Solid-State Synthesis and Photocatalytic Activity of Polyterthiophene Derivatives/TiO2 Nanocomposites

Poly(3,4-propylenedioxy-2,2′:5′,2″-terthiophene)/TiO2 and poly(3,4-(2,2-dimethylenepropylenedioxy)-2,2′:5′,2″-terthiophene)/TiO2 nanocomposites were synthesized by a simple solid-state method. Additionally, the poly(3,4-propylenedioxy thiophene)/TiO2 and poly(3,4-2,2-dimethylenepropylenedioxythiophene)/TiO2 nanocomposites were synthesized in a similar manner for comparison. The structure and morphology were characterized by Fourier transform infrared (FTIR), ultraviolet-visible (UV-Vis) absorption spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM). The photocatalytic activities of the nanocomposites were examined through the degradation processes of a methylene blue (MB) solution under UV light and sunlight irradiation. The results of FTIR and UV-Vis spectra showed that the composites were successfully synthesized by solid-state method and the poly(3,4-propylenedioxy-2,2′:5′,2″-terthiophene)/TiO2 and poly(3,4-(2,2-dimethylenepropylenedioxy)-2,2′:5′,2″-terthiophene)/TiO2 nanocomposite had a higher oxidation degree and conjugation length than others. The results also indicated that the TiO2 had no effect on the crystallinity of composites, but was well embedded in the polymer matrix. Additionally, the highest degradation efficiency of 90.5% occurred in the case of the poly(3,4-propylenedioxy-2,2′:5′,2″-terthiophene)/TiO2 nanocomposite.


Introduction
The use of semiconducting materials in environmental decontamination as photocatalysts has attracted a great deal of attention in recent years [1]. When semiconducting materials are exposed to light with energy larger than their band gaps, electron-hole pairs can be created, which can initiate photocatalytic reactions [2]. Most photocatalytic systems are based on TiO 2 , which is chemically stable and has a long life-time of electron-hole pairs generated by optical excitation. However, the band gap of TiO 2 is ~3.2 eV, which is in the ultraviolet regime, and therefore, visible light, which comprises most of the solar light, cannot be absorbed by TiO 2 [3]. To overcome this problem, some strategies have been investigated, including noble metal deposition, the doping of metal or nonmetal ions, mixing with another metal oxide, surface photosensitization with dye and preparing composites with a conducting polymer [4][5][6]. Conducting polymers have attracted considerable attention, because of their interesting semiconducting, electronic and optical properties [7]. Among conducting polymers, polyaniline and polythiophene are widely used for the fabricating of conducting polymer/TiO 2 hybrid materials [8,9]. Polythiophene and its derivatives show many advantages in combining with TiO 2 , and the large internal interface area in the polythiophene/TiO 2 composite enables an efficient separation of charge, which is very important for photovoltaic application [10,11]. At the present time, numbers of reports have been published on the preparation of polythiophene and its derivatives/TiO 2 composites, including original in situ photopolymerization [12], electrochemical polymerization [13] and the chemical solution method [14]. Although polythiophene has become the focus of considerable interest, polythiophene suffers from the occurrence of undesired α,β-and β,β'-couplings during polymerization, which deteriorate their electronic and optical properties [15,16]. As derivatives of polythiophene, polyterthiophene-type conjugated polymers have rapidly gained considerable attention, due to the preexistence of α,α'-linkages in their monomers, which makes the whole polyterthiophene-type chain grow regularly and leads to very interesting electronic, electrochromic and optical properties [17][18][19]. Additionally, they can be prepared by electrochemical and chemical methods. Recently, we have demonstrated a novel room-temperature solid-state oxidative method for the polymerization of ethylenedioxy-substituted terthiophene, and we have found that solid-state polymerization was an effective method for the polymerization of the terthiophene-type monomer [20].

Ultraviolet-Visible (UV-Vis) Absorption Spectra
The UV-Vis spectra of solid-state synthesized PProDOT/TiO 2 , PProDOT-Me 2 /TiO 2 , poly(TPT)/TiO 2 and poly(TMPT)/TiO 2 in N-methylpyrrolidone(NMP) are shown in Figure 2. As shown in Figure 2, PProDOT/TiO 2 displays a broad peak raging from ~430 nm to 610 nm with several shoulders at ~430, ~450, ~500, ~560 and ~610 nm, while the PProDOT-Me 2 /TiO 2 , poly(TPT)/TiO 2 and poly(TMPT)/TiO 2 display relatively sharp peaks raging from ~460 nm to ~610 nm with several shoulders at ~460, ~500, ~554 and ~610 nm. The absorption peaks and shoulders at ~430-560 nm are assigned to the π → π* transition of the thiophene ring [30][31][32][33], while the peaks at ~610 nm are the p-doping peaks of the polythiophene molecular chains [34]. Comparing with the absorption spectra of others, more shoulders appear in the PProDOT/TiO 2 nanocomposite, which can be considered as the absorption peaks arising from conjugated segments having different conjugation lengths [35]. Furthermore, the peak of the PProDOT-Me 2 /TiO 2 nanocomposite at ~500 nm is red shifted to ~523 nm in the poly(TMPT)/TiO 2 nanocomposite. Generally, the red shift of the absorption spectrum shows the increase of the conjugated chain length [36]. Moreover, the peak of poly(TPT)/TiO 2 appears at a relatively lower wavelength compared to the poly(TMPT)/TiO 2 , which is mostly originated from the strong interaction between the TiO 2 particles and the poly(TMPT) [37].    Figure 3 shows the XRD patterns of solid-state synthesized PProDOT/TiO 2 , PProDOT-Me 2 /TiO 2 , poly(TPT)/TiO 2 , poly(TMPT)/TiO 2 and nano-TiO 2 , respectively. As seen from Figure 3, the composites possess rather broad diffraction peaks, suggesting a small degree of crystallinity, but an amorphous structure, which is similar to other thiophene derivatives [38], and its diffraction peaks are located at about 2θ ~ 24.4° (associated with the intermolecular π → π* stacking or assigned to the (020) reflection) [39] and 2θ ~ 37.5° (assigned to the (111) reflection [40,41]). The XRD patterns of composites shows the presence of the characteristic diffraction peaks of rutile TiO 2 (~27.6°, 36.1°, 41.3° 54.4°, 56.7° and 69.5°), suggesting the successful incorporation of rutile TiO 2 . Figure 3 implies that the poly(TMPT)/TiO 2 nanocomposite has lower crystallinity than others. This can be associated with the introduction of dimethyl substituents in the PProDOT or poly(TPT/TiO 2 ), which may decrease the crystallinity with its separation effect on the polymer main chains [42]. However, the intensity of the characteristic diffraction peaks of TiO 2 is lower in the case of the PProDOT/TiO 2 and poly(TPT)/TiO 2 nanocomposites than that of PProDOT-Me 2 /TiO 2 and poly(TMPT)/TiO 2 , suggesting that the TiO 2 particles are uniformly embedded in the polymer matrix of PProDOT/TiO 2 and poly(TPT)/TiO 2 [43]. The energy dispersive X-ray spectroscopy (EDX) analysis on each composite is presented in Figure 4. As shown in Figure 4, the elements of C, O, S, Cl, Ti and Fe are observed. Additionally, the quantitative analysis of the result of EDX indicates that the wt% of Ti in each nanocomposite is 1.55%, 2.20%, 3.34% and 3.12%, respectively, which implies that poly(TPT)/TiO 2 and poly(TMPT)/TiO 2 have a higher wt% of TiO 2 than that of PProDOT/TiO 2 and PProDOT-Me 2 /TiO 2 .  Figure 5 represents the transmission electron microscopy (TEM) images of solid-state synthesized PProDOT/TiO 2 , PProDOT-Me 2 /TiO 2 , poly(TPT)/TiO 2 and poly(TMPT)/TiO 2 . As shown in Figure 5, all the nanocomposites display a sponge-like morphology, except from the PProDOT/TiO 2 nanocomposite. Additionally, the TiO 2 particles (dark-shaded nanoparticles) are found to be entrapped in polymer (light shaded) matrix. These results reveal that the TiO 2 particles are not simply mixed up or blended with the polymer, suggesting that the TiO 2 particles are embedded in the polymer matrix, which is quite in accordance with the results of the XRD analysis.  Figure 6 shows the photocatalytic degradation of methylene blue (MB) dye with the presence of the PProDOT/TiO 2 , PProDOT-Me 2 /TiO 2 , poly(TPT)/TiO 2 and poly(TMPT)/TiO 2 nanocomposites and pure nano-TiO 2 as catalysts under UV light at different irradiations time, while Figure 7 indicates the photocatalytic degradation of MB dye in the presence of the above composites and the pure nano-TiO 2 as the catalyst under natural sunlight at different irradiations time. Generally, the adsorption/desorption equilibrium is an important preliminary step in the photocatalytic degradation process. Therefore, prior to irradiation, the MB solution was magnetically stirred in the dark for 30 min to ensure the establishment of the adsorption/desorption equilibrium. As can be seen in Figures 6 and 7, the diminished characteristic band of MB dye at ~660 nm after 7 h under UV light and natural sunlight indicates that the MB has been degraded by all the nanocomposites and pure nano-TiO 2 . Additionally, the degradation efficiency of each nanocomposite is 32.5%, 29.6%, 51.5% and 44.4% under UV light for PProDOT/TiO 2 , PProDOT-Me 2 /TiO 2 , poly(TPT)/TiO 2 and poly(TMPT)/TiO 2 , respectively, while the degradation efficiency of each nanocomposite is 79.6%, 62.8%, 90.5% and 84.6% under natural sunlight for PProDOT /TiO 2 , PProDOT-Me 2 /TiO 2 , poly(TPT)/TiO 2 and poly(TMPT)/TiO 2 , respectively. In addition, the degradation efficiency of nano-TiO 2 under UV light and natural sunlight is 27% and 11.6%, respectively. It is easy to see that the degradation efficiency of the MB in the presence of poly(TPT)/TiO 2 and poly(TMPT)/TiO 2 is higher than that of PProDOT/TiO 2 and PProDOT-Me 2 /TiO 2 . Meanwhile, the degradation efficiency increased up to two and three times compared to PProDOT/TiO 2 and PProDOT-Me 2 /TiO 2 , respectively, under the same conditions, which suggests that the poly(TPT)/TiO 2 and poly(TMPT)/TiO 2 nanocomposites are more effective photocatalysts for the degradation of MB than the PProDOT/TiO 2 and PProDOT-Me 2 /TiO 2 nanocomposites.

Photocatalytic Properties
Based on previous reports, the photocatalytic degradation of MB under UV or visible light irradiation is mainly related to the generation of reactive hydroxy and hydroperoxy radicals [44][45][46]. Herrmann et al. investigated the TiO 2 /UV photocatalytic degradation of MB in aqueous heterogeneous suspensions, and a detailed degradation pathway was determined by a careful identification of intermediate products, in particular aromatics, whose successive hydroxylations led to the aromatic ring opening. Furthermore, they found that TiO 2 /UV-based photocatalysis was simultaneously able to oxidize the MB with an almost complete mineralization of carbon and of nitrogen and sulfur heteroatoms into CO 2 , NH 4 + , NO 3 − and SO 4 2− , respectively [47]. According to the reports, the hydroxy and hydroperoxy radicals produced by the action of photocatalyst are capable of oxidizing MB molecules at the surface layer of photocatalyst. MB molecules tend to be adsorbed by the surface of the catalyst. This process can be enhanced by controlling the surface charge of the photocatalyst. Moreover, MB is a cationic dye, and a negatively charged photocatalyst surface accelerates the adsorption of the MB and, consequently, the photo degradation process [44,45].
As can be seen in Figure 7, almost complete degradation of the MB has been achieved 7 h under natural light irradiation using nanocomposites as the photocatalyst, except from the PProDOT-Me 2 /TiO 2 nanocomposites. In addition, the degradation efficiency of both poly(TPT)/TiO 2 and poly(TMPT)/TiO 2 are higher than that of PProDOT/TiO 2 and PProDOT-Me 2 /TiO 2 . To summarize, the poly(TPT)/TiO 2 and poly(TMPT)/TiO 2 nanocomposites had higher degradation efficiencies than the PProDOT/TiO 2 and PProDOT-Me 2 /TiO 2 nanocomposites under both light sources. Moreover, the degradation efficiency of all nanocomposites under sunlight are much higher than under UV light, which indicates that the nanocomposites are more effective photocatalysts for the degradation of MB under the higher intensity of sunlight irradiation than UV irradiation.   Figure 8 shows the schematic mechanism of MB dye degradation to explain the photocatalytic activity of the nanocomposite catalyst under sunlight. According to the previous report, TiO 2 particles can absorb UV light to create electrons (e − ) in the conduction band and holes (h + ) in the valence band [48], respectively. If the electrons and holes cannot be captured in time, they will recombine with each other within a few nanoseconds, which will reduce the photocatalytic efficiency of TiO 2 . However, in the case of composites, due to the existence of the interface between polymer and TiO 2 , separated electrons and holes have little possibility to recombine again. This ensures higher charge separation efficiency and better photo-oxidation capacity for the nanocomposite. In addition, the polymer can absorb the visible light and produces an electron (e − ) that transfers to the conduction band of TiO 2 [48,49]. The amount of • OH and O 2 •− formed in the case of composites is more than that with TiO 2 alone as a photocatalyst. Moreover, MB molecules can transfer from solution to the catalyst's surface and be adsorbed with an offset face-to-face orientation via π-π conjugation between MB and aromatic regions of the polymer, and therefore, the adsorptivity of MB on polymer increases compared to that of MB on bare TiO 2 , which makes the composites have a higher efficiency in the photodegradation of MB compared to TiO 2 alone as a photocatalyst.

Preparation of Composites
A typical solid-state synthesis procedure of the composite was as follows: a mixture of 0.3 g (0.9 mmol) 3,4-propylenedioxy-2,2':5',2"-terthiophene (TPT) and 15 mg TiO 2 in 3 mL chloroform were ultrasonicated for 30 min to facilitate the monomer in adsorbing onto the surface of TiO 2 . After ultrasonication, the mixture was allowed to evaporate the chloroform, then the mixture was put in a mortar, and 0.61 g (3.6 mmol) anhydrous iron(III) chloride (FeCl 3 ) were added to this mixture, then ground for 1 h. Then, the mixture was washed with chloroform, ethanol and distilled water, respectively, until the filtrate was colorless, finally drying the powder under vacuum at 60 °C for 48 h.

Structure Characterization
The Fourier transform infrared (FTIR) spectra of the composite were obtained by using a BRUKERQEUINOX-55 Fourier transform infrared spectrometer (Billerica, MA, USA) (frequency range: 4000-500 cm −1 ). The UV-Vis spectra and photocatalytic activity of the samples were recorded on a UV-Visible spectrophotometer (UV4802, Unico, Franksville, WI, USA). X-ray powder diffraction (XRD) patterns were obtained by using a Bruker AXS D8 diffractometer (Bruker AXS Co., Karlsruhe, Germany), with a monochromatic Cu-Ka radiation source (λ = 0.15418). The scan range (2θ) was 5°-80°. Transmission electron microscopy (TEM) experiments were carried out in a Hitachi 2600 electron microscope (Tokyo, Japan). The samples for TEM measurements were prepared by placing a few drops of sample ethanol suspension on copper supports.

Measurement of Photocatalytic Activities
The photocatalytic activities of nanocomposites were performed using MB as degraded materials in quartz tubes under UV light and natural sunlight irradiation. FSL MW1-Y15 (Royal Dutch Philips Electronics Ltd., Amsterdam, The Netherlands) was used as the irradiation source (λ = 254 nm) located in a light infiltrated chamber. Before irradiation, the suspension was stirred magnetically for 30 min in dark conditions until an adsorption-desorption equilibrium was established. Then, the suspensions were irradiated by light sources. Under natural sunlight investigations, all experiments were done inside a laboratory in an open atmosphere in the month of September. The photodegradation efficiency (R, %) was calculated by use of the following equation: R = [C 0 − C/C 0 ] (where C 0 represents the concentration of the dye before illumination and C denotes the concentration of dye after a certain irradiation time, respectively).

Conclusions
In this paper, the polyterthiophene derivatives/TiO 2 nanocomposites, such as poly(3,4propylenedioxy-2,2':5',2"-terthiophene)/TiO 2 nanocomposite (poly(TPT)/TiO 2 ) and poly(3,4-(2,2dimethylenepropylenedioxy)-2,2':5',2"-terthiophene)/TiO 2 nanocomposite (poly(TMPT)/TiO 2 ), were synthesized by a simple solid-state method. The poly(3,4-propylenedioxythiophene)/TiO 2 nanocomposite (PProDOT/TiO 2 ) and poly(3,4-2,2-dimethylenepropylenedioxythiophene)/TiO 2 nanocomposite (PProDOT-Me 2 /TiO 2 ) were synthesized by the same method for comparison. The results showed that the higher oxidation degree and the strong interaction between TiO 2 and polymer occurred in the case of poly(TPT)/TiO 2 and poly(TMPT)/TiO 2 compared with that of PProDOT/TiO 2 and PProDOT-Me 2 /TiO 2 . This phenomenon mainly resulted from the linear growth tendency of terthiophene-type monomers, which made the whole polyterthiophene-type chain grow regularly and led to an enhancement of the electronic and optical properties of the nanocomposites. The results also indicated that TiO 2 had no effect on the crystallinity of the polymer, but TiO 2 can be embedded in the polymer matrix, which implied that the solid-state method can be an effective method for preparing such composite materials. Furthermore, the UV-Vis photodegradation efficiency of MB by these composites showed that the poly(TPT)/TiO 2 and poly(TMPT)/TiO 2 nanocomposites had higher degradation efficiency than that of the PProDOT/TiO 2 and PProDOT-Me 2 /TiO 2 nanocomposites, which resulted from the positive effect of the higher oxidation degree of the polyterthiophene derivatives/TiO 2 nanocomposites, as well as the strong interaction between TiO 2 and the polyterthiophene derivatives than that of PProDOT/TiO 2 and PProDOT-Me 2 /TiO 2 . The photodegradation studies suggested that the polyterthiophene derivatives/TiO 2 nanocomposites were more effective photocatalysts for the degradation of MB under UV and sunlight irradiation. All nanocomposites were more effective photocatalysts as compared to bare TiO 2 , because the polymers acted as light-harvesting species, and the polymers displayed a positive role in charge separation, except the adsorption effect of polymers for MB via π-π conjugation between MB and aromatic regions of the polymer.