Sidewall functionalization of single-walled carbon nanotubes with organic peroxides

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: [email protected]

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.


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.


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


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.


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.


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.


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.

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Sidewall functionalization of single-walled carbon nanotubes with organic peroxides

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