Carbon 42 (2004) 1147–1151

www.elsevier.com/locate/carbon

Synthesis and characterization of carbon nanotubes–TiO2

nanocomposites

A. Jitianu 1, T. Cacciaguerra, R. Benoit, S. Delpeux, F. B�eguin, S. Bonnamy *

Centre de Recherche sur la Mati�ere Divis�ee, UMR 6619, CNRS––Universit�e d’Orl�eans, 1B rue de la F�erollerie, 45071 Orl�eans Cedex 02, France

Available online 11 February 2004

Abstract

The main objective of this paper is to coat carbon multiwall nanotubes surface with TiO2 as anatase in view of photocatalytic

application for these nanocomposites. Carbon nanotubes were produced by catalytic decomposition of acetylene at 600 �C. Thecoating was performed by a sol–gel method using classical alkoxides as Ti(OEt)4 and Ti(OPri)4 and by hydrothermal hydrolysis of

TiOSO4, leading to different TiO2 morphologies. In using the sol–gel method, nanotubes are coated either with a continuous TiO2

thin film when the precursor is Ti(OEt)4, or with TiO2 nanoparticles when the precursor is Ti(OPri)4. By hydrothermal treament,

more compact and crystalline nanocomposites are obtained.

� 2004 Elsevier Ltd. All rights reserved.

Keywords: A. Carbon multiwall nanotubes, Anatase-type TiO2; B. Sol–gel; C. HRTEM

1. Introduction

The photocatalytic degradation of organic pollutants

on TiO2-anatase surface becomes a very active field of

research nowadays due to the big problem of pollution.This application is based on semiconducting properties

of TiO2-anatase [1]. Most of the studies were realized on

TiO2 aerogels powders, especially for the decontamina-

tion of water polluted with phenols and salicylic acid [1–

3]. More recently, also TiO2 thin films deposited on

different substrates were used in the photocatalytic

processes of maleic acid [4].

Carbon multiwall nanotubes (MWNTs) could beconsidered as a good support for materials with photo-

catalytic properties due to their high mechanical [5] and

chemical [6] stability and their mesoporous character

which favors the diffusion of reacting species. On the

other hand, a dispersion of TiO2 on the MWNTs surface

could create many active sites for the photocatalytic

degradation. Different techniques have already been

*Corresponding author. Tel.: +33-2-38-25-53-66; fax: +33-2-38-25-

53-76.

E-mail addresses: ajitianu@yahoo.com (A. Jitianu), bonn-

amy@cnrs-orleans.fr (S. Bonnamy).1 Present address: Centre for Advanced Materials Processing,

Clarkson University, 8 Clarkson Avenue, Box 5814, Potsdam, NY

13699-5814, USA.

0008-6223/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.carbon.2003.12.041

used to coat carbon supports for specific applications.

The first TiO2 coating by hydrolysis of TiCl4 was real-

ized on active carbon fibers [7] in order to obtain

absorptive materials for NH3. TiO2 has been also

deposited by sol–gel method on carbon fibers in order toincrease their thermal stability [8]. By electrodeposition,

Zhithomirsky et al. [9] succeeded to deposit a TiO2 thin

layer on a graphite support which is an important

component of PZT solid solutions. Activated carbon

spheres coated with TiO2 by hydrothermal treatment

demonstrated very good performance in the photocat-

alytic degradation of methylene blue [10,11].

Seeger et al. [12] and Hernadi et al. [13] were the firstto coat carbon nanotubes with oxide layers as SiO2 and

Al2O3, respectively, by an alkoxydic way. On the other

hand, Vincent et al. [14] showed that TiO2 thin films

obtained by sol–gel method could be reinforced using

carbon MWNTs. The mechanism of TiO2 formation by

hydrolysis–polycondensation of classical titanium alk-

oxides and titanium organically modified alkoxides was

elucidated by Livage and coworkers [15,16]. The mostimportant studies to evidence TiO2 thin films formation

were presented by Yoldas [17,18].

The present paper reports the anatase-type TiO2

coating of carbon MWNTs by a sol–gel method using

classical alkoxides as Ti(OEt)4 and Ti(OPri)4 and by

hydrothermal hydrolysis of TiOSO4 in sulfuric acid

under elevated pressure at 120 �C. To compare the

1148 A. Jitianu et al. / Carbon 42 (2004) 1147–1151

two different ways, the corresponding nanocomposites

were characterized by X-ray diffraction (XRD), X-ray

photoelectron spectroscopy (XPS), thermal analysis,

nitrogen adsorption at 77 K and high resolution trans-mission electron microscopy (HRTEM).

2. Experimental

The multiwall nanotubes were produced by catalytic

decomposition of acetylene at 600 �C on a CoxMgð1�xÞOsolid solution as catalyst, according to the conditions

reported in a previous paper [19].

Carbon MWNTs coating from sol–gel solutions

(SGS). The SGS were prepared using two alkoxides

precursors: titanium tetra-ethoxide Ti(OEt)4 or titanium

tetra-isopropoxide Ti(OPri)4 (Merck), and also absolute

ethanol (Prolabo), isopropanol (Carlo-Erba), nitric acid

(64%) (Prolabo) and distilled water. The molar ratio forthe two SGS preparation is: Ti(OR)4:ROH:H2O:HNO3

¼ 1:70:1.9:0.2 [17,18]. The solutions were homogenized

under reflux at 80 �C for 1 h using a magnetic stirrer.

For each sample, about 0.9 g MWNTs are mixed with

80 ml of SGS and stirred with a magnet in close vials for

3 h. The impregnated MWNTs are separated from the

solution by filtration.

Hydrothermal deposition. The hydrothermal deposi-tion of anatase-type TiO2 on the MWNTs surface was

realized in a teflon–steel home-made autoclave. For this

process, the precursor consists of 15 wt.% TiOSO4 (Al-

drich) in diluted H2SO4. MWNTs (0.5 g) are added to 50

ml TiOSO4 solution in water (0.1 mol/l) and the hydro-

thermal treatment is performed in the autoclave at 120

�C for 5 h. After cooling down, the carbon nanotubes are

separated by filtration, and washed with water on a filteruntil neutral pH of the washing solution.

Both kind of impregnated nanotubes were dried in an

oven at 80 �C for 12 h in air and then thermally treated

at 300 �C for 1 h giving coated MWNTs. The coating

process has been repeated up to three times in the same

conditions, in order to favor the formation of a con-

tinuous coating on the nanotubes surface.

The thermal behavior of the nanocomposites wasanalyzed with a TG-DTA 92-18 Setaram apparatus, at 5

�C/min heating rate, under air flow (70 ml/min). XRD

study was performed with a PW 3020 Philips diffracto-

meter (CuKa ¼ 0.15418 nm). XPS data were recorded on

Table 1

Elemental composition, BET specific surface area and micropore volume of

Sample Precursors C (%)

1 Carbon MWNTs >98

2 MWNTs+Ti(OEt)4 37.5

3 MWNTs+Ti(OiPr)4 31.2

4 MWNTs+TiOSO4 23.6

a The micropore volume was determined using the Dubinin–Radushkevich

a VG ESCALAB 250 spectrometer using a AlKa

monochromator source (15 kV, 15 mA) and a multi-

detection analyzer, under 10�8 Pa residual pressure. A

small spot lens system allowed to analyze less 1 mm2

area. Nitrogen adsorption isotherms at 77 K were re-

corded using a Micromeritics ASAP 2000 apparatus.

Before adsorption the samples were out-gassed at 150 �Cfor 12 h, reaching a final pressure of 10�6 mbar. The

pristine and coated MWNTs were characterized by

(HRTEM) using a Philips CM-20 operating at 200 kV.

3. Results and discussion

The elemental composition and the BET specific

surface area of the samples are given in Table 1. It shows

that the TiO2 amount deposited on the carbon nano-tubes depends on the type of precursor and also on the

method used for the deposition. The later fact could be

explained by different deposition mechanisms. As a

consequence of increasing TiO2 deposition, the specific

surface area and the micropore volume decrease from

samples 1 to 4 due to the blockage of the MWNTs mi-

cropores by the oxide layer. All the adsorption iso-

therms recorded were identified to be type IV isotherms,confirming that the mesoporous character of the nano-

tubular substrate is preserved after the deposition of

TiO2. Although that the TiO2 layer might contribute to

some extent to the observed value of the specific surface

area, the lowest value measured for sample 4 (150 m2/g)

could be due to the fact that the hydrothermal treatment

takes place under pressure and leads to a more compact

sample.The thermal behavior and stability under air of the

different samples are summarized in Table 2. Although

thermo-gravimetric analyses were performed after each

coating, only the data obtained after the third deposi-

tion and before the last thermal treatment at 300 �C are

given in Table 2. The oxidation of the pristine MWNTs

starts at 350 �C and the exothermal effect is maximum

at 475 �C. In the case of samples 2 and 3, three stepswere observed during the weight loss. The first one,

expressed by a weak endothermic effect, is assigned to

water and solvent removal. The second corresponds to

the removal of structural water, i.e. OH groups. The

last one, attributed to the MWNTs combustion, occurs

the pristine and coated MWNTs samples

TiO2 (%) SBET (m2/g) V a (cm3/g)

– 300 0.13

51.2 280 0.11

59.2 187 0.07

61.4 150 0.07

equation.

Table 2

Thermal stability of the samples according to differential thermal analysis and thermo-gravimetric analysis

Sample Temperature range (�C) Thermal effect (�C) Weight loss (%) Assignment

Endo. Exo.

1 350–600 – 475 100 Carbon nanotubes combustion

2 25–165 75 – 10.4 Water and solvent evolution

165–320 – – 4.8 Structural water evolution

320–600 – 486 36.6 Carbon combustion

3 25–165 73 – 9.1 Water and solvent evolution

165–360 – – 5.7 Structural water evolution

360–600 – 480 31.2 Carbon combustion

4 25–175 70 – 8.3 Water evolution

175–420 – – 5.7 Structural water evolution

420–650 – 537 17.6 Carbon combustion

20 40 60 800

100

200

300

400

500

600

700

800

(4)

(3)

(2)

(1)

(100)

(100)

(100)(002)

(224)(215)(220)(116)(204)

(211)(105)(200)(004)

(101)

Cou

nts

[a.u

.]

2θ o

Fig. 1. X-ray diffraction patterns: sample 1, pristine MWNTs; samples

2–4, MWNTs coated three times with TiO2 and annealed at 300 �C in

air: sample 2, prepared with Ti(OEt)4 by sol–gel method; sample 3,

prepared with Ti(OPri)4 by sol–gel method; sample 4, prepared with

TiOSO4 by hydrothermal method.

A. Jitianu et al. / Carbon 42 (2004) 1147–1151 1149

between 340 and 600 �C and is marked by an exo-thermal effect with a maximum at 480 �C. For sample

4, the oxidation step of carbon is shifted to higher

temperature. As indicated by the lower value of specific

surface area, the surface activity of oxygen is lower

with this material. Moreover, the weight loss decreases

from samples 2 to 4 as a consequence of the increasing

TiO2 loading on the carbon nanotubes. Taking into

account these thermo-gravimetric data, all the sampleswere thermally treated at 300 �C in order to crystallize

anatase on the nanotubes surface, without their

destruction.

The XRD patterns of the thermally treated samples

are given in Fig. 1. The most intense peaks of MWNTs

correspond to the (0 0 2) reflection and (1 0) band. The

additional peaks present in all the diffractograms cor-

respond to the anatase form of TiO2. The (0 0 2) reflec-tion due to MWNTs overlaps the anatase (1 0 1)

reflection. It is worth to notice that the intensity of

anatase diffraction peaks increases from sample 2 to 4

and the width at half height of the peaks decreases. This

is consistent with the increasing amount of TiO2 from

samples 2 to 4, that favors more extended crystallized

domains on the surface.

The C1s XPS spectra are presented in Fig. 2. For all

the samples a main peak is observed at 284.6 eV due to

the C–C bonds. For samples 2 and 3 additional peaks

are present, located at 285.8 and 286.8 eV and respec-

tively attributed to C–O and C@O bonds. They expressa slight oxidation of the nanotubes surface during the

sol–gel process leading to TiO2 coating. For sample 4 a

more drastic MWNTs surface oxidation occurs during

hydrothermal process since three intense peaks due to

carbon–oxygen bonds are found, the third one located

at 289.3 eV is assigned to COO groups. The two other

peaks, those due to C–O and C@O bonds, are shifted to

the higher binding energy in comparison with previousspectra. Furthermore the plasmon peak has disap-

peared. All these data express a more pronounced oxi-

dation due to the fact hydrothermal process, taking

place in the presence of sulfuric acid, is known to

damage MWNTs surface.

TEM images of the nanocomposites are presented in

Fig. 3. The low magnification image of sample 2 pre-

pared by the sol–gel method with Ti(OEt)4 (Fig. 3a)shows an homogeneous sample with only individual

nanotubes covered with TiO2, without any jam-like

aggregates between nanotubes and TiO2. Higher mag-

nification shows that the nanotube surface (Fig. 3b and

inset) is coated with a very thin TiO2 film (1–2 nm in

thickness). For the sample 3 prepared by the sol–gel

method using the Ti(OPri)4 precursor (Fig. 3c), all the

nanotubes surface is homogeneously covered with TiO2

nanoparticles having a diameter ranging from 3 to 5 nm

and no TiO2 aggregates are observed. For the sample 4

prepared by the hydrothermal process (Fig. 3d), the

material is macroscopically more compact and more

crystalline as seen in XRD, but the coating is not so

uniform in comparison with the samples obtained by the

sol–gel method. In this case nanoparticles of anatase

forms irregular coating and aggregates link the nano-tubes resulting in a more dense material. We noticed

Fig. 2. C1s XPS spectra: sample 1, pristine MWNTs; samples 2–4, MWNTs coated three times with TiO2 and annealed at 300 �C in air: sample 2,

prepared with Ti(OEt)4 by sol–gel method; sample 3, prepared with Ti(OPri)4 by sol–gel method; sample 4, prepared with TiOSO4 by hydrothermal

method.

1150 A. Jitianu et al. / Carbon 42 (2004) 1147–1151

that the hydrothermal treatment damages a little the

nanotubes surface.

4. Conclusion

We succeeded to coat the carbon MWNTs surface

with anatase TiO2 and to get different TiO2 morpho-

logies depending on the coating technique. By the sol–

gel method, MWNTs surface was coated either with a

continuous TiO2 thin film when Ti(OEt)4 precursor was

used or with TiO2 nanoparticles when the precursor was

Ti(OPri)4. With the hydrothermal method, a quite good

coating is also obtained but the nanotubes surface ispartially damaged due to the oxidizing medium of

deposition. From this point of view our results recom-

mend the sol–gel method for future TiO2 coating on

MWNTs.

Fig. 3. TEM images: (a) sample 2, overall view of MWNTs covered with TiO2 (from Ti(OEt)4 by sol–gel method); (b) sample 2, MWNTs surface

covered with TiO2 thin film (from Ti(OEt)4 by sol–gel method) and in inset higher magnification; (c) sample 3, MWNTs surface covered with TiO2

nanoparticles (from Ti(OPri)4 by sol–gel method); (d) sample 4, MWNTs covered with TiO2 (from TiOSO4 by hydrothermal method).

A. Jitianu et al. / Carbon 42 (2004) 1147–1151 1151

Taking into account the semiconducting properties of

TiO2, these nanocomposites can be applied for photo-

catalytic decomposition of aromatic pollutants in

aqueous medium under UV irradiation. This is toevaluate the extent of beneficial role played by the

nanotubular support.

Acknowledgements

This work was supported by the European contract

NANOCOMP, HPRN-CT-2000-00037 (2000–2003) andby ‘‘ACI-CNRS Mat�eriaux’’ contract (2001–2003).

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