Immobilization of a TiO2–PEDOT:PSS hybrid heterojunction photocatalyst for degradation of organic effluents

Heterojunction photocatalysts have recently emerged for use in degradation of organic pollutants, typically being suspended in effluent solution to degrade it. Post degradation, the catalyst must be removed from the treated solution, which consumes both energy and time. Moreover, the separation of nano catalysts from the treated solution is challenging. In the present work, we explore fabrication of immobilized TiO2–PEDOT:PSS hybrid heterojunction catalysts with the support of a PVA (polyvinyl alcohol) matrix. These photocatalytic films do not require any steps to separate the powdered catalyst from the treated water. While the PVA-based films are unstable in water, their stability could be significantly enhanced by heat treatment, enabling efficient removal of organic effluents like methylene blue (MB) and bisphenol-A (BPA) from the aqueous solution under simulated sunlight irradiation. Over 20 cycles, the heterojunction photocatalyst maintained high photocatalytic activity and showed excellent stability. Hence, an immobilization of the TiO2–PEDOT:PSS hybrid heterojunction is suggested to be beneficial from the viewpoint of reproducible and recyclable materials for simple and efficient wastewater treatment.


Introduction
2][3][4][5] In conventional wastewater treatment processes, TiO 2 nanoparticles (NPs) are directly dispersed in the effluent solution, providing a high catalyst surface area which reacts with the pollutant. 6However, this mechanism possesses some practical technical challenges.Separation of TiO 2 nanoparticles from the solution post-treatment remains a major obstacle, because of the required time, energy, and high cost, which limits the conventional photocatalytic degradation process for practical usage. 6,7For industrial applications, the photocatalysts are expected to be recyclable, for use in multiple cycles and with no loss of TiO 2 nanoparticles during the long-term process of recycling. 8Immobilizing TiO 2 nanoparticles on a variety of substrates is a promising method to overcome the current limitations. 9,10Furthermore, immobilization of the catalyst with a polymer matrix support could be advantageous over the other methods, since hydroxyl groups or carboxyl groups on the polymers may form chemical bonding with the hydroxyl groups on the surface of the TiO 2 nanoparticles. 112][13][14][15][16] Polyvinyl alcohol (PVA) is a hydrophilic polymer with good exibility and low cost and could also be potential candidate to immobilize TiO 2 . 17,18][24][25][26] The wide bandgap of TiO 2 (∼3.2 eV) poses another limitation to its absorption capability of incident light.8][29][30][31] Forming a heterojunction with another lower bandgap semiconductor could be one of the alternatives to reduce the effective bandgap, so that it can absorb more sunlight. 32A heterojunction between two semiconductors with matching electronic band structures can signicantly reduce the effective bandgap and enhance the separation of photogenerated electrons (e − ) and holes (h + ), thereby considerably improving the photocatalytic performance. 32,33The organic conjugated polymer PEDOT:PSS (poly(3,4ethylenedioxythiophene:polystyrene sulfonate)) which has been used as hole transport layer or as p-type material in organic electronics, 34,35 may serve as a potential candidate for heterojunction semiconductors.PEDOT:PSS provides a good hole transport layer, with a low band gap (∼1.6 eV). 36It is thus expected that PEDOT:PSS (p-type) and TiO 2 (n-type) can form a hybrid heterojunction due to their electronic band structure compatibility. 37The photoexcited electrons and holes in TiO 2 -PEDOT:PSS hybrid heterojunction can be localized at one semiconductor having more cathodic conduction band (CB) level and the other semiconductor possessing more anodic valence band (VB) level respectively.It provides Z-scheme charge transfer through heterostructures with superior reducing and oxidizing ability for enhancing the photocatalytic degradation performance. 38n this work, we demonstrate the fabrication of a TiO 2 -PEDOT:PSS hybrid heterojunction photocatalyst, to enhance the optical and electronic properties of an immobilized catalyst with the support of a PVA matrix.The photocatalytic performance of as fabricated TiO 2 -PEDOT:PSS and bare TiO 2 hybrid lms was investigated by degrading organic effluents methylene blue (MB) and bisphenol-A (BPA) under simulated sunlight.

Experimental
2.1 Preparation of TiO 2 -PEDOT:PSS hybrid heterojunction lms TiO 2 (P25) nanoparticles (100 mg) were initially dispersed in 10 ml of DI water and ultrasonicated for 2 h 1 ml of PEDOT:PSS (Sigma Aldrich-1.3%)was mixed with the TiO 2 /H 2 O suspension aer rst being stirred magnetically at room temperature for 3 h 1 g of PVA was subsequently added into the TiO 2 -PEDOT:PSS solution under continuous stirring at 90 °C for 1 h, followed by stirring at 60 °C for 2 h.The solution was le in the beaker to avoid air bubbles and bring it down to room temperature.The PVA/TiO 2 -PEDOT:PSS solution was casted on Petri dishes and le overnight.For comparative purposes, bare PVA/TiO 2 and PVA/PEDOT:PSS lms were also prepared following the same procedure.The as-prepared dried lms were heated at an optimized temperature of 140 °C for 2 h in vacuum.The experimental conditions like TiO 2 to PVA weight ratio and thermal treatment temperature were systematically varied.It was found that 1 : 10 and 140 °C (2 h) were optimal conditions for fabricating stable and efficient hybrid heterojunction lms.Furthermore, aer identifying these optimal conditions, PEDOT:PSS Sigma Aldrich (1.3%) content was varied from 0.5 ml to 3 ml in 100 mg/10 ml TiO 2 -PVA suspension.We found that 1 ml of PEDOT:PSS in 10 ml of TiO 2 -PVA suspension was optimal for the formation of stable TiO 2 -PEDOT:PSS lms, and showed better photocatalytic performance over other compositions.PVA/PEDOT:PSS, PVA/TiO 2 and PVA/TiO 2 -PEDOT:PSS lms were designated as P, T and PT respectively.Pictures of the as-prepared lms are shown in Fig. 1.T-lm appears in white colour and P-lm appears in dark blue colour.Post mixing different weight percentages of TiO 2 and PEDOT:PSS in order to form PT heterostructure, the colour of the heterostructure changes to whitish blue.Here the colours T and PT lms are from precursor solution.

Characterization
The surface morphology and cross-sectional thickness of the as prepared lms were examined using scanning electron microscopy (SEM, Jeol, 20 kV).The structural properties of the P, T and PT lms were analyzed by X-ray diffraction (XRD, Bruker D2 X-ray Diffractometer, Cu Ka source, 2q = 10-80°).The chemical structure and interaction between the PVA, TiO 2 and PEDOT:PSS components were conrmed using Fourier Transfer Infra-Red Spectroscopy (FTIR, JASCO-5300 FT-IR spectrometer, within the range 200 to 4000 cm −1 ) and Raman spectroscopy (LabRAM HR Evolution, HORIBA, excitation wavelengths of 532 nm).During the photocatalytic degradation of the organic effluents, the effluent concentration was analyzed using UVvisible spectroscopy (Jasco V-730).

Photocatalytic degradation experiments
The photocatalytic degradation of methylene blue (MB) and bisphenol-A (BPA) was studied using PVA/TiO 2 and PVA/TiO 2 -PEDOT:PSS hybrid lms under simulated sunlight at standard test conditions (STC) (300 W, Xe-lamp source with 1000 W m −2 (1 sun) irradiation, AM1.5).Initially, 10 ppm of MB dye and BPA solutions were prepared, and 1.5 cm × 1.5 cm lms were dipped into 20 ml of effluent solution and kept in the dark for 1 h to achieve an adsorption-desorption equilibrium.At every 1 h time interval, 10 ml of effluent solution was collected.The concentration of MB and BPA was then detected by measuring the maximum absorbance at 664 nm and 275 nm respectively, using UV-visible absorption spectroscopy (Jasco, V-730).The hybrid lm aer the degradation was washed and dried before another photodegradation test.The reusability of the photocatalytic lm was tested for 20 cycles.

Results and discussion
Fig. 2(a-f) shows the surface morphology and corresponding cross-sectional images of the P, T and PT lms.The surface morphology of the P lm shows a uniform dispersion of PEDOT:PSS and the corresponding thickness of lm is ∼14 mm.An aggregation of TiO 2 NPs is observed in T and PT lms with a cross sectional thickness of ∼12 mm and ∼17 mm respectively.All lms have a uniform microporous surface structure due to heat treatment.The Energy Dispersive Spectroscopy (EDS) mapping shows (Fig. S1) a uniform distribution of Ti, O and S elements in the PT lm.The XRD patterns of the P, T and PT lms are shown in Fig. 3.In the XRD pattern of the P lm, diffraction peaks corresponding to PEDOT:PSS are overlapped by the stronger peaks of PVA, while the XRD data for the T and PT lms only show the characteristic peaks of PVA and TiO 2 .For complementary analysis, the existence of PEDOT:PSS and crosslinking of TiO 2 and PVA in the PT lm was conrmed by FTIR and Raman spectroscopy.
FTIR studies (Fig. 4(a)) were performed on P, T and PT lms, to conrm the crosslinking of TiO 2 NPs and PEDOT:PSS in a PVA matrix in order to immobilize the TiO 2 -PEDOT:PSS hybrid heterojunction catalyst.The stronger peaks corresponding to PVA overlap the corresponding peaks of PDOT:PSS.Hence, the signature peaks of PEDOT:PSS could not be identi-ed in the FTIR spectra from the P and PT lms.However, signicant crosslinking between PVA and TiO 2 was observed.The peak appearing at 1244 cm −1 in T and PT is from the vibration of Ti-O-C bonds, 11,39 whereas corresponding peak in sample P is absent.Other smaller peaks at 1024 cm −1 and 829 cm −1 are attributed to O-C-C and C-C, respectively. 40It is expected that the PVA chains absorb heat and rearrange to form crystalline regions, leading to the appearance of a small peak at 1145 cm −1 , corresponding to the C-C stretching vibration of the PVA crystals. 41These characteristic peaks indicate the crystallinity of the heat-treated P, T and PT samples.It also demonstrates the formation of covalent Ti-O-C bonds between PVA and TiO 2 nanoparticles in the T and PT samples, which leads to effective immobilization of TiO 2 nanoparticles in a highly stable PVA matrix.Under heat treatment, active OH groups located on the surface of nano-sized TiO 2 and OH groups in PVA molecular chains can form covalent Ti-O-C bonds, 11 which keeps the PVA matrix stable in solution.While some of the characteristic peaks of TiO 2 , PVA and PEDOT:PSS overlap in FTIR spectrum, the peaks at 955 cm −1 and 915 cm −1 can be assigned to the C-S bond of the thiophene ring in PEDOT (the S]O and O-S-O signals of the PSS chains are hidden by strong PVA polymer absorption). 42The peaks at 2910 cm −1 and 3230 cm −1 indicate C-H stretching and O-H functional groups in PVA. 43These peaks are broader in P samples when compared to those appearing in T and PT, pointing to the fact that heat treated T and PT lms are more  Raman spectroscopy was performed to improve the understanding and to provide structural information of the P, T and PT lms.Fig. 4(b) reveals the vibrational modes of the lms.In the Raman spectrum of the P sample, the vibrational mode at 990 cm −1 was assigned to the deformation of oxyethylene rings, while the vibrational modes at 1100 cm −1 and 1218 cm −1 were attributed to the PSS component and PEDOT C a -C a ′ inter ring stretching vibrations, respectively. 44The peaks around 1369 cm −1 and 1434 cm  520 cm −1 , and 638 cm −1 , respectively. 45All the PEDOT:PSS and TiO 2 characteristic peaks were observed in the PT-lm, which implies successful formation of a TiO 2 -PEDOT:PSS hybrid heterojunction in a PVA matrix.
3.1 Photocatalytic activity of the PVA/TiO 2 and PVA/ PEDOT:PSS + TiO 2 hybrid lms Model wastewater containing organic effluents like MB and BPA could be degraded using PVA/TiO 2 and PVA/TiO 2 -PEDOT:PSS hybrid lms under simulated sunlight.The photocatalytic performance of a PVA/TiO 2 -PEDOT:PSS hybrid heterojunction lm was analyzed and compared with a PVA/TiO 2 lm.Initially 10 ppm of MB dye solution was prepared, and 1.5 cm × 1.5 cm lms were dipped in 20 ml of effluent solution and kept in the dark for 1 h to achieve an adsorption-desorption equilibrium.Film T showed a photodegradation efficiency of about 78%, whereas lm PT exhibited about 99% photodegradation under light irradiation (1000 W m −2 , AM1.5) for 5 h.The concentration and degradation prole of MB using T and PT are shown in Fig. 5(a and b).The enhanced photocatalytic performance of the PT lm is due to the formation of a heterojunction between the wideband gap TiO 2 (∼3.2 eV) and the lower bandgap PEDOT:PSS (∼1.6 eV).Formation of a heterojunction enables the absorption of a broader-spectrum light as well as effective charge separation, thus mitigating the electron-hole pair recombination.Immobilization of TiO 2 -PEDOT:PSS through use of a PVA matrix enables recycling for multiple treatment cycles without any need of powder extraction.The as prepared lms are highly stable over 20 cycles and the photodegradation efficiency is consistent in all cycles for over 100 h.The degradation performance of the T and PT lms for multiple cycles is shown in Fig. 5(c and d).Aer 20 cycles the degradation efficiency drops to 69% and 94% for the T and PT lms, respectively.PVA matrix retained TiO 2 and PEDOT:PSS for several cycles, hence the lms are extremely stable, which is very important for efficient photocatalytic activity for multiple cycles.Another organic effluent, bisphenol-A (10 ppm), was also degraded using T and PT lms.The degradation performance of the lms along with their respective cyclic stability is shown in Fig. 6(a and b).The degradation efficiency of T and PT are 75% and 93%, respectively.Aer 20 cycles the degradation efficiency has dropped to 70% and 87% for the T and PT lms, respectively.The lms showed almost consistent performance in all the cycles for degrading BPA.
A plausible mechanism to explain the degradation is shown in Fig. 7.In principle, photocatalytic degradation occurs when a TiO 2 -PEDOT:PSS heterojunction catalyst is irradiated with light and produces electron-hole (e − -h + ) pairs, which react with H 2 O, OH − and O 2 to generate oxidizing species like hydroxyl radicals (cOH), superoxide radicals (O 2 c − ) and hydrogen peroxide (H 2 O 2 ), etc. 46 According to previous reports, the positions of the highest occupied molecular orbital (HOMO) level and lowest unoccupied molecular orbital (LUMO) level of PEDOT:PSS are located at 0.28 eV and −1.32 eV, respectively, 47 while the estimated positions of the valence band (VB) and conduction band (CB) of TiO 2 are approximately 2.91 eV and −0.31 eV, respectively. 48The corresponding band alignments of the two components are illustrated in Fig. 7. Due to the wellmatched energy levels, photo-generated holes of TiO 2 can be easily transferred to the HOMO level of PEDOT:PSS.Being an excellent hole conductor, PEDOT:PSS can transport the holes to the surface of the photocatalyst quickly, which hinders their recombination with electrons and participate in the oxidation of dye molecules.On the other hand, the electrons of PEDOT:PSS can immigrate to the conduction band of TiO 2 .Since the conduction band edge of TiO 2 (−0.31 eV) is more negative than the reduction potential of O 2 /O 2 c − (0.13 eV), the electrons from the conduction band can react with oxygen molecules and produce superoxide radicals (O 2 c − ) which subsequently take part in the dye degradation.These redox reactions decompose the organic effluents into harmless byproducts such as CO 2 , H 2 O, etc. Excessive OH − groups on the PVA surface also serve as active species for efficient degradation of the organic effluents.

Conclusion
PVA/TiO 2 -PEDOT:PSS lms were successfully fabricated by a simple and convenient method combining solution casting followed by heat treatment.The photocatalyst showed good stability due to strong interaction and Ti-O-C covalent bond formation between the PVA matrix and TiO 2 , originating from the heat treatment.The addition of PEDOT:PSS establishes a hybrid heterojunction with TiO 2 (PVA/TiO 2 -PEDOT:PSS), enhancing the light absorption as well as charge separation, which improves the photocatalytic performance over PVA/TiO 2 .The photodegradation efficiency of these lms remains stable during twenty subsequent cycles, for over 100 h, which puts forth the immobilized PVA/TiO 2 -PEDOT:PSS hybrid heterojunction photocatalysts as promising candidates for industrial wastewater treatment.

Fig. 2
Fig. 2 Surface morphology and corresponding cross-sectional images for the films P (a and b), T (c and d) and PT (e and f).

Fig. 3
Fig.3XRD data for the P, T and PT films.
Fig. 4 (a) FTIR spectra, (b) Raman spectra of P, T and PT films.

Fig. 5
Fig. 5 (a) Concentration (C/C 0 ) profile of MB using T and PT films for degradation, (b) degradation profile of MB using a PT film, (c) reusability of the T and PT films for degradation of MB for twenty cycles, and (d) degradation of MB with cycling.

Fig. 6 Fig. 7 A
Fig. 6 (a) Concentration (C/C 0 ) profile of BPA using T and PT films for degradation, and (b) degradation of BPA with cycling.