Jianling Zhang, Yan Yao, Haigang Yang, Yuyan Yu, Shoubin Xu, Long Jiang,* Yi Dan*
State Key Laboratory of Polymer Materials Engineering of China (Sichuan University), Polymer Research Institute of Sichuan University, Chengdu 610065, China
Proceedings of the Nature Research Society, Volume 1, Article Number 01003, 2017
Published Online: 27 September 2017 (Article)
Strengthening the interface between conjugated polymers and TiO2 is of vital importance for promoting electron transfer and enhancing photocatalytic activity, thus providing conjugated polymers/TiO2 hybrid photocatalysts a promising future. Herein, carboxyl-functionalized P3HT was synthesized via copolymerization of 3-thiophenic acid with 3-hexylthiophene, and the resultant polymer was subsequently composited with TiO2, aiming to enhance the interfacial bonding strength through dipolar-dipolar interaction between carboxyl groups (-COOH) and the polar groups (-OH) on the surface of TiO2. Dissolution test suggested that the interfacial bonding increased with the 3-thiophenic acid content within our conditions, and photocatalytic test revealed that carboxyl-functionalized P3HT/TiO2 composites with the highest 3-thiophenic acid content (15%) exhibited the best photocatalytic performance, which is 2.2 and 5.3 times as great as that of unfunctionalized polymer/TiO2 composites and pristine TiO2 for degrading methyl orange, respectively. The significant enhancement of photocatalytic activity can be ascribed to the favored electron injection and facilitated separation of photogenerated carriers owing to the strengthened interfacial electronic coupling interaction between the carboxyl-functionalized P3HT and TiO2 according to the results of PL spectra and trapping experiments. This suggests that carboxyl-functionalization of P3HT via a facile copolymerization of 3-HT with 3-thiophenic acid might be a general and practical strategy for improving photocatalytic degradation activity in P3HT-based hybrid photocatalysts.
Hybrid composites of conjugated polymers and inorganic nanocrystals have attracted intense attention for various applications in photocatalysis, photovoltaics and other optoelectronic devices due to their versatile and synergistic optical and electronic properties [1-4]. Especially, conjugated polymers/TiO2 hybrids combining the advantages of the conjugated polymer and the inorganic semiconductor, moreover, overcoming the serious drawbacks of fast charge recombination and limited visible light absorption of TiO2 [5-7], have been used extensively as photocatalysts in water splitting [4,8,9] and pollutant degradation [10-16]. The composites not only exhibit efficient visible light activity but also possess improved ultraviolet light activity. Benefiting from its high absorption coefficients  and high mobility of charge carriers, the conjugated polymers extends the response spectrum of TiO2 to visible light region and facilitates the separation of photogenerated carriers. Meanwhile, the conjugated polymers offer several other advantages over inorganic semiconductors for rational designing photocatalytic heterojunctions, such as low cost, mechanical flexibility, and ease of fabrication. Particularly, their physicochemical properties (optical bandgap [18-20] and reactive sites) can be precisely tailed by various synthetic strategies, including end-group functionalization [21,22],side-chain engineering [23,24] and introduction of electron donor-acceptor structures into the polymer backbone [25,26], offering great potential for optimizing the photo-conversion efficiency of conjugated polymer hybrids.
As is well known, the photocatalytic activity of conjugated polymers/TiO2 photocatalysts under visible light is determined by the visible light absorption of conjugated polymers and the followed electron transfer from the excited polymers to the conduction band (CB) of TiO2 [4,27-29]. The conduction band electrons can subsequently transport to the surface of photocatalyst and react with absorbed O2 and H2O to generate O2·−and ·OH, which are the most potent oxidizing agents to degrade organic pollutants. The electron transfer on the interface between conjugated polymers and TiO2 is essential for the photocatalytic efficiency and strong interfacial interaction is favorable for the interfacial charge transfer, leading to suppressed charge recombination and enhanced photocatalytic activity [9,24,28,30]. Our very recent investigation has revealed that the photocatalytic activity of P3HT/TiO2 hybrid composites could be dramatically enhanced by binding cyanoacrylic acid end-groups functionalized P3HT to the surface of TiO2 . The enhanced interfacial bonding facilitates electron injection from P3HT to TiO2, and consequently improved the separation efficiency of photogenerated carriers and the photocatalytic activity. In consideration of the great importance of interface engineering, conjugated polymers with various reactive groups including carboxylate [18,24,31-33], phosphonate [34,35], silane [36,37] and so on have been exploited to strengthen the interface binding with their inorganic counterpart for fabrication of high performance conjugated polymers-based hybrids. However, the current methods are mostly related to tedious multi-step synthesis and convenient methods are of highly desired.
Poly(3-hexylthiophene) (P3HT) is known as one of the best choice for p-type donor materials in bulk heterojunctions, because of its high charge carrier mobility, dissolubility, processability, long-term stability and strong absorption in visible region . It has also been widely incorporated with TiO2 to form effective composite photocatalysts [11,39-43]. In the study at hand, we synthesized a new carboxyl-functionalized P3HT, where the carboxyl group was distributed in the middle of the polymer chains, by a simple oxidative copolymerization of 3-hexylthiophene and 3-thiophenic acid and the carboxyl-functionalized P3HT was subsequently composited with TiO2, resulting in strengthened interface between them, as illuminated in Scheme 1. Significant photocatalytic enhancement deriving from the strengthened interface was expected and investigated systematically. The results in this work may give a useful enlightenment for the preparation of more effective and practical conjugated polymer/TiO2 photocatalysts.
Scheme 1. Schematic diagram of carboxyl-functionalized HT/TiO2 composites.
3-Hexylthiophene was purchased from Chengdu Aike chemical technology Co., Ltd. 3-Thiophenic acid was obtained from Zhengzhou Alfa chemical Co., Ltd. Anhydrous iron(III)-chloride (FeCl3) and methyl orange (MO) were bought from Chengdu Kelong Chemical Reagents Factory while titanium dioxide was obtained from Degussa (P25, particle diameter 20 nm, surface area 50 m2/g). Chloroform (CHCl3) was purified by washing with deionized water thrice to remove traces of alcohol, and then distilled over CaCl2. All reagents were used without further purification unless stated otherwise.
1H-NMR spectra were recorded on a Bruker Avance Ⅱ-400MHz specteometer. FTIR spectra were measured on a Nicolet IS-10 spectrometer (Thermo Fisher Scientific, America), using KBr as reference sample. Fluorescence spectra were investigated on an F-4600 spectrometer (Hitachi, Japan). UV-vis absorption spectra were analyzed on a UV-2300 spectrophotometer (Hitachi, Japan) and UV-vis diffuse reflectance spectra were recorded on a UV-2600 spectrophotometer equipped with an integrating sphere attachment (Shimadzu, Japan). X-ray photoelectron spectroscopy measurements were carried out with a XSAM-800 spectrometer (KRATOS, Britain). High-resolution transmission electron microscopy was studied on a Tecnai G2 F20 electron microscopy instrument (FEI, America). The EDX analyses were performed with a scanning electron microscopy equipped with an energy-dispersive X-ray spectrometer (FEI, QUANTA250).
Synthesis and preparation
Carboxyl-functionalized P3HT was synthesized by a typical chemical oxidative copolymerization of 3-hexylthiophene and 3-thiophenic acid in CHCl3. 0.01 mol mixed monomers with different molar ratio of 3-thiophenic acid (5%, 10% and 15%) and 50 mL CHCl3 was added to a 250 three-necked, round-bottom flask equipped with a magnetic, Teflon-coated stirrer. Subsequently, 0.03 mol anhydrous FeCl3 was added to the mixture under constant stirring. The reaction was carried out in an ice bath under nitrogen for 4 h. Then, the mixture was precipitated in methanol and the resultant product was extracted in methanol to remove iron ion. The obtained copolymers were labeled as CP-5, CP-10 and CP-15 respectively. P3HT was synthesized as the same method without adding of 3-thiophenic acid as the reference, denoted as CP-0.
The composites were prepared by the following procedure: a certain amount of TiO2 particles (dried at 120 °C for 4 h before use) was added to chloroform and sonicated for 1 h to ensure totally dispersion before 20 mL of polymer chloroform solution (5 mg/mL) was added into the above suspension. The mixture was stirred and sonicated for 1 h respectively in the dark until the solvent was removed by rotary evaporation under vacuum. The composites were obtained after further dried in an oven at 40 oC under vacuum for 12 hours, labeled as CPT-0, CPT-5, CPT-10 and CPT-15 correspondingly.
Evaluation of photocatalysis
The photocatalytic performance of the composites was studied by the degradation of MO under visible light. 10 mg of photocatalyst and 20 mL of MO aqueous solution (10 mg/L) were added to a 50-mL beaker which was surrounded by circulated water to keep temperature at 25 °C. A 100W LED lamp (Zhongshan Balin Light Co., Ltd. China), used as the VL irradiation source, was positioned over the solution to maintain on irradiance of 45 mW/cm2 and a 420 nm cut-off filter was used to cut off UV light below 420 nm. The suspension was stirred in the dark for 60 min before illumination, to ensure the establishment of adsorption–desorption equilibrium between the photocatalyst and MO. A portion of the samples were then withdrawn at regular times, filtered immediately for separation of any suspended solid and kept in the dark. The change in the concentration of MO was monitored by measuring the absorbance at λmax (464 nm) with a UV-2300 UV-Vis spectrophotometer. Ammonium oxalate (AO), isopropanol (IP) and benzoquinone (BQ) were used as captures for hole (h+), hydroxyl radical (•OH) and superoxide radical (O2•-) respectively to investigate the active species generated in the photocatalytic degradation process.
Results and discussion
Characterization of carboxyl-functionalized P3HT
Given that carboxyl has been widely utilized and shown to bind efficiently to TiO2 [18,24,31-33,44], carboxyl-functionalized P3HT was synthesized by a simple oxidative copolymerization of 3-hexylthiophene and 3-thiophenic acid in CHCl3. The successful incorporation of 3-thiophenic acid is confirmed by the results of 1H-NMR spectra and FTIR spectra, as shown in Figure 1. Compared to CP-0, a small signal at 8.03 ppm belonging to the β-hydrogen atoms of the 3-thiophenic acid for CP-15 appears, indicating the existence of 3-thiophenic acid in the copolymer. However, due to its low activity of polymerization, the quantity of 3-thiophenic acid is so low that no signal is found in the 1H-NMR spectra of CPT-5 and CPT-10. As displayed in Figure 1b, the FTIR spectra of CP-5, CP-10 and CP-15 show a new signal of C=O vibration at 1650 cm-1, which is absent in CP-0, providing further evidence for the successful carboxyl-functionalization of P3HT by copolymerization.
Figure 1. (a) 1H-NMR (400 MHz) spectra of CP-0 and CP-15. (b) FTIR spectra of CP-0, CP-5, CP-10 and CP-15.
The incorporation of carboxyl groups as the side chains may disturb the conjugated polymers’ regioregularity, cause torsion in the backbone and decrease the planarity of the conjugated backbone, resulting in change of the photophysical properties. Therefore, we investigate their photophysical properties, which are of paramount importance for conjugated polymers/TiO2 photocatalysts’ ability to absorb and utilize phonons. The normalized UV-vis absorption and fluorescence mission spectra of all polymers are displayed in Figure 2. Solution of CP-0 exhibits an unstructured absorption profile with an onset at 532 nm and a maximum at 439 nm while its film shows a broadening absorption with an onset at 642 nm (corresponding to 1.93 eV) and a maximum at 503 nm, with typical characteristics of P3HT. The absorption spectra of CP-5, CP-10 and CP-15 are nearly identical to that of CP-0, exhibiting typical π–π* transition in the range of visible light from 400 nm to 700 nm, both in solutions and films. Due to enhanced polymer backbone planarity and efficient energy migration to low energy sites, the polymer films show strong red-shifted absorption and emission spectra relative to the molecular solutions. While the fluorescence emission spectra of CP-5, CP-10 and CP-15 solutions bear very strong similarity to that of CP-0, the films of CP-5, CP-10 and CP-15 show different emission spectra with blue-shifted maximum (652, 649, 648 and 646 nm for CP-0, CP-5, CP-10 and CP-15) and less structured feature relative to CP-0. It is reported that photoexcitation can induce a change of polymer conformation from a more flexible ground state to a more rigid planar excited state geometry. Therefore, PL spectra are of more structured with vibronic peaks or shoulders than absorbance spectra when polymers become planar after photoexcitation [45,46]. The blue shifted and structureless PL indicates a more coil-like conformation of carboxyl-functionalized P3HT. This is because that the carboxyl of copolymers can form hydrogen bonds, fix the polymer chain in a disordered conformation and hinder the π – π stacking, resulting in a decrease in the 0-1 emission intensity relative to the 0-0 transition [31,47,48]. However, the absorption spectra are not influenced due to the very low quantity of 3-thiophenic acid in the copolymers. Overall, the photophsical properties of CP-0, CP-5, CP-10 and CP-15 are similar and the carboxyl-functionalizations do not sacrifice the original optic properties of P3HT.
Figure 2. Normalized (a) UV-vis absorption and (b) fluorescence mission spectra of CP-0, CP-5, CP-10 and CP-15 in chloroform solutions (dashed lines) and films on quartz (solid lines).
Characterization of the P3HT/TiO2 composites
TEM analyses were employed to characterize the morphology of P3HT/TiO2 composites, as presented in Figure 3. Both TiO2 and P3HT/TiO2 composites are composed morphology of globules, with a diameter of about 20-50 nm. The distance between TiO2 lattice planes is measured to be 0.35 nm, corresponding to the interplanar spacing of anatase TiO2 (101) plane and the incorporation of P3HT has rarely effect on the crystal structure and size of TiO2. A polymer layer with the thickness of 1-2 nm can be seen on the surface of TiO2 for P3HT/TiO2 composites. The SEM images of CPT-15 composites is shown in Figure S1a and the results are in good agreement with that of TEM, showing morphology of globules with a diameter of about 20-50 nm. The polymer layer may not be seen very clearly because the very low content. Therefore, the energy dispersive X-ray (EDX) and elemental mapping analysis was measured to further elucidate the composition and morphology of the composites, especially the distribution of the polymer. Ti, O, C and S elements are all identified on the surface of P3HT/TiO2 composites, indicating the definite existence of P3HT and TiO2. The corresponding elemental mapping images (Figure S1c-f) of P3HT/TiO2 composites (Figure S1b) show that C and S elements distribute homogeneously while O and Ti elements are more intensive and almost everywhere in the view. Moreover, the FTIR spectra of the composites (Figure 3d) show not only absorption bands originating from TiO2 (680 cm-1 for Ti-O-Ti bond and 3000-3500 cm-1 for –OH groups) but also absorption bands at 2956 cm-1, 2925 cm-1 and 2855 cm-1 belonging to the C-H stretching vibration of hexyl group, which come from the polymers. Even more, the CPT-15 composites were pressed into a slice and ATR-FTIR spectra were measured in ten different spots on the slice. The ATR-FTIR spectra measured in ten different spots (Figure S2) are almost the same, implying that the composites are kind of homogeneous. These results can complement and confirm each other, verifying that the conjugated polymers were successfully incorporated with TiO2 by stacking on its surface to form P3HT/TiO2 composites and P3HT spreads universally on the surface of TiO2.
Figure 3. High-resolution TEM images of (a) TiO2 and (b) CPT-15 composites. (c) Energy dispersive X-ray (EDX) analysis of CPT-15 composites. (d) FTIR spectra of TiO2 and P3HT/TiO2 composites.
The high-resolution Ti2p XPS spectra of P3HT/TiO2 composites were analyzed to look into the interaction between P3HT and TiO2 (Figure 4a). The binding energy of Ti 2p in TiO2 can be deconvoluted into two peaks centering at 458.58 (Ti2p3/2) and 464.23 eV (Ti2p1/2) while the peaks of P3HT/TiO2 composites all shift slightly to the lower binding energy. This result indicates the charge re-distribution between the conjugated polymers and TiO2, which is beneficial to the synergistic effect of them. To verify the strengthened interface between carboxyl-functionalized P3HT and TiO2, dissolution tests were done to characterize the interfacial strength of P3HT/TiO2 composites. In a typical test, 10 mg of P3HT/TiO2 composites (CPT-0, CPT-5, CPT-10 and CPT-15) were added to 5 ml chlorobenzene respectively and the mixtures were stirred for 12 h to dissolve polymers from composites. The UV-vis absorption spectra of P3HT solutions dissolved from P3HT/TiO2 composites are shown in Figure 4b. CP-0 was the easiest to be dissolved from CPT-0, followed by CP-5 and CP-10 while CP-15 was the hardest to be dissolved from CPT-15, illustrating that the interfacial interaction strength between P3HT and TiO2 follows the order of CPT-15＞CPT-10≈CPT-5＞CPT-0. The results demonstrate that the carboxyl-functionalization of P3HT does strengthen its interface with TiO2 and the interfacial interaction strength increases with the increase of carboxyl groups. CPT-5 and CPT-10 show no apparent difference in interfacial strength maybe because that CP-5 and CP-10 bear similar composition. The strengthened interface can potentially facilitate the electron injection from excited polymers to TiO2, promote the interfacial charge-transfer efficiency and enhance the photocatalytic activity of P3HT/TiO2 composites [24,30,49].
Figure 4. (a) High resolution XPS spectra of Ti2p in TiO2 and P3HT/TiO2 composites. (b) UV-vis absorption spectra of P3HT solutions dissolved from P3HT/TiO2 composites.
Absorption of light is the prerequisite to energy conversion by photocatalysts. The UV-Vis diffuse reflectance spectra (Figure 5), exhibiting an inverse relationship between reflectivity and absorbance, where a high reflectivity corresponds to a low absorbance, of TiO2 and P3HT/TiO2 composites were measured to evaluate their ability of harvesting light. As expected, TiO2 can only absorb UV light and reflects most light with a wavelength ranging from 400 nm to 800 nm due to its wide band gap. However, for P3HT/TiO2 composites, a new absorption band in range of 400-700 nm can be observed, which can undoubtedly be attributed to the electron transition from the valence bond to the antibonding polaron state of the incorporated P3HT. The photo images of the P3HT/TiO2 composites (Figure S3) show their color of aubergine，resulting from the absorption band of 400-700 nm. In accordance with the absorption spectra of polymers (Figure 2a), the UV-Vis diffuse reflectance spectra and colors of CPT-0, CPT-5, CPT-10 and CPT-15 are very similar with each other, indicating their almost identical capabilities in harvesting solar energy.
Figure 5. UV-vis diffuse reflectance spectra of TiO2 and P3HT/TiO2 composites.
The photocatalytic performance of TiO2 and P3HT/TiO2 composites CPT-0, CPT-5, CPT-10 and CPT-15 was evaluated by using MO as target contaminant. The degradation ratio of MO was calculated as (1-Ct/C0)×100%, where C0 is the concentration of MO after adsorption equilibrium, Ct is the concentration of MO at time t during the photocatalytic experiments. As displayed in Figure 6a, the degradation ratio of MO over TiO2, CPT-0, CPT-5, CPT-10 and CPT-15 is 25%, 45 %, 42%, 51% and 75% respectively after 180 min of visible light irradiation. TiO2 can photocatalytic degrade MO in a certain degree under visible light because of its mixed-phase structure while P3HT/TiO2 composites show more effective photocatalytic activity. CPT-5 and CPT-10 exhibit no much difference in photocatalytic performance of degrading MO with CPT-0 while CPT-15 exhibits significant photocatalysis enhancement. The inset UV-vis absorption spectra of MO with different irradiation time in the presence of CPT-15 show not only decrease in the absorbance but also an blue-shift of the maximum absorption peak, confirming the degradation of MO molecules. To give quantifiable proof, the degradation of MO is found to accord with pseudo-first-order kinetics by linear transforms of ln(C0/Ct) = kt, where C0’ is the concentration of MO after adsorption, Ct is the concentration of MO at time t and k is kinetic constant. The obtained degradation rate constants of MO over TiO2 and P3HT/TiO2 composites are presented in Figure 6b. The rate constants of MO degradation over TiO2, CPT-0, CPT-5, CPT-10 and CPT-15 are 0.0014, 0.0033, 0.0027, 0.0038 and 0.0074 min-1 respectively, indicating the advantage of conjugated polymers/TiO2 hybrids for the degradation of organic pollutants under visible light. Although CPT-5 and CPT-10 show comparative photocatalytic activity with CP-0, CPT-15 exhibits significantly improved photocatalytic activity, with degradation rate about 2.2 and 5.3 times as that of CPT-0 and TiO2. In the case of CPT-15, the obviously enhanced photocatalytic performance should drive from the strengthened interface between carboxyl-functionalized P3HT and TiO2 (as demonstrated by the results of dissolution test) and the resulted favorable interfacial electronic coupling interaction. Based on the above results, it is quite spontaneous and reasonable for us to suppose that the incorporation of more carboxyl groups in P3HT than that of CP-15 will further enhance the photocatalytic performance of P3HT/TiO2 composites. Unfortunately, the obtained polymer was insoluble when we attempt to introduce 20 mol% 3-thiophenic acid to the comonomer to prepare CP-20. This should be induced by too much introduction of 3-thiophenic acid, whose structure is quite rigid without flexible alkyl chain. Therefore, it was not possible to prepare corresponding P3HT/TiO2 composites (CPT-20) by the current dissolving-blending method and further investigate the influence of the carboxyl groups content on their photocatalytic performance.
Figure 6. (a) Degradation ratio of MO vs. irradiation time under visible light in the presence of TiO2 and P3HT/TiO2 composites. Inset is the UV-vis absorption spectra of MO with different irradiation time in the presence of CPT-15. (b) The rate constants of MO degradation over TiO2 and P3HT/TiO2 composites.
Mechanism of photocatalytic activity enhancement
Photoluminescence (PL) spectra originating from the recombination of free charge carries was utilized to characterize the processes of charge migration, transfer and separation of the composites and look into their correlation of energy levels. The PL spectrum of TiO2 (Figure 7a) shows a broad asymmetric band with a maximum intensity at 392 nm (corresponding to 3.16 eV), which is attributed to the radiative recombination of free exciton charges. .The emission spectra of CP-0, CP-5, CP-10 and CP-15 composites are quite quenched, indicating that the process of radiative charge recombination is greatly suppressed in P3HT/TiO2 composites with the localization of electrons in TiO2 and holes in P3HT on the other side by the band offsets. The lower emission intensity of CPT-5, CPT-10 and CPT-15 compared to that of CPT-0 should imply their stronger charge localization resulting from the carboxyl-functionalization of P3HT. PL spectra of P3HT/TiO2 composites under 400 nm excitation are presented in Figure 7b. Two distinct emission peaks centering at 470 and 555 nm, which are absent for pristine conjugated polymers are observed while the emission pecks of pure conjugated polymers at 650 and 708 nm (Figure 2b) almost disappear, suggesting the strong charge communication between P3HT and TiO2. The first band at 470 nm (2.64 eV) presumably originates from emission of the localized states at P3HT-TiO2 interface to the valence band (VB) of TiO2. The second band at 555（2.23 eV）can be ascribed to an emission process from electrons in the π orbital of the P3HT to the VB of the TiO2, indicating the forming of an electronically active interface between P3HT and TiO2. In consideration of the bandgaps of P3HT and TiO2, the energy level of the lowest unoccupied molecular orbital (LUMO) for P3HT is calculated as 1.00 eV above the conduction band (CB) of TiO2, agreeing with the previous reports.  The relative positions of the electronic energy levels of P3HT and TiO2 make it possible for the electron injection from LUMO of P3HT to the CB of TiO2. This can be further confirmed by the PL quenching of pristine P3HT because it provides an alternative pathway for relaxation of excited electrons and suppresses the irradiative emission of the excitons. To get insight into the mechanism of their photocatalysis for degrading pollutants under visible light, the positions of the electronic energy levels of P3HT and TiO2 are figured out and illustrated in Figure 7f for the CB potential of TiO2 is reported as -0.5 eV vs NHE . When P3HT is excited by visible light irradiation, electrons are simultaneously excited from the highest occupied molecular orbital (HOMO) into LUMO, followed by the electron transfer from LUMO of P3HT to CB of TiO2. Since the CB potential of TiO2 is more negative than the standard redox potential E0(O2/O2·−) (-0.16 eV vs. NHE) , the accumulated electrons in the CB of TiO2 can react readily with dissolved O2 to produce O2·−. Nevertheless, the photoinduced holes of P3HT cannot oxidize the adsorbed H2O or –OH to ·OH because its more negative HOMO potential relative to the standard redox potential E0(·OH/H2O) (2.77 eV vs. NHE) and E0(·OH /OH-) (1.99 eV vs. NHE). The O2·− can react with H2O to generate H2O2, which can be further reduced to·OH. The obtained radicals (O2·− and ·OH) bear strong oxidizing power and can lead to complete mineralization of organic pollutants. Meanwhile, the photoinduced holes of P3HT (h+) can oxide the adsorbed pollutant molecules directly.
Figure 7. PL spectra of P3HT/TiO2 composites under (a) 270 nm and (b) 400 nm excitation. Photocatalytic degradation curves of MO in the presence of (c) CPT-0 and (d) CPT-15 with different scavengers. (e) The rate constants of MO degradation over CPT-0 and CPT-15 with the addition of scavengers. (f) Schematic diagram of photocatalytic mechanism for P3HT/TiO2 composites under visible light.
The trapping experiments of active species for CPT-0 and CPT-15 were performed by using isopropanol (IP) as a hydroxyl radical scavenger, ammonium oxalate (AO) as a hole scavenger and benzoquinone (BQ) as a superoxide radical scavenger  to further confirm the mechanism. As displayed in Figure 7c-e, both CPT-0 and CPT-15 have almost lost their abilities to photocatalytic degrade MO by the addition of the hole capture agent (AO) and superoxide radical scavenger (BQ), indicating that h+ and O2·− are the main oxidative species for P3HT/TiO2 composites. Hydroxyl radical also plays definite role in the photocatalytic degradation of MO for P3HT/TiO2 composites because CPT-0 and CPT-15 show partly decrease in photocatalytic activity after the addition of hydroxyl radical scavenger (IP). Given the fact that ·OH cannot be generated by the oxidization of adsorbed H2O or –OH, the hydroxyl radical must be produced by the further reactions of superoxide radical. CPT-15 shows much more decrease in the photocatalytic efficiency for the rate constant of MO degradation over CPT-15 and CPT-0 decreases by 51.4% and 27.3%, respectively. This may be explained as follows: the carboxyl groups of CP-15 lead to strong interfacial interaction with TiO2 due to multiple dipolar-dipolar and hydrogen bonding interactions between –COOH and –OH on the surface of TiO2. The strong interfacial electronic coupling interaction between them will facilitate the electron injection from LUMO of P3HT to CB of TiO2. As a result, more superoxide radicals can be obtained by the reduction of O2, leading to the generation of more hydroxyl radicals. In summary, as illustrated in Fig 7f, an electronically active interface can generate between P3HT and TiO2 for P3HT/TiO2 composites. After excited by visible light, the electrons in the LUMO of P3HT can be injected to the CB of TiO2 and subsequently are transported to the surface of the photocatalyst to react with adsorbed O2 and H2O to generate O2·−and ·OH, who can oxidize the pollutant molecules together with the holes remained on the HOMO of P3HT. The strengthened interface between carboxyl-functionalization P3HT and TiO2 can facilitate the interfacial electron injection and promote the separation of photogenerated carriers, resulting in the significant photocatalytic enhancement of CPT-15.
Carboxyl-functionalized P3HT was synthesized by a simple oxidative copolymerization of 3-hexylthiophene and 3-thiophenic acid without sacrificing the original optic properties of the conjugated polymer. In spite of the very low quantity of 3-thiophenic acid due to its low activity, P3HT/TiO2 composites with strengthened interface were obtained by hybridizing carboxyl-functionalized P3HT with TiO2. When the feed ratio of 3-thiophenic acid reached 15%, the carboxyl of the corresponding copolymer (CP-15) could strengthen the electronically active interface with TiO2 most effectively, resulting in P3HT/TiO2 composites (CPT-15) with significantly enhanced visible light photocatalytic activity. The generating of electronically active interface between P3HT and TiO2 and their relative positions of the electronic energy levels were figured out by the results of PL spectra. And the proposed possible interfacial reactions accordingly were demonstrated by the results of trapping experiments. By analyzing the results of PL spectra and trapping experiments, the enhancement of photocatalytic activity can be ascribed to the favored electron injection and facilitated separation of photogenerated carriers owing to the strengthened interfacial interaction between the carboxyl-functionalized P3HT and TiO2. This work provides a facile but robust interfacial engineering strategy for constructing P3HT-based hybrid materials with broad light adsorption and high efficiency.
The authors are grateful to the National Natural Science Foundation of China (Grant No. 51573109) for the support of this research.
The authors declare no competing financial interest.
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