Polymer Hybrid for Efficient

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Environ. Sci. Technol. 2010, 44, 3481–3485

Photonic Crystal Coupled TiO2/ Polymer Hybrid for Efficient Photocatalysis under Visible Light Irradiation GAOZU LIAO, SHUO CHEN, XIE QUAN,* HUAN CHEN, AND YAOBIN ZHANG Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education, China), School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China

Received December 18, 2009. Revised manuscript received April 5, 2010. Accepted April 6, 2010.

Inverse TiO2 opal photonic crystal coupled TiO2/poly(3hexylthiophene) (bilayer TiO2/P3HT) was structured on FTO substrate for efficient photocatalysis under visible light irradiation (λ > 400 nm). We expected that the photocatalytic capability of this hybrid photocatalyst could be enhanced by the efficient visible light absorption owing to the photonic crystal structure and effective charge separation owing to the unique heterojunction built between TiO2 and P3HT. The bilayer TiO2/ P3HT photocatalyst was prepared first by depositing inverse TiO2 opalonFTOsubstrateviareplicatingpolystyreneopal,followed by spin coating a layer of TiO2 nanoparticles on the inverse TiO2 opal. The as prepared bilayer TiO2 was modified by P3HT via dipping method. Environmental scanning electron microscopy (ESEM) images demonstrated that the as prepared photocatalyst was composed of inverse TiO2 opal layer and TiO2 nanoparticles layer. The UV-vis diffuse reflectance spectra showed that the optical absorption for bilayer TiO2/P3HT was more intensive than for pristine TiO2 nanoparticle/P3HT (NP-TiO2/ P3HT) in the range of 400-650 nm. The enhanced generation of photocurrent under visible light irradiation (λ > 400 nm) was observed using the bilayer TiO2/P3HT. The results of photocatalytic experiments under visible light irradiation revealed that the pseudofirst-order kinetic constant of photocatalytic degradation of methylene blue using the bilayer TiO2/P3HT was 2.08 times as great as that using NP-TiO2/P3HT, showing the advantage of the unique structure in the bilayer TiO2/P3HT for efficient photocatalysis.

Introduction TiO2 has been proved to be an excellent photocatalytic material due to its nontoxicity, good stability, and excellent photocatalytic activity (1, 2). However, the wide band gap (3.2 eV) of TiO2 limits the utilization of visible light which occupies a large part of solar light. Many attempts have been carried out to improve the utilization of solar light by extending the photoresponse of TiO2 to the visible region, such as metal ion doping (3), nonmetal doping (4, 5), noble metal deposition (6), narrow band gap semiconductors coupling (7), and dye sensitization (8, 9). * Corresponding author phone: +86-411-84706140; fax: +86-41184706263; e-mail: [email protected]. 10.1021/es903833f

 2010 American Chemical Society

Published on Web 04/13/2010

Recently, conducting polymers, such as polyaniline, polythiophene, polypyrrole, and their derivatives have been reported as promising sensitizers in bulk TiO2 photocatalysts. Modifying bulk TiO2 with conducting polymers has been explored to extend the spectral response of TiO2 to visible light effectively (10, 11). The photocatalytic activity of conducting polymers modified bulk TiO2 under visible light irradiation resulted from the visible light absorption of conducting polymers and effective charge separation of photogenerated carriers owing to the heterojunction built between TiO2 and the conducting polymers. Zhang et al. (12) prepared polyaniline modified TiO2 nanoparticles photocatalyst via chemisorption approach. Under visible light irradiation, the synergetic effect between polyaniline and TiO2 enhanced the photocatalytic performance to degrade methylene blue and rhodamine B. Wang et al. (13) reported that poly(3-hexylthiophene) (P3HT) modified TiO2 nanoparticles photocatalyst exhibited excellent photocatalytic ability to degrade methyl orange under visible light irradiation. However, the visible light harvest efficiency of these conducting polymers modified bulk TiO2 were still very low. In order to achieve high photocatalytic capability, it is necessary to intensify the light absorption for the conducting polymers modified bulk TiO2 photocatalysts. Photonic crystals are periodic optical materials or structures that are designed to affect the motion of photons in optoelectronic devices (14) and waveguide (15). The periodic modulation of photonic crystals leads to a photonic band gap that can affect the propagation of light with certain frequencies (16-19). To be specific, the light within the wavelength range of photonic band gap could not propagate in the photonic crystals due to the Bragg diffraction and scattering on lattice planes (termed as band gap scattering effect) (16, 20). In other words, photonic crystals act as a dielectric mirror to scatter the light in the wavelengths range of photonic band gap. Because of this property, photonic crystals could be used to enhance the light absorption of photoresponsive material by multiple scattering. For example, Silvia et al. (21) reported that light absorption of dyesensitized solar cells is amplified by introducing onedimensional photonic crystals as a back reflector. Aside from the band gap scattering effect, the light at longer wavelength side (red edge) of the photonic band gap will propagate with strongly reduced group velocity (termed as slow photon effect) (22-24). These slow photons could enhance the interaction of light with photoresponsive material, so as to amplify the optical absorption and photochemical reaction. Chen et al. (25, 26) demonstrated the amplified UV absorption and enhanced photocatalytic ability of TiO2 when it was structured as inverse opals with energies close to the band gap, which resulted from the effective harvest of slow photons in TiO2. Nishimura et al. (27) reported that the light harvesting efficiency of dye-sensitized photoelectrode was enhanced by coupling a conventional TiO2 nanocrystalline film to TiO2 photonic crystal layer. The photocurrent of the photoelectrode increased by about 26% compared with the conventional nanocrystalline TiO2 electrode. They illustrated that the TiO2 photonic crystal act as not only a dielectric mirror for wavelengths corresponding to the photonic band gap, but also a medium for enhancing light absorption on the red edge of the photonic band gap. Considering the superior ability of photonic crystals to confine, control and manipulate photons, they could be utilized to enhance the photocatalytic efficiency of heterojunction photocatalyst by improving the optical absorption. In our previous work, enhanced photocatalytic efficiency VOL. 44, NO. 9, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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under UV light irradiation was achieved by structuring an inverse TiO2/Pt opal photonic crystals Schottky heterostructure photocatalyst (28). In present study, we focused on constructing a visible light photocatalyst with photonic crystal architecture for enhanced photocatalysis. For this purpose, P3HT was employed as the visible light absorber to modify TiO2. The heterojunction built between P3HT and TiO2 will benefit the separation of photogenerated carriers. Inverse TiO2 opal photonic crystals were coupled to the photocatalyst for enhancing the visible light absorbance. Through abovementioned approach, we constructed an inverse TiO2 opal photonic crystals coupled TiO2/P3HT heterojunction photocatalyst (bilayer TiO2/P3HT) on FTO substrate for efficient visible light photocatalysis. Methylene blue (MB) were used as model substance for photocatalytic degradation reactions to evaluate the photocatalytic capability of the as prepared bilayer TiO2/P3HT photocatalyst under visible light irradiation (λ > 400 nm).

Materials and Methods Chemicals. Monodisperse polystyrene latex spheres (192 and 365 nm, 2 wt % in water) were purchased as a suspension from Nano-Micro Technology Co., Ltd. Poly(3-hexylthiophene) (P3HT) was obtained from Synwit Technology Co.,Ltd. and used without purification. All of the other reagents (analytical grade purity) were purchased from Tianjin Kermel Chemical Reagents Development Centre and were used without further purification. FTO glass with 2.2 mm thickness and 15 Ω/square sheet resistance was purchased from Geao Co., China. Fabrication of Inverse TiO2 Opal. FTO substrates were washed in ultrasonic bath with ethanol and deionized water (DI) for 10 min, respectively, and then followed by rinsing with DI water and drying in air. Colloidal crystals polystyrene films were prepared via solvent evaporation method. The suspension of monodisperse polystyrene spheres (d ) 192 nm) was diluted with DI water to a concentration of 0.1 wt %, and then was sonicated in a glass vial for 30 min. The FTO substrates were immersed vertically in the suspension and the water was evaporated at 55 °C overnight in an oven. To obtain the inverse TiO2 opal, liquid-phase deposition (LPD) method was used to infiltrate titanium dioxide into voids of the template. First, the colloidal crystal films were immersed vertically in a solution of 0.15 wt % titanium isopropoxide and 0.015% HNO3 in ethanol for 5 min. After dried in air, it was submerged vertically in an aqueous solution of 0.1 M ammonium hexafluorotitanate and 0.3 M boric acid at 60 °C. The pH of the solution was adjusted to about 3 by adding 1 M hydrochloric acid. After 25 min, the samples were washed thoroughly with DI water and dried in air at room temperature. The polystyrene latex spheres were removed by calcination in muffle furnace at 500 °C for 2 h with heating rate of 2 °C/min. After calcination, monodisperse polystyrene spheres film result in a highly ordered inverse opal TiO2. Synthesis of TiO2 Nanoparticles. TiO2 nanoparticles were synthesized by hydrothermal method. First, 20 mL of titanium isopropoxide were added to 36 mL of DI water and stirred for 2 h. The hydrolysis product of titanium isopropoxide was filtered and washed with DI water. Then the product was placed in a Teflon reactor containing 3.9 mL tetramethylammonium hydroxide aqueous solution (0.6 M). Finally the reactor was kept in oven at 120 °C for 3 h. After cooling to room temperature, a uniformly dispersed colloidal suspension of anatase TiO2 nanoparticles was obtained, which was confirmed by TEM image and XRD patterns (Supporting Information (SI) Figure S4). Preparation of Bilayer TiO2/P3HT Nanocomposites. TiO2 nanoparticles were loaded on the inverse opal TiO2 via spin coating at 500 rpm for 30 s. After several cycles, the bilayer TiO2 film was annealed at 450 °C for 2 h in order to crystallize 3482

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and sinter the TiO2 nanoparticles. In the end, the bilayer TiO2 film was immersed in a tetrahydrofuran (THF) solution with 0.3 g/L of P3HT for 12 h and dried in oven at 80 °C for 1 h. The as prepared inverse TiO2 opal coupled TiO2/P3HT nanocomposites are termed as bilayer TiO2/P3HT. For comparison, a pristine TiO2 nanoparticle film and pristine inverse opal TiO2 was employed to deposit P3HT, which are termed as NP-TiO2/P3HT and PC-TiO2/P3HT, respectively. Disordered TiO2 coupled TiO2/P3HT was fabricated from mixed polystyrene spheres template film (192 nm: 365 nm ) 1:1), which are termed as Dis-TiO2/P3HT. All other processes were the same as that of bilayer TiO2/P3HT. Characterization. The morphology of the samples was characterized by environmental scanning electron microscopy (ESEM, Quanta 200 FEG) with an accelerating voltage of 30.0 kV and a transmission electron microscopy (TEM FEI-Tecnai G2 F30 S-Twin). The structural information of the samples was measured by Fourier transform spectrophotometer (FT-IR, Prestige-21, Shimadzu) with KBr as the reference sample. The crystal structure of the samples was investigated using an X-ray diffractometer (XRD, Shimadzu LabX XRD-6000) with Cu KR radiation. The optical absorption property of the samples was investigated through diffuse reflectance spectra (DRS) by UV-vis spectrophotometer (Shimadzu, UV-2450). Photoelectrochemical Measurements. The photocurrent measurements, using 0.04 M potassium hydrogen phthalate and 0.1 M potassium thiocyanate as electrolyte, were carried out in a standard three-electrode cell with the P3HT modified TiO2 electrode as photoanode, Pt sheet as cathode and SCE as reference electrode. They were connected to a CHI electrochemical analyzer (CH Instruments 650B, Shanghai Chenhua). A high pressure xenon short arc lamp (CHF-XM35500W, Beijing Changtuo) with illumination intensity of 70 mW/cm2 provided full spectrum illumination; a filter (ZUL0400, Asahi Spectra Co. whose transmission spectra are displayed in the Supporting Information section) was used to allow visible light (λ > 400 nm) passes through. Radiometers of model FZ-A (Photoelectric Instrument Factory Beijing Normal University, tested spectral region: 400-1000 nm) were used to measure the incident visible light intensity. Photocatalytic Experiment. The photocatalytic activities of the samples were evaluated by the degradation of methylene blue (MB) in a single compartment photo reactor (2 ×2 × 3 cm). A high pressure xenon short arc lamp (CHFXM35-150W, Beijing Changtuo Co.) was served as the visible light source, a glass filter was added to allow visible light (λ > 400 nm) to pass through. The illumination intensity was 50 mW/cm2, which was measured as follows: First, the probe of the radiometers of model FZ-A was put in the center of photo reactor, and then tuned the distance between the photo reactor and high pressure xenon short arc lamp until the 50 mW/cm2 was achieved. The initial concentrations of MB solution were 10 mg/L. During the photoreaction, samples were collected at different time intervals for analysis. The concentration of MB was determined by UV-vis spectrophotometer (UV-721, Xinmao) at 664 nm. To evaluate the photostability of the photocatalyst, the photocatalytic processes were repeated 3 times for the photodegradation of MB.

Results and Discussion Morphology of Bilayer TiO2/P3HT. The procedure for preparing bilayer TiO2/P3HT was illustrated in Scheme 1. Polystyrene (PS) opals template on FTO was prepared by solvent evaporation method. The inverse TiO2 opals were made by replicating the polystyrene opals template. Liquidphase deposition (LPD) method was used to infiltrate TiO2 nanoparticles into the voids of the PS template. After the PS opals were removed by calcination, inverse TiO2 opals were

SCHEME 1. Schematic Procedure for Preparing Bilayer TiO2/P3HT

formed. Subsequently, TiO2 nanoparticles were covered on the inverse TiO2 opals by spin coating. The bilayer TiO2/ P3HT was obtained after the bilayer TiO2 was modified by P3HT via dipping method. The top view SEM images of inverse TiO2 opals were shown in Figure 1a. We can see the monodisperse polystyrene spheres were replicated successfully via the LPD method. Inverse TiO2 opal is composed of uniform spherical voids with a diameter of 150 nm. Compared with the original size of the polystyrene spheres, the diameter of the voids was decreased by about 20%. The inset in Figure 1a is the cross section image of inverse TiO2 opals. The thickness of inverse TiO2 opal is about 2.5-3 µm. Figure 1b displayed the cross section SEM image of bilayer TiO2/P3HT. As shown in Figure 1b, the bilayer TiO2/P3HT was constructed with inverse TiO2 opal layer at the bottom, and TiO2 nanoparticle layer on the top. The thickness of the TiO2 nanoparticle layer is about 4-5 µm. The loading amount of TiO2 and P3HT on FTO was 12.4 mg/cm2 and 0.3 mg/cm2 respectively, which could be calculated by subtracting the weight of FTO. Crystal Structure of Bilayer TiO2/P3HT. SI Figure S1 shows the FT-IR spectra of NP-TiO2, P3HT and bilayer TiO2/ P3HT. The wide peak at 480-800 cm-1 in the spectra of NPTiO2 corresponds to the Ti-O bending mode of TiO2. The spectra of P3HT mainly includes the characteristic absorption bands as following: the peak at 820 cm-1 is assigned to CsH out-of-plane stretching vibration, the peaks at 1508 and 1454 cm-1 represent the antisymmetric CdC stretching and symmetric stretching vibration on the thiophene ring, respectively, the peaks in 2800-3100 cm-1 can be attributed to the aliphatic CsH stretching vibrations. Within the spectrum of bilayer TiO2/P3HT, we can see the characteristic peaks of P3HT at 2800-3100 cm-1, 1508 cm-1, and 1454 cm-1 also appear in the nanocomposite, which proved the existence of P3HT in bilayer TiO2/P3HT. The X-ray diffraction patterns (XRD) of NP-TiO2 and bilayer TiO2/P3HT are presented in SI Figure S2. For the bilayer TiO2, It can be seen that a dominant TiO2 peak was observed at 2θ ) 25.2° corresponding to anatase (101), and all other peaks presented in the graph are ascribed to anatase TiO2. The XRD patterns of bilayer TiO2/P3HT hardly changed in peak positions and shapes compared with pristine bilayer TiO2, indicating that the modification with P3HT did not influence the lattice structure of TiO2, which will be very beneficial for photocatalysis of the as prepared hybrid photocatalyst. Optical Absorption. In order to obtain the optical property of as prepared inverse TiO2 opal, the transmittance spectra

of inverse TiO2 opal was characterized as shown in Figure 2a and b. We can see the photonic band gap of inverse TiO2 opal photonic crystals is at 410 nm. The red edge of the photonic band gap is in the vicinity of 550 nm. These characteristics are beneficial for efficient visible absorption of bilayer TiO2/P3HT. To be specific, when the incident light penetrate the nanoparticle TiO2 layer and get to the interface of inverse TiO2 opal, the inverse TiO2 opal first act as a back reflector to scatter the light around 410 nm into the outside nanoparticle TiO2 layer, which bring on the increase of optical path length in the bilayer TiO2/P3HT. Moreover, the light around 550 nm will propagate with reduced group velocity in the inverse TiO2 opal layer. These slow photons can also increase the absorption of P3HT. Consequently, the cooperation of band gap scattering effect and slow photons effect will enhance the visible absorption of bilayer TiO2/P3HT, which was proved by the UV-vis DRS of bilayer TiO2/P3HT as shown in Figure 2c and d. The bilayer TiO2/P3HT exhibited increased absorption in 400-650 nm compared with NPTiO2/P3HT, indicating that the visible absorption of TiO2/ P3HT was enhanced by inverse TiO2 opal coupling. Photoelectrochemical Measurements. Figure 3 shows the photocurrent densities of NP-TiO2, NP-TiO2/P3HT and bilayer TiO2/P3HT under visible light illumination (λ > 400

FIGURE 1. SEM image of (a) inverse TiO2 opal, and (b) bilayer TiO2/P3HT. Inset shows the cross-sectional view of inverse TiO2 opal.

FIGURE 3. Photocurrent Densities of (a) NP-TiO2, (b) NP-TiO2/ P3HT, (c) Bilayer TiO2/P3HT under Visible Light Irradiation (λ > 400 nm).

FIGURE 2. UV-vis Diffuse Reflectance Spectra (DRS) of (a) NP-TiO2, (b) Inverse TiO2 Opal, (c) NP-TiO2/P3HT, (d) Bilayer TiO2/P3HT.

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FIGURE 4. Process of photocatalytic degradation of MB under visible light irradiation (λ > 400 nm) (a) bilayer TiO2/P3HT in dark, (b) direct photolysis, (c) NP-TiO2, (d) PC-TiO2/P3HT, (e) NP-TiO2/P3HT, (f) Dis-TiO2/P3HT, (g) bilayer TiO2/P3HT. nm). It could be seen that the photocurrent of pristine TiO2 was near zero under visible light irradiation due to the limitation of wide band gap. The NP-TiO2/P3HT exhibited increased photocurrent under visible light irradiation compared with NP-TiO2. It was ascribed to the heterojunction built between P3HT and TiO2, which impelled the photogenerated carriers injecting from P3HT to TiO2. The photocurrent of bilayer TiO2/P3HT was 2.76 times as great as that of NP-TiO2/P3HT under visible light irradiation. This enhancement resulted from the increased visible absorption owing to the inverse TiO2 opal coupling, which lead to the increase of photogenerated carriers. This experiment confirms that bilayer TiO2/P3HT possesses enhanced photoresponse than NP-TiO2/P3HT. This experiment confirms that bilayer TiO2/P3HT possesses enhanced photoresponse than NP-TiO2/P3HT. Photocatalytic Experiments. The photocatalytic capability of NP-TiO2, NP-TiO2/P3HT and bilayer TiO2/P3HT under visible light illumination (λ > 400 nm) was evaluated by photocatalytic degradation of MB as presented in Figure 4. The adsorption of MB by bilayer TiO2/P3HT in dark and direct photolysis of MB served as control. During 6 h illumination, the NP-TiO2 showed very low photocatalytic ability under visible light irradiation because of the limitation of wide band gap. The NP-TiO2/P3HT exhibited much higher visible light photocatalytic activity than NPTiO2. 59.7% of MB was photodegraded using NP-TiO2/ P3HT, implying that P3HT extend the photocatalytic ability of NP-TiO2 to visible light effectively. Among these photocatalysts, the bilayer TiO2/P3HT exhibited the most excellent photocatalytic ability. 86.0% of MB was photodegraded by bilayer TiO2/P3HT. This is ascribed to the enhanced visible light absorption by inverse TiO2 opal coupling and photogenerated charge separation owing to the heterojunction between TiO2 and P3HT. In order to approve this enhancement further, the photocatalytic ability of PC-TiO2/P3HT and Dis-TiO2/P3HT was investigated. Corresponding curves were shown in Figure 4d and f. In 6 h, 50.1% and 62.8% of MB was photodegraded, respectively, which confirmed the unique photonic crystal structure to the enhancement of photocatalysis further. It is found that the degradation of MB accords with pseudofirst-order kinetics by linear transforms ln(C0/Ct) ) kt, where C0 is the initial concentration of MB, Ct is the concentration of MB at time t, and k is kinetic constant. The corresponding kinetic constants (k) and regression coefficients (R2) were calculated and given in SI Table S1. Under the same experimental conditions, the kinetic constant of MB photodegradation with bilayer TiO2/P3HT nanocomposites electrode was 2.08 times as great as that of the NP-TiO2/P3HT electrode, which showed the advantage of the unique structure in the bilayer TiO2/P3HT 3484

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for efficient photocatalysis. The photostability of bilayer TiO2/P3HT photocatalyst was investigated by performing three recycled photocatalytic experiments. After three repeated experiments, the degradation efficiency decreased 10% approximately, which was comparable to the photostability of other sensitized photocatalysts (29, 30). Discussion on Photocatalytic Mechanism of Bilayer TiO2/P3HT. The photocatalytic reaction can be considered as the process of the generation, transfer, and consumption of the photogenerated carriers. Any approach that could enhance each step in the process may improve photochemical reactions. Here, the efficient photocatalytic capability of bilayer TiO2/P3HT resulted from the enhanced photogenerated carriers generation and transfer. The mechanism could be explained as following: Under visible light irradiation, the incident light first arrived at the outside nanoparticle TiO2/P3HT layer, and partial photons will be consumed by the P3HT in this layer. The other photons will propagate further to the interface of inverse TiO2 opal. Because the photonic band gap of inverse TiO2 opal is at 410 nm, the light around 410 nm will be scattered multiply into the outside nanoparticle TiO2 layer due to the back reflection of inverse TiO2 opal, which caused an increase of optical path length in the nanoparticle TiO2 layer. As a result, the absorption of P3HT in the nanoparticle TiO2 layer was enhanced. Moreover, because the red edge of photonic band gap is in the vicinity of 550 nm, the light around 550 nm propagated with reduced group velocity in the inverse TiO2 opal. When the group velocity decreased, the interaction between the photons and P3HT would be reinforced. Consequently, the absorption of P3HT in this region was enhanced. The cooperative enhancement of visible absorption increased the generation of excited-state electrons. The heterojunction built between TiO2 and P3HT impelled the excited-state electrons injecting to the conduction band of TiO2 and increased the yield of hydroxyl, super oxide radicals and positive carbon radicals, which are responsible for the enhanced photodegradation of MB (SI Scheme S1) (8-13). In summary, remarkable enhancement of photocatalytic capability under visible light irradiation was achieved by coupling inverse TiO2 opal photonic crystals to the TiO2/ P3HT heterojunction photocatalyst. This enhanced photocatalytic capability benefited from the enhanced visible light absorption owing to photonic crystal structure and facilitated separation of photogenerated carriers owing to the heterojunction built between TiO2 and P3HT. We believe that the configuration and fabrication method of the photocatalyst in this work provide a valuable knowledge for the development of highly efficient photocatalysts.

Acknowledgments This work was supported by National Natural Science Foundation of China (No. 20837001, No.20525723), Education Department of Liaoning Province (No.2008T224) and PCSIRT (IRT0813).

Supporting Information Available Detailed information includes the FT-IR spectrum of P3HT, NP-TiO2 and bilayer TiO2/P3HT (FIGURE S1), XRD patterns of NP-TiO2 and bilayer TiO2/P3HT (FIGURE S2), transmission spectrum of the ZUL0400 optical filter (FIGURE S3), TEM image and XRD patterns of TiO2 nanoparticles (FIGURE S4), Schematic diagram of the relative energy level of P3HT (πorbital and π*-orbital) and TiO2 (SCHEME S1), the kinetic constants and regression coefficients of photocatalytic degradation processes of MB under visible light irradiation (λ > 400 nm) (TABLE S1). This material is available free of charge via the Internet at http://pubs.acs.org.

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