of carbon nanotube films of ca.150 nm thickness deposited on rough Au supports. For wavenumbers greater than 2000 cm 21 the Raman signal displayed in spectrum 2 was multiplied by ten.

The third group, ranging from 100 to 350 cm 21 , which does not exist in graphite, is associated to the radial breathing modes (RBM); its peak position is related to the tube diameter according to the relation y (cm 21 )5223.75 / d (nm) [16]. Bands belonging to this group are very sensitive to the excitation wavelength; the intensity of each band is enhanced when the photon energy of the excitation light corresponds to a transition between the van Hove singularities (Eii ) in the valence and conduction bands of all possible nanotubes [25]. According to the above equation the two strongest bands 164 and 176 cm 21 observed for excitation wavelengths of 1064 nm and 676.44 nm indicate that the resonance occurs over a narrow range of diameters around of 1.36 and 1.27 nm, respectively. Further, using the calculated data regarding the allowed optical transitions for nanotubes of various diameters and helicities [26], these bands are identified with transitions E S22 and E M 11 for semiconducting and metallic tubes, respectively. One can see that the values of the transition energies, evaluated with Eii 5 2ia C – Cg0 /d [27] are E S22 (1.13 and E M 11 (1.94

eV, where g0 is the nearest-neighbor electronic overlap integral and i51,2,4,5,... for semiconducting (g0 52.7 eV), and i53,6,... for metallic tubes (g0 52.9 eV). This is close enough to the excitation energies of 1.165 eV (1064 nm) and 1.83 eV (676.4 nm) to have a resonant Raman process. In the case of the nanotube samples used in this study, the SERS spectra show that each Raman band, at 164 and 176 cm 21 , is accompanied by a weak band shifted towards high frequencies by |14 cm 21 , i.e. at ca.178 and 190 cm 21 . Any attempt to explain the origin of these bands as above is not satisfactory. As a rule, the tube diameter distribution broadens the radial bands inhomogeneously, often giving rise to many peaks of weaker intensities. In these circumstances the SERS method is of great advantage since the enhanced Raman signals make visible bands which are difficult to detect by the regular Raman technique. Such a case is presented in the inset of Fig. 1a, where we note three weak bands at ca.110, 102 and 85 cm 21 . The two former bands, related to the tubes of 2.03 and 2.2 nm diameter, are

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attributed by us to the metallic tubes whose calculated energies E M 11 of 1.23 and 1.14 eV, respectively, are not so far from the excitation energy 1.165 eV ( lexcit 51064 nm). The third band, appearing as a shoulder at ca. 85 cm 21 could be considered evidence for the transition E S33 51.15 eV for a semiconducting tube. The others peaks at 264, 279 and 328 cm 21 are combinations and overtone of the radial spectrum. The chemical transformation of the nanotubes is well illustrated in Fig. 2 by the changes in the radial and

tangential bands as a function of the excitation wavelength and the concentration of sulfuric acid. The chemical transformation means in the context of this work that a SWNT film deposited on an Au plate was dipped for 5 min into an aqueous solution of H 2 SO 4 , and later the SERS spectra were recorded with the dried samples. For the spectra shown in Fig. 2, before all other variations we note an important decrease of the nanotube characteristic Raman signal which signifies less scattering units as result of their transformation into a graphite salt [28,29]. Because

Fig. 2. The chemical transformation of the carbon nanotubes with H 2 SO 4 and KOH aqueous solution evidenced by SERS spectroscopy. The films of SWNTs of ca.150 nm thickness deposited on an Au plate were dipped for 5 min into 0.5 M H 2 SO 4 (curves 2 in a 1 , a 2 ,c 1 ,c 2 ) and 10 M H 2 SO 4 (curves 2 in b 1 ,b 2 ,d 1 ,d 2 ). The curves (3) illustrate the recovery process achieved by a subsequent reaction of 5 min with 0.5 M KOH. The curves (1) correspond to the initial state.

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such a process has to be more dependent on the chemical properties of the nanotubes than on their diameters, one expects the Raman spectra to reveal specifically the behavior of metallic and semiconducting tubes. Such a behavior is observed for the Raman bands associated to tangential vibration modes. Thus, at lexcit 51064 nm the intensity of the TM bands group, with the strongest band at about 1593 cm 21 , decreases uniformly without any modifications in shape and position. At lexcit 5676.44 nm, its variation is not uniform: It consists of a decrease or even loss of the low energy side, i.e. the component peaking at |1540 cm 21 , which represents the contribution of metallic nanotubes and a lesser diminishment at the high energy side with the maximum at about 1593 cm 21 . The chemical transformation of the nanotubes under action of the sulfuric acid is reversible to some extent if the sample is afterwards dipped into an aqueous 0.5 M KOH solution for a few minutes. As is expected, the restoration is not complete, it depends on reaction time and concentration of the chemical reagents, and appears different at the two excitation wavelengths. Fig. 2 is representative for the modification of the Raman spectra of carbon nanotubes by the action of sulfuric acid under 0.1 M and 10 M; no significant differences are observed if one works in the absence of oxygen, i.e. under an inert gas (Ar or N 2 ). The analogy with graphite intercalation from liquid phase indicates that H 2 SO 4 is a sufficiently strong oxidizing agent for the carbon nanotubes, although an oxidation reaction in sulfuric acid does not occur with graphite without addition of a strong oxidizing agent like K 2 Cr 2 O 7 , KMnO 4 , or CrO 3. For graphite an equation like nC graphite 1 (1 1 y)H 2 SO 4 2 e → (nC graphite ) (HSO 4 ) (H 2 SO 4 ) y 1 H 2

1

2

1

(1)

establishes the coexistence of neutral H 2 SO 4 molecules in the intercalate layers with bisulfate ions as a result of a charge transfer between the host carbon lattice and intercalated guests. A similar reaction may be considered for the carbon nanotubes reacting with sulfuric acid: [SWNT] 1 (1 1 y)H 2 SO 4 2 e 2 → [SWNT] 1 (HSO 4 )2 ? (H 2 SO 4 ) y 1 H 1

(2) 1

2

The formula [SWNT] (HSO 4 ) ?(H 2 SO 4 ) y , signifies a salt of the hydrogen sulfate type similar to that resulting from the graphite–sulfuric acid interaction. The formation of this compound means a decrease of the nanotubes’ number in the investigated sample and consequently a weaker specific Raman signal. Later on, the reaction of neutralization with an aqueous solution of KOH is described by the reaction (3): 2[SWNT] 1 (HSO 4 )2 ? (H 2 SO 4 ) y 1 2(2y 1 1) KOH → [SWNT] 1 [SWNT]O 1 2yK 2 SO 4 1 2KHSO 4 1 (4y 1 1) H 2 O

(3)

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Reaction (3) suggests a partial recovery of the nanotubes; a fraction is converted to an oxidized form with the oxygen bound on the nanotube surface. Normally, the portion of oxidized tubes increases with the oxidizing power of the chemical reagent which occurs by an increase of the concentrations of H 2 SO 4 or by addition of a strong oxidant like K 2 Cr 2 O 7 as shown by spectra 2 in Fig. 2(b 1 ,b 2 ,d 1 ,d 2 ) and Fig. 3, respectively. The supplementary modifications of the Raman spectra which appear in the latter case are better revealed for relative low concentrations of H 2 SO 4 , i.e. 0.1 M and 0.5 M. The transformation of carbon nanotubes by a chemical process occurring in a solution of 0.5 M H 2 SO 4 enriched with 3?10 22 M K 2 Cr 2 O 7 is illustrated by the spectra 2 in Fig. 3. In comparison with Fig. 2, one observes significant modifications of the Raman spectra at lexcit 51064 nm: (i) Absence of the characteristic radial band at 164 cm 21 indicates almost a total transformation of the nanotubes to other scattering units; (ii) the bands group associated to tangential vibration modes by change of profile, widening and up-shifts at |1600 cm 21 becomes similar with that observed on amorphous carbon; (iii) the relative increase of the D band intensity accompanied by the change of its profile from Lorentzian to Gaussian reveals a transition towards disordered state or a defect state. Similarly, at lexcit 5676.4 nm, the bands group associated to tangential vibration modes is replaced by a bands group with its maximum at |1605 cm 21 and the D band increases and gets a Gaussian profile with peak position at ca. 1330 cm 21 , very close to 1324 cm 21 observed for the D band with lexcit 51064 nm. This result has an important significance if one takes into consideration that the D band, present as a rule in the Raman spectra of graphitic materials, behaves resonantly at the change of the excitation wavelength; shifts its peak from 1350 to 1275 cm 21 (65 cm 21 ) when lexcit varies between 457.9 and 1064 nm [3]. Such a shift is observed even in the Raman spectra of amorphous carbon (dashed line in Fig. 3a 2 ,b 2 ) which can be considered the last stage attained of transformed nanotubes which still maintains the features of graphitic particles. The non-resonant behavior of the D band, revealed in spectra 2 in Fig. 3 by the same peak position at 1064 and 676.4 nm excitation wavelength, indicates that it is rather associated to a new compound which behaves differently than graphitic particles. The subsequent restoration reaction with 0.5 M KOH is still less complete, the smaller recovery factor, among |0.2 and |0.3 for the radial and tangential bands groups, demonstrates that a greater portion of tubes was irreversibly transformed. For this, an explanation is that by the reaction of SWNTs with (H 2 SO 4 1K 2 Cr 2 O 7 ) two compounds are formed, [SWNT] 1 (HSO 4 )2 ?(H 2 SO 4 ) y and [SWNT]O, the latter representing the oxidized nanotubes, formed by the action of the atomic oxygen developed in H 2 SO 4 solution by the addition of K 2 Cr 2 O 7 , for which the restoration reaction with 0.5 M KOH does not work.

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Fig. 3. Evidence by SERS spectra of the transformation of carbon nanotubes with 0.5 M H 2 SO 4 aqueous solution enriched with 3?10 22 M K 2 Cr 2 O 7 , spectra (2). The curves (3) illustrate the recovery process achieved by a subsequent reaction of 5 min with 0.5 M KOH. The curves (1) correspond to the initial state.

In principle, the reaction (3) implies that the bond of the oxygen to the tube surface occurs by formation of different species such as carboxyl groups (–COOH), hydroxyl (– COH), carbonyl (–CO) and epoxide groups ( ), the latter two being also met in the chemistry of fullerenes [30]. An analysis of the Raman spectra in terms of the modifications induced by the attachments of these species on the tube surface concludes that the product [SWNT]O could be an epoxide. The following facts support this: (i) If the carboxylic acid group is attached to the tube surface it has to generate a characteristic Raman band at 1650– 1675 cm 21 which is associated with C=O stretching vibration mode of carboxylic acids [31]. This band is not observed in our spectra, in consequence we believe that the carboxylic acid group is not present; (ii) the hydroxyl and carbonyl groups on tube surface must be noted by the Raman bands associated to the O–H and C=O vibration stretching modes at 3340–3380 cm 21 and 1725 cm 21 , respectively, which also are absent [31]. In addition, the

presence of C=O groups has to be related to a quinoid ring 21 featured by a Raman band at 600–630 cm [32], which also is absent in the Raman spectra after the reaction of the nanotubes with H 2 SO 4 and KOH. This result is still more significant if one takes into account that the band at 600–630 cm 21 can be intentionally created by the reaction of the SWNTs with a diluted aqueous solution of KMnO 4 (10 23 M), see the inset in Fig. 4. It is well known that quinoid rings and C=O groups are formed by the oxidation of higher polynuclear aromatic hydrocarbons with H 2 CrO 4 or KMnO 4 ; in consequence the appearance of the band at 600–630 cm 21 in Fig. 4 which increases gradually with increasing reaction time can be ascribed to the quinoid rings which are gradually formed. This band shows a non-resonant behavior at the change of the excitation wavelengths, 1064 and 676.4 nm, which is normal because the quinoid structure, once formed, is not specifically related to the semiconducting or metallic tubes. Fig. 4 demonstrates also that the variations of the Raman bands associated to the radial and tangential vibration modes are

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Fig. 4. Evidence by SERS spectra of the quinoid ring formation in carbon nanotube structure as a consequence of the presence of C=O groups. The films of SWNTs of ca. 150 nm thickness deposited on an Au plate were dipped into 10 23 M KMnO 4 for 3 h (dashed line) and 18 h (dotted line). The solid lines correspond to the initial state and Fig. 3c shows the SERS spectrum of KMnO 4 .

not necessarily correlated. A striking result of Fig. 4, which will be analyzed in a forthcoming paper, concerns the behavior of the TM bands group, which is very different for the metallic and the semiconducting component. Fig. 4c proves that the band at 600–630 cm 21 does not belong to KMnO 4. (iii) The absence in the Raman spectrum of a specific signature due to the epoxide species attached to the nanotube surface is not surprising if one draws an analogy to the fullerenes for which no differences between the Raman spectra of C 60 and C 60 O were observed [33]. Returning to Fig. 2 we find:

• The changes of the radial band intensities is the main indication for a chemical transformation of the SWNTs. The significant decrease of these bands after the reaction with H 2 SO 4 illustrates that there are fewer nanotubes in the measured sample because the largest part has been transformed to compounds mentioned in the reaction (2). The similar values for the relative decrease of the radial bands, 0.1 and 0.12 in Fig. 2a 1 and 2a 2 , show that the percentage of destroyed tubes is approximately the same for the semiconducting and metallic tubes. The subsequent reaction with 0.5 M KOH lead to a partial recovery of the radial Raman spectrum, of about 0.6Iinitial . This means that the

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difference, representing ca. 40% of the nanotubes, was transformed to other particles which have no specific Raman bands located in this spectral range. According to reaction (3), these particles could be the oxidized nanotubes. Normally, the portion of oxidized tubes increases with the oxidizing power of the chemical reagent; this occurs at higher concentrations of H 2 SO 4 , as one sees in Fig. 2b 1 ,d 1 , or by addition of K 2 Cr 2 O 7 . The others radial bands (|102 and |110 cm 21 ) vary similarly with the band at 164 cm 21 . Beside the decrease of intensity, the chemical reaction with sulfuric acid leads also to an up-shift of about 4 cm 21 of the radial bands. This shift suggests a decrease of the tubes’ diameter which results from the insertion between the tubes of the (HSO 4 )2 and (SO 4 )22 ions. An up-shift of the radial bands has been also observed when nanotubes were submitted to an increasing hydrostatic pressure [34]. • The TM bands group behaves differently. The striking result, observed at lexcit 5676.44 nm, is that by subsequent reaction with H 2 SO 4 the low energy side of the TM band, representing the contribution of nanotubes of metallic type, practically no longer exists. A more illustrative analysis can be obtained by comparing the variation in area rather than in intensity of the TM bands group. The reaction with 0.5 M H 2 SO 4 produces a decrease of the Raman signal associated to the TM bands group area by a factor of |0.3, that is the same for both excitation wavelengths. However, this variation reveals a significant fact, namely that at lexcit 5676.44 the decrease of the TM bands group is not uniform over the whole spectral range, it is more important for the components around |1540 cm 21 and less important, only |0.6, for the components peaking at about 1593 cm 21 . This reveals the two different contributions to the Raman spectrum, one due to metallic carbon nanotubes which is an asymmetric band of BWF profile peaking |1540 cm 21 , and another one at ca. 1593 cm 21 characteristic for the graphite and other graphitic particles. • The restoration of the tangential bands groups subsequent to the reaction with 0.5 M KOH is also different. At lexcit 51064 nm, it almost regains its initial intensity, |0.95Iinitial , without changes of profile and peak position, while at lexcit 5676.44 nm, the recovery mainly concerns the BWF component, with an increase of the whole area of TM bands group up to |0.56Iinitial without significant changes in intensity of the component peaking at about 1593 cm 21 , spectra 3 in Fig. 2a 2 ,c 2 . The similar values of the recovery factors of radial bands, |0.6 and |0.55 for lexcit 51064 and 676.44 nm, respectively, demonstrate that the reaction with H 2 SO 4 destroys in equal proportion the semiconducting and metallic tubes, Fig. 2a 1 ,a 3 . The recovery factor of |0.56 of the area of tangential bands group at lexcit 5676.44 nm is suggestive, it allows an

estimate of the portion of tubes destroyed, that is about 40% for each type. These particles, of graphitic type, resulting from the chemical transformation of the nanotubes are seen in the Raman spectrum by the TM Raman bands group with the main maximum at ca. 21 1593 cm and on the basis of reaction (3) we consider that they represent the oxidized tubes. This explains the greater value of the recovery factor, of |0.95, observed at lexcit 51064 nm at which the Raman spectra associated to tangential vibration modes are quite similar for the carbon nanotubes and graphitic particles like highly oriented pyrolytic graphite (HOPG) [2]. The chemical transformation of the carbon nanotubes and the way in which this is related to variations of the Raman spectra is an inciting subject. The Raman scattering by carbon nanotubes is a resonant process characterized by the enhancement of some Raman bands, appearance of a second-order Raman spectrum, and an anti-Stokes spectrum not related by a Maxwell–Boltzmann distribution to its corresponding Stokes spectrum. The last one has focused a great interest. An analysis of the Stokes and anti-Stokes resonant Raman spectra showed major differences in the tangential band spectra; one can excite separately only semiconducting or only metallic nanotubes in the Stokes and anti-Stokes spectra by an excitation energy selected in the range 1.49–2.19 eV [26]. This behavior is unique to SWNTs in comparison to other sp 2 -hybridized carbon-base materials and arises from the differences in the one-dimensional density of electronic states for metallic and semiconducting tubes. Systematic studies of the Stokes and anti-Stokes spectra of isolated SWNTs showed large asymmetries between the Stokes and anti-Stokes components for the radial breathing modes which, analyzed in the framework of resonant Raman theory, provide a reliable method for the determination of transition energy between van Hove singularities, Eii [35– 37]. Our recent SERS studies of carbon nanotubes have shown that the asymmetry between the Stokes and antiStokes components of the radial bands depends on film thickness, i.e. it is different for isolated and packed tubes. In this context the behavior of the packed tubes appears clearer on thicker films. An adequate thickness is ca. 150 nm for which still exist an electromagnetic enhancement of the Raman signal. The Stokes and anti-Stokes Raman spectra recorded on SWNT films of ca.150 nm thickness at excitation wavelengths of 1064 and 676.44 nm are shown in Fig. 5. The doted curves represent the anti-Stokes spectra calculated from the Maxwell–Boltzmann distribution: IaS 5 IS [(v0 2 v ) /(v0 1 v )] 4 exp(2hv /kT )

(4)

where v0 is frequency of the excitation light, h and k are the Planck and Boltzmann constants, respectively, T (K)

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Fig. 5. Effect of the sulfuric acid (0.5 M H 2 SO 4 ) on Stokes and anti-Stokes SERS spectra (radial bands) of the SWNT film at excitation wavelengths 1064 nm and 676.44 nm. The dotted curves display the anti-Stokes lines calculated with Eq. (4).

temperature and IStokes is the measured Stokes Raman profile [38]. We found that the two intense radial bands at 164 and 176 cm 21 which appear at lexcit 51064 nm and 676.4 nm, respectively, behave differently: The former, attributed to the semiconducting tubes shows an anti-Stokes spectrum different for the calculated spectrum from the Maxwell– Boltzmann distribution, and the latter, attributed to the metallic tubes, behaves as in a normal Raman scattering process. The deviation from the normal anti-Stokes replica is not related to a change of the sample temperature in the focused laser spot, this being verified by Stokes and anti-Stokes Raman spectra on sulfur. Also, some inhomogeneities on the measured sample cannot be invoked; using a normalized scale, the ratio Ianti-Stokes /IStokes remains almost unchanged everywhere on the sample surface. The reaction with sulfuric acid and subsequent KOH does not

change the anti-Stokes replica of the radial bands at 164 and 176 cm 21 . If one considers that the asymmetry between anti-Stokes and Stokes spectra is an indication of a resonant Raman process, it follows from the above data that such a process takes place rather for the semiconducting tubes. A supplementary argument is the appearance of the strongest second-order Raman spectrum located in the high frequencies range from 1700 to 3500 cm 21 . The fact that the intensities of the second-order Raman bands of the metallic tubes ( lexcit 5676.4 nm) are at least ten times weaker than the intensities of the corresponding bands of the semiconducting tubes ( lexcit 51064 nm), Fig. 1, suggests rather a normal Raman scattering process at lexcit 5 676.4 nm. Coming back to radial bands at 164 and 176 cm 21 , observed at the two excitation wavelengths, we note that each one has a shoulder up-shifted by ca. 14 cm 21 i.e. at

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about 178 and 190 cm 21 , whose variation resulted from the reaction of nanotubes with sulfuric acid is different in comparison with the main bands at 164 and 176 cm 21 . Analyzing the variation of the bands at 164 and 178 cm 21 observed at lexcit 51064 nm, and taking the original intensity as unity, after reactions with 0.5 M H 2 SO 4 and 0.5 M KOH we found their intensities in the ratios of 1 / 0.08 / 0.6 and 1 / 0.55 / 1, respectively. The same findings were made with the bands at 176 and 190 cm 21 observed at lexcit 5676.4 nm. Another significant result is that the bands at 178 and 190 cm 21 behave similarly on the anti-Stokes side; their intensity is higher than that calculated from the Maxwell–Boltzmann distribution. All these facts make it difficult to explain the origin of these bands on the basis of the same reasoning as for the radial bands at 164 and 176 cm 21 . A certain significance could have the fact that for the same lot of nanotubes and the same films thickness the SERS spectra show that the relative intensities of these bands depend on the dispersing quality of the solvent used in the films’ preparation; this suggests the involvement of the packed tubes in the bundles. Not long time ago, the equation y (cm 21 )5223.75 /d (nm) which relates the position of the radial band to tube diameter has been amended taking into account the intertubule interaction within a bundle y (cm 21 )5223.75 /d (nm)114 (cm 21 ) [39]. If these equations work separately on isolated and packed tubes then the difference of about 14 cm 21 between the two radial bands, at 164 and 178 cm 21 , and 176 and 190 cm 21 becomes very suggestive. In these circumstances, the different variation of intensities of the bands at 164 and 178 cm 21 , and 176 and 190 cm 21 , observed at lexcit 51064 nm and 676.4 nm, respectively, is explainable; it is consequence of the fact that a reaction with a chemical reagent has to be more energetic for the isolated tubes than the tubes packed in bundles which leads to a different variation rate of the intensities of corresponding Raman bands. In connection with this arise two questions: why these bands are enhanced in the anti-Stokes side and whether it is not possible that the tubes, metallic and semiconducting, packed together in the bundles form a different scattering system. A SERS process, due to the high density of the electromagnetic energy at the interface metal / dielectric, has the features of a resonant process: Enhanced Raman signal, asymmetry between Stokes and anti-Stokes spectra [40,41] and overtones [42]. In certain circumstances, when a diffraction grating is used as optical coupler, the SERS becomes a stimulated Raman process [42]. Although, a theoretical prediction of a SERS type process on carbon nanotube bundles in which are packed together metallic and semiconducting tubes, is missing yet, an analogy with a SERS system featured by the enhancement of the Raman scattering at the interface metal / dielectric could be drawn. Such a process is attempted to be invoked for the origin of the bands at |178 and |190 cm 21 which appears at lexcit 51064 nm and 676.4 nm, respectively.

4. 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, respectively. Chemical transformation of nanotubes was monitored by variations of the Raman bands group, ranging about 100–200 and 1100–1600 cm 21 , associated to radial (RBM) and tangential vibration (TM) modes, respectively. Using a semiquantitative analysis of the variations of SERS spectra, we have found that the reaction with H 2 SO 4 destroys in equal proportion the semiconducting and metallic tubes. The particles resulting from the irreversible transformation of nanotubes are oxidized nanotubes, whose characteristic Raman spectrum is formed of a bands group with the maximum at ca. 1593 cm 21 , similar to the spectrum of the highly oriented pyrolitic graphite (HOPG). Special attention was paid to variations of the radial bands, at 164 and 176 cm 21 and their 14 cm 21 up shifted replica at 178 and 190 cm 21 which are observed for the excitation wavelengths of 1064 and 676.44 nm, respectively. We have found for these bands a different anti-Stokes spectrum: the band at 164 cm 21 , attributed to the semiconducting tubes shows a weaker intensity in the anti-Stokes side than in a normal Raman scattering process and the band at 176 cm 21 , attributed to the metallic tubes, behaves as in a normal Raman process, i.e. the anti-Stokes intensity is equal to that calculated from the Maxwell–Boltzmann distribution. The up-shifted replicas of these bands, peaking at |178 and |190 cm 21 , behave similarly in the anti-Stokes side, both bands show a greater anti-Stokes intensity than in a normal Raman process. The action of the sulfuric acid induces also a different variation of the relative intensity for the bands at 164 and 176 cm 21 in comparison to their replica at 178 and 190 cm 21 , respectively. These facts seem to indicate two different resonant Raman light scattering processes: One, sensitive to tube diameter, occurs on the isolated tubes, metallic or semiconducting, and the other one, characterized by an increased value of the Ianti-Stokes /IStokes ratio compared with the ratio featuring a normal Raman process which rather takes place on the bundles. In the latter case an analogy is drawn to a SERS process, which can take place on metallic and semiconducting tubes packed together in the bundles.

Acknowledgements The authors thank the Groupe de Dynamique des Phases ´ of the Montpellier University for providing the Condensees

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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 Institute of Materials Physics, Bucharest.

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