Reactive & Functional Polymers 70 (2010) 282–287

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Reactive & Functional Polymers

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Photocatalytic degradation of methyl orange usingpolythiophene/titanium dioxide composites

Yunfeng Zhu, Shoubin Xu, Dan Yi *

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 a b s t r a c t

Article history:Received 28 September 2009Received in revised form 29 January 2010Accepted 31 January 2010Available online 4 February 2010

Keywords:PT/TiO2 compositesMethyl orangeAdsorptionPhotocatalysisVisible light

1381-5148/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.reactfunctpolym.2010.01.007

* Corresponding author. Tel.: +86 28 85407286; faxE-mail address: danyichenweiwei@163.com (D. Yi

Polythiophene/titanium dioxide (PT/TiO2) composites were prepared via in situ chemical oxidative poly-merization, and the obtained composites were characterized by X-ray photoelectron spectroscopy (XPS),ultraviolet–visible (UV–Vis) diffuse reflectance spectroscopy (DRS) and transmission electron microscopy(TEM). Using methyl orange (MeO) as a target pollutant, the adsorption capacities and the photocatalyticactivities of the resulting composites were investigated. The results indicate that PT/TiO2 compositeshave good adsorption capacities due to the electrostatic attraction between the positively charged com-posite particles’ surfaces and MeO; the incorporation of PT into the composites enhances the photocata-lytic degradation activity for MeO under both UV and visible light.

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1. Introduction

Since 1976 and 1977 when Carey et al. [1] first reported theresults of photocatalytic degradation of biphenyl and chlorobi-phenyls using TiO2 and Frank and Bard [2,3] first examined thepossibilities of using TiO2 to decompose cyanide in water,respectively, there has been increasing interest in the environ-mental application of TiO2 photocatalysis [4]. Many investiga-tions have been carried out with the aim of understanding thefundamental processes and enhancing photocatalytic efficiencies,especially for water [5,6] and air [7] pollution control. In thepast decades, TiO2 has proven to be effective for photocatalyticdegradation of organic pollutants [8,9]. However, because ofthe rather high band gap (3.2 eV, anatase) of TiO2, it can only ab-sorb and be excited by UV light (which accounts for <5% of sun-light) with a wavelength below 387 nm [10]; this intrinsiccharacteristic of TiO2 limits its visible light harvesting and conse-quently restricts its practical application. Moreover, the highrecombination rate of electron–hole pairs of TiO2 causes lowphotocatalytic activity. Many measures have been taken to solvethe above problems; these measures include dye-sensitization[11–15], ion doping [16,17] and semiconductor coupling[18,19]. In particular, dye-sensitized photocatalytic materials ex-hibit high efficiency in degrading organic pollutants and utilizingvisible light. Dyes adsorbed on the surface of TiO2 are excited by

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visible light and inject an electron into the conduction band ofTiO2. This electron is then captured by oxidants to produce ac-tive radicals (e.g. �O�2 , �OOH, and �OH) and these radical speciesattack organic pollutant molecules, which results in their ulti-mate degradation [20]. However, the possible desorption ofloosely attached dyes and the poor stability of dyes with lowmolecular weight could result in further environmental pollutionand would limit the long-term performance of the dye-sensitizedphotocatalytic materials [21]. Substituting conjugated polymerssuch as polythiophene, polypyrrole, polyaniline and poly(1,4-phenylenevinylene) for dyes is an efficient method to resolveproblems of dye-sensitized photocatalytic materials by takingadvantage of the high absorption coefficients of conjugated poly-mers in the visible light region and bringing together the func-tions of light absorption, charge (hole) transport and betterstability in a single material [22–26].

Very recently we have reported [27] the technique for syn-thesizing the polythiophene/titanium dioxide (PT/TiO2) compos-ites by in situ chemical oxidative polymerization of thiopheneonto the TiO2 particles (using FeCl3 as an oxidant in CHCl3)as well as the structure characterization and a pilot study ofits photocatalytic properties under UV light. As a consecutivework, in this paper, a further study of the adsorption capaci-ties in the dark and the photocatalytic activity of the PT/TiO2

composites under UV and visible light was performed andthe influence of PT contents on the photocatalytic activitiesof composite particles and photocatalytic mechanism are dis-cussed in detail.

Scheme 1. Schematic representation of the photocatalytic reactor for degradingorganic pollutants with visible light: (1) halogen lamp; (2) water jacket; (3) K2CrO4

solution; (4) glass vessel; (5) magnetic stirrer.

Table 1Surface composition of TiO2 and PT/TiO2 composites with different PT contents.

Element Percentage (%)

TiO2 PT(0.5%)/TiO2 PT(1.0%)/TiO2 PT(2.0%)/TiO2

Ti 25.5 21.7 20.8 20.0O 44.0 41.7 40.1 38.6C 30.5 36.6 38.4 40.2S – – 0.7 1.2

Y. Zhu et al. / Reactive & Functional Polymers 70 (2010) 282–287 283

2. Experimental

2.1. Materials

Titanium dioxide (HR-3), with purity ca. 99.5%, was purchasedfrom Zhoushan Mingri Nanometre Material Co., Ltd. (Zhoushan,China). It is in the anatase form with a specific surface area (BET)of 240 ± 30 m2/g, corresponding to a mean particle size of ca.15 nm, nonporous. The thiophene monomer (99+%), anhydrousiron (III)-chloride (FeCl3), chloroform (max. water 0.03%), methylorange (MeO) (C14H14N3SO3Na) and methanol were bought fromChengdu Kelong Chemical Reagents Factory (Chengdu, China) andused without further purification.

2.2. Preparation of PT/TiO2 composites

The PT/TiO2 composites were prepared by a typical in situchemical oxidative polymerization of thiophene in the presenceof TiO2 particles. The preparation method has been described in de-tail in our previous work [27]. The nanocomposite materials with0, 0.5, 1.0, and 2.0 wt.% of PT (thiophene monomer dosage basedon the content of TiO2) were prepared and labeled as PT,PT(0.5%)/TiO2, PT(1.0%)/TiO2, and PT(2.0%)/TiO2 composites.

2.3. Characterization methods

UV–Vis diffuse reflectance spectra were measured on a TU-1901spectrophotometer equipped with an integrating sphere attach-ment (IS 19–1). A baseline correction was performed using a cali-brated sample of barium sulphate. X-ray photoelectronspectroscopy (XPS) measurements were carried out on a spectrom-eter (XSAM-800, KRATOS Co.) with an Mg Ka anticathode.Transmission electron microscopy (TEM) investigations were per-formed on a Hitachi H-600 transmission electron microscope withan accelerating voltage of 80 kV. The samples were prepared bydepositing a small droplet of the suspension (PT/TiO2 compos-ites/ethanol) on a copper mesh. The zeta potential of the catalystswere measured with a suspension concentration of 1.0 g/L with aZetasizer nano-ZS90 zeta potential measurement (Malvern). ThepH value of the suspensions was adjusted by 0.5 M HCl and0.5 M NaOH. The isoelectric points of the catalysts were obtainedby measuring the zeta potential. The pH value of the suspensioncontaining 1.0 g/L catalysts was measured with a PHS-25 pH meterat ambient temperature.

2.4. Evaluation of the photocatalysis

An aqueous solution of MeO was used as a model contaminantfor studying the adsorption performance and photocatalytic activ-ity of the prepared materials. The initial MeO concentration, C0,was 40 mg/L. The adsorption experiments in the dark were per-formed by adding 10 mL of contaminant solution and 10 mg ofphotocatalyst in a glass tube wrapped tightly with black rubber-ized fabric that would prevent the suspension from being irradi-ated by light. The glass reactor was sealed and the suspensionwas magnetically stirred throughout the experiments.

Photocatalysis experiments under UV-irradiation were carriedout in a self-regulating UV-irradiation instrument; the structureof the instrument and the experimental method has been de-scribed in detail in our previous work [27].

Photocatalysis experiments under visible light irradiation werecarried out in a self-regulating visible light irradiation instrument(see Scheme 1). A 360-W linear halogen lamp was used as an irra-diation source and was placed at about 30 cm away from the reac-tor. A glass vessel (U15 cm � 10 cm) with a circulating water

jacket was placed between the irradiation source and the reactor.The glass vessel was filled with potassium chromate solution to re-move radiation below 450 nm [28] and to ensure illumination onlyby visible light. In a typical experiment, 300 mL of MeO aqueoussolution was mixed with catalysts (1.0 g/L) and continuously stir-red with a magnetic stirrer in the dark for 1 h to establish anadsorption–desorption equilibrium. Then photocatalysis wasstarted by turning on a halogen lamp. The first sample was takenout at the end of the dark adsorption period, just before the lightwas turned on, to determine the MeO concentration in solution,which is hereafter considered as the initial concentration (C 00) afterdark adsorption. A portion of samples were then withdrawn regu-larly from the reactor at various time intervals and centrifuged at5000 rpm for 10 min (TDL-5-A centrifuge) immediately to separateany suspended solid. The change in the concentration of MeO wasmonitored by measuring the absorbance at kmax = 464 nm with aUV-240 UV–vis spectrophotometer (Shimadzu Co., Japan). In thispaper, the decolorization ratio of contaminant was calculated by:

g ¼ A0 � At

A0� 100% ð1Þ

where At is the absorbance of the contaminant solution at differentreaction time t, and A0 is the absorbance of the initial contaminantsolution.

3. Results and discussion

3.1. Structure and light absorption characteristics of the PT/TiO2

composites

To investigate the surface structure of the resulting composites,XPS analysis was carried out. The relative ratios of elements on thesurface of PT/TiO2 composites with different content of PT areshown in Table 1 [29]. From this table, it can be seen that onlyTi, O and C elements (C element comes from the testing environ-

Fig. 2. UV–Vis diffuse reflectance spectra of (a) TiO2, (b) PT(0.5%)/TiO2, (c) PT(1.0%)/TiO2, (d) PT(2.0%)/TiO2 composites, and (e) PT.

Fig. 3. Relative concentration of MeO (Ct/C0) vs. time during the adsorptionexperiments performed using PT/TiO2 composites with different PT contents.

284 Y. Zhu et al. / Reactive & Functional Polymers 70 (2010) 282–287

ment) were detected on the surface of TiO2 particles, while the Selement was not detected. However, S was detected in the caseof PT/TiO2 composites. The S element is attributed to PT. Moreover,with an increase in the PT content, the relative content of Ti and Oelements on the surface of the PT/TiO2 composites decreases, butthe relative content of S increases. The results indicate that thePT/TiO2 composites are formed from polythiophene-covered tita-nium dioxide. The fine nanostructures of TiO2 particles andPT(1.0%)/TiO2 composites were further investigated as demon-strated in Fig. 1. The TEM images of PT(1.0%)/TiO2 composites illus-trate the similar morphology with that of TiO2 particles; theparticle size is about 10–20 nm. The polymer layer is not clearlyobserved possibly because of the low amount and the thinness ofthe PT layer.

The UV–Vis diffuse reflectance spectrum characterizes the lightabsorption characteristics of a photocatalyst. The UV–Vis diffusereflectance spectra of (a) TiO2, (b) PT(0.5%)/TiO2, (c) PT(1.0%)/TiO2, (d) PT(2.0%)/TiO2 composites and (e) PT are presented inFig. 2 [29]. As expected, TiO2 showed its characteristic spectrumwith its absorption rising at �400 nm, although TiO2 reflects mostlight with a wavelength ranging from 400 to 800 nm. The UV–Visspectrum of PT indicates that PT with maximum absorption at awavelength of about 500 nm can absorb most light with a wave-length ranging from 200 nm to 800 nm. From the spectra of PT/TiO2 composites, it can be seen that there are two absorption bandsfrom 200 to 400 nm and from 400 to 800 nm, respectively. Theband from 200 to 400 nm can be assigned to the characteristicabsorption of TiO2 and the PT absorption band in the UV light re-gion. The second band are attributed to the electron transitionfrom the valence bond to the antibonding polaron state (p–p* type)of PT [30]. With an increase in the PT content, the absorbency forPT/TiO2 composites increases in the visible light region. Comparedwith the spectrum of TiO2, the presence of PT significantly changesthe spectrum of TiO2 in the visible light range, as the PT/TiO2 com-posites absorb much more visible light than TiO2. The results indi-cate that PT/TiO2 composites are promising materials that can beexcited by visible light.

3.2. Photocatalytic degradation of methyl orange

3.2.1. Adsorption of MeO on the surface of PT/TiO2 composites in thedark

Fig. 3 shows the relative concentration of MeO, Ct/C0, for differ-ent periods of time during darkness adsorption. It can be seen thata negligible decolorization of MeO was observed within 120 min inthe presence of pure PT or pure TiO2 particles. However, the incor-poration of PT into the composites provides an apparently additiveeffect on their adsorption capacities, and the adsorption equilib-rium can be reached after 60 min. The adsorption capacity of PT/TiO2 composites increases with an increase in PT content. The

Fig. 1. TEM images of TiO2 particles (a) and

adsorption percentages of MeO onto the surface of the TiO2 andPT(2%)/TiO2 composites corresponding to the initiative MeO were2.2% and 63.8%, respectively. The increase in the content of PT from0% to 2% favoring the adsorption capacity of PT/TiO2 compositesmay be because of the surface property changes of the composites.To further investigate the adsorption properties of PT/TiO2 com-posites, the zeta potential of the TiO2 and PT/TiO2 compositeswas determined. The zeta potential of the TiO2 and PT(2%)/TiO2

PT(1.0%)/TiO2 composite particles (b).

Fig. 4. Plots of the zeta potential as a function of pH for (a) TiO2 and (b) PT(2.0%)/TiO2 composite suspensions (0.5 g/L) in deionized water.

Y. Zhu et al. / Reactive & Functional Polymers 70 (2010) 282–287 285

composites changing with the pH values of the solution is shown inFig. 4. As shown in Fig. 4, the PZC (point of zero charge) of TiO2 andPT(2%)/TiO2 composites are about 5.75 and 6.3 pH units, respec-tively. Because of the amphoteric behavior of semiconducting tita-nium dioxide, the surface charge properties of TiO2 changes withpH values according to the following reactions [31]:

pH < pHPZC : TiOHþHþ $ TiOHþ2 ð2ÞpH > pHPZC : TiOHþ OH� $ TiO� þH2O ð3Þ

The actual pH values of the dispersions of TiO2 and PT(2%)/TiO2

composites (1.0 g/L), which were obtained by adding 10 mg of asample into 10 mL of MeO (40 mg/L), were determined to be 6.2and 3.3, respectively. The result indicates that in the dispersionthe surface of the TiO2 is negatively charged but the surface ofthe PT(2%)/TiO2 composites is positively charged. Thus, due to Cou-lombic repulsion, MeO is scarcely adsorbed on the surface of theTiO2. However, the positively charged surface of the PT(2%)/TiO2

composites can strongly adsorb MeO as shown in Scheme 2. Thezeta potential investigation results are consistent with the adsorp-

Scheme 2. Schematic representation of MeO ad

tion percentages of MeO onto the surface of the TiO2 and PT/TiO2

composites. The adsorption of organic contaminants would con-tribute to a high photodegradation efficiency because it facilitatesan efficient interfacial charge transfer [32]. Therefore, the strongadsorption properties of PT/TiO2 composites could possibly in-crease the consequent photocatalytic degradation rate of MeO.

3.2.2. Photocatalytic degradation of MeO by PT/TiO2 composites underUV-irradiation

A blank experiment was carried out with UV illumination butno catalysts. No appreciable decolorization of MeO (lower than5.0%) was observed during 3 h of continuous stirring under theexperimental conditions. Consequently, MeO is kinetically stable.

Fig. 5 shows the decolorization ratio of MeO, g, for differenttime periods during UV-irradiation. UV light irradiation starts after60 min of adsorption in the dark, during which it reaches a con-stant value. After the sample is placed under UV illumination, thedecolorization ratios of MeO for the PT have no significant change.Conversely, a continuous decrease of the relative concentration (Ct/C0) and hence a continuous increase of the decolorization (g%) ofMeO are observed for TiO2 and PT/TiO2 composites under UV lightirradiation. The decolorization ratios of MeO after 150 min of UV-irradiation in the presence of TiO2, PT(0.5%)/TiO2, PT(1.0%)/TiO2

and PT(2.0%)/TiO2 composites are 77.6%, 92.5%, 96.4%, and 85.1%,respectively, which indicates that the PT/TiO2 composites are effec-tive photocatalysts to degrade MeO under UV-irradiation.

The linear fitting curves between Ln(C00/Ct) and irradiation time(Fig. 6 [29]) were consistent with first-order kinetics, Ln(C00/Ct) = kt,where C00 is the concentration of MeO after adsorption, and Ct is theconcentration of MeO over time.

The correlation constants (r2) for the fitted lines were calculatedto be between 0.991 and 0.999, which indicates that the photocat-alytic degradation of MeO can be described by a first-order kineticmodel. The apparent first-order rate constant (k) first increases andthen decreases with an increase in the PT content. The pure TiO2,PT(1.0%)/TiO2 and PT(2.0%)/TiO2 composites give apparent rateconstants of 1.02 � 10�2 min�1, 1.94 � 10�2 min�1, and 0.67 �10�2 min�1, respectively. The increase in the PT percentage from0.5% to 1.0% favors the photodegradation of MeO, which can be ex-plained by the fact that the polythiophene with its p-conjugatedstructure has high electron mobility [33], which will facilitate theseparation of the electron–hole pairs generated under UV-irradia-tion. Thus, the recombination of electron–hole pairs of TiO2 isinhibited, and photocatalytic activity is enhanced. The decreasein activity of PT/TiO2 composites when the PT content is higherthan 1.0% is considered to be related to increased absorption andscattering of photons by the large amount of PT on the surface ofTiO2.

3.2.3. Photocatalytic degradation of MeO by PT/TiO2 composites undervisible light irradiation

The blank experiments showed that no obvious photo-bleach-ing was observed when the solution of MeO in the absence of cat-alyst was irradiated by visible light for 10 h, which implies thatMeO is stable to visible light. After 60 min of adsorption in thedark, the visible light starts to irradiate the reactor. Fig. 7 showsthe decolorization ratio of MeO as a function of experiment time.From Fig. 7 it can be seen that only a slight decolorization was ob-served in the presence of pure TiO2 and PT. While PT/TiO2 compos-

sorbed on the surface PT/TiO2 composites.

Fig. 5. Decolorization ratio of MeO (g) vs. time in typical photocatalytic experi-ments performed using PT/TiO2 composites with different PT contents under UV-irradiation.

Fig. 6. Apparent first-order linear transform Ln(C00/Ct) = f(t) of MeO degradationkinetic plots for PT/TiO2 composites with different PT contents.

Fig. 7. Decolorization ratio of MeO (g) vs. time in a typical photocatalyticexperiment performed using PT/TiO2 composites with different PT contents undervisible light irradiation.

286 Y. Zhu et al. / Reactive & Functional Polymers 70 (2010) 282–287

ites show much higher photocatalytic activities compared to pureTiO2 and PT, the decolorization ratio of MeO after 10 h of visiblelight irradiation in the presence of PT(0.5%)/TiO2 compositesreached 80.3%. Aside from the effect of adsorption in the dark(25.8%), 54.5% of MeO was degraded by PT(0.5%)/TiO2 compositesunder visible light irradiation. Similar results appeared for theother PT/TiO2 composites with a different PT content: 42.8% and12.0% of MeO was degraded by PT(1.0%)/TiO2 and PT(2.0%)/TiO2

composites under visible light irradiation, respectively. The higherphotocatalytic activities of PT/TiO2 composites compared withpure TiO2 can be interpreted by the difference of UV–Vis diffusereflection spectra between the PT/TiO2 composites and pure TiO2.As shown in Fig. 2, pure TiO2 can only absorb UV light with a wave-length below 387 nm to generate electron–hole pairs but cannot beexcited by visible light, while PT/TiO2 composites exhibit a strongabsorbance in the visible light region. PT has a strong and broadabsorbance within the visible light region. When the compositesare irradiated by visible light, the PT adsorbed on the surface ofthe TiO2 is excited, a photon is absorbed, and an electron is simul-taneously excited from the ground state into the excited state.Since the reductive potential of PT is lower than that of TiO2

[23], the polymer layer electrons were easily injected into the con-duction band of the TiO2. The conduction band electron subse-quently reacts with O2 adsorbed on the surface of TiO2 toproduce �O�2 radicals and further reacts with H2O to generate �OOHand �OH, �O�2 and �OH, which are potent oxidizing agents to degradeorganic pollutants. According to the earlier report on the photocat-alytic degradation of organic pollutants using organic dyes andpolymer-modified TiO2 under visible light [13,24], a series of pos-sible reactions using PT/TiO2 composites to degrade MeO could berepresented as follows:

PT=TiO2 þ hv ! �PT=TiO2 ð4Þ�PT=TiO2 ! þPT=TiO2 þ e�CB ð5ÞþPT� TiO2 þM! PT=TiO2 þMþ� ð6Þe�CB þ O2 ! �O�2 ð7ÞH2Oþ �O�2 ! �OOHþ OH� ð8Þ2�OOH! O2 þH2O2 ð9Þ�OOHþH2Oþ e�CB ! H2O2 þ OH� ð10ÞH2O2 þ e�CB ! �OHþ OH� ð11ÞH2O2 þ �O�2 ! �OHþ OH� þ O2 ð12Þ�OH=�O�2 =

�OOHþM=Mþ� ! Degraded products ð13Þ

(ht = visible light; M = methyl orange).Additionally, Fig. 7 shows that with the increase of PT content,

the photocatalytic activities first increase and then decrease, whichmay be explained by the fact that the PT has a dual function in thecomposites as both sensitizer and charge transport layer. Photoex-citations created initially in the bulk of the antenna layer must beable to diffuse within their lifetime to the semiconductor interface[34]; when the PT content in the composites increases past the dis-tance that the exciton can diffuse from the bulk to the interface,the exciton will decay during the long diffusion period and henceresult in a decrease in photocatalytic activity.

4. Conclusion

The PT/TiO2 composites prepared in the present study can ab-sorb both UV light and visible light. The introduction of PT intothe composites provides an apparently additive effect on theiradsorption capacities. A strong adsorption of MeO on the compos-ite particles results from the electrostatic attraction of the posi-tively charged composite particle surfaces with the MeOmolecules. Under UV light irradiation, PT/TiO2 composites with a

Y. Zhu et al. / Reactive & Functional Polymers 70 (2010) 282–287 287

certain content of PT exhibit higher photocatalytic activities thanTiO2 because of the high electron mobility of PT with its p-conju-gated structure. Under visible light irradiation, PT adsorbed onthe surface of TiO2 acts as a sensitizer and charge transport layer;PT/TiO2 composites absorb visible light and photocatalytically de-grade MeO. The photocatalytic activities of PT/TiO2 compositeswith PT contents lower than 1.0% show good photocatalytic activ-ity for the decomposition of MeO under visible light.

Acknowledgement

The authors are grateful to the National Natural Science Foun-dation of China (Grant No. 20374036 and 50573052) for the sup-port of this research.

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