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Sidewall Functionalization of Single-WalledCarbon Nanotubes by OrganometallicChromium-Centered Free Radicals
A. S. Lobach
Institute of Problems of Chemical Physics RAS, Chernogolovka,
Moscow Region, Russia
R. G. Gasanov
Nesmeyanov Institute of Organoelement Compounds RAS,
Moscow, Russia
E. D. Obraztsova
Natural Sciences Center of General Physics Institute RAS,
Moscow, Russia
A. N. Shchegolikhin
Institute of Biochemical Physics RAS, Moscow, Russia
V. I. Sokolov
Nesmeyanov Institute of Organoelement Compounds RAS,
Moscow, Russia
Abstract: The interaction of organometallic chromium-centered free radicals
generated by the homolytic dissociation of (pentamethylcyclopentadienyl)chromium-
tricarbonyl dimer in toluene with single-walled carbon nanotubes (SWNT) was inves-
tigated using ESR spectroscopy. Low values of g-factors of the radical species formed
from chromium-centered free radicals and SWNT as well as invariability of disorder
mode (D band) intensity in Raman spectra of pristine and functionalized SWNT
after this reaction indicated that chromium-centered free radicals added to the
Received 24 February 2005, Accepted 28 April 2005
Address correspondence to A. S. Lobach, Institute of Problems of Chemical Physics
RAS, 142432 Chernogolovka, Moscow Region, Russia. E-mail: lobach@icp.ac.ru
Fullerenes, Nanotubes, and Carbon Nanostructures, 13: 287–297, 2005
Copyright # Taylor & Francis, Inc.
ISSN 1536-383X print/1536-4046 online
DOI: 10.1080/15363830500237085
287
surface of nanotubes through rather oxygen atoms than to sidewall carbon atoms. This
is the first chromium-derivatization of carbon nanotubes.
Keywords: Single-walled carbon nanotubes, chromium-centered free radical, func-
tionalized SWNT, ESR spectroscopy, Raman spectroscopy
INTRODUCTION
Single-walled carbon nanotubes have unique structure-dependent physical
properties, which permit them to find a broad potential application in various
areas of science and technology (1). Chemical reactivity of nanotubes
attracts great attention since the controlled covalent sidewall functionalization
of SWNT can widen perspectives of their participations in chemical reactions
in solutions (2–5). Some reactions of SWNT with coordination compounds
(6–8) and salts of metals are known (9). It was found (6) that raw and
oxidized carbon nanotubes reacted with the Vaska’s complex, trans-
IrCl(CO)(PPh3)2, in two pathways: h2-coordination with raw nanotubes and
coordination through oxygen atoms with oxidized nanotubes to form
covalent nanotube-metal complexes. Oxidized carbon nanotubes also reacted
with the Wilkinson’s complex, RhCl(PPh3)3, by Rh coordination through
oxygen atoms of sidewall SWNT to form a hexacoordinate structure around
the rhodium atom (7). Vanadyl salen complexes were covalently anchored to
the mercapto-modified SWNT through a radical chain mechanism (8). The
interaction of lanthanide salts with oxidized SWNT was found (9) to occur
to form ionic-type bonding. Thus, binding of metals with tubes through
oxygen atoms appear to be quite usual for oxidized carbon nanotubes.
Recently we have reported the addition of the organometallic chromium-
centered free radicals to fullerenes C60 and C70 and found theh2-structure of the
spin-adducts stable in solution from ESR spectroscopy and quantum-chemical
calculations (10, 11). In this paper, we wish to describe the extension of this
approach for SWNT to afford the first chromium-derived nanotubes.
Free radicals were prepared by the homolytic dissociation of the (penta-
methylcyclopentadienyl)chromiumtricarbonyl dimer, [Cp�Cr(CO)3]2. For
very recent review of chemistry of these compounds (12). A number of
methods: UV-Vis-NIR spectroscopy, ESR, thermogravimetric analysis
(TGA), transmission electron microscopy (TEM), scanning electron
microscopy (SEM), and Raman spectroscopy provided a direct evidence for
a chemical attachment of functional groups to the tubes.
EXPERIMENTAL
The experiments described here were performed using purified high pressure
carbon monoxide decomposition (HiPco) SWNT. The raw materials of HiPco
SWNT were purchased from Carbon Nanotechnologies, Inc. and purified
A. S. Lobach et al.288
according to existing purification protocols modified by us (13, 14). The metal
content after purification was measured by TGA analysis in air and was deter-
mined to be approximately 3 wt.%.
Chemical functionalization of purified HiPco SWNT (sample A) is based
on a radical addition to sidewalls of oxidized carbon nanotubes. The source of
radicals was a (pentamethylcyclopentadienyl)chromiumtricarbonyl dimer,
[Cp�Cr(CO)3]2, which dissociates at room temperature in solution into two
equivalent metal-centered radicals as shown:
½Cp�CrðCOÞ3�2 �! 2�Cp†CrðCOÞ3
Functionalization Procedures
Sample A (2.5 mg) was suspended (ultrasonic disruptor, 150 W, 1 hour) in
toluene (5 ml) saturated with argon, and a solid sample of the
[Cp�Cr(CO)3]2 dimer (1.8 mg) was added in an argon flow (1 : 62 mol C/mol dimer). The heterogeneous reaction mixture was stirred at 158C for 7
hours. The reaction products were separated from the solution by vacuum fil-
tration through a track membrane (0.2mm; JINR, Dubna, Russia), washed
with solvents and dried in air. Black solid films of the resulting materials
were easily peeled off from the filter, dried for 8 hours in vacuum at
T ¼ 1008C and weighed (sample B).
The Raman spectra were acquired on a Jobin Yvon S-3000 spectrometer.
The spectra were excited by irradiation of Arþ-ion laser (514.5 nm). The UV-
VIS-NIR absorption spectra of SWNT dispersed in DMF were taken with a HP
8453 spectrophotometer. The suspension was treated with ultrasonic disruptor
(1 hour, power 100 W, 35 kHz) to disintegrate the sample. The upper fraction
of the solution was taken for measurements. TGA measurements were
performed on a Perkin Elmer TGA Pyris 1 instrument with a heating rate of
258C min21 up to 10008C under air flow of 14 L/min. TEM samples were
obtained by drying sample droplets from methanolic dispersion onto a
300-mesh Cu grid coated with a laced carbon film. All micrographs were
taken at accelerating voltage of 120 kV on a Philips TEM EM-208 instrument.
SEM images were also obtained on Cu grids at accelerating voltage of 12.0 kV
on a Philips SEM XL-30.
RESULTS AND DISCUSSION
ESR Measurements
Suspension of A in toluene saturated with argon was treated with solid
[Cp�Cr(CO)3]2, then transferred into an ESR ampoule in the vacuum line,
subjected to several “freeze–thaw” cycles and finally sealed under argon.
The ESR spectra were recorded on a Varian E-12A spectrometer. Temperature
Organometallic Chromium-Centered Free Radicals 289
was controlled using a “Unipan” device; g-factors were determined using a
Varian standard with g-factor equal to 2.0028.
Sample B such prepared exhibited yellow colour and two persistent ESR
signals with g1 ¼ 1.9929 and g2 ¼ 1.9938 with satellites due to magnetic
nucleus a1(53Cr) 16.5 G and a2(53Cr) 15.0 G, respectively. During several
days the intensity of the first signal increased, while that of the second one
faded (Figure 1a, 1b).
Initially, when beginning this work, we supposed that h2-addition of�Cp†Cr(CO)n (n ¼ 2 or 3) radical occurred across double bonds C55C of
SWNT to give stable radical-adducts as in the case of fullerenes (10). It
happened that the radical-adduct was quite stable, indeed. However, both
signals in the ESR spectrum of the Cr-modified SWNT had g-factors much
lower than those observed for the fullerene radical-adducts (g ¼ 2.0134)
(11). This suggests greater transfer of electron density from metal to
nanotube, making the product more like cation-radicals (e.g., g-factor for
bis(benzene)chromium cation, (C6H6)2Crþ, g ¼ 1,9863, a(53Cr) 19.0 G)
(15). In fact, in the reaction of the same Cr-complex with nitroso
compounds ONMe3 or 2,3,5,6-Me4C6HNO wherein Cr was obviously
coordinated to oxygen, ESR signals were observed with the following
parameters: g ¼ 1.9992, a(53Cr) ¼ 18.6 G, a(15N) ¼ 7.3 G) and g ¼ 1.9998,
a(53Cr) ¼ 18.3 G, a(15N) ¼ 6.0 G), respectively.
This finding strongly supports the conclusion that chromium atom in the
free radical �Cp†Cr(CO)3 attacks irreversibly oxygen atoms in the oxidized
nanotube (which may be carbonyl, carboxyl, or epoxide) with substantial
transfer of electron density from chromium to oxygen.
Figure 1. a) The ESR spectrum of radical adducts [�Cp†Cr(CO)3]xSWNT of the
reaction of [�CpCr(CO)3]2 with SWNT in toluene solution. T ¼ 293 K. The lines
labelled � are due to 53Cr isotope in �Cp†Cr(CO)3 radicals; b) the ESR spectrum
measured after 2 days.
A. S. Lobach et al.290
Raman Spectroscopy
The Raman spectrum of A (Figure 2a, 2b) displays two strong bands: so-called
radial breathing (267 cm21) and tangential (1588 cm21) (G band) modes.
The multiple peaks seen in the radial breathing mode (RBM) are due to the
distribution of tube diameters in a sample.
The RBM is highly sensitive to diameters of tubes, being inversely
proportional to the RBM position (16). A weaker band centered at
Figure 2. Raman spectra (lex ¼ 514.5 nm) of A and B: a) in the tangential (G band)
and disorder mode (D band) regions; b) in the radial breathing mode region.
Organometallic Chromium-Centered Free Radicals 291
ca. 1334 cm21 (D band) is attributed to disorder or sp3–hybridized carbons in
the hexagonal framework of the nanotube walls. The comparison of Raman
spectra of A and B evidences no substantial changes. The G band maximum
at 1588 cm21 remains almost unchanged for (A) and (B) samples, its
intensity (I) being slightly increased, and the increase in the G bandwidth
for B sample by 10 cm21 is observed. The D band intensity increases for
B sample, but ID/IG ratio for B decreases (�10%) relative to starting
sample A. This is in contradiction with previously obtained results when
covalent C-addition of aryl radicals to the sidewall of nanotubes leads to the
increase in ID/IG ratio (17). However, it was reported in (9) that a ID/IG
ratio decreased upon the formation of oxygen-metal-functionalized adducts
of SWNT.
The increase in the intensity of the D band in functionalized tubes is
usually associated with the formation of a covalent bond between a functional
group and a sidewall of nanotubes, which results in a conversion of a signifi-
cant amount of sp2–hybridized carbon to sp3–hybridized carbon (18). The
absence of this effect upon the addition of chromium-centered free radicals
to nanotubes indicates that these radicals add via rather an oxygen atom (of
carboxylic acid, ester, or carbonyl groups on the nanotubes surface) than a
carbon atom of the tube surface. The formation of carboxylic acid, ester and
quinone groups on surfaces of nanotubes purified by acidic and air
oxidation of raw SWNT was proved by IR spectroscopy (19, 20). A slight
upshift (�1–4 cm21) of RBM bands and a change in relative intensity of
the two bands (247 and 267 cm21) were observed in the RBM region
(Figure 2b). The increase in the intensity of the band at 247 cm21 and the
decrease in the intensity of the bands at 267 cm21 in functionalized tubes of
sample B can be attributed to greater reactivity of nanotubes of smaller
diameter and, therefore, to a higher degree their of functionalization.
In general, the Raman experiments show that chromium-centered free
radicals preferentially interact with SWNT of smaller diameter. Constant
intensity of the D band (ID/IG ratio values) in Raman spectra of the carbon
tubes functionalized via the addition of chromium-centered free radicals is
evidence of that these radicals add to non-carbon atoms on the nanotube
surface.
UV-Vis-NIR Absorption Spectra
Figure 3 shows the UV-Vis-NIR absorption spectra of A and B suspended in
dimethylformamide. The spectra of B show that the fine structure is indistinct
as compared to that of A and optical density of a solution decreases for B. As
compared to the spectrum of A, that of B exhibits no absorption bands at 824
and 663 nm or they are shifted (896, 745, 565, and 514 nm) and a new intense
band appears at 985 nm. Since the absorption bands in the spectrum of A cor-
respond to electron transfer within the van Hove singularities for tubes of
A. S. Lobach et al.292
different diameters, chirality and electrical conductivities (metallic, semicon-
ducting tubes), one could suggest the disappearance and shifts of some bands
in the spectrum of B to be due to a selective interaction of chromium-centered
free radicals with nanotubes. This differs from functionalization of nanotubes
via the reaction with aryl radicals when the C–C bond is formed between a
radical and carbon of a sidewall of nanotube, and the total loss of the elec-
tronic structure of nanotubes is observed (17). The origin of the band at
985 nm is not quite clear. Thus, the interaction of chromium-centered free
radicals results in partial changes in the electronic structure of nanotubes.
Thermal Gravimetric Analysis
Figure 4 shows TGA loss weight curves in air, their temperature derivative
curves and 3 Gaussian fits of the derivative curves for the corresponding
A and B samples. TGA of A and B showed that the temperature of the
beginning of weight loss significantly decreased in the chromium derivative:
A—4208C, B—2508C. Lower temperature in B can be associated with the loss
of a ligand (CO or/and cyclopentadienyl) from the chromium atom bound to
the nanotube structure. A comparative analysis of the TGA curves and their
temperature derivatives for A and B shows that they have the same
character: weight loss is a three-step process and relative values of these
steps (the area under the Gauss curve) are equal. However, the maxima of
temperatures of weight loss for B are shifted by 1608C to lower temperatures.
Figure 3. UV-Vis-NIR absorption spectra of A and B suspended in dimethylforma-
mide (1 mg sample in 5 ml solution, 2 mm cell). (A) purified pristine HiPco SWNT, (B)
HiPco SWNT functionalized by the reaction with Cp�Cr†(CO)3.
Organometallic Chromium-Centered Free Radicals 293
Lower temperatures of combustion of B can be a result of a catalytic action of
metallic chromium remaining on the nanotube surface after complex decompo-
sition. The reverse phenomenon was observed in the course of nanotube
purification from metal catalyst: lower metal content in a sample results in
lower temperature of combustion point (14). The loss weight curve of B
allows one to determine chromium content in functionalized tubes. An incom-
bustible material remainder, which consists of iron and chromium oxides is
16 w/w%, the content of chromium being equal to 7 w/w%.
Figure 4. TGA loss weight curves (solid line), the derivative curves (solid line) and
3 Gaussian fits (dotted lines) of the derivative curves of A and B. 258C/min, air.
A. S. Lobach et al.294
TEM and SEM Analysis
Figure 5a shows the TEM image of A, which consists of single bundles of the
size varying from 20 to 35 nm. The profiles of the bundles are straight and the
bundles are long. It is seen from the TEM image that material B also appears as
bundles, however, with fuzzy lateral surfaces. It is seen that there is a disorder
in the bundles of functionalized tubes, which is absent in starting SWNT. The
TEM images of the materials evidence that their rope network morphologies
are different. The SEM images of A and B samples (Figure 5b) justify the
TEM data on different morphology of these materials. Both materials consist
of fibers, those of functionalized tubes being thicker forming denser packing,
and the material of functionalized tubes is purer than starting tubes. One
could suggest that the addition of chromium-centered free radicals to
nanotube surface changes the nature of the surface that results in their better
interweaving. This is justified by the formation of dense rigid films from
functionalized tubes on a track membrane upon filtering their suspension in
toluene.
CONCLUSION
ESR spectroscopy investigations of the interaction of organometallic
chromium-centered free radicals with purified HiPco SWNT strongly
Figure 5. a) TEM images of samples A and B; b) SEM images of samples A and B.
Organometallic Chromium-Centered Free Radicals 295
support the conclusion that chromium atom in the �Cp†Cr(CO)3 free radical
attacks irreversibly oxygen atoms in the oxidized nanotube with substantial
transfer of electron density from chromium to oxygen. Raman experiments
show that chromium-centered free radicals interact preferentially with
SWNT of a smaller diameter. Raman spectra show that the D band intensity
(ID/IG ratio) in the spectra of pristine and functionalized SWNT is invariable
that indicates chromium-centered free radicals to add not by carbon atoms of
the nanotubes surface. The addition of chromium-centered radicals to SWNT
results in partial changes in the electronic structure of nanotubes, which
are supported by UV-Vis-NIR spectra of functionalized nanotubes. TGA
showed functionalized nanotubes to contain about 7% chromium, which
lowers the temperature of the beginning of nanotube material combustion,
the stages of weight loss upon combustion being similar for both pristine
and functionalized nanotubes. TEM and SEM analyses showed different
morphology of SWNT in starting and functionalized materials.
Such sidewall-functionalized carbon nanotubes involving the addition of
chromium metal complex can be useful for the development of new supported
catalysts with interesting catalytic properties.
ACKNOWLEDGMENTS
The [Cp�Cr(CO)3]2 sample was kindly provided by Prof. Lai Yoong Goh
(National University of Singapore) to whom we express our gratitude. This
work was supported by the RFBR grants #03-03-32727, 04-02-17618,
Program of Russian Academy of Sciences Chemistry Division, #OX-1 and
the Ministry Program, Contract #02.434.11.2023.
REFERENCES
1. Dresselhaus, M.S., Dresselhaus, G., and Eklund, P.C. (1996) Science of Fullerenesand Carbon Nanotubes; Academic Press: New York.
2. Niyogi, S., Hamon, M.A., Hu, H., Zhao, B., Bhowmik, P., Sen, R., Itkis, M.E., andHaddon, R.C. (2002) Chemistry of single-walled carbon nanotubes. Acc. Chem.Res., 35 (12): 1105–1113.
3. Hirsch, A. (2002) Functionalization of single-wall carbon nanotubes. Angew.Chem. Int. Ed., 41 (11): 1853–1859.
4. Bahr, J.L. and Tour, J.M. (2002) Covalent chemistry of single-wall carbonnanotubes. J. Mater. Chem., 12: 1952–1958.
5. Lobach, A.S., Solomentsev, V.V., Obraztsova, E.D., Shchegolikhin, A.N., andSokolov, V.I. (2004) Reaction of single-wall carbon nanotubes with radicals.In Electronic Properties of Synthetic Nanostructures, XVIII InternationalWinterschool/Euroconference on Electronic Properties of Novel Materials,Kirchberg, Tirol, Austria, March 6–13, 2004; Kuzmany, H., Fink, J.,
A. S. Lobach et al.296
Mehring, M., and Roth, S., eds.; AIP Conference Proceedings 723: New York,209–212.
6. Banerjee, S. and Wong, S.S. (2002) Functionalization of carbon nanotubes with ametal-containing molecular complex. Nano Lett., 2 (1): 49–53.
7. Banerjee, S. and Wong, S.S. (2002) Structural characterization, optical properties,and improved solubility of carbon nanotubes functionalized with Wilkinson’scatalyst. J. Am. Chem. Soc., 124: 8940–8948.
8. Baleizao, C., Gigante, B., Garcia, H., and Corma, A. (2004) Vanadyl salencomplexes covalently anchored to single-wall carbon nanotubes as heterogeneouscatalysts for the cyanosilylation of aldehydes. J. Catal., 221: 77–84.
9. Hemraj-Benny, T., Banerjee, S., and Wong, S.S. (2004) Interactions of lanthanidecomplexes with oxidized single-walled carbon nanotubes. Chem. Mater., 16:1855–1863.
10. Gasanov, R.G., Tumanskii, B.L., Sokolov, V.I., and Goh, L.Y. (2003) (Cyclopen-tadienyl)chromiumtricarbonyl dimers as a source of metal-centered free radicals toreact with fullerenes, Book of Abstracts, 6th Biennial International Workshop,Fullerenes and Atomic Clusters, St. Petersburg, Russia, June 30–July 4; P109.
11. Sokolov, V.I., Gasanov, R.G., Goh, L.Y., Weng, Z., Chistyakov, A.L., andStankevich, I.V. (2005) (Cyclopentadienyl)chromiumtricarbonyl dimers as asource of metal-centered free radicals to form stable h2-bonded spin-adductswith fullerenes. J. Organomet. Chem., 690: 2333–2338.
12. Weng, Z. and Goh, L.Y. (2004) Homolytic cleavage and aggregation processes incyclopentadienylchromium chemistry. Acc. Chem. Res., 37 (3): 187–199.
13. Chiang, I.W., Brinson, B.E., Smalley, R.E., Margrave, J.L., and Hauge, R.H.(2001) Purification and characterization of single-wall carbon nanotubes.J. Phys. Chem. B., 105 (6): 1157–1161.
14. Lobach, A.S., Spitsina, N.G., Terekhov, S.V., and Obraztsova, E.D. (2002)Comparative analysis of various methods of purification of single-walled carbonnanotubes. Phys. Solid. State., 44 (3): 475–477.
15. Vetchinkin, S.I., Solodovnikov, S.P., and Chibrikin, V.M. (1960) Distribution ofspin density in bis(benzene)chromium cation. Optika and Spectroskopiya(Russ.), 8 (1): 37.
16. Rao, A.M., Chen, J., Richter, E., Schlecht, U., Eklund, P.C., Haddon, R.C.,Venkateswaran, U.D., Kwon, Y.-K., and Tomanek, D. (2001) Effect of van derWaals interactions on the Raman modes in single walled carbon nanotubes.Phys. Rev. Lett., 86 (17): 3895–3898.
17. Bahr, J.L., Yang, J., Kosynkin, D.V., Bronicowski, M.J., Smalley, R.E., andTour, J.M. (2001) Functionalization of carbon nanotubes by electrochemicalreduction of aryl diazonium salts: a Bucky paper electrode. J. Am. Chem. Soc.,123 (27): 6536–6542.
18. Holzinger, M., Abraham, J., Whelan, P., Graupner, R., Ley, L., Hennrich, F.,Kappes, M., and Hirsch, A. (2003) Functionalization of single-walled carbonnanotubes with (R-) oxycarbonyl nitrenes. J. Am. Chem. Soc., 125 (28):8566–8580.
19. Kuznetsova, A., Mawhinney, D.B., Naumenko, V., Yates, Jr J.T., Liu, J., andSmalley, R.E. (2000) Enhancement of adsorption inside of single-wallednanotubes: opening the entry ports. Chem. Phys. Lett., 321: 292–296.
20. Feng, X., Matranga, C., Vidic, R., and Borguet, E.J. (2004) A vibrational spectro-scopic study of the fate of oxygen-containing functional groups and trapped CO2 insingle-walled carbon nanotubes during thermal treatment. J. Phys. Chem. B.,108 (52): 19949–19954.
Organometallic Chromium-Centered Free Radicals 297