Solar Energy Materials & Solar Cells 94 (2010) 1658–1664

Contents lists available at ScienceDirect

Solar Energy Materials & Solar Cells

0927-02

doi:10.1

n Corr

E-m

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

Photocatalytic activity of poly(3-hexylthiophene)/titanium dioxidecomposites for degrading methyl orange

Yunfeng Zhu, Yi Dan n

State Key Laboratory of Polymer Materials Engineering of China (Sichuan University), Polymer Research Institute of Sichuan University, Chengdu 610065, China

a r t i c l e i n f o

Article history:

Received 25 November 2009

Received in revised form

6 May 2010

Accepted 9 May 2010Available online 20 May 2010

Keywords:

P3HT/TiO2 composites

Methyl orange

Photocatalysis

Visible light

Degradation pathways

48/$ - see front matter & 2010 Elsevier B.V. A

016/j.solmat.2010.05.025

esponding author. Tel.: +86 28 85407286; fa

ail address: danyichenweiwei@163.com (Y. D

a b s t r a c t

Poly(3-hexylthiophene) (P3HT) was synthesized by chemical oxidative polymerization method using

FeCl3 as an oxidant in CHCl3 and poly(3-hexylthiophene)/titanium dioxide (P3HT/TiO2) composites

were prepared by a convenient polymer/inorganic blending technique. The resulting P3HT/TiO2

composites were characterized by Fourier transform infrared (FTIR) spectroscopy, thermogravimetric

analysis (TGA), ultraviolet–visible diffuse reflectance spectroscopy (UV–vis DRS), and X-ray photoelec-

tron spectroscopy (XPS). The results indicated that P3HT/TiO2 composites were formed from poly(3-

hexylthiophene) covered titanium dioxide. UV–vis DRS measurements showed that P3HT/TiO2

composites have a broad and strong absorption in visible range, indicating that the incorporation of

P3HT onto the surface of TiO2 can extend the photoresponse range of TiO2. P3HT/TiO2 composites can

adsorb methyl orange (MeO) to a certain extent. Both under UV and visible light irradiation, P3HT/TiO2

composites were more efficient in removing dye from solution than pure TiO2. Using P3HT/TiO2

composites as photocatalysts, the different degradation pathways of MeO were found under UV and

visible light irradiation. The chromophoric groups of MeO molecules were dominantly cleaved under

UV light irradiation, while under visible irradiation, the competitive photodegradation reactions

between the formation of intermediates with chromophoric group and degradation of MeO occured in

the photocatalysis system.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

In the past decades, the fascinating inorganic semiconductortitanium dioxide (TiO2) has attracted extensive attention inphotocatalytic area for decomposition of organic compounds,sterilization, cancer treatment, etc. due to its high photocatalyticactivity, abundant resource, biological and chemical inertness,and nontoxicity [1]. However, due to the intrinsic structurecharacteristics and broad band gap of TiO2 (ca. 3.2 eV, anatase),two disadvantages to its effective utilization are obvious. First, theconcentration of photo-generated electron–hole pairs is reducedby the inherent recombination process, leading to the destructionof active electron–hole pairs and further resulting in lowphotocatalytic activity [2]. Another disadvantage lies in ineffec-tive use of sunlight or visible light as irradiation source, for TiO2

can only absorb and be excited by UV light with wavelengthbelow 387 nm (which is less than 5% of the sunlight) [3]. To solvethe above problems, several different approaches have beenproposed, such as dye sensitization[4–8], metal ion [9] and non-metal atoms doping [10], and semiconductor coupling [11,12],

ll rights reserved.

x: +86 28 85402465.

an).

especially, dye-sensitized photocatalytic materials exhibit highefficiency in degradation of organic pollutants and utilization ofvisible light. However, the stability of dye with low molecularweight has been suggested as one of the critical factors limitingthe long-term performance of the dye-sensitized photacatalyticmaterials.

Conjugated polymers such as polythiophenes, polypyrroles,polyanilines, poly(p-phenylenevinylene), and their derivates areextensively employed in photovoltaic devices as antenna layersfor photovoltaic conversion of solar energy [13–17] due to theirhigh absorption coefficients in the visible part of spectrum, highmobility of charge carriers, and excellent stability [18]. As ourprevious work reported [19], conjugated polymers also can beapplied in photocatalytic area; in recent years, there is increasinginterest in using polythiophenes [18,19], polyanilines [20–22],polypyrroles [23], and their derivates [24–27] to sensitize TiO2

and produce polymer/TiO2 photocatalytic materials.Poly(3-hexylthiophene) (P3HT) has a higher charge carrier

mobility, dissolubility and processability, long-term stability anda broad and strong absorption in visible region (with band gap of1.9–2.0 eV) [28]. Therefore, it should be a good candidate for asensitizer of TiO2. In present study, P3HT/TiO2 composites weredescribed for their application in degrading organic dyes. Methylorange (MeO), a typical azo dye, was used as the target pollutant

Y. Zhu, Y. Dan / Solar Energy Materials & Solar Cells 94 (2010) 1658–1664 1659

in aqueous media to assess the photocatalytic activities of theP3HT/TiO2 composites. The adsorption capacities in the dark andthe photocatalytic activities of the P3HT/TiO2 composites underUV and visible light irradiation were studied, the influence ofP3HT contents on the photocatalytic activities of composites anddegradation pathway of pollutant were also discussed.

Fig. 1. TGA curves of (a) P3HT, (b) TiO2, and (c) P3HT(10%)/TiO2 composites.

2. Experimental

2.1. Sample preparation

3-hexylthiophene (3-HT) monomer (1H NMR dH (400 MHz)0.89 (t, 3H), 1.25–1.63 (m, 8H), 2.62 (t, 2H), 6.9 (m, 2H), 7.22 (dd,1H)) was synthesized by coupling n-hexylmagnesium bromidewith 3-bromothiophene in the presence of a nickel catalyst [29].Poly(3-hexylthiophene) was synthesized by chemical oxidativepolymerization method using anhydrous FeCl3 as an oxidant inCHCl3. 3-hexylthiophene (0.01 mol), FeCl3 (0.02 mol), and CHCl3

(50 mL) were added into a dried flask, the resulting mixture wasmagnetically stirred in an ice bath for 4 h. Then, solid wasprecipitated in methanol, filtered with a Buchner funnel, andextracted with refluxing methanol until a colorless washing wasobtained to remove the residual FeCl3. During this procedure, thecolor of the solid changed from black to red. Finally, the solid waswashed with THF and the soluble part was collected and dried in avacuum at room temperature to obtain P3HT.

A commercially available anatase titanium dioxide (TiO2, HR-3,Zhoushan Mingri Nanometre Material Co., Ltd.), with a specificsurface area (BET) of 240730 m2 g�1, corresponding to a meanparticle size of ca.15 nm, nonporous, was used to preparenanocomposites with different P3HT contents: 1.0%, 2.0%, 4.0%,and 10%. The P3HT/TiO2 composites were prepared by thefollowing procedure: a certain amount of TiO2 particles (dried at100 1C for 2 h before use) were dispersed in anhydrous alcoholunder ultrasonic vibrations (SC-I, Chengdu Jiuzhou UltrasonicTechnology Co.) at room temperature for 10 min. The TiO2 slurrywas then diverted into a single-necked, round-bottom flaskequipped with a magnetic and Teflon-coated stirrer. P3HT wasdissolved in THF and added dropwise to the TiO2 suspension andthe mixture was then stirred for 1 h in the dark. Subsequently, theP3HT/TiO2 composites were obtained by removing the solvent ina vacuum at 50 1C. The composites are labeled as P3HT(X%)/TiO2,where X corresponds to the P3HT content in the composites.

2.2. Characterization techniques

FT-IR spectra of the purified samples were measured on aNicolet 560 FT-IR spectrometer. The KBr pellets were preparedwith dried samples and the spectrum was collected in the rangefrom 4000 to 400 cm�1. Thermal analysis of all samples wasperformed with an SDT Q600 (TA Instruments Co., Ltd.) thermalanalysis instrument. The samples were heated from 35 to 800 1Cat a rate of 20 1C min�1 in a nitrogen atmosphere, at a flow rate of100 mL min�1. UV–vis diffuse reflectance spectra were recordedon a TU-1901 spectrophotometer equipped with an integratingsphere attachment (IS 19-1). XPS measurements were carried outon a spectrometer (XSAM-800, KRATOS Co.) with a Mg Kaanticathode, 12 kV, 11 mA, FRR mode.

2.3. Evaluation of photocatalysis

2.3.1. Adsorption experiments

An aqueous solution of methyl orange (MeO) was used toinvestigate the adsorption performance and photocatalytic

activities of the prepared materials. The initial MeO concentra-tion, C0, was 40 ppm. The adsorption experiments were per-formed by adding 10 mL of MeO solution and 10 mg ofphotocatalyst in a glass tube wrapped closely by black rubberizedfabric, which can prevent the suspension from being irradiated bylight. The glass reactor was sealed and the suspension wasmagnetically stirred throughout the experiments. A portion ofsamples were then withdrawn regularly from the reactor andcentrifuged at 5000 rpm for 10 min (TDL-5-A centrifuge) imme-diately for separation of any suspended solid. The change in theconcentration of MeO was monitored by measuring the absor-bance at lmax¼464 nm with a UV-240 UV–vis spectrophotometer(Shimadzu Co. Japan). In this paper, the decolorization ratio ofcontaminant was calculated as follows:

Z¼ A0�At

A0� 100% ð1Þ

where At is the absorbance of the contaminant solution atdifferent reaction time t, A0 is the absorbance of the initialcontaminant solution.

2.3.2. Experiments performed under UV and visible light irradiation

Photocatalysis experiments under UV and visible light irradia-tion were carried out in two different self-regulating instruments,the structure of the instruments and the experimental methodshave been described in detail elsewhere [19,30]. The change in theconcentration of MeO was monitored by the same methoddescribed in adsorption experiments section. The first samplewas taken out at the end of the dark adsorption period, just beforethe lamps were turned on, in order to determine the MeOconcentration in solution, which was hereafter considered as theinitial concentration ðC00Þ after dark adsorption.

3. Results and discussion

3.1. Characterization of the P3HT/TiO2 composites

Fig. 1 shows the TGA curves of (a) P3HT, (b) TiO2, and (c)P3HT(10%)/TiO2 composites. From Fig. 1a, it can be seen that thereis a weight loss of 72% in the range of 30–800 1C; the onset andend temperature of polymer decomposition is �250 and�490 1C, respectively. Fig. 1b shows a weight loss of �6.9%,

Fig. 3. XPS survey spectra for the surface of (a) TiO2, (b) P3HT, and (c) P3HT(10%)/

TiO2.

Table 1Surface composition of P3HT, TiO2, and P3HT/TiO2 composites.

Element Percentage (%)

TiO2 P3HT(4.0%)/TiO2 P3HT(10%)/TiO2 P3HT

Ti 25.4 17.4 18.2 Not detected

O 44.0 34.2 32.9 7.7

C 30.6 48 47.7 88.2

S Not detected 0.5 1.2 4.1

Y. Zhu, Y. Dan / Solar Energy Materials & Solar Cells 94 (2010) 1658–16641660

which is attributed to a small amount of water in the sample, theTiO2 particles are stable in the experimental temperature range. Asmall amount of weight decrease of the P3HT/TiO2 composites(Fig. 1c) in the range of 30–200 1C is attributed to the waterevaporation. In the range of 200–800 1C, there is an obviousweight loss of ca. 10%, which is attributed to the decomposition ofP3HT adsorbed on the surface of TiO2 particles. The result is ingood agreement with the P3HT dosage, which suggests that P3HTis distributed uniformly on the surface of TiO2 particles.

Fig. 2 displays the FTIR spectra of (a) TiO2, (b) P3HT, and (c)P3HT(10%)/TiO2 composites. The bands attributed to the presenceof P3HT in the composites are confirmed by the C–H stretchingmode of thiophene ring, C–H stretching vibration of hexyl groupand symmetric CQC stretching mode of thiophene ring at ca.2926, 2854, and 1459 cm�1, respectively. The spectra of P3HT/TiO2 composites also show the characteristic broad absorptionband of Ti–O–Ti at around 400–500 cm�1 and –OH groupsabsorption band at 3000–3500 cm�1.

The XPS analysis spectra for the surface of (a) TiO2, (b) P3HT,and (c) P3HT(10%)/TiO2 composites are shown in Fig. 3. It can beseen that Ti, O, C, and S elements were detected on the surface ofP3HT(10%)/TiO2 composites, the Ti and O elements assign to TiO2,the C and S elements come from P3HT. The relative atomicconcentration of the individual elements (Ci) can be calculated asfollows [31]:

Ci ¼Ai=Si

Pm

j

Aj=Sj

ð2Þ

where Ai is the photoelectron peak area of element i, Si is thesensitivity factor of element i, and m is the number of measuringelements. The relative atomic ratios of C, O, Ti, and S on thesurface of P3HT, TiO2, and P3HT/TiO2 composites are representedin Table 1, the relative atomic ratios of Ti and O elements on thesurface of the P3HT/TiO2 composites decrease in comparison withthat of TiO2 particles because of the coverage of P3HT on thesurface of the TiO2 particles.

Fig. 4 shows the UV–vis diffuse reflectance spectra of (a) TiO2,(b) P3HT(1.0%)/TiO2, (c) P3HT(2.0%)/TiO2, (d) P3HT(4.0%)/TiO2,and (e) P3HT(10%)/TiO2 composites. TiO2 can only absorb UV lightwith wavelength lower than 387 nm because of its wide band gap.P3HT/TiO2 composites can absorb both UV light and visible light(see Fig. 4(b)–(e)); moreover, there are two absorption bands from200 to 400 nm and from 400 to 700 nm, respectively. The bandfrom 200 to 400 nm can be assigned to the characteristicabsorption of TiO2 and the P3HT absorption band in the UVlight region. The second band is attributed to the electron

Fig. 2. FTIR spectra of (a) TiO2, (b) P3HT, and (c) P3HT(10%)/TiO2 composites.

Fig. 4. UV–vis diffuse reflectance spectra of (a)TiO2 and P3HT/TiO2 composites

with different contents of P3HT: (b) 1.0%, (c) 2.0%, (d) 4.0%, and (e) 10%.

transition from the valence bond to the antibonding polaronstate (p�pn type) of P3HT. With an increase of P3HT content, theabsorbency for P3HT/TiO2 composites increases in the visible lightregion. Compared with the spectrum of TiO2, the presence of P3HTsignificantly changes the spectrum of TiO2 in the visible light

Fig. 6. Decolorization ratio of MeO (Z) vs. time in typical photocatalytic

experiment performed using P3HT/TiO2 composites with different P3HT contents

under UV light irradiation. Initial MeO concentration was 40 mg L�1. Catalyst

concentration was 1.0 g L�1.

Y. Zhu, Y. Dan / Solar Energy Materials & Solar Cells 94 (2010) 1658–1664 1661

range, as the P3HT/TiO2 composites absorb much more visiblelight than TiO2. The results indicate that P3HT/TiO2 compositesare promising materials which can be excited by visible light.

3.2. Photocatalytic degradation of methyl orange

3.2.1. Adsorption of methyl orange on the surface of P3HT/TiO2

composites in the dark

Fig. 5 shows the decolorization ratio of MeO, Z, for differentinterval time during darkness adsorption. It can be seen that anegligible decolorization of MeO was observed within 120 min inthe presence of pure P3HT or pure TiO2 particles. However, theincorporation of P3HT into the composites provides an apparentlyadditive effect on their adsorption capacities, the adsorptionpercentages of MeO onto the surface of the TiO2 and P3HT(10%)/TiO2 composites corresponding to the initiative MeO were 2.2%and 17%, respectively. The adsorption equilibrium can beestablished after 60 min.

3.2.2. Photocatalytic degradation of MeO by P3HT/TiO2 composites

under UV-irradiation

UV light illumination begins after 60 min of adsorption in thedark. Fig. 6 shows the decolorization ratio of MeO, Z, for differentinterval time during UV-irradiation. In Fig. 6, it can be seen that nomore than 5% of decolorization occurred in the presence of pureP3HT after 150 min of UV-irradiation, which can be attributed tothe adsorption and photolysis of MeO, P3HT can hardly degradeMeO molecules. However, TiO2 and P3HT/TiO2 composites candegrade MeO under UV-irradiation. After 150 min of UV-irradiation, the decolorization ratios of MeO for TiO2,P3HT(2.0%)/TiO2, P3HT(4.0%)/TiO2, and P3HT(10%)/TiO2

composites are 77.6%, 90.1%, 92.7%, and 86.8%, respectively,indicating that the P3HT/TiO2 composites with P3HT contentshigher than 2.0% exhibit higher decolorization ratios than that ofTiO2; moreover, with an increase in the P3HT content of P3HT/TiO2 composites, the decolorization rate first increases and thendecreases. In this heterogeneous catalytic system, thephotocatalytic activities of P3HT/TiO2 composites are affected bythe following factor. First, the intensity of UV light absorbed byTiO2 particles can obviously affect the photocatalytic activities ofP3HT/TiO2 composites. Second, P3HT with p-conjugated structurehas high electron mobility, which can facilitate the separation of

Fig. 5. Decolorization ratio of MeO (Z) vs. experimental time during the

adsorption experiments.

the electron–hole pairs generated under UV-irradiation, a certaincontent of P3HT can improve the photocatalytic activities ofP3HT/TiO2 composites. Third, high adsorption of organiccontaminants would increase the reaction rates, though theadsorption of some organic contaminants is not requisite for thereaction since the reactive dOH radicals and other oxidizingspecies can diffuse into the bulk solution to react with organicpollutants [32], adsorption capacity of P3HT/TiO2 composites isanother important factor to affect the photocatalytic activities.The final photocatalytic activities are combined performance ofthe three factors. When the P3HT/TiO2 composites were subjectedto UV-irradiation with energy equal to or greater than the bandgap of TiO2, electrons were excited from the valence band to theconduction band and electron–hole pairs were generated and theseparated electrons and holes can migrate to the interface of P3HTand TiO2 and transfer to P3HT. In competition with chargetransfer is intrinsic and interfacial electrons and holesrecombination. The electron transfer process is more efficientbecause of the intrinsical p-conjugated structure of P3HT and thepreadsorption of P3HT on the surface of TiO2 [33]. When thecontent of P3HT is lower than 2.0%, P3HT not only cannot exhibitits noticeable effect on transfer charge but also decrease the UVlight intensity that TiO2 can absorb; therefore, P3HT(1.0%)/TiO2

composites exhibit lower decolorization ratio than that of TiO2.With the content of P3HT increasing, P3HT gradually exhibits itscontribution on charger transfer, and the adsorption capacities ofcomposites was enhanced, P3HT(4.0%)/TiO2 composites exhibitthe highest decolorization ratio. With a further increase in P3HTcontent, the decrease in activity of P3HT/TiO2 composites isconsidered to be related to the increased absorbing and scatteringof photons by a large amount of P3HT adsorbed on the surface ofTiO2, even the high adsorption capacity cannot offset theinfluence of the decrease in UV intensity.

The linear fitting curves between LnðC 00=CtÞ and irradiationtime were according to apparent first-order kinetics,LnðC00=CtÞ ¼ kt, where C00 is the concentration of MeO afteradsorption and Ct the concentration of MeO at time t, and areshown in Fig. 7. The correlation constants (r2) for the fitted lineswere calculated to be between 0.991 and 0.999 (see Table 2),which indicates that the photocatalytic degradation of MeO canbe described by the first-order kinetic model.

Fig. 7. Apparent first-order linear transform ln ðC00=Ct Þ ¼ f ðtÞ of MeO degradation

kinetic plots for P3HT/TiO2 composites with different P3HT contents.

Table 2Pseudo-first order apparent constant values of MeO degradation for P3HT/

TiO2.composites with different contents of P3HT.

Catalyst Kapp�102 (min�1) C00 (mg L�1) Correlation coefficient (r2)

TiO2 1.02 38.84 0.999

P3HT(1.0%)/

TiO2

0.69 39.56 0.996

P3HT(2.0%)/

TiO2

1.59 39.1 0.998

P3HT(4.0%)/

TiO2

1.86 36.5 0.997

P3HT(10%)/TiO2 1.37 33.37 0.993

Fig. 8. UV–vis absorption spectral changes of MeO as a function of UV light

irradiation time in the presence of P3HT(10%)/TiO2 composites.

Table 3Surface composition of P3HT(10%)/TiO2 composites (a) as-prepared, (b) after

adsorption of MeO, and (c) after UV light irradiation for 5 h.

Element Percentage (%)

a b c

Ti 18.2 13.9 17.7

O 32.9 28.5 32.5

C 47.7 56.3 48.5

S 1.2 1.3 1.3

Y. Zhu, Y. Dan / Solar Energy Materials & Solar Cells 94 (2010) 1658–16641662

Fig. 8 shows UV–vis spectral changes of MeO afterphotocatalytic degradation. During the photocatalytic periods,the separated holes and electrons were captured by H2O and O2

dissolved in water to generate dOH and superoxides ( dO�2 )radicals. Attacking of dOH and superoxides radicals on MeO

moleculars resulted in decolorization of MeO. The color of MeO isdetermined by azo bond (–NQN–) and its associatedchromophores and auxochromes, which shows a peakwavelength at 464 nm in visible range. The data of Fig. 8indicate only a decrease in absorbance with the increasingirradiation time and no peak wavelength shifts for the P3HT/TiO2/MeO system, suggesting that the cleavage of azo bond tookplace and MeO molecules turned into smaller pieces. Acontinuous irradiation will further oxidize the smaller piecesuntil they completely mineralized to CO2, H2O, NO�X , and SO2�

4 .To further study the change on the surface of catalysts before

and after the photocatalytic reactions, using P3HT(10%)/TiO2

composites as photocatalysts, the adsorption experiments andphotocatalysis experiments were performed, subsequently thesolid catalysts was separated and analyzed by XPS, and the resultsare shown in Table 3. It can be seen that after adsorption processthe relative atom contents of Ti and O elements on the surface ofcatalysts decrease while C content increases in comparison withthat of as-prepared catalysts, which can be attributed to thecoverage of adsorbed MeO on the surface of catalysts. However,after 5 h of UV-irradiation, the relative atom contents of Ti and Oelements on the surface of catalysts increase while C contentdecreases again, and the relative atom contents of Ti, O, and Celements recover to the level of that of as-prepared catalysts,indicating that the adsorbed MeO was degraded completely,which is consistent with the results that after 5 h UV-irradiationthe decolorization ratio MeO reaches to almost 100%.

3.2.3. Photocatalytic degradation of MeO by P3HT/TiO2 composites

under visible light irradiation

Control experiments show that the aqueous solution of MeO isfairly stable to visible light irradiation in absence of catalysts, noobvious photo-bleaching was observed after 10 h of irradiation(data was not shown). Photocatalytic reaction started after 60 minof adsorption in the dark, as being discussed above, the adsorptionequilibrium has been established. UV–vis absorption spectra ofMeO with different reaction time under visible light irradiation inthe presence of pure TiO2 particles and P3HT(4.0%)/TiO2 compo-sites are illustrated in Figs. 9 and 10, respectively. No significantchanges in absorbance and no peak wavelength shifts can be seenin Fig. 9, indicating that MeO cannot be degradedin the presence of TiO2 under experimental conditions.However, visible light irradiation leads to a continuous decreasein absorbance of MeO in the presence of P3HT(4.0%)/TiO2

composites.P3HT has a strong and broad absorbance within visible light

spectrum, under visible light irradiation, P3HT adsorbed on thesurface of the TiO2 is excited, electrons were simultaneouslyexcited from the highest occupied molecular orbital (HOMO) intothe lowest unoccupied molecular orbital (LUMO), the excitedelectrons in the bulk of the polymer layer can reach the interfaceby exciton diffusion [34]. In the P3HT molecular, the hexyl groupwas electron donor, which was helpful to increase the effectiveconjugation length of thiophene ring backbone [35] and enhance

Fig. 9. UV–vis absorption spectral changes of MeO as a function of visible light

irradiation time in the presence of TiO2 particles.

Fig.10. UV–vis absorption spectral changes of MeO as a function of visible light

irradiation time in the presence of P3HT(4.0%)/TiO2 composites.

Scheme 1. Schematic representation of interfacial charge separation in P3HT/TiO2

composites under visible light irradiation.

Y. Zhu, Y. Dan / Solar Energy Materials & Solar Cells 94 (2010) 1658–1664 1663

electron mobility. Since the energy level of LUMO for P3HT wasabout �3.5 eV [36], which is above the bottom of the conductionband of TiO2 (�4.2 eV) [37], it is feasible for electrons to beinjected from the LUMO of the P3HT to the conduction band ofTiO2. The conduction band electron subsequently reacts with O2

adsorbed on the surface of TiO2 to produce dO�2 radicals. Theresulting dO�2 radicals further react with H2O to generate dOOHand dOH radicals. The dO�2 and dOH radicals are most potentoxidizing agents to degrade organic pollutants. The possibleinterfacial charge separation processes of P3HT/TiO2 compositesunder the irradiation of visible light are shown in Scheme 1.

It is worth noting that visible light irradiation not only leads toa decrease in absorbance but also leads to a hypsochromic shift ofmaximal absorption wavelength of MeO in the presence ofP3HT(4.0%)/TiO2 composites. It is possible that the decrease ofmaximal absorption wavelength is due to active oxygen radicals(e.g. dO�2 , dOOH, dOH) in the bulk solution attack principally atthe aromatic ring [3] and end groups (sulfonic group and methyl).A considerable persistence of yellowish color in the solution wasobserved even in the presence of very low concentrations of MeO,

indicating that the main intermediates of MeO retain thechromophoric group, especially the azo bond. Bianco Prevotet al. [38,39] reported the main intermediates and possiblereaction mechanism for the photocatalytic degradation of MeOinduced by simulated sunlight, hydroxylation, and demethylationprocess on aromatic chromophore ring was presumed to beresponsible for the formation of intermediates. Similar results arefound in the case of photocatalytic degradation of other dyes byTiO2 under visible light irradiation [40–42]. Whatever pathwaythe photodegradation of MeO follows, the over diminution ofabsorbance was achieved with increasing irradiation time,indicating that a further degradation of the residual intermediatestook place until an ultimate dye mineralization was obtained.Even though the structure of intermediates should be furtherstudied, the results suggest that the competitive photodegrada-tion reactions between the formation of intermediates withchromophoric group and degradation of MeO occured in thephotocatalysis system, which is different from that of photo-catalysis under UV light irradiation.

4. Conclusions

The results of our research indicated that the introduction ofP3HT onto the surface of TiO2 surface extended the photoresponserange of TiO2, and the P3HT/TiO2 composites were proven to beeffective to degrade MeO both under UV and visible lightillumination. Using P3HT/TiO2 composites as photocatalysts, thedifferent degradation pathways of MeO were found under UV andvisible light irradiation. The azo bond of MeO molecule wasdominantly cleaved under UV light irradiation, while undervisible irradiation, not only the cleavage of azo bond but alsothe formation of intermediates with chromophoric group oc-curred. Further study should be focused on the photocatalyticactivity of the composites in degrading other pollutants especiallygaseous pollutants and structure analysis of intermediates.

Acknowledgement

The authors are grateful to the National Natural ScienceFoundation of China (Grant nos. 20374036 and 50573052) forsupport of this research.

References

[1] A. Fujishima, T.N. Rao, D.A. Tryk, Titanium dioxide photocatalysis, J.Photochem. Photobiol. C 1 (2000) 1–21.

Y. Zhu, Y. Dan / Solar Energy Materials & Solar Cells 94 (2010) 1658–16641664

[2] T.L. Thompson, J.T. Yates Jr., Surface science studies of the photoactivation ofTiO2—new photochemical processes, Chem. Rev. 106 (2006) 4428–4453.

[3] Y. Zhao, C. Li, X. Liu, F. Gu, Highly enhanced degradation of dye with well-dispersed TiO2 nanoparticles under visible irradiation, J. Alloys Compd. 440(2007) 281–286.

[4] P.V. Kamat, M.A. Fox, Photosensitization of TiO2 colloids by erythrosin B inacetonitrile, Chem. Phys. Lett. 102 (1983) 379–384.

[5] B. Patrick, P.V. Kamat, Photosensitization of large-bandgap semiconductors.Charge injection from triplet excited thionine into ZnO colloids, J. Phys. Chem.96 (1992) 1423–1428.

[6] D. Chatterjee, A. Mahata, Photoassisted detoxification of organic pollutants onthe surface modified TiO2 semiconductor particulate system, Catal. Commun.2 (2001) 1–3.

[7] D. Chatterjee, A. Mahata, Demineralization of organic pollutants on the dyemodified TiO2 semiconductor particulate system using visible light, Appl.Catal. B 33 (2001) 119–125.

[8] D. Chatterjee, A. Mahata, Visible light induced photodegradation of organicpollutants on dye adsorbed TiO2 surface, J. Photochem. Photobiol. A 153(2002) 199–204.

[9] W.Y. Choi, A. Termin, M.R. Hoffmann, The role of metal ion dopants inquantum-sized TiO2: correlation between photoreactivity and charge carrierrecombination dynamics, J. Phys. Chem. 98 (1994) 13669–13679.

[10] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Visible-light photocatalysisin nitrogen-doped titanium oxides, Science 293 (2001) 269–271.

[11] Y. Bessekhouad, D. Robert, J.V. Weber, Bi2S3/TiO2 and CdS/TiO2 heterojunc-tions as an available configuration for photocatalytic degradation of organicpollutant, J. Photochem. Photobiol. A 163 (2004) 569–580.

[12] X.Z. Li, F.B. Li, C.L. Yang, W.K. Ge, Photocatalytic activity of WOx–TiO2 undervisible light irradiation, J. Photochem. Photobiol. A 141 (2001) 209–217.

[13] I. Gonzalez-Valls, M. Lira-Cantu, Vertically-aligned nanostructures of ZnO forexcitonic solar cells: a review, Energy Environ. Sci. 2 (2009) 19–34.

[14] B. Kippelen, J. Bredas, Organic photovoltaics, Energy Environ. Sci. 2 (2009)251–261.

[15] F. Padinger, R.S. Rittberger, N.S. Sarciftci, Effects of postproduction treatmenton plastic solar cells, Adv. Funct. Mater. 13 (2003) 85–88.

[16] P. Ravirajan, S.A. Haque, J.R. Durrant, D.D.C. Bradley, J. Nelson, The effect ofpolymer optoelectronic properties on the performance of multilayer hybridpolymer/TiO2 solar cells, Adv. Funct. Mater. 15 (2005) 609–618.

[17] M. Helgesen, R. Søndergaard, F.C. Krebs, Advanced materials and processesfor polymer solar cell devices, J. Mater. Chem. 20 (2010) 36–60.

[18] H. Liang, X. Li, Visible-induced photocatalytic reactivity of polymer-sensitizedtitania nanotube films, Appl. Catal. B 86 (2009) 8–17.

[19] Y. Zhu, S. Xu, L. Jiang, K. Pan, Y. Dan, Synthesis and characterization ofpolythiophene/titanium dioxide composites, React. Funct. Polym. 68 (2008)1492–1498.

[20] F. Wang, S.X. Min, TiO2/polyaniline composites: an efficient photocatalyst forthe degradation of methylene blue under natural light, Chin. Chem. Lett. 18(2007) 1273–1277.

[21] X. Li, D. Wang, G. Cheng, Q. Luo, J. An, Y. Wang, Preparation of polyaniline-modified TiO2 nanoparticles and their photocatalytic activity under visiblelight illumination, Appl. Catal. B 81 (2008) 267–273.

[22] M.A. Salem, A.F. Al-Ghonemiy, A.B. Zaki, Photocatalytic degradation of allurared and quinoline yellow with polyaniline/TiO2 nanocomposite, Appl. Catal. B91 (2009) 59–66.

[23] D. Wang, Y. Wang, X. Li, Q. Luo, J. An, J. Yue, Sunlight photocatalytic activity ofpolypyrrole–TiO2 nanocomposites prepared by ‘in situ’ method, Catal.Commun. 9 (2008) 1162–1166.

[24] C. Wen, K. Hasegawa, T. Kanbara, S. Kagaya, T. Yamamoto, Visible light-induced catalytic degradation of iprobenfos fungicide by poly(3-octylthio-phene-2,5-diyl) film, J. Photochem. Photobiol. A: Chem. 133 (2000) 59–66.

[25] L. Song, R. Qiu, Y. Mo, D. Zhang, H. Wei, Y. Xiong, Photodegradation of phenolin a polymer-modified TiO2 semiconductor particulate system under theirradiation of visible light, Catal. Commun. 8 (2007) 429–433.

[26] B. Muktha, D. Mahanta, S. Patil, G. Magras, Synthesis and photocatalyticactivity of poly(3-hexylthiophene)/TiO2 composites, J. Solid State Chem. 180(2007) 2986–2989.

[27] D. Wang, J. Zhang, Q. Luo, X. Li, Y. Duan, J. An, Characterization andphotocatalytic activity of poly(3-hexylthiophene)-modified TiO2 for degradationof methyl orange under visible light, J. Hazard. Mater. 169 (2009) 546–550.

[28] D.E. Motaung, G.F. Malgas, C.J. Arendse, S.E. Mavundla, C.J. Oliphant,D. Knoesen, Thermal-induced changes on the properties of spin-coatedP3HT: C60 thin films for solar cell applications, Sol. Energy Mater. Sol. Cells93 (2009) 1674–1680.

[29] C.V. Pham, H.B. Mark Jr., H. Zimmer, Convenient synthesis of 3-alkylthio-phenes, Synth. Commun. 16 (1986) 689–696.

[30] Y. Zhu, S. Xu, Y. Dan, Photocatalytic degradation of methyl orange usingpolythiophene/titanium dioxide composites, React. Funct. Polym. 70 (2010)282–287.

[31] S.H. Kim, S.-Y. Kwak, T. Suzuki, Photocatalytic degradation of flexible PVC/TiO2 nanohybrid as an eco-friendly alternative to the current waste landfilland dioxin-emitting incineration of post-use PVC, Polymer 47 (2006)3005–3016.

[32] N. Guettaı, H.A. Amar, Photocatalytic oxidation of methyl orange in presenceof titanium dioxide in aqueous suspension. Part II: kinetics study, Desalina-tion 185 (2005) 439–448.

[33] A.L. Linsebigler, G. Lu, J.T. Yates Jr., Photocatalysis on TiO2 surfaces: principles,mechanisms, and selected results, Chem. Rev. 95 (1995) 735–758.

[34] J.E. Kroeze, T.J. Savenije, M.J.W. Vermeulen, J.M. Warman, Contactlessdetermination of the photoconductivity action spectrum, exciton diffusionlength, and charge separation efficiency in polythiophene-sensitized TiO2

bilayers, J. Phys. Chem. B 107 (2003) 7696–7705.[35] J. He, Z. Su, B. Yan, L. Xiang, Y. Wang, The effects of different side groups on

the properties of polythiophene, J. Macromol. Sci., Pure Appl. Chem. 44 (2007)989–993.

[36] M. Al-Ibrahim, H. Roth, U. Zhokhavets, G. Gobsch, S. Sensfuss, Flexible largearea polymer solar cells based on poly(3-hexylthiophene)/fullerene, Sol.Energy Mater. Sol. Cells 85 (2005) 13–20.

[37] J. Ji, Y. Lin, H. Lu, L. Wang, S. Rwei, Synthesis and photoluminescence ofpoly(3-hexylthiophene)/titania nanostructured hybrids, Thin Solid Films511–512 (2006) 182–186.

[38] C. Baiocchi, M.C. Brussino, E. Pramauro, A. Bianco Prevot, L. Palmisano,G. Marc�ı, Characterization of methyl orange and its photocatalytic degrada-tion products by HPLC/UV–vis diode array and atmospheric pressureionization quadrupole ion trap mass spectrometry, Int. J. Mass Spectrom.214 (2002) 247–256.

[39] A. Bianco Prevot, A. Basso, C. Baiocchi, M. Pazzi, G. Marcı, V. Augugliaro, L.Palmisano, E. Pramauro, Analytical control of photocatalytic treatments:degradation of a sulfonated azo dye, Anal. Bioanal. Chem. 378 (2004)214–220.

[40] C. Chen, W. Zhao, J. Li, J. Zhao, H. Hidaka, N. Serpone, Formation andidentification of intermediates in the visible-light-assisted photodegradationof sulforhodamine-B dye in aqueous TiO2 dispersion, Environ. Sci. Technol. 36(2002) 3604–3611.

[41] T. Wu, G. Liu, J. Zhao, H. Hidaka, N. Serpone, Photoassisted degradation of dyepollutants. V. Self-photosensitized oxidative transformation of rhodamine Bunder visible light irradiation in aqueous TiO2 dispersions, J. Phys. Chem. B102 (1998) 5845–5851.

[42] X. Li, G. Liu, J. Zhao, Two competitive primary processes in the photode-gradation of cationic triarylmethane dyes under visible irradiation in TiO2

dispersions, New J. Chem. 23 (1999) 1193–1196.