Highly-efficient photocatalytic degradation of methylene blue by PoPD-modified TiO2 nanocomposites due to photosensitization-synergetic effect of TiO2 with PoPD

Poly-o-phenylenediamine modified TiO2 nanocomposites were successfully synthesized via an ‘in situ’ oxidative polymerization method. The modified nanocomposites were characterized by BET, XRD, TEM, FT-IR, TGA, XPS, EA and UV-Vis DRS. The photocatalytic degradation of methylene blue was chosen as a model reaction to evaluate the photocatalytic activities of TiO2 and PoPD/TiO2. The results indicated that PoPD/TiO2 nanocomposites exhibited good photocatalytic activity and stability. The photocatalytic activity of PoPD/TiO2 increased as the initial pH increased because of electrostatic adsorption between the photocatalyst and MB as well as the generation of ·OH, whereas it exhibited an earlier increasing and later decreasing trend as the concentration of the photocatalyst increased owing to the absorption of visible light. The photocatalytic stability of the PoPD/TiO2 nanocomposite was dependent on the stability of its structure. Based on radical trapping experiments and ESR measurements, the origin of oxidizing ability of PoPD/TiO2 nanocomposites on photocatalytic degradation of MB was proposed, which taking into account of ·OH and ·O2 − were the first and second important ROS, respectively. The possible photocatalytic mechanism and photocatalytic activity enhanced mechanism has been proposed, taking into account the photosensitization effect and synergetic effect of TiO2 with PoPD.

In the field of environmental chemistry, the use of semiconductors as photocatalysts has been the focus of recent attention since it aims at the destruction of contaminants in water and air 1, 2 . Among the semiconductors, titanium dioxide (TiO 2 ) is an excellent photocatalyst because it is an effective, photostable, reusable, inexpensive, non-toxic and easily available catalyst [3][4][5] . However, the wide band gap (3.2 eV) of TiO 2 only allows it to absorb ultraviolet light (<387 nm), which limits the utilization of solar light since UV light represents less than 5% of solar light 6,7 .
To extend the photoresponse of TiO 2 to the visible region, many modification methods, such as metal ion doping 8,9 , non-metal doping 10,11 , noble metal deposition 12 , composite semiconductors 13,14 and surface dye sensitization 15,16 have been reported. Recently, a large body of work has been produced based on using conjugated polymer-modified TiO 2 to degrade organic pollutant because nanocomposites of conductive polymers and inorganic particles show interesting physical properties and application potential [17][18][19] . Some studies have been published on the combination of conductive polymers and TiO 2 to improve performance under UV light and sunlight conditions. For example, Zhang et al. 20 reported that PANI-modified TiO 2 nanocomposites showed a higher photocatalytic activity than TiO 2 under ultraviolet light and visible light, and the enhancement was attributed to the synergetic effect between TiO 2 and PANI. As a typical conducting polymer, poly-o-phenylenediamine (PoPD) has attracted considerable attention since its discovery 21,22 . Taking advantage of the unique electrical, optical and photoelectric properties of PoPD, we expect that the combination of PoPD with TiO 2 may induce an interesting charge transfer and thus enhance the photocatalytic activity of TiO 2 under visible light irradiation. However, the photocatalytic activity enhanced mechanism has not been studied.
In our studies, PoPD/TiO 2 nanocomposites were synthesized via an 'in situ' oxidative polymerization method. The modified photocatalysts were characterized by BET Test, X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), Elemental Analysis (EA) and UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS). The results indicated that the PoPD exists on the surface of TiO 2 , the presence of PoPD does not impact on the lattice structure and grain size of TiO 2 , and the presence of PoPD enhances the visible light response. The photocatalytic degradation of methylene blue (MB) was chosen as a model reaction to evaluate the photocatalytic activities of TiO 2 and PoPD/TiO 2 . The results indicated that PoPD/TiO 2 nanocomposites exhibited good photocatalytic activity (apparent first-order rate constants of 0.0021 min −1 for TiO 2 and 0.0033 min −1 for P/T(1/4)) and stability (recycled 5 times, 15 hours of operation). The photocatalytic activity of PoPD/TiO 2 was influenced by the initial pH and concentration of the photocatalyst. Based on radical trapping experiments and electron spin resonance (ESR) measurements, the origin of oxidizing ability of PoPD/TiO 2 nanocomposites on photocatalytic degradation of MB was proposed, which taking into account of ·OH and ·O 2 − were the first and second important ROS, respectively. The photocatalytic activity enhanced mechanism has been proposed, taking into account the photosensitization effect (UV-Vis DRS) and synergetic effect of TiO 2 with PoPD (synergetic factor and electrochemical impedance spectroscopy (EIS)).

Experimental
Reagents and materials. TiO 2 was purchased from Degussa with a BET specific surface area of 291.380 m 2 /g. The o-phenylenediamine (oPD) and ammonium persulfate (APS) were purchased from Tianjin Kermel Chemical Reagent Co., Ltd. Ethyl alcohol was purchased from Tianjin Fuyu Fine Chemical Co., Ltd. Hydrochloric acid was purchased from Sinopharm Chemical Reagent Co., Ltd. Methylene blue was purchased from Tianjin Guangcheng Chemical Reagent Co., Ltd. All of the chemicals were of analytical grade and used without further purification. Deionized water was used for the preparation of all of the solutions.
Preparation of PoPD/TiO 2 nanocomposites. The typical synthesis of the PoPD/TiO 2 nanocomposites is described below.
An appropriate amount of oPD was dissolved in 90 ml of a 1.2 mol/L hydrochloric acid solution followed by the addition of 0.512 g of TiO 2 . The solution was ultrasonicated for 15 min to ensure uniform mixing. After dissolution, the solution was labeled A. An appropriate amount of APS was dissolved in 30 ml of a 1.2 mol/L hydrochloric acid solution, and this solution was labeled B. After solution A was transferred to a 250 ml round-bottom flask, a magneton was added, and the solution was stirred with a magnetic stirrer. Then, solution B was transferred to a 100 mL constant pressure funnel, and solution A was added dropwise at approximately 1 drop/second with stirring. The reaction was continued for 24 hours at room temperature. The final products were filtered and washed with deionized water and ethanol followed by drying at 80 °C for several hours in a vacuum oven. In the experiment, different initial molar ratios of oPD to TiO 2 (from 1/6 to 4/1) were employed to obtain TiO 2 nanocomposites deposited by PoPD. In this fashion, a series of PoPD/TiO 2 nanocomposites with various initial molar ratios of oPD to TiO 2 (i.e., 1/6, 1/5, 1/4, 1/2, 1/1, 2/1, 3/1, and 4/1) were prepared, and these nanocomposites are referred to as P/T(1/6), P/T(1/5), P/T(1/4), P/T(1/2), P/T(1/1), P/T(2/1) P/T(3/1), and P/T(4/1), respectively. To confirm the effect of PoPD in the composites, the TiO 2 nanocomposites were treated with the same procedure as that used for the composites without the addition of oPD.
Characterizations of PoPD/TiO 2 nanocomposites. The surface texture of TiO 2 and PoPD/TiO 2 nanocomposite was examined by N 2 adsorption at 77 K (Quantachrome instruments Quadrasorb SI). The specific surface area was calculated from the N 2 adsorption isotherm using the BET equation. X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation. Transmission electron microscopy (TEM) was performed on a JEM-2100 transmission electron microscope. The Fourier-transform infrared spectra (FT-IR) of the samples were recorded on Vertex 70 spectrometer in a range from 4000 to 400 cm −1 . Thermogravimetric analyses (TGA) of all of the samples were performed with a Q500 thermal analysis instrument. The samples were heated from 35 to 800 °C at a rate of 10 °C min −1 in air. The X-ray photoelectron spectroscopy (XPS) measurements were performed using a Thermo ESCALAB 250Xi system with an Al Kα X-ray source. All of the binding energies were referenced to the C1s peak at 284.8 eV for the surface adventitious carbon. Elemental Analysis (EA) was performed on a vario MACRO cube elemental analyzer. Ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS) was performed using a UV-2550PC ultraviolet and visible spectrophotometer from 200 to 800 nm with BaSO 4 as the background. Electrochemical impedance spectroscopy (EIS) was s performed on a CHI660D VersaSTAT. TiO 2 and PoPD/TiO 2 nanocomposites were deposited as a film on a 1 cm × 1 cm indium-tin-oxide conducting glass to obtain the working electrode. The saturated calomel electrode and a Pt electrode served as the reference and counter electrodes, respectively. The electrolyte was 0.1 mol L −1 NaClO 4 solution.
Scientific RepoRts | 7: 3973 | DOI:10.1038/s41598-017-04398-x Photocatalytic activity test. The photocatalytic activities of the samples were evaluated based on the degradation of MB in an aqueous solution under a 1000 W xenon lamp (BL-GHX-V photochemical reactions instrument). Aqueous suspensions of MB (30 ml, 40 mg/L) were placed in a quartz tube, and 30 mg of the PoPD/ TiO 2 nanocomposites were added. Prior to irradiation, the suspensions were magnetically stirred in the dark for approximately 1 h. The suspensions were maintained under constant air-equilibrated conditions before and during illumination. At certain time intervals, 1 ml of the liquor was sampled and centrifuged to remove the particles. The filtrates were analyzed by recording variations in the maximum absorption band (664 nm for MB) using a UV-2550PC ultraviolet and visible spectrophotometer. This process was repeated five times to confirm the stability of the nanocomposites.

Results and Discussion
Characterization results of PoPD/TiO 2 nanocomposites. It was well-known that the BET surface area of sample was an essential parameter for enhanced photocatalytic activity. The adsorption and desorption isotherms of N 2 at 77 K on TiO 2 and P/T(1/4) nanocomposites are shown in Fig. 1. Like the P25 TiO 2 , the P/T(1/4) nanocomposite also displays a Type II isotherm characteristic of a mesoporous material. Clearly the total pore volume and surface area of PoPD/TiO 2 nanocomposite are much less than those of TiO 2 . However, the PoPD/ TiO 2 nanocomposite showed the higher photocatalytic activity than TiO 2 , indicating that the surface area of photocatalyst is only an index to character the physicochemical properties, not the decisive index to ensure the photocatalytic activity.
The XRD patterns of TiO 2 and P/T(1/4) are compared in Fig   The FT-IR spectra of TiO 2 , PoPD, and P/T(1/4) are shown in Fig. 4. The main characteristic bands of PoPD are assigned as follows: the intensive peaks between 3500 cm −1 and 3200 cm −1 can be attributed to the N-H stretching vibrations of the -NH 2 and -NH-groups. The peak at 1629 cm −1 is associated with C=N stretching vibration, and the strong absorption band at 1523 cm −1 is ascribed to the C=C stretching vibrations in the benzene ring. The weak peaks at 1328 cm −1 and 1238 cm −1 are correspondingly assigned to the =C-N stretching on the benzene ring 24 . The FT-IR spectrum of the P/T(1/4) contains the same main characteristic bands as that of PoPD but with a shift to higher wavenumbers 25 . The results show that there is a strong interaction between PoPD and the TiO 2 nanoparticles, and the PoPD deposits and forms a shell on the surface of the TiO 2 nanoparticles (2350 cm −1 , 2850 cm −1 , and 2925 cm −1 ). The deposition of PoPD on the surface of the TiO 2 nanoparticles not only constrains the motion of the PoPD chains but also restricts the vibration mode in the PoPD molecule. It can be observed that the characteristic band of TiO 2 near 500 cm −1 occurred in the PoPD/TiO 2 nanocomposite and the band is so wide that it hides the figure peak in the PoPD/TiO 2 nanocomposite.
The thermal behavior of TiO 2 and P/T(1/4) was investigated by TGA, and the results are shown in Fig. 5. In Fig. 5, black curve indicates that TiO 2 is very stable in air, and no decomposition occurred in the 30-800 °C range, and red curve indicates that P/T(1/4) has different showing. The first weight loss was observed at 80 °C owing to desorption of the water that was absorbed on the PoPD. This curve also indicates that a sharp weight loss occurs at approximately 450 °C and continues up to 550 °C. This weight loss was due to decomposition of the skeletal PoPD chain structure 26 .
X-ray photoelectron spectroscopy (XPS) is an important tool for studying the electronic structure of condensed matter and is widely used for quantitative surface analysis. According to the XPS survey spectra of TiO 2 and P/T(1/4), as showing in Fig. 6(a-h), Ti and O were present in TiO 2 based on the two peaks at binding energies of 458.5 and 529.8 eV. In addition, the C, O, Ti and N elements existed in the P/T(1/4) based on the four peaks with binding energies of 284.8, 529.8, 458.5 and 400.3 eV, which are related to C1s, O1s, Ti2p and N1s, respectively 27 . The atomic percentages of C, O, Ti and N were 54.57%, 23.66%, 6.17% and 15.6%, respectively, suggesting that PoPD exists on the TiO 2 surface 28 . Meanwhile, as the Fig. 6(e) and 6(f) showing the O1s, it was obviously that there was one peak on O1s of TiO 2 at 528.8 eV (Ti-O), but there were two peaks on O1s of P/T(1/4) at 530.1 eV (Ti-O) and 532.3 eV (O-PoPD), the results indicated that the PoPD existed on the surface of TiO 2 and there was an interaction between TiO 2 and PoPD. The same results were also obtained via Elemental Analysis as Table 1 showing.
The UV-Vis diffuse reflectance spectra (UV-Vis DRS) of TiO 2 , PoPD and P/T(1/4) are shown in Fig. 7(a). The absorption of both ultraviolet light and visible light by PoPD is similar to that of PoPD/TiO 2 nanocomposites. In  comparison to TiO 2 , the absorption of the P/T(1/4) nanocomposite increases over the entire visible light range but decreases in the UV range. The results indicate that our method is effective for extending the absorption of TiO 2 to the visible light range 29 . As shown in Fig. 7(b-d), the band gap energies (E g ) of TiO 2 , PoPD and PoPD/ TiO 2 nanocomposites, which were obtained from the wavelength values corresponding to the intersection point of the vertical and horizontal portions of the spectra using hc/λ = E g , where E g is the band gap energy, h is Planck's constant, c is the speed of light (m/s), and λ is the wavelength (nm), were determined to be 3.1 eV, 1.89 eV and 2.45 eV, respectively. Therefore, the PoPD/TiO 2 nanocomposites can be excited to produce more electron-hole pairs under visible light illumination, which may result in higher photocatalytic activities.
Photocatalytic activity and stability. The photocatalytic activity was investigated based on the degradation of MB in an aqueous solution under 1000 W xenon lamp irradiation. MB has a maximum absorption of approximately 664 nm. Figure 8(a) shows the degradation of MB in the presence of TiO 2 and PoPD/TiO 2 with different initial ratios of oPD to TiO 2 . The kinetics plots are shown as the apparent first-order linear transform -ln(C/ C 0 ) = k app t in Fig. 8(b). The activity of the TiO 2 and PoPD/TiO 2 photocatalysts can be evaluated by comparing the apparent first-order rate constants (k app ) shown in Table 2. The TiO 2 and P/T(1/4) nanocomposites have apparent rate constants of 0.0021 min −1 and 0.0033 min −1 , respectively. However, not all of the PoPD/TiO 2 nanocomposites exhibited higher photocatalytic activity, and an optimal molar ratio between oPD and TiO 2 exists. P/T(1/4) can obviously enhance photocatalytic activity. The degradation rate of MB exhibited an up-down-up-down trend as the initial molar ratios of oPD to TiO 2 changed from 1/6 to 4/1. Therefore, the photocatalytic activity was influenced by at least two key factors including solar absorption and charge separation.
To evaluate the photocatalytic activity of the P/T(1/4) nanocomposites with different initial pH values, the degradation of MB with an initial pH of 3.21, 8.07 and 11.35 is shown in Fig. 9(a). The results indicated that the photocatalytic activity of P/T(1/4) increased as the initial pH increased, and the first-order rate constant was 0.0033 min −1 for a pH of 3.61 and 0.0113 min −1 for a pH of 11.41. The higher photocatalytic activity at a higher initial pH was due to electrostatic adsorption between the photocatalyst and MB as well as the generation of ·OH 30 . On the one hand, a higher pH led to the photocatalyst with negative electricity, and MB was a typical cationic dye with a positive charge. Therefore, the electrostatic interaction was beneficial for enhancing the adsorptive property of P/T(1/4), which was also beneficial for the photocatalytic activity. On the other hand, a higher pH indicates rich OH − , and ·OH was generated by h + and OH − . Therefore, a higher initial pH was beneficial for the generation of ·OH and enhanced photocatalytic activity.
To evaluate the photocatalytic activity of the P/T(1/4) nanocomposites with different photocatalyst concentrations, the degradation of MB with PoPD/TiO 2 concentrations of 0.5 g/L, 1.0 g/L, 1.5 g/L, and 2.0 g/L is shown in Fig. 9(b). The results indicate that the photocatalytic activity of P/T(1/4) increased as the concentration of PoPD/ TiO 2 increased from 0.5 g/L (0.0021 min −1 ) to 1.5 g/L (0.0035 min −1 ), but it decreased as the concentration of PoPD/TiO 2 increased from 1.5 g/L (0.0035 min −1 ) to 2.0 g/L (0.0031 min −1 ). This result indicates that the optimal concentration of the PoPD/TiO 2 nanocomposite was 1.5 g/L. The photocatalytic activity with different PoPD/ TiO 2 concentrations was influenced by the absorption of visible light 31 . When the concentration of P/T(1/4) was low, the absorption of visible light increased as the P/T(1/4) concentration increased, which was beneficial for the generation of ROS. However, when the concentration of P/T(1/4) was high, the absorption of visible light decreased as the P/T(1/4) concentration increased owing to the obstructive effect from the excess photocatalyst, which was detrimental for the generation of ROS.
In addition, some experiments were carried out to confirm the photocatalytic stability of the PoPD/TiO 2 photocatalysts. It has been confirmed that PoPD/TiO 2 exhibited good photocatalytic stability under irradiation conditions and continued to maintain its photocatalytic activity after five cycles, as shown in Fig. 10(a). The slight decrease in the photocatalytic activity during each cycle is due to a slight aggregation of the nanocomposites during the photocatalytic process. The FT-IR spectra of the PoPD/TiO 2 nanocomposites before and after the reaction are shown in Fig. 10(b). The shape of the composite FT-IR spectrum after the photocatalytic experiment is similar to that of the particles prior to the experiment, which indicates that the structure of PoPD/TiO 2 does not change during the photocatalytic process 32 . Therefore, the stability of the photocatalytic activity is dependent on the stability of the structure.

Origin of oxidizing ability of PoPD/TiO 2 nanocomposites on photocatalytic degradation of MB.
Some experiments were carried out to confirm the origin of oxidizing ability of PoPD/TiO 2 nanocomposites on photocatalytic degradation of MB. On the one hand, to further evaluate the role of these active species such as ·OH and ·O 2 − , different types of active species scavengers are added in catalyst system. Figure 11a shows    after 180 min of light irradiation. Since the PoPD itself is not only electron donors, but also the hole acceptors, a special kind of similar to the circulatory system between the PoPD and TiO 2 will form when P/T is exposed to light. The benzoquinone (BQ) has the ability to trap ·O 2 − by a simple electron transfer mechanism. The addition of BQ (1 mmol, 2 mmol) provokes partial inhibition of the MB degradation as shown in Fig. 11(a), and the related results of ESR are shown in Fig. 11(b) and 11(c). A combination of the results of ESR and the addition of BQ indicates that ·O 2 − plays an important role in the photocatalytic process with decolourization ratio of MB decreasing from 44.5% to 34.6% (BQ 1 mmol) and 33.2% (BQ 2 mmol) and the first-order rate constant decreasing from 0.0033 min −1 to 0.0027 min −1 (BQ 1 mmol) and 0.0022 min −1 (BQ 2 mmol). Meanwhile, After 1 mL and 2 mL of tert-butyl alcohol (TBA) as a ·OH-scavenger are added into the reaction system, the rate for degradation of MB

Photocatalysts
-ln(C/C 0 ) = k app t k app (min −1 ) R 2  Table 2. Apparent first-order rate constants (k app ) of MB degradation and linear regression coefficients from a plot of -ln(C/C 0 ) = k app t.  Scientific RepoRts | 7: 3973 | DOI:10.1038/s41598-017-04398-x over P/T(1/4) is remarkably decreased. A combination of the results of ESR and the addition of TBA indicates that ·OH plays an more important role in the photocatalytic process with decolourization ratio of MB decreasing from 44.5% to 12.5% (TBA 1 mL) and 15.0% (TBA 2 mL) and the first-order rate constant decreasing from 0.0033 min −1 to 0.0008 min −1 (TBA 1 mL) and 0.0009 min −1 (TBA 2 mL).
Photocatalytic mechanism and photocatalytic activity enhanced mechanism of the PoPD/TiO 2 nanocomposites. The basic mechanism of PoPD/TiO 2 nanocomposites was well established. TiO 2 nanocomposite is irradiated with UV light to generate electron-hole pairs, which can react with water to yield hydroxyl and superoxide radicals, which oxidize and mineralize the organic molecules. However, the band gap of TiO 2 is 3.11 eV, meaning that only UV light can excite the TiO 2 nanocomposites to generate electron-hole pairs. One solution to overcome this shortcoming was to use a dye with a narrow band gap as a sensitizer to enhance the response of TiO 2 to visible light. PoPD has a band gap of 1.89 eV, which is narrower than that of TiO 2 (3.   The photocatalytic mechanism of PoPD/TiO 2 is as follows 37 : Although the photocatalytic activity of the photocatalyst was influenced by many factors, two key factors were identified based on the photocatalytic mechanism. In the primary reaction process, TiO 2 is excited under light irradiation to generate electron-hole pairs. In addition, in the secondary reaction process, the ROS is produced to degrade organic pollutants. Therefore, the two key factors include solar absorption and charge separation. For PoPD/TiO 2 , because PoPD is a photosensitizer with a narrow level spacing, PoPD/TiO 2 inserts the energy level of PoPD into the energy level of TiO 2 to enhance its response to visible light. On the other hand, owing to the synergetic effect of the well-matched energy levels of TiO 2 and PoPD, PoPD/TiO 2 hinders the recombination of the hole and electron to generate more ROS that degrade MB. Therefore, we can explain that the photocatalytic activity enhanced mechanism is based on a photosensitization effect as well as a synergetic effect 38, 39 . Photosensitization effect to enhance the response to visible light. For a crystalline semiconductor, the optical absorption near the band edge follows the Kubelka-Munk function 40 (see Supporting Information Equation 1), and the results are shown in Fig. 7. The band gap energies for TiO 2 , PoPD and PoPD/TiO 2 were determined to be 3.10 eV, 1.89 eV and 2.45 eV, respectively, which indicated that PoPD/TiO 2 is a better photocatalyst than the unmodified TiO 2 owing to PoPD being a photosensitizer. The excitation between the HOMO and LUMO in PoPD is much lower because the benzene rings are conjugated through an imine linkage and the sulphonyl is an electron-withdrawing group. The experimental absorption spectrum for PoPD indicates a band gap of 1.89 eV, which is interpreted as excitations to the polaron band. In addition, PoPD is an efficient electron donor and hole transporter upon light excitation 41 . In the combined system with PoPD and TiO 2 , the response of light expands from 400 nm (UV) to 506 nm (visible light), which were obtained from the wavelength values corresponding to the intersection point of the vertical and horizontal portions of the spectra using hc/λ = E g , where E g is the band gap energy, h is the Planck's constant, c is the light velocity (m/s), and λ is the wavelength (nm). Therefore, because of the photosensitization effect of PoPD, the PoPD/TiO 2 nanocomposites exhibit a stronger response to visible light, and the photocatalytic activity of PoPD/TiO 2 was enhanced owing to the enhanced response to visible light.
Synergetic effect to enhance the generation of ROS. The synergetic effect between TiO 2 and PoPD on the photocatalytic degradation of MB exists in PoPD/TiO 2 because of the well-matched energy levels, and the optimum synergetic effect was observed for P/T(1/4). The energy levels of TiO 2 and PoPD are as follows: 42 . Under irradiation, the electrons are excited from the HOMO to the LUMO to the CB. Then, the holes transfer from the VB to the HOMO owing to PoPD being a hole transporter. Therefore, electrons and holes gather in the CB of TiO 2 and the HOMO of PoPD, respectively. This behavior is favorable for enhancing the quantum efficiency as the separation efficiency between the hole and electron increases. The synergetic factor (f) can be calculated based on the apparent first-order kinetics (see Supporting Information Equation 2) as = f k k C T T / , where k C/T is the first-order rate constant of PoPD/TiO 2 and k T is the first-order rate constant of TiO 2 43 .
Based on the apparent first-order kinetic constants of the degradation of MB from Table 2, the synergetic factors of the PoPD/TiO 2 nanocomposites are 0.86 for P/T(1/6), 1.10 for P/T(1/5), 1.57 for P/T(1/4), 0.95 for P/T(1/2), 0.38 for P/T(1/1), 1.00 for P/T(2/1), 1.38 for P/T(3/1), and 0.81 for P/T(4/1). Therefore, the photocatalytic activity of PoPD/TiO 2 was enhanced by increasing the quantum efficiency. Moreover, electrochemical impedance spectroscopy (EIS) as a common electrochemical method has been widely used in evaluating the interface charge transfer efficiency and separation of photogenerated electron-hole pairs over the photocatalyst. As shown in Fig. 13(a) and 13(b), it is so clearly that the radius of the arc on the EIS Nynquist plot of P/T(1/4) is smaller than that of the TiO 2 , which reflects that P/T(1/4) possesses the faster interfacial charge transfer, and the impedance on the EIS Bode plot of P/T(1/4) is smaller than that of the TiO 2 , which reflects that P/T(1/4) possesses the higher photocatalytic activity. The results of EIS test is well correspond to that of photocatalytic experiments. According to the EIS, it is indicated that the presence of PoPD in the P/T(1/4) nanocomposites is capable of improving separation efficiency and effectively inhibit the electron-hole pair recombination.

Conclusions
PoPD/TiO 2 nanocomposites were synthesized via an 'in situ' oxidative polymerization method. The modified photocatalysts were characterized by BET, XRD, TEM, FT-IR, TGA, XPS, EA and UV-Vis DRS. The results indicated that the PoPD exists on the surface of TiO 2 , the presence of PoPD does not impact on the lattice structure and grain size of TiO 2 , and the presence of PoPD enhaces the visible light response. The photocatalytic degradation of methylene blue (MB) was chosen as a model reaction to evaluate the photocatalytic activities of TiO 2 and PoPD/TiO 2 . The results indicated that PoPD/TiO 2 nanocomposites exhibited good photocatalytic activity (apparent first-order rate constants of 0.0021 min −1 for TiO 2 and 0.0033 min −1 for P/T(1/4)) and stability (recycled 5 times, 15 hours of operation). The photocatalytic activity of PoPD/TiO 2 was influenced by the initial pH and concentration of the photocatalyst. Based on radical trapping experiments and ESR measurements, the origin of oxidizing ability of PoPD/TiO 2 nanocomposites on photocatalytic degradation of MB was proposed, which taking into account of ·OH was the first important ROS and ·O 2 − was the second important ROS. The photocatalytic activity enhanced mechanism has been proposed, taking into account the photosensitization effect (UV-Vis DRS) and synergetic effect of TiO 2 with PoPD (Synergetic Factor and EIS).