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A facile and novel approach towards carboxylic acid functionalization

of multiwalled carbon nanotubes and efcient water dispersion

Ata-ur Rehman a, Syed MustansarAbbas a,b,n, Haz Muhammad Ammad a,Amin Badshah a,n,1, Zulqar Ali b,c, Dalaver Hussain Anjum d

a Department of Chemistry, Quaid-e-Azam University, Islamabad, Pakistanb Nanoscience and Catalysis Division, National Centre for Physics, Islamabad, Pakistanc College of Earth and Environmental Sciences, University of Punjab, Lahore, Pakistand Imaging and Characterization Lab, King Abdullah University of Science and Technology, Thuwal, Saudi Arabia

a r t i c l e i n f o

Article history:

Received 1 May 2013

Accepted 2 July 2013Available online 10 July 2013

Keywords:

Carbon nanotubes

Oxidation

Surfaces

FTIR

Defects

a b s t r a c t

A convenient, cheap and mild covalent functionalization route for multiwalled carbon nanotubes

(MWCNTs) have been developed for the rst time. The MWCNTs were treated with wet chemical

oxidants (NaNO2/HCl, HNO3/H2O2) in order to modify MWCNTs with carboxyl groups. Surface function-

ality groups and morphology of MWCNTs were analyzed by FTIR, TGA, SEM and TEM. The results

consistently conrmed the formation of carboxyl functionalities on MWCNTs, while the structure of

MWCNTs has remained relatively intact. Functionalized MWCNTs showed good dispersion in aqueous

media than untreated MWCNTs. Results show that NaNO2/HCl treatment is best suited for the chemical

functionalization, giving optimum surface carboxyl groups and minimum length shortening of MWCNTs.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Multiwalled carbon nanotubes (MWCNTs) are extraordinary

nanomaterials due to their desirable properties. The sp2 carbon

structure of their concentric cylindrical graphene layers provides

them with excellent electronic and thermal conductivity[1],high

modulus [2] and large surface area [3]. Although MWCNTs are

extraordinary by themselves, only a few applications have been

reported since most applications require dispersion in solvents or

in high-viscosity matrices [4]. These operations are hindered by

the chemical inertness of the MWCNTs surface. Therefore, surface

functionalization of nanotubes is an approach often used to over-

come these problems[5].

MWCNTs are often modied by covalent functionalization in

order to improve their dispersion in solvents. In recent years, variousfunctionalities such as amines [6], hydroxyl[7], carboxyl[8], aryl

diazonium salt[9] and halogens[10]have been applied onto CNTs

surface using covalent functionalization. Among the various func-

tional groups, carboxyl-functionalized MWCNTs attracted numer-

ous research interests [11]. Carboxyl-functionalized MWCNTs are

mostly prepared by wet chemical oxidation [8]. Wet chemicaloxidation using concentrated HNO3, HNO3/H2O2, H2SO4/HNO3and

H2SO4/KMnO4as oxidants not only leads to a large weight loss but

also produce defects on MWCNTs surface. In order to overcome

these shortcomings, it is an urgent priority to nd a favorable

method for attaching carboxyl groups to the outer sidewalls and

ends of MWCNTs by covalent bonds.

In this work, two novel approaches (NaNO2/HCl and HNO3/

H2O2) for covalent attachment of carboxylic acid groups to the

sidewalls of MWCNTs are presented. Moreover, our methodology

also does not comprise severe acid treatment, which is common in

the conventional methods. The MWCNTs samples were character-

ized from the viewpoint of functionality and morphology. Mean-

while, the dispersibility of functionalized MWCNTs in various

solvents is also studied.

2. Experimental

MWCNTs with a purity of495%, outer diameters of o10 nm

and lengths ofo1 mm were supplied by ShenZhen Nanotech Port

Co., Ltd., China and used as received.

2.1. Purication of MWCNTs

To remove any amorphous carbon contents, MWCNTs were heated

for 15 min at 400 1C in a furnace. Purication begins with a 3 h reux

Contents lists available at SciVerse ScienceDirect

journal homepage: www.elsevier.com/locate/matlet

Materials Letters

0167-577X/$- see front matter& 2013 Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.matlet.2013.07.009

n Corresponding authors at: Department of Chemistry, Quaid-e-Azam University,

Islamabad, Pakistan. Tel.: +92 3218539839; fax: +92 512077395.

E-mail addresses:qau_abbas@yahoo.com (S.M. Abbas),aminbadshah@yahoo.com

(A. Badshah).1 Tel.: +92 051 90642131.

Materials Letters 108 (2013) 253256

http://www.sciencedirect.com/science/journal/0167577Xhttp://www.elsevier.com/locate/matlethttp://dx.doi.org/10.1016/j.matlet.2013.07.009mailto:qau&underscore;abbas@yahoo.comhttp://-/?-http://-/?-http://dx.doi.org/10.1016/j.matlet.2013.07.009http://dx.doi.org/10.1016/j.matlet.2013.07.009http://dx.doi.org/10.1016/j.matlet.2013.07.009http://dx.doi.org/10.1016/j.matlet.2013.07.009http://-/?-http://-/?-mailto:qau&underscore;abbas@yahoo.comhttp://crossmark.dyndns.org/dialog/?doi=10.1016/j.matlet.2013.07.009&domain=pdfhttp://crossmark.dyndns.org/dialog/?doi=10.1016/j.matlet.2013.07.009&domain=pdfhttp://crossmark.dyndns.org/dialog/?doi=10.1016/j.matlet.2013.07.009&domain=pdfhttp://dx.doi.org/10.1016/j.matlet.2013.07.009http://dx.doi.org/10.1016/j.matlet.2013.07.009http://dx.doi.org/10.1016/j.matlet.2013.07.009http://www.elsevier.com/locate/matlethttp://www.sciencedirect.com/science/journal/0167577X

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in 6 M HCl to remove metal catalyst particles. Following reux the

black solution is centrifuged at 12,000 rpm leaving black sediment at

the bottom and sides of the centrifuge tube and a clear, brownish

yellow supernatant acid, which is decanted off. The sediment still

contains substantial trapped acid which is removed by repeatedly re-

suspending the sediment in deionized-water (by shaking vigorously),

centrifuging and decanting the supernatant liquid.

2.2. Functionalization of MWCNTs

In a typical experiment p-MWCNTs (100 mg) were added to a

1:1.5 mixture of NaNO2(6 M) and HCl (6 M) in a two-neck round-

bottomedask and were sonicated for 8 h at 0 1C. The paste was

diluted with deionized-water and ltered through a cellulose

nitrate membrane lter (0.22 mm pore size). The collected solid

was washed several times with deionized-water, followed by

ltration until the liquid ltrate came out colorless. The resulting

functionalized MWCNTs-1 were vacuum-dried overnight at 80 1C.

The effect of NaNO2 is thought to initiate a semi-stable diazonium

ion by receiving an electron from MWCNTs, which then results in a

radical reaction with MWCNTs sidewalls.

In another experiment, p-MWCNTs (100 mg) were dispersed in

a 1:1 mixture of HNO3(5 M) and H2O2(30%), in a two-neck round-

bottomed ask, equipped with a reux condenser. Next, the

mixture was sonicated and reuxed at 70 1C for 8 h. After cooling

to 25 1C, the paste was diluted with deionized-water and ltered

through the membrane lter. The collected solid was washed with

copious amount of deionized-water, followed by ltration until the

ltrate became colorless. The resulting functionalized MWCNTs-2

were dried in a vacuum oven overnight.

2.3. Sample characterization

Morphology of the samples was examined by SEM (JSM5910-

JEOL) and TEM (FEI-TITAN). FTIR spectra were recorded on (NICO-

LET-6700) and to measure degree of functionalization TGA was

carried out using a (METTLER-TOLEDO) at a heating rate of

10 1C min1 in a high-purity nitrogen atmosphere.

3. Results and discussion

FT-IR spectra of p-MWCNTs, MWCNTs-1 and MWCNTs-2 are

shown in Fig. 1a. As expected, p-MWCNTs is devoid of any

identiable functional groups, while MWCNTs-1 and MWCNTs-2

samples show the signal of carboxyl molecules. But the intensities

of groups introduced on the surface of treated MWCNTs are much

weaker, due to the high absorbance of the backbone of MWCNTs

and the low concentration of groups. The peaks at 1665 cm1 and

1655 cm1 are consistent to bending and stretching vibration of

CQC group in MWCNTs-1 and MWCNTs-2 respectively. Peaks at

1718 cm1 and 1732 cm1 can be assigned to CQO stretching

vibration of carboxyl groups in MWCNTs-1 and MWCNTs-2

respectively. These peaks are not sharp as that of normal strong

characteristic peaks of CQO stretch of carboxyl acids. Also, peaks

appearing at 1213 cm1 and 1255 cm1 are in agreement with

stretching vibration of CO in MWCNTs-1 and MWCNTs-2 respec-

tively. Sharp stretching vibrations at 3622 cm1and 3597 cm1 in

MWCNTs-1 and MWCNTs-2 respectively can be assigned to free

OH groups of carboxylic acids. All these peaks conrm acid oxidation

of p-MWCNTs.

TGA curve of the p-MWCNTs and MWCNTs-1 and MWCNTs-2 is

shown in Fig. 1b. Both, MWCNTs-1 and MWCNTs-2 start to

Fig. 1. (a) FTIR spectra of samples and (b) TGA analysis of samples.

NaNO2 HCl+ OH N O NaCl+

OH N O + H+ OH N O

H

+H2O

N O+

N O+

+

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decompose earlier than the untreated MWCNTs. The curve of

p-MWCNTs shows a small mass loss in a temperature range of

0500 1C which signify their structural stability. Weight loss

around 150 1C in curves of MWCNTs-1 and MWCNT-2 is due tophysio-adsorbed water, while weight loss from 150 1C to 350 1C

may be attributed to loss of grafted carboxyl groups on the surface

of MWCNTs [12]. Similarly weight loss of MWCNTs by both

treatments in the range of 350500 1C is due to elimination of

hydroxyl groups [13]. Especially in case of MWCNTs-1 there is

huge weight loss of nearly 28% which can be rendered to prior

damage to the graphitized structure of MWCNTs.

Fig. 2ac presents SEM images of the p-MWCNTs, MWCNTs-1

and MWCNTs-2. The cylindrical cross-section of MWCNTs is clearly

observed. The SEM image of functionalized MWCNTs clearly

indicate the removal of amorphous carbon content with minimal

breakdown of the MWCNTs due to less severe acid treatments

applied. In addition, p-MWCNTs show low entanglement in

comparison with functionalized samples which could be

attributed to carboxyl functionalization of MWCNTs [14]. In fact,

the carboxyl molecules act cross-links adjacent MWCNTs.

Fig. 2df presents TEM images of the p-MWCNTs, MWCNTs-1 and

MWCNTs-2. The higher roughness of functionalized MWCNTs implies,the part of sp2 hybridized carbons has been changed to sp3 hybridiza-

tion [15]. Furthermore, the functionalized MWCNTs appear slightly

denser than p-MWCNTs, which support the idea of an additional

agglomeration of the MWCNTs due to the stacking interactions

between carboxyl groups attached onto neighboring bundles.

Fig. 3shows the dispersion behavior of p-MWCNTs, MWCNTs-1

and MWCNTs-2 in water (0.1 mg mL1) after 45 days. Unsurpris-

ingly, the treated MWCNTs could easily disperse in water and form

a homogenous and stable solution. In addition, treated MWCNTs

were also dispersed in acetone, ethanol and dimethylforamide to

form homogenous suspensions after 5 min of sonication. The

suspensions of MWCNTs-1 can remain stable for 2 days in acetone,

for 5 days in ethanol and for 90 days in dimethylforamide while

MWCNTs-2 show lower dispersion period. When the

Fig. 2. SEM image of (a) p-MWCNTs (b) MWCNTs-1 and (c) MWCNTs-2, TEM image of (d) p-MWCNTs (e) MWCNTs-1 and (f) MWCNTs-2.

Fig. 3. Photographs of (a) p-MWCNTs in distilled-water (b) p-MWCNTs in DMF (c) MWCNTs-2 in distilled-water (d) MWCNTs-1 in distilled-water, (0.1 mg mL1) after 45 days.

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concentrations of treated MWCNTs in polar solvents increased up

to 0.9 mg mL1, MWCNTs entangle and precipitate due to the low

density of carboxyl groups and the high content of MWCNTs.

The easily-miscible carboxyl functionalities may explain the higher

dispersion of the treated MWCNTs in polar solvents.

4. Conclusion

Considering the importance of carboxylic acid moieties on the

surface of the MWCNTs, we have introduced a novel and applied

strategy to attach this moiety to MWCNTs. Both types of treat-

ments enhanced the dispersion of MWCNTs in water with max-

imum dispersion and carboxyl concentration for NaNO2 treated

MWCNTs. Carboxyl groups of treated MWCNTs can be used as

active sites for further reactions in order to expand the function of

MWCNTs in practical applications.

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