Preparation and characterization of nanostructured

MWCNT-TiO2 composite materials for photocatalytic

water treatment applications

Wendong Wang a,c, Philippe Serp b, Philippe Kalck b, Claudia Gomes Silva a,Joaquim Luıs Faria a,*

a Laboratorio de Catalise e Materiais, Departamento de Engenharia Quımica, Faculdade de Engenharia da Universidade do Porto,

Rua Dr. Roberto Frias s/n 4200-465 Porto, Portugalb Laboratoire de Chimie de Coordination UPR 8241 CNRS, Ecole Nationale Superieure d’Ingenieurs en Arts Chimiques Et Technologiques,

118 Route de Narbonne Toulouse Cedex 31077, Francec Department of Materials Science and Engineering, University of Science and Technology of China,

Hefei 230026, People’s Republic of China

Received 7 April 2007; received in revised form 16 April 2007; accepted 25 April 2007

Available online 1 May 2007

Abstract

Nanoscale composite materials containing multi-walled carbon nanotubes (MWCNT) and titania were prepared by using a

modified sol–gel method. The composites were comprehensively characterized by thermogravimetric analysis, nitrogen adsorp-

tion–desorption isotherm, powder X-ray diffraction, scanning electron microscopy with energy dispersive X-ray analysis,

transmission electron microscopy, X-ray photoelectron spectroscopy and UV–vis absorption spectroscopy. The analysis revealed

the presence of titania crystallites of about 7.5 nm aggregated together with MWCNT in particles of 15–20 nm of diameter. The

photoactivity of the prepared materials, under UV or visible irradiation, was tested using the conversion of phenol from model

aqueous solutions as probe reaction. A synergy effect on the photocatalytic activities observed for the composite catalysts was

discussed in terms of a strong interphase interaction between carbon and TiO2 phases by comparing the different roles of MWCNT

in the composite materials.

# 2007 Elsevier Ltd. All rights reserved.

Keywords: A. Nanostructures; A. Composites; B. Sol–gel chemistry; C. Photoelectron spectroscopy; D. Catalytic properties

1. Introduction

Since the discovery of carbon nanotubes (CNT) efforts have been made to explore their applications using various

approaches [1], as they are in fact one of the most remarkable emergent materials. In a recent review, attention has been

drawn to the fact that CNT can compete with activated carbon as catalyst supports due to the combination of their

electronic, adsorption, mechanical and thermal properties [2].

www.elsevier.com/locate/matresbu

Materials Research Bulletin 43 (2008) 958–967

* Corresponding author. Tel.: +351 2 25 081 645; fax: +351 2 25 081 449.

E-mail address: jlfaria@fe.up.pt (J.L. Faria).

0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2007.04.032

On the other hand, titanium dioxide has been widely used as photocatalytic material for solving environmental

problems, especially for removing toxic chemicals from waste waters [3]. The sol–gel route is well established as an

excellent method to prepare the TiO2-based materials, including modification by incorporating foreign ions [4], TiO2

film by a dip-coating process [5] and titania/carbon composites [6].

Composite materials containing CNT and TiO2 are believed to provide many applications and exhibit cooperative

and synergetic effects between the metal oxides and carbon phases. Some recent works have emphasized on the

preparation of these new hybrid materials and a few examples are worth noticing. A sol–gel method has been used to

prepare the inclusion of CNT into an inorganic film of TiO2 matrix [7]. MWCNT-based titania composite material has

been prepared by an impregnation method, which provides a homogeneous inorganic cover layer on the surface of

MWCNT [8]. Rutile TiO2 has been immobilized on the sidewall of MWCNT by a simple one-step scheme, which

produces three distinct morphologies of hybrid MWCNT at different reaction temperatures [9]. Coating MWCNT

surface with TiO2 has also been performed by a sol–gel method using different alkoxides and by hydrothermal

hydrolysis of TiOSO4, which can lead to different morphologies [10]. However, the reported study with a low CNT

content (1.5 wt%) failed to confirm the predicted synergetic effect in phenol degradation under UV illumination [11].

In the present work, we aim at the preparation of MWCNT-TiO2 composite materials through an acid-catalyzed

sol–gel method to be used as photocatalysts in water treatment processes. We take advantage of the unique properties

of MWCNT to optimize the surface and catalytic properties of the resulting composite materials. The structural

characterization of the materials is given in detail. We used UV or visible light irradiation for activating the catalysts

and probe their photo-efficiency in the reaction of phenol removal from model aqueous solutions.

2. Experimental

2.1. Catalyst preparation

High purity MWCNT were synthesized by a catalytic chemical vapor deposition (CCVD) method in a fluidised bed

reactor over Fe/Al2O3 catalyst, and then purified according to a standard sulphuric acid washing procedure [12].

MWCNT-TiO2 composite catalysts were prepared using a modified acid-catalyzed sol–gel method starting from

alkoxide precursors. The preparation was performed at room temperature as follows. Firstly, 0.1 mol of Ti(OC3H7)4

(Aldrich 97%) was dissolved in 200 mL of ethanol. The solution was stirred magnetically for 30 min, and then

1.56 mL of nitric acid (65 wt%) was added. Subsequently, certain amount of MWCNT was introduced into the

Ti(OC3H7)4 ethanol solution. The mixture was loosely covered and kept stirring until a homogenous MWCNT-

contained gel formed. The gel was aged in air for several days, and then the obtained xerogel was crushed into a fine

powder. The powder was calcined at 400 8C in a flow of N2 for 2 h to obtain MWCNT-TiO2 composite materials.

Degussa TiO2 P25 was used as standard for comparison purposes whenever necessary.

2.2. Characterization methods

Thermogravimetric analysis (TGA) of the composite materials was carried out with a Mettler TA 4000 system at a

heating rate of 10 8C min�1 under air flow. N2 adsorption–desorption isotherms were measured by using a Coulter

Omnisorp 100 CX apparatus and the surface area was calculated by a BET method. Powder X-ray diffraction (XRD)

patterns were obtained on a Philips X’Pert MPD diffractometer (Cu Ka = 0.15406 nm). X-ray photoelectron

spectroscopy (XPS) was performed with a VG Scientific ESCALAB 200A spectrometer using a monochromatized Mg

K X-ray radiation (15 keV, 300 W). The composite catalysts were characterized by scanning electron microscopy

(SEM) using a JEOL JSM-6301F equipped with an X-ray detector for energy dispersive X-ray analysis (EDX) and

also by transmission electron microscopy (TEM) using a Phillips CM12 operating at 120 kV. The UV–vis spectra of

the powder solids were measured on a JASCO V-560 UV–vis spectrophotometer, equipped with an integrating sphere

attachment (JASCO ISV-469).

2.3. Photocatalytic reaction

Photocatalytic removal of phenol from model aqueous solutions under UV and visible light irradiation was used to

probe the activity of the photocatalysts. An immersion cylindrical batch reactor of 1000 mL was used in all

W. Wang et al. / Materials Research Bulletin 43 (2008) 958–967 959

experiments and the geometry of the system was kept similar for all the radiation sources. As UV radiation source we

used a Heraeus TNN 15/32 low pressure mercury vapor lamp (3 W of the radiant flux), located axially to the reactor

and held in a quartz immersion tube. The visible light irradiation source was a Heraeus TQ 150 medium pressure

mercury vapor lamp, again located axially and in a Duran 50 immersion tube. For the visible light source, a circulating

water jacket was employed to cool the lamp and cancel the infrared radiation, thus preventing any heating of the

suspension.

The experiments were carried out in a glass immersion photochemical reactor charged with 800 mL of suspension/

solution. The initial phenol concentration ðC00Þwas 50 mg/L. The amount of suspended photocatalyst was kept at 1 g/L

of TiO2, with the corresponding amount of carbon being calculated accordingly for the composite catalysts. Before

turning on illumination, the suspension containing phenol and photocatalyst was magnetically stirred in the dark for

1 h, allowing the adsorption–desorption equilibrium to be reached. Then, the suspension was irradiated with UV or

visible light at constant stirring speed. The first sample was taken out at the end of the dark adsorption period, just

before the light was turned on, in order to determine the phenol concentration in solution, which was hereafter

considered as the initial concentration (C0) after dark adsorption. Samples were then withdrawn regularly from the

reactor and immediately centrifuged to separate any suspended solid. The clean transparent solution was analyzed by

UV–vis spectroscopy (JASCO V-560). The full spectrum (200–800 nm) for each sample was recorded and the

absorbance at characteristic band 270 nm was followed to determine the phenol concentration. Repetition tests were

made to ensure the reproducibility.

Product analysis was performed by HPLC using a Hitachi Elite LaChrom liquid chromatograph equipped with an

L-2450 diode array detector. The stationary phase consisted in a Purospher Star RP-18 endcapped column

(250 mm � 4.6 mm, 5 mm particles) working at room temperature. The mobile phase was a mixture of water and

methanol with a gradient concentration at a flow rate of 1 mL min�1. For known compounds, the relationship between

area and concentration was determined using standards.

3. Results and discussion

3.1. Characterization of MWCNT-TiO2 composite materials

The thermal stabilities of different solids are shown in TG curves (Fig. 1A). It is apparent that no obvious mass loss

occurs for neat TiO2 up to 800 8C, while neat MWCNT may be completely gasified in an air flow within this range. The

composite material MWCNT-TiO2, containing 20% weight ratio of MWCNT to the unity weight basis of neat TiO2,

exhibit a mass loss of 16.4% due to the carbon gasification, which agrees very well with the calculated value from

initial ratio. This result indicates that there is no appreciable loss of MWCNT during the preparation procedure, where

a calcination temperature of 400 8C in a N2 flow was used for 2 h.

At the same time, a shift to lower temperature is observed in carbon gasification of the composite material with

relation to MWCNT as shown in DTG profiles (Fig. 1B). The temperature at which the maximum carbon gasification

rate occurs is 554 8C for the neat MWCNT, in agreement with the reported 550–650 8C range values [2]. In the case of

MWCNT-TiO2 composite material, the DTG curve features one intense band centered at 493 8C. The cause for the

shift of the maximum carbon gasification temperature can be explained in that the presence of TiO2 in the composite

catalysts could affect the concentration of MWCNT surface defect sites via chemical adsorption between carboxylic

acid groups of MWCNT and surface hydroxyl groups on TiO2 particles [13], which may catalyze carbon gasification,

thus lowering the temperature.

The N2 adsorption–desorption isotherms (not shown) for TiO2, MWCNT, and MWCNT-TiO2 composite materials

can be mainly ascribed to type IV, which suggests a mesoporous pore texture. The BET surface areas of neat TiO2 and

MWCNT are 107 and 169 m2/g, respectively. It is interesting to note that the surface area of the composite material

(139 m2/g) is higher than the value (119 m2/g) calculated in proportion to the TiO2 and MWCNT content. This result is

also observed for the composite materials with different initial MWCNT amounts, suggesting that some porosity is

developed during the treatment.

Average pore sizes of different solids are also obtained from N2 isotherm. Pores in MWCNT include narrowly

distributed inner hollow cavities of 3.4 nm and widely distributed aggregated pores of 20 nm formed by interaction of

isolated MWCNT [2]. TiO2 presents mono-pore size distribution centered at 3.2 nm. The composite material reveals

two different pores of adjacent diameters with different distribution ratio: 3.2 nm with high distribution can be

W. Wang et al. / Materials Research Bulletin 43 (2008) 958–967960

attributed to TiO2, while 3.7 nm with low distribution to inner hollow cavities in MWCNT. The latter increased with

the carbon content of the composite materials. It is worth noticing that the aggregated pores in MWCNT of 20 nm are

absent for all the composite materials.

It seems that MWCNT introduced into TiO2 can prevent TiO2 particles from agglomerating, consequently

increasing its surface area of the composite materials. Meanwhile, the absence of MWCNT aggregated pores in the

composite catalysts suggests a homogeneous coverage of TiO2 over MWCNT. This effect is also supported by SEM

observation in Fig. 2. The morphologies of MWCNT-TiO2 composite materials feature relatively homogeneous TiO2

supported on MWCNT without apparent agglomeration of TiO2 particles. Additionally, EDX spectra analysis of

MWCNT-TiO2 composite materials confirms only the presence of C, O and Ti elements.

The examination of XRD patterns (Fig. 3) of different solids reveals that only TiO2 in anatase phase can be

identified for TiO2 and MWCNT-TiO2 composite materials. The rutile and brookite phases of TiO2 are not observed. It

is noteworthy that the characteristic peaks of MWCNT can hardly been identified from the patterns of composite

materials with different initial MWCNT weight ratio varied from 1 to 40%. It is observed that the peaks width broaden

slightly and gradually with the increase in MWCNT amount for the composite materials. TiO2 crystallite sizes are

estimated from the line broadening by using Scherrer’s equation. The crystallite size for neat TiO2 of 8.5 nm is

obtained, while the size decreases to 7.4 nm for MWCNT-TiO2 composite material containing 20% initial weight ratio

of MWCNT. Furthermore, this decrease in TiO2 crystallite size is also systematically observed for all the composite

materials with different initial MWCNT ratio varied from 1 to 40%. This result is as well confirmed by TEM

inspection of MWCNT-TiO2 composite material in Fig. 4. The image presents an overall view of MWCNT, with

external diameters ranging from 15 to 20 nm. The average particle size of TiO2 determined from the TEM images is

about 7.5 nm, which is consistent with those calculated from the XRD peak broadening.

W. Wang et al. / Materials Research Bulletin 43 (2008) 958–967 961

Fig. 1. TG (A) and DTG (B) curves of (a) TiO2, (b) MWCNT-TiO2 and (c) MWCNT.

XPS patterns of Ti 2p and O 1s for the MWCNT-TiO2 composite material are shown in Fig. 5 to probe the chemical

environment of the elements in the near surface range.

It is observed that the binding energies of Ti 2p3/2 and 2p1/2 are centered at 459.6 and 465.4 eV, respectively, in

agreement with those of neat TiO2. This clearly indicates that all the titanium cations in the composite material are in

the oxidation state IV. However, O 1s core level spectra of TiO2 and the composite materials are quite different. TiO2

mainly exhibits only one peak centered at 530.8 eV, which is due to the lattice Ti–O bond of TiO2 phase. In the case of

MWCNT-TiO2 composites, the curve fitting of O 1s spectra basically indicates three components centered at 530.8,

532.8 and 534.6 eV. The latter two peaks are commonly ascribed to the surface oxygen complexes of carbon phase

[6,14].

Based on the characterization mentioned above, the fact that surface areas of composite materials are higher than

the calculated values suggests that there may be a strong interphase structural effect between the carbon and metal

oxide phases. This is also supported by the appearance of surface oxygen complexes of carbon phase in the XPS

spectra of the composite materials. The disappearance of MWCNT characteristic peaks in the XRD patterns of the

composite materials is in line with a homogeneous coverage of TiO2 on MWCNT, which is additionally supported by

W. Wang et al. / Materials Research Bulletin 43 (2008) 958–967962

Fig. 2. SEM image of MWCNT-TiO2 composite material.

Fig. 3. XRD patterns of (a) TiO2, (b) MWCNT-TiO2 and (c) MWCNT.

the absence of MWCNT aggregated pores in the composite materials. On the other hand, the introduction of MWCNT

into TiO2 favors less extended crystallized TiO2 domains on MWCNT surface, and thus avoiding TiO2 particles

agglomeration. All factors account for the increase in surface areas of the composite materials.

The diffuse reflectance UV–vis spectra of the different solids are given in Fig. 6 where Kubelka-Munk equivalent

absorption units are used. In comparison with Degussa TiO2 P25 characteristic spectrum with its fundamental

absorption sharp edge rising at 400 nm, the absorption spectrum of neat TiO2 prepared by the sol–gel method shifts to

the red, leading to an enhanced absorption of the composite materials in longer wavelengths. This shift corresponds to

a decrease in the band gap energy, which should be related to the loss of crystallinity of the samples rather than to

change of crystal structure since only anatase is found in the composite materials. A correlation between the

MWCNT amount and the UV–vis spectrum change features the enhancements of absorption increasing with

MWCNT amounts of the composite materials. This result also suggests that there might be an increment of surface

electric charge of the oxides in the composite materials due to the introduction of MWCNT, which may lead to

modifications of the fundamental process of electron/hole pair formation while applying for the photocatalytic

process under illumination.

3.2. Photocatalytic removal of phenol under different irradiation sources

The photocatalytic removal of phenol from model aqueous solutions was investigated using UV and visible light

irradiation sources. We observed that the phenol decomposition in the presence of neat MWCNT, as well as the direct

photolysis without any solid, is negligible within the considered reaction times. The complete removal of phenol

(>95%) is observed in 6 h for TiO2 under UV light, which is also obtained for commercially available Degussa P25

under the same reaction conditions and referred elsewhere [15,16] despite different experimental systems. By contrast,

phenol removal after 5 h of irradiation (X5 h), the >95% removal time (t95) and phenol removal rate (r), are compared

in Table 1. The introduction of MWCNT into TiO2 apparently induces a synergy effect on phenol removal. The most

favorable effect is presented by the composite catalyst with 20% weight ratio of MWCNT, which can achieve the

complete disappearance of phenol in 4 h. A detailed kinetic study can be found elsewhere [17].

In the case of visible light irradiation, this synergy effect is more remarkable in that the full removal of phenol is

accomplished in 5 h and more than double phenol removal rate is observed. Neat TiO2 can only reach 40.6% of

removal within the same reaction time, and the complete elimination of phenol was not attained after 10 h.

W. Wang et al. / Materials Research Bulletin 43 (2008) 958–967 963

Fig. 4. TEM image of MWCNT-TiO2 (1:5 w/w) composite material.

W. Wang et al. / Materials Research Bulletin 43 (2008) 958–967964

Fig. 5. XPS spectra of Ti 2p and O 1s for MWCNT-TiO2 composite material.

Fig. 6. Diffuse reflectance UV–vis spectra of Degussa P25, neat TiO2 and MWCNT-TiO2 composite.

In order to further explore this synergy effect of the composite materials, a suspended mechanical mixture of TiO2

and 20% MWCNT (MWCNT+TiO2, Table 1) was prepared by merely stirring and its photocatalytic behaviors are

compared with TiO2 and the composite catalysts under UV and visible light irradiation, respectively. The irradiated

mechanical mixture exhibits less synergy effect on phenol removal than the composite catalyst in the cases of both

irradiation sources, which is due to a reduction of the interphase interaction between MWCNT and TiO2 phases in the

mixture as anticipated.

The initial phenol concentration (C0) after 1 h dark adsorption is similar for all materials (Table 1), indicating

equivalent adsorption capacities for the different solids. Unlike the adsorption behavior of TiO2 and activated carbon

systems [15,18], where the adsorption property of activated carbon is the dominant effect on the phenol removal, the

introduction of MWCNT into the composites does not offer a significant enhancement on their adsorption capacities as

an adsorbent. Additionally, MWCNT physically acting as a dispersing agent is not regarded as a major factor

accounting for the observed synergy effect due to a relatively poor photocatalytic activity of neat TiO2 especially under

the visible light irradiation.

3.3. Product analysis

HPLC analysis was used in order to detect and quantify the main intermediates of the photocatalytic degradation of

phenol. Hydroquinone and catechol were detected as main reaction intermediates. This is in agreement with the

probability of adduct formation by insertion of the hydroxyl radical. Compounds like benzoquinone and 1,2,4-

benzenetriol were detected at very low concentrations. Therefore, a simplified reaction mechanism is proposed

(Scheme 1).

Initial rates (r0) for phenol consumption, catechol and hydroquinone formation, were obtained assuming that all

reactions follow first order kinetics (Table 2).

Results show that the synergy effect is more pronounced when the composite catalyst is irradiated under visible

light which is in agreement with the increase of the absorption of the composite catalyst in this spectral range and a

concomitant decrease in the bandgap energy.

W. Wang et al. / Materials Research Bulletin 43 (2008) 958–967 965

Table 1

Photocatalytic removal of phenol using different materials and lights sources, expressed as function of initial concentration after dark adsorption

(C0), in terms of 5 h conversion (X5 h), time for >95% removal (t95) and phenol removal rate (r)

Catalyst Irradiation source C0 (mg/L) X5 h (%) t95 (h) r ð�10�3 mmol g�1TiO2

min�1Þ

TiO2 UV 47.9 69.2 6 1.17

MWCNT+TiO2 UV 46.5 85.3 5.5 1.41

MWCNT-TiO2 UV 45.9 98.7 4 1.61

TiO2 Visible 47.5 40.6 >10 0.68

MWCNT+TiO2 Visible 46.5 71.2 8 1.17

MWCNT-TiO2 Visible 45.4 96.1 5 1.55

Scheme 1. Simplified mechanism for phenol degradation.

Accordingly, it is more reasonable to ascribe this synergy effect to MWCNT as a photosensitizer [19,20].

Considering the semiconducting property of carbon nanotubes, MWCNT may absorb the irradiation and inject the

photo-induced electron into TiO2 conduction band, which can trigger the formation of very reactive radicals,

superoxide radical ion O2�� and hydroxyl radical HO�, both responsible for the degradation of the organic compound.

The suggested electron transfer between carbon and TiO2 was experimentally supported by the observed enhanced

photocurrent of the composite materials in other investigations [19,21–23].

4. Conclusion

The MWCNT-TiO2 composite materials prepared through an acid modified sol–gel route, display a strong

interphase structure effect between MWCNT and TiO2, derived from the existence of an intimate contact of both

phases in the composite material.

By this method there is a homogeneous coverage of TiO2 over MWCNT and less agglomeration of TiO2

nanoparticles on MWCNT surface.

Consistent with the shift of the UV–vis absorption of the composites to longer wavelengths, there is a synergy effect

on the photocatalytic removal of phenol, which is more noticed in the case of the reaction activated by visible light

irradiation source. This effect was explained in terms of MWCNT acting as photosensitizer rather than adsorbent and

dispersing agent.

The effect is stronger for the composite material than for the mechanical mixture of MWCNTand TiO2 constituting

in the same ratio, which supports the existence of the interphase structure effect.

The reported results put in evidence the unique properties on the resulting MWCNT-TiO2 composite materials.

Acknowledgements

This work was supported by Fundacao para a Ciencia e a Tecnologia, Programa Operacional (POCTI/POCI), co-

supported by FEDER (projects POCI/EQU/58252/2004, POCTI/1181/2003 and grants SFRH/BPD/11598/2002,

SFRH/BD/16966/2004) and by CRUP (F-12/03). Thanks to Conference des Presidents d’Universite for financial

support ‘‘Action integree Luso-Francaise’’. This work was also partly supported by China Postdoctoral Science

Foundation.

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Table 2

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