Facile synthesis of novel hierarchical TiO2@Poly(o-phenylenediamine) core–shell structures with enhanced photocatalytic performance under solar light

Journal of Environmental Chemical Engineering 1 (2013) 620–627

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Facile synthesis of novel hierarchical TiO2@Poly(o-phenylenediamine) core–shell structures with enhanced photocatalytic performance under solar light Pandi Muthirulan, Chenthamarai Kannan Nirmala Devi, Mariappan Meenakshi Sundaram * Centre for Research and Post-Graduate Studies in Chemistry, Ayya Nadar Janaki Ammal College (Autonomous), Sivakasi 626 124, India

A R T I C L E I N F O

A B S T R A C T

Article history: Received 7 May 2013 Accepted 28 June 2013

The present work describes a novel contribution to the development of a new photocatalyst based on titanium dioxide (TiO2)@poly(o-phenylenediamine) (PoPD) core–shell nanocomposites for the mineralization of Rhodamine B (RB) dye under sun light irradiation. The core–shell feature of the nanocomposites was proved by Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM). Ultraviolet–visible Diffuse Reflectance Spectra (UV–vis DRS) revealed that TiO2@PoPD core–shell composites show broad and stronger absorption than TiO2 in the whole range of visible light indicating the sensitizing effect of PoPD. Structure of the TiO2@PoPD nanocomposite was confirmed by Fourier Transform Infrared spectroscopy (FT-IR) and X-ray diffraction (XRD) pattern. The hybrid photocatalysts possess dramatic photocatalytic activity for the degradation of the RB under solar light irradiation. A plausible pathway and mechanism for the photocatalytic degradation of RB was also discussed. ß 2013 Elsevier Ltd All rights reserved.

Keywords: TiO2@PoPD core shell structure Transmission electron microscopy Rhodamine B Solar irradiation Photocatalysis

Introduction Conjugated polymers with extended p-conjugated electron systems such as polyaniline, polythiophene, and polypyrrole have shown great promise, due to their high absorption coefficients in the visible part of the spectrum, high mobility of charge carriers, and good environmental stability [1]. Moreover, many conjugated polymers also are efficient electron donors and good hole transporters upon visible-light excitation. Therefore, conjugated polymers with wide band gap inorganic semiconductors are receiving attention for optical, electronic, photocatalytic and photoelectric conversion applications [2,3]. Recently, some studies have been published on the combination of conductive polymers and TiO2 to improve their performance of UV light and sunlight activities [4–12]. In addition to the photocatalytic properties, a successful candidate for a global scale catalyst material needs to be non-toxic, inexpensive, stable and widely available. Poly(o-phenylenediamine), PoPD, is a polyaniline derivative containing 2,3-diaminophenazine or quinoraline repeating unit [13], has variable conductivity, strong electroactivity, good optical and magnetic activity, high environmental and thermal stability [14]. Thus, the preparation of a PoPD based nanocomposite is receiving attention since it can exhibit unique properties arising

* Corresponding author. Tel.: +91 9486028616; fax: +91 4562254970. E-mail addresses: [email protected], [email protected] (M.M. Sundaram). 2213-3437/$ – see front matter ß 2013 Elsevier Ltd All rights reserved. http://dx.doi.org/10.1016/j.jece.2013.06.025

from the electrically conductive PoPD film and metal/semiconducting nanoparticles [15–17]. To the best of our knowledge, work has not been published on the photocatalytic degradation of Rhodamine B dye using TiO2@PoPD core–shell nanocomposites as a photocatalyst. In this paper, effective TiO2@PoPD core–shell nanocomposite photocatalysts have been prepared by in situ chemical oxidative polymerization using ammonium persulfate (APS) as an initiator. The combination of the synergetic and complementary behaviors of PoPD with TiO2 nanoparticles provides a potential photocatalyst for an efficient degradation of RB dye. Materials and methods Chemicals The TiO2 nanoparticles and o-phenylenediamine obtained from Sigma–Aldrich and ammonium persulfate (APS) received from Merck are used as received. All other chemicals and reagents were of analytical grade and used without further purification. Synthesis of TiO2@PoPD core–shell nanocomposites In situ chemical oxidative polymerization of oPD was performed in the presence of negatively charged TiO2 nanoparticles (different wt% i.e., 0.5, 1.0, 1.5, 2.0 and 2.5 wt%) using ammonium persulphate as oxidant. A typical procedure is outlined: oPD (1 g) was added to 200 mL of deionized water

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containing 0.5 wt% of TiO2 particles with constant stirring. Dilute HCl solution was added to adjust the pH into 3 and the solution was cooled at 5 8C. Ammonium persulphate (1.14 g) was then added slowly into the mixture with constant stirring to initiate the polymerization reaction. The solution quickly turned purple-red and became turbid in 5 s. The reaction mixture was then subjected to ultrasonication for 45 min at room temperature and the product was centrifuged and washed several times with double-distilled water.

followed by stirring for 30 min in dark to attain the adsorption– desorption equilibrium. After a setup exposure time, 10 mL suspension was sampled, centrifuged and the supernatant was taken out for the UV–vis absorption measurements. The extent of removal of the RB dye, in terms of the values of percentage removal of dye has been calculated using the following relationship:

Instrumentation

where C0 is the initial concentration of dye and Cf is the final concentration of dye at given time.

UV–vis spectrum was recorded using ‘‘TECHCOMP’’ UV–visible spectrometer model 8500. FT-IR spectrum was recorded using ‘‘BRUKER’’ (model TNSOR 27). The crystallographic structures of the materials were determined by a powder X-ray diffractometer (model RICH SIEFRT & CO) equipped with Cu-Ka radiation (l = 1.5406  1010 m). The surface microstructure of the samples was investigated by transmission electron microscope (model: Hitachi H-800) and Atomic Force Microscope (Nanosurf AG, Grammetstrasse 14, CH-4410 Liestal, Switzerland). Thermogravimetry (TGA) analysis results were recorded using a Perkin Elmer Thermal Analyzer over a temperature range of 0–1000 8C in an inert atmosphere at a heating rate of 10 8C min1. Photocatalysis 50 ml aqueous solution of RB dye with concentration of 50 mgL1 was mixed with adequate concentration of photocatalyst

Percentage removal ¼ 100 

C0  Cf Ci

(1)

Results and discussion TiO2@PoPD core–shell structures formation mechanism Scheme 1 illustrates the formation mechanism of (A) TiO2@PoPD core–shell nanocomposites and (B) PoPD polymer film by chemical oxidative polymerization method. Since oPD has a known pKa of 4.63, it is expected to be primarily positively charged at pH below this value. oPD monomers are converted into ph-NH2+ cations (phenylenium ion) in acidic conditions with a pH of 3 and adsorbed strongly onto the negatively charged TiO2 surface through electrostatic interactions [18]. The titania cores serve as templates for adsorption of oPD monomers as well as counter-ions for doping of the synthesized PoPD. During the addition of APS, the adsorbed ph-NH2+ cations polymerized immediately and electrostatically complexed on the surface of the TiO2 [16,18–20]. This

Scheme 1. Representation for the (A) preparation of TiO2@PoPD core–shell nanocomposites by in situ chemical oxidative polymerization process and (B) reaction pathway of PoPD polymer film formation.

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Fig. 1. AFM images of (A) PoPD film, (B) 1.5 wt% TiO2@PoPD core–shell nanocomposites and (C) TEM image of 1.5 wt% TiO2@PoPD core–shell nanocomposites.

simple approach could provide an opportunity to design novel core–shell nanostructures with diverse functionality and high colloidal stability. Surface characterizations The surface morphologies and core@shell features of the 0.5 wt% TiO2@PoPD core–shell nanocomposites were confirmed by AFM and TEM analyses. The striking differences in morphology of the PoPD and TiO2@PoPD core–shell features become very clear in AFM images. Fig. 1A and B corresponds to three-dimensional (3D) images and Inset corresponds to two-dimensional (2D) images of PoPD and TiO2-PoPD core–shell nanocomposites. The 3D image of the PoPD film reveals rough topography consisting of orderly arranged continuous large spherical shaped clusters of molecules that are homogeneously distributed and this topography is being consequence of the linear growing of polymer chains. In contrast, the 3D image of the TiO2@PoPD composite film reveals relatively smooth topography consisting of small spherical sized TiO2 granules dispersed uniformly in the polymer matrix. The existence of nano-sized particles in the TiO2@PoPD composite matrix is more clearly reflected in its 2D AFM image. It can be noted from Fig. 1B inset that the composite film prepared in the presence of TiO2 nanoparticles does not contain any noticeable clusters; instead, it contains globular entities. The TEM observation revealed that the TiO2@PoPD core–shell composites were nearly monodisperse and well dispersed in the TEM grid (Fig. 1C). The TiO2-PoPD core–shell nanocomposites were produced with pre-aggregated or glued titanium in the cores (inside black components) and polymer in the shells (outside white components), the entire surface of the TiO2 particles were

surrounded by uniform thin PoPD film layer and the TiO2 nanoparticles are well dispersed in the polymer matrix. Moreover, the outer shell of the particle exhibited a fine increment in brightness compared with the dark inner core, which confirmed the formation of core–shell feature of the TiO2–PoPD nanocomposites. The formation of PoPD encapsulated TiO2 core–shell nanocomposites was attributed to the strong electrostatic interaction between PoPD and TiO2. These results are in good agreement with the PoPD and PANI based nanocomposites [17,19,20]. Thus, TEM results totally agree with those from AFM measurements, thereby confirming the core–shell features of the TiO2@PoPD nanocomposite. Structural and thermal analysis Fig. 2A shows the UV–vis diffuse reflectance spectra of TiO2, PoPD and TiO2@PoPD core–shell nanocomposites with different wt% of TiO2 (0.5, 1.5, 2.5), respectively. TiO2 can only absorb UV light with wavelength lower than 387 nm because of its wide band gap. The UV–vis absorption spectrum of PoPD revealed that the absorption maxima at 657 nm and 350 nm are originating from the charge-transfer-excitation-like transition from the highest occupied molecular orbital (HOMO) energy level to the lowest unoccupied molecular orbital (LUMO) energy level and the p– p* transition. TiO2@PoPD core–shell nanocomposites show broad and stronger absorption than TiO2 under the whole range of visible light that is due to the sensitizing effect of PoPD. This indicates that the incorporation of PoPD on to the surface of TiO2 can extend the photo response range of TiO2. With an increase of TiO2 content up to 1.5 wt%, the absorbency for TiO2@PoPD core–shell nanocomposites increases in the visible light region and then decreases (for 2.0

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A

Absorbance (a.u)

30

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4000

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Wavenumber (cm ) Fig. 2. (A) UV–vis DRS and (B) FTIR of PoPD film and TiO2@PoPD core–shell nanocomposites with different compositions.

and 2.5 wt% of TiO2@PoPD nanocomposites), implying that the TiO2@PoPD core–shell composites hybrid composite can be used as visible-light-driven photocatalyst [13–18]. The molecular structures of PoPD and TiO2@PoPD core–shell nanocomposites (0.5, 1.5 and 2.5 wt%) were characterized by FTIR. Fig. 2B gives the typical FTIR spectra of PoPD polymer, the bands of stretching vibrations of free N–H (3100–3500 cm1) suggested the presence of secondary amine groups. Two strong bands at 1530 and 1626 cm1 are ascribed to the stretching vibrations of the C5 5C and C5 5N in the phenazine ring, respectively. The peaks at 1239 and 1367 cm1 are associated with the C–N stretching in the benzenoid and quinoid imine units. Moreover, the bands at 754 and 587 cm1, which are characteristic of C–H out-of-plane bending vibrations of benzene nuclei in the phenazine skeleton of the ladder structured PoPD [13–15]. The FTIR spectrum of the TiO2@PoPD core–shell nanocomposites with different wt% of TiO2 represents the identical characteristic peaks of the PoPD polymer. However, the corresponding peaks are shifted to the higher wave numbers; besides, the peak intensities are reduced after the TiO2 nanoparticles addition. This indicates that the hydrogen bonding in the PoPD becomes stronger due to the formation of PoPD-TiO2 composite and therefore strong interaction may exist between the PoPD and TiO2. The results of FTIR spectra of PoPD agree well with previous reports [16–18].

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Fig. 3 depicts XRD pattern of PoPD, TiO2 and TiO2@PoPD core– shell nanocomposites with different compositions. The peak at 2u = 258 for PoPD polymer film should be assigned to the scattering from the periodicity parallel and perpendicular to PoPD chains, which specifies that the PoPD polymer film has some degree of crystallinity. For TiO2, the peaks at 2u = 25.38, 37.88, 48.08, 53.98, and 55.18 can be assigned to the diffractions of the (1 0 1), (0 0 4), (2 0 0), (1 0 5), and (2 1 1) crystal planes of anatase phase. The characteristic diffraction peaks observed at 27.48 and 36.18 can be attributed to the (1 1 0) and (1 0 1) faces of rutile phase in TiO2 [35,36]. It is also clear from the XRD pattern that the TiO2@PoPD composite is similar to well-ordered TiO2 with reduced peak intensity followed by the disappearance of diffraction peak of PoPD. According to the indexation of peaks, the TiO2 nanoparticle thus mainly contains both anatase and rutile phases. For the TiO2@PoPD nanocomposites, it can be seen that the peak intensity of rutile quickly decreases and the anatase peak at 2u = 25.358 becomes smaller and broader compared to that of the TiO2. A more critical trend can be confirmed for the TiO2@PoPD nanocomposites as with the increased amount of PoPD (i.e., 0.5 wt% of TiO2) the diffraction peak of anatase (1 0 1) became smaller and peaks attributed to rutile phase (1 1 0) disappeared completely. This is probably because of the amorphous state of the PoPD polymer deposited on/in TiO2 attenuated the characteristic diffraction of TiO2, thus affecting the detection of the crystal diffraction of TiO2. Similar observation was reported by Liang and Li [21] for polythiophene–TiO2 nanotube nanocomposite for the degradation of 2,3-dichlorophenols. In addition, it is clear from XRD analysis that the organic complex stabilizes titania anatase phase and limits the transformation to rutile phase which will be another cause for increasing the photocatalytic reactivity of the hybrid catalyst. The TGA exploration of the PoPD film and TiO2@PoPD core–shell nanocomposite, having different concentration of TiO2 nanoparticles, is shown in Fig. 4. It can be observed from the figure that the losses of weight of PoPD occur around three temperature periods. The first two weight losses are mainly attributed to residual water, elimination of impurities, and some oligomers. The major weight loss occurs between 200 to 250 8C indicating the structural decomposition of the PoPD back bone, and thereafter a plateau appears up to about 300 8C. In the temperature range of 300– 700 8C, a gradual weight loss occurred, which is attributed to the degradation of the skeletal PoPD chain structure [17]. In contrast, TiO2@PoPD core–shell nanocomposites possess greater thermal stability, and improvement in the thermal stability of the nanocomposite increases with an increase in TiO2 content than pure PoPD film. The onset of thermal degradation is shifted toward higher temperature by about 40 8C for the composites, having highest concentration of TiO2 (2.5 wt%). The improved thermal stability of the nanocomposites can be attributed to the reduced mobility of the polymer chains that in turn suppresses the free radical transfer via inter-chain reactions. Assessment of photocatalytic enactment and mechanism UV–vis absorption spectrum of RB with different reaction time under solar light irradiation in the presence 1.5 wt% TiO2@PoPD core–shell nanocomposite was illustrated in Fig. 5A and the structure of RB is given in Fig. 5A inset. Solar light irradiation leads to a continuous decrease in absorbance of RB in the presence of PoPD-TiO2 composites and the decrease of the absorption band intensities of the dye indicated that dye has been degraded. As can be seen from this figure, the disappearance of the characteristic band of RB dye at 550 nm after 60 min. under solar light irradiation indicates that RB has been degraded completely. The degradation experiments by solar light irradiation of RB dye containing photocatalysts TiO2 and TiO2@PoPD core–shell

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2θ (deg.) Fig. 3. XRD pattern of (A) TiO2; (B) 0.5 wt%; (C) 1.5 wt% and (D) 2.5 wt% of TiO2@PoPD core–shell nanocomposites. (A) Inset – XRD pattern of PoPD film.

ln

C0 ¼ ka p p t Cf

(2)

where C0 is the initial concentration of dye solution; Cf is the final concentration of dye solution at time t; kapp is the apparent first order rate constant; t is the irradiation time. The value of ln(C0/Ct) is plotted against time (min) and the plots were found to be linear (Fig. 5B). The rate constants of the degradation of RB for the catalysts are k = 0.0230 min1 for TiO2, k = 0.0419 min1 for 0.5 wt% of TiO2@PoPD, k = 0.0683 min1 for 1.0 wt% of TiO2@PoPD, k = 0.1379 min1 for 1.5 wt% of TiO2@PoPD, k = 0.1151 min1 for 2.0 wt% of TiO2@PoPD and k = 0.1130 min1 for 2.5 wt% of TiO2@PoPD respectively. It can be seen that the 1.5 wt% TiO2 content behaves the highest photocatalytic activity compared with other TiO2 loaded composites. It is noteworthy that 2.0 and 2.5 wt% TiO2 loaded TiO2@PoPD core shell nanocomposites shows a lower photocatalytic activity than that of 1.5 wt%. It is because that, when the TiO2 content is too high, such TiO2 loaded composites, just like bare TiO2 and will not behave the advantages of the supports. That is, it could not identify and select the organic dye molecules effectively. These results are in accordance with UV–vis DRS and XRD analysis [27,28]. Photodegradation pathway and mechanism Identification of possible intermediate products during the photocatalytic reaction is the best way to understand the

photocatalytic degradation reaction mechanism. The mineralization of RB dye under solar irradiation using TiO2@PoPD core– shell nanocomposite photocatalyst was confirmed by ESI-MS analysis (Figure not shown). The possible degradation pathway for the RB dye was given in Scheme 2. Major intermediates during the degradation process were proposed by using m/z values of the mass spectra. The photocatalytic degradation of RB dye by the photogenerated active species such as OH and hole

100 90 80 70

% Weight Loss

nanocomposite follows first-order kinetics with respect to the irradiation time (t) [22].

60 50 40 30

Pure PoPD 0.5wt% TiO2-PoPD 1.5wt% TiO2-PoPD 2.5wt% TiO2-PoPD

20 10 0 100

200

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o

Temperature ( C) Fig. 4. TG spectrum of PoPD film and TiO2@PoPD core–shell nanocomposites with different compositions.

P. Muthirulan et al. / Journal of Environmental Chemical Engineering 1 (2013) 620–627 (C2H5)2N

O

625

N(C2H5)2 COOH

m/z = 443

O

C2H5HN

(C2H5)2N

N(C2H5)2

O

NHC2H5 COOH

COOH

m/z = 415

C2H5HN

O

NHC2H5

m/z = 415

H2N

O

COOH

O

C2H5HN

O

NH2 COOH

COOH

m/z = 387

m/z = 387

m/z = 387

C2H5HN

N(C2H5)2

H 2N

NH2

NHC2H5 C2H5HN

O

O

NH2 COOH

COOH

COOH

m/z = 359

m/z = 359

m/z = 359

O

H 2N

NH2

COOH

m/z = 331

H2N

O COOH

m/z = 258

O

NH2

O COOH

COOH

COOH

m/z = 316

m/z = 230

m/z = 224

Cleavage of chromophore

OH

OH OH

OH

m/z = 112

O

O

O

HO

O

OH OH

OH

OH

OH

m/z = 166

O

m/z = 155

m/z = 90

m/z = 90

Mineralization CO2 + H2O + NH4+ + NO3Scheme 2. Plausible pathway for the photocatalytic degradation of RhB dye.

could attack the central carbon of RB to degrade the dye and further degraded via, N-deethylation process. From the mass spectra, the main intermediates with m/z value of 443, 415, 387, and 359 were corresponding to RB, N-deethylated intermediates

such as N,N-diethyl-N-ethylrhodamine, N,N-diethyl rhodamine, N-ethyl-N-ethylrhodamine and N-ethylrhodamine respectively. Ndeethylated intermediates were degraded into possible intermediate corresponding to the m/z values of 331. This can be further degraded

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TiO2 0.5wt% TiO2-PoPD 1.0wt% TiO2-PoPD 1.5wt% TiO2-PoPD 2.0wt% TiO2-PoPD 2.5wt% TiO2-PoPD

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-lnC0/Ct

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3

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1 10

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30 40 Irradiation time/min.

50

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Fig. 5. (A) UV–vis absorbance traces 1.5 wt% TiO2@PoPD core–shell nanocomposites for RB dye degradation. Inset: Structure of RB dye; (B) Apparent first-order linear transforms ln(C/C0) vs. t of RB dye (dye concentration: 8  105M) and catalyst dose: 0.5 gL1).

into m/z values of 316 and 258. The formed intermediates were further degraded into the m/z values of 244 and 230. Further the as formed intermediates were oxidized into benzoic acid, phthalic acid, 3,4-dihydroxybenzoic acid, adipic acid and butane-1,3-diol. Finally the formed oxidized products were mineralized into CO2, H2O, NO3 and NH4+. Our results are in good agreement with earlier reports for the degradation of RB dye in various semiconductor mediated photocatalysts [29–32]. On the basis of the results of photocatalytic studies, the schematic mechanism of charge separation and photocatalytic reaction over the TiO2-PoPD core shell nanocomposites photocatalyst is shown in Fig. 6. When a TiO2@PoPD nanocomposite is illuminated under natural light, both TiO2 and PoPD absorb photons and then charge separation occurs at the interface. The conduction band of nanocomposites and the lowest unoccupied molecular orbital (LUMO) level of PoPD are well matched for the charge transfer. The generated electrons from PoPD can be transferred to the conduction band of nanocomposites, whereas holes in the valence band of TiO2 are transferred into the PoPD, and enhancement in the charge separation occurs and promoting the photocatalytic activity of photocatalyst. The conduction band electron subsequently reacts with O2 adsorbed on the surface of TiO2 to produce O2 radicals. The resulting O2 radicals further react with H2O to generate OOH and OH radicals. The O2 and  OH radicals are most potent oxidizing agents to degrade organic pollutants [4–11,23–26]. The valence band (VB) position of TiO2 was lower than the highest HOMO of PoPD, so the later could act as an acceptor for the photogenerated holes in the hybrid photocatalysts. When TiO2 absorbed UV light to generate electron-hole pairs, the holes in VB could directly transfer to the HOMO of PoPD. Furthermore, PoPD was a good material for transporting holes: the holes transferred easily to the surface and oxidized the adsorbed contaminations directly. Electrons moved in the opposite direction from holes, reducing the recombination of photogenerated electrons and holes and making charge separation more efficient; then the recombination of electrons and holes in TiO2-PoPD core shell nanocomposites was greatly suppressed, leading to a higher photocatalytic activity [4–11,23–26].

Fig. 6. Schematic illustration of photocatalytic activity of TiO2@PoPD core–shell nanocomposites for RB degradation under solar light illumination.

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Conclusions In situ chemical oxidative polymerization method was adopted first time for the preparation of TiO2@PoPD core–shell nanocomposites. The TEM and AFM analyses confirm the core–shell features of TiO2@PoPD nanocomposites. UV–vis DRS spectra showed that TiO2-PoPD core–shell nanocomposites have a broad and strong absorption in visible range, indicating that the incorporation of PoPD onto the surface of TiO2 can extend the photo response range of TiO2. TGA studies showed that the TiO2-PoPD nanocomposite possesses more thermal stability when compared to PoPD. Photocatalytic activities of PoPD@TiO2 nanocomposites on RB were remarkably improved than TiO2 in solar light. Based upon the synergetic effect between PoPD and TiO2, a rapid charge separation and slow charge recombination came true in both the visible and ultraviolet radiations. It is hoped that our work could provide valuable information on the design of polymer modified semiconductor with more excellent properties and set the foundation for the further industrial application. Acknowledgment The authors gratefully acknowledged University Grants Commission, Government of India for providing financial assistance in the form of Major Research Project Grant. References [1] X. Zhao, L. Lv, B. Pan, W. Zhang, S. Zhang, Q. Zhang, Polymer-supported nanocomposites for environmental application: a review, Chem. Eng. J. 170 (2011) 381–394. [2] Q. Li, C. Zhang, J. Li, Photocatalysis and wave-absorbing properties of polyaniline/ TiO2 microbelts composite by in situ polymerization method, Appl. Surf. Sci. 257 (2010) 944–948. [3] R. Gangopadhyay, A. De, Conducting polymer nanocomposites: a brief overview, Chem. Mater. 12 (2000) 608–622. [4] J. Hou, R. Cao, S. Jiao, H. Zhu, R.V. Kumar, PANI/Bi12TiO20 complex architectures: controllable synthesis and enhanced visible-light photocatalytic activities, Appl. Catal. B: Environ. 104 (2011) 399–406. [5] S. Xu, L. Gu, K.H. Yang, Y. Song, L. Jiang, Y. Dan, The influence of the oxidation degree of poly(3-hexylthiophene) on the photocatalytic activity of poly(3-hexylthiophene)/TiO2 composites, Sol. Energy Mater. Sol. Cells 96 (2012) 286–291. [6] H. Zhang, R.L. Zong, J.A. Zhao, Y.F. Zhu, Dramatic visible photocatalytic degradation performances due to synergetic effect of TiO2 with PANI, Environ. Sci. Technol. 42 (2008) 3803–3807. [7] F. Denga, Y. Li, X. Luob, Y. Lixia, X. Tu, Preparation of conductive polypyrrole/TiO2 nanocomposite via surface molecular imprinting technique and its photocatalytic activity under simulated solar light irradiation, Colloid Surf A: Physicochem. Eng. Aspects 395 (2012) 183–189. [8] L. Zhang, P. Liu, Z. Su, Preparation of PANI/TiO2 nanocomposites and their solidphase photocatalytic degradation, Polym. Degrad. Stab. 91 (2006) 2213–2219. [9] X.Y. Li, D.S. Wang, G.X. Cheng, Q.Z. Luo, J. An, Y.H. Wang, Preparation of polyaniline-modified TiO2 nanoparticles and their photocatalytic activity under visible light, Appl. Catal. B: Environ. 81 (2008) 267–273. [10] D.S. Wang, Y.H. Wang, X.Y. Li, Q.Z. Luo, J. An, J.X. Yue, Sunlight photocatalytic activity of polypyrrole – TiO2 nanocomposites prepared by ‘in situ’ method, Catal. Commun. 9 (2008) 1162–1166.7.

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