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Functionalization of carbon nanotubes by potassium permanganate assisted with phase

transfer catalyst

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2002 Smart Mater. Struct. 11 962

(http://iopscience.iop.org/0964-1726/11/6/318)

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INSTITUTE OF PHYSICS PUBLISHING SMART MATERIALS AND STRUCTURES

Smart Mater. Struct. 11 (2002) 962–965 PII: S0964-1726(02)55164-8

Functionalization of carbon nanotubes bypotassium permanganate assisted withphase transfer catalyst

Nanyan Zhang, Jining Xie and Vijay K Varadan

Center for the Engineering of Electronic and Acoustic Materials and Devices, Pennsylvania

State University, University Park, PA 16802, USA

Received 24 June 2002Published 4 November 2002Online at stacks.iop.org/SMS/11/962

AbstractAn improved process is presented to functionalize carbon nanotubes bypotassium permanganate with the help of phase transfer catalyst (PTC). ThePTC helps to extract potassium permanganate from the solid phase to anorganic solvent phase and improves the efficiency of nanotube oxidation.The higher reaction efficiency as well as mild reaction conditions leads to ahigher yield of functional nanotube preparation. X-ray photoelectronspectroscopy confirms the functional groups attached to the nanotubesurface. A preliminary comparison is given of the potassium permanganateoxidation of nanotubes with and without PTC. This method is believed to bea potential economic method for large-scale functional nanotubepreparation.

1. Introduction

It is well known that carbon nanotubes have been expected

to have versatile potential applications due to their novel

structures and remarkable mechanical, thermal and electrical

properties [1, 2]. Unfortunately, experimental confirmation

of these applications has been hindered mainly by the poor

processibility of carbon nanotubes. Because of the pure

carbon element and their stable structure, carbon nanotubes

are insoluble in any organic solvents, which makes it

extremely difficult to explore their properties and applications.

For example, carbon nanotubes are regarded as promising

filler materials for a new generation of high performancenanocomposites because of their exceptionally high Young

modulus [3], bending strength and low density. However, this

composite processing is still limited to bench-top processing

and has been hampered by high viscosity of available matrix

materials, lack of dispersion and excessive porosity.

To overcome this problem, chemical modification on

the carbon nanotube surface is regarded as the best method.

Chemical functionalization of carbon nanotubes is expected

to play an essential role in tailoring the properties of

material. After functionalization, with the help of functional

groups attached to the surfaces, carbon nanotubes could react

readily with other chemical reagents and form homogenous

dispersions or even well aligned materials.

Graphite will undergo fluorination and oxidation

completely under certain conditions, which leads to the loss

of its most interesting properties. To meet the requirement

of nanotube functionalization, chemical reaction occurring on

the surface must be controlled in a particular manner. In other

words, we need thechemical modification of nanotubes as well

as their initial properties. In this case, a reagent is desired to

selectivelyattack some of theπ-bonds without bringing a total

destruction of the graphene structures of the nanotubes.

It is well documented that the chemistry of the fullerenes

is characterized by addition reactions. The fullerenes undergo

such reactions with relative ease because the conversion of

trigonal carbon atoms to tetrahedral carbon atoms servesto release the tremendous strain present in the spheroidal

geometry [4]. For carbon nanotubes, because of the greater

curvature, the caps are the sites of preferential reactivity. This

is thereason whymanyresearchers haveused reactions to open

the caps and insert various species into the nanotubes. As for

chemical functionalization, functional groups are attached to

the caps much more easily than to the walls. Therefore, the

nondestructive attachment of functional groups to the walls of

nanotubes presents a further challenge to experiment.

Permanganate has been used for decades as an oxidizer

in organic chemical manufacturing [5]. The primary redox

reaction under low pH for permanganate is given as MnO−4 +

8H+ + 5e− → Mn+2 + 4H2O. With excess permanganate,

0964-1726/02/060962+04$30.00 © 2002 IOP Publishing Ltd Printed in the UK 962

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Functionalization of carbon nanotubes by potassium permanganate assisted with phase transfer catalyst

Mn2+ can be oxidized subsequently as 3Mn+2 + 2MnO−4 +

2H2O → 5MnO2(s) + 4H+. Permanganate oxidation of many

organic compounds can be achieved without cleavage of the

carbon framework.

In 1995, Hiura et al [6] reported the functionalization

of carbon nanotubes during the purification process. In that

experiment, potassium permanganate was used as the oxidant

in the presence of sulfuric acid. This oxidation changes

the chemical make-up of the reactive edge of the tips and

perhaps even the outer (and the inner) layer of the nanotube.

X-ray photoelectron spectroscopy (XPS) was used to studied

the surface modification and indicated that about 15% of the

carbonconstituting thenanotubes is bound to eitherhydroxy or

carboxy groups. Clearly, not only the tips but also the surface

of the outer layer were found to be covered with these species.

Besides potassium permanganate, other oxidant reagents

were tried to attach functional groups to nanotube surfaces by

Satishkumar and his group in India [7], such as concentrated

HNO3, concentrated H2SO4, aqua regia, superacid HF/BF3,

aqueous OsO4, OsO4–NaIO4 etc. The concentration of the

surface acid groups was found to be in the range of 2 × 1020–

10× 1020 acid sites pergramof nanotubes, as shownin table 1.

Liu etal [8]reporteda functionalizationmethod forsingle-

walled nanotubes by using a mixture of concentrated sulfuric

and nitric acids as the oxidant. Another purpose of this process

is to cut nanotubes into short pieces. They also assumed the

open ends to be terminated with many carboxylic acid groups.

However, the methods mentioned above all suffered

from low yield of the functionalization process. In this

paper, we present an improved procedure to functionalize

carbon nanotubes by potassium permanganate with the help

of phase transfer catalyst (PTC). The higher yield of this

functionalization process is the major advantage.

2. Experiment

The nanotube source used in this experiment was gained from

the microwave CVD method. This method is based on the

pyrolysis of acetylene by microwave heating on nano-sized

cobalt particlesembeddedin zeolite, whichservesas a catalytic

support. The average diameter of these multi-walled carbon

nanotubes ranges from 20 to 30 nm [9]. After hydrofluoric

acid treatment and air oxidation, the as-prepared nanotubes

were purified and ready for the functionalization process.

For comparison, the functionalization of nanotubes by

potassium permanganate without PTC was also performed.

Reaction 1. 0.12 g of purified nanotubes were dispersed

in 20 ml of 0.5 M sulfuric acid by ultrasonic vibration in a

two-necked flask equipped with a condenser and a dropping

funnel. The suspension was refluxed in an oil bath at 120 ◦C

with magnetic stirring. Meanwhile, 1.98 g of potassium

permanganate was dissolved in 20 ml of 0.5 M sulfuric acid

and this solution was added to the flask drop by drop. Then

the reaction was kept at 120 ◦C for 3 h. After that, the

resulting suspension was filtered, washed with concentrated

hydrochloric acid and deionized water and then dried.

Reaction 2. 0.12 g of purified nanotubes and 25 ml

dichloromethane (CH2Cl2) were added to a 100 ml flask and

the suspension was vibrated ultrasonically for 0.5 h. About

1.0 g of phase transfer agent (Aliquat 336, from Aldrich) was

Figure 1. Micrograph of SEM of carbon nanotubes afterfunctionalization.

added, followed by 5 g of powdered potassium permanganate

in small portions during a period of 2 h. 5 ml of acetic acid was

also added. The mixture was then stirred vigorously overnight

at room temperature. As the final step, the resulting material

was gained afterfiltering, concentric HCl acid treatment, water

washing and drying.

A Hitachi 3000N scanning electron microscope (SEM)

was used to investigate the morphology of the materials after

functionalization. X-ray photoelectron spectroscopy (XPS)

spectrum of f-CNTs wasobtained by using a Kratos Analytical

XSA800 pci under 10−8–10−9 Torr vacuum.

3. Results and discussion

In the functionalization reaction, the colour of the suspension

changes from dark purple to dark brown, indicating the

transformation of Mn+7 to Mn+4. The functionalized

nanotubes gained from both methods are dark in colour.

The yield of the functionalized nanotubes in the reaction

without PTC is about 35–40% in terms of the total weight of

starting material, which is consistent with the result reported

by Hiura [6]. For the reaction with PTC, the yield of

functionalized nanotubes is about 65–70%, much higher than

the yield of reaction 1.

Scanning electron microscopy was used to investigate the

carbon nanotube sample before and after functionalization. As

shown in figure 1, the morphology of carbon nanotubes after

functionalization preserves. No destruction was observed,

which means carbon nanotubes are strong enough towithstand

the functionalization process.

We also found that thefunctionalizednanotubes from both

methods could be well dispersed in many common organic

solvents, such as ethanol, methanol, methyl ethyl ketone

(MEK), dichloromethane, toluene etc. This phenomenon of

uniform nanotubedispersion in solvents is oneof theproperties

of functionalized nanotubes. It has been reported that due to

the nanoscale size, the high surface energy of nanotubes gives

them a strong tendency to agglomerate. Even with the help of

ultrasonic vibration, the untreated nanotubes may not remain

in any solvent in quiescent suspension. However, appropriate

functionalized nanotubes can dramatically raise the stability

of suspensions [10]. The functionalization process induces a

negatively charged surface, particularly through the ionization

of acidic surface groups. The resulting electrostatic repulsion

leads to the stable uniform colloidal dispersion.

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N Zhang et al

Table 1. The concentration of the surface acid groups in the nanotubes opened by different oxidants [7]

HNO3 H2SO4 Aqua regia KMnO4 (acid) KMnO4 (alkali) OsO4–NaIO4

2.5 ×1020 6.7×1020 7.6×1020 8.3×1020 10×1020 5.2×1019

(a)

(b)

Figure 2. XPS spectra of functionalized nanotubes. (a) XPSspectrum of functionalized nanotubes from reaction 1; (b) XPSspectrum of functionalizednanotubes from reaction 2.

(This figure is in colour only in the electronic version)

XPS, as oneof thesurfaceanalytical techniques, is capable

of readily providing information about chemical bonding on

the surface. XPS was performed on each functionalized

nanotube sample to get the information of functional groups on

the nanotube surface. Thus, the C 1s main peak was scanned

in high resolution. In the curve fitting, the overall peak in the

range of 283–289 eV can be fitted by a superposition of fourpeaks. The main peak (∼284.7 eV) is attributed to the C 1s,

while the other three peaks are assigned to –C–O (286.1 eV),

–C=O (287.5 eV) and –COO− (288.8 eV) respectively. The

XPS spectra are shown in figure 2.

As the qualitative result, XPS spectra show that both

functional nanotubes have functional groups. The semi-

quantitative analysis gives the rough atomic concentration of

carbon atoms functionalized. Table 2 gives the comparison of

semi-quantitative analysis of functional nanotubes.

For f-nanotubes from reaction 1, the result indicates that

about 12% of carbon atoms are bonded with an –OH group

and 7.45% with a –COOH group. The f-nanotubes from reac-

tion 2 have a higher concentration of –OH groups (∼23%) and

a lower concentration of –COOH groups (∼3.8%). It is nec-

essary to point out in this paper that this quantitative analysis

of functional groups attached to nanotubes is at best semi-

quantitative. These values can only give us rough information

about the functional groups. The concentrations of the –OH

group are higher than the values reported [6]. The reason may

be the energy-loss tail in the C 1s peak. Also the trace amount

of zeolite material in the purified nanotube material may be re-

lated to the higher concentration of –OH groups. Much more

accurate quantitative analysis of functionalized nanotubes is

going to be performed in the near future.

The higher yield of functionalized nanotubes is the major

advantage of oxidation by potassium permanganate with PTC.

Untreated nanotubes are hydrophobic and do not dispersewell in aqueous solvent. On the other hand, potassium

permanganate is water soluble. In reaction 1, the hydrophobic

property of nanotubes leads to the low efficiency of contact

between nanotubes and potassium permanganate dissolved in

water. Thus high reaction temperature, 120 ◦C, is necessary

to increase the reaction rate of functionalization. However, at

high temperature, potassium permanganate is a strong oxidant,

which may destroy nanotube structure. This may be the reason

for the low yield of functionalization.

Phase transfer catalysis is based on the ability of catalytic

amounts of the transfer agents to increase therate of a chemical

reaction between reagents located in different phases of a

reaction mixture [11]. In reaction 2, potassium permanganatecould be extracted into organic solvent (dichloromethane,

where the nanotubes were suspended) by phase transfer agent.

This process could substantially increase the reaction rate and

efficiency. Therefore, the functionalization can be carried out

undermild conditions. Fewer nanotubes being destroyed leads

to higher yield. The function of acetic acid in the reaction is

to neutralize the hydroxide ions formed to promote a more

complete reaction [12]. We observed that without acetic acid,

the resulting suspension is still dark purple after reaction and

the quality of functionalized nanotubes is not acceptable.

4. Conclusion

Carbon nanotubes should be functionalized to explore their

unique properties and potential applications. In this paper,

potassium permanganate is used as an oxidant to functionalize

multi-walled carbon nanotubes derived from the microwave

CVD method. Unlike the conventional oxidation, PTC

is introduced into the functionalization reaction, which

drastically increases the efficiency of oxidation by potassium

permanganate. The yieldof functionalization is improvedfrom

∼35to∼65%. XPS confirms the functional groups attached to

the nanotube surface. This improved oxidation with the help

of PTC could be a potential economic method to prepare high

quality functional nanotubes on a large scale.

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Functionalization of carbon nanotubes by potassium permanganate assisted with phase transfer catalyst

Table 2. Semi-quantitative analysis of functionalized nanotubes. (Reaction 1, functionalization reaction by potassium permanganatewithout PTC; reaction 2, functionalization reaction by potassium permanganate with PTC.)

f-nanotubes from reaction 1 f-nanotubes from reaction 2

Peak Position (eV) Atomic conc. (%) Position (eV) Atomic conc. (%)

C 1s 284.66 75.96 284.71 68.35–C–OH 286.33 12.63 286.12 23.60–COOH 289.00 7.45 288.91 3.82–C=O 287.62 3.96 287.54 4.23

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