Article pubs.acs.org/IECR
Novel Floating Ag+‑Doped TiO2/Polystyrene Photocatalysts for the Treatment of Dye Wastewater Seema Singh, Pramod Kumar Singh, and Hari Mahalingam* Jaypee University of Engineering and Technology (JUET), A.B. Road, Raghogarh, Guna, Madhya Pradesh 473226, India ABSTRACT: Two different solvent-cast techniques, namely, impregnation and strewing, were used to immobilize silver-iondoped titanium dioxide particles into and onto a polystyrene matrix. The developed floating photocatalysts were characterized by X-ray diffraction, optical microscopy, scanning electron microscopy, energy-dispersive spectroscopy, and Fourier transform infrared spectroscopy. The doped sheets were evaluated for their photocatalytic activity by the photodiscoloration of methylene blue dye in aqueous solutions at its natural pH under both ultraviolet (UV) and solar irradiation employing nonanchored and nonstirred conditions. The maximum color removal efficiency achieved by the doped strewn photocatalyst was about 94% and 83% under UVC (254 nm) light and solar irradiation, respectively. For the doped impregnated photocatalyst, the maximum photodiscoloration observed under UV light and sunlight was around 86% and 68%, respectively. The prepared photocatalysts can be easily recovered, and the photocatalytic activity was found to be sustained after three consecutive runs. reported23,24 that the modified photocatalysts are active in the visible light. However, because of the recombination of charge carriers, the activity of TiO2 seems to be reduced. Therefore, this calls for the development of a photocatalyst that works efficiently in the visible light. Keeping the above-mentioned points in mind, the current study focuses on improving the PCA of titania for visible light by doping TiO2 with silver ion (Ag+) and supporting the resulting Ag+-doped TiO2 particles on a polymer matrix by a simple and inexpensive technique. It may be noted that the deposition of noble metals like gold (Au), silver (Ag), and platinum (Pt) on the surface of titania has been tried to enhance the PCA under visible light. The noble metals are said to act like electron traps, directing electrons away from the titania surface and thereby delaying the electron−hole recombination.25 In the present work, Ag+-doped TiO2 particles have been prepared by the liquid impregnation method,26 and these particles have been anchored on a polystyrene (PS) matrix using two different solvent-cast techniques, namely, impregnation and strewing. These simple and inexpensive methods have been explored in this work to fabricate buoyant-doped titania photocatalysts. The preparation of buoyant Ag+-TiO2/PS photocatalysts using the above-mentioned methods has not been reported in the literature to date. From the literature survey, the only research work that has employed the method of strewing for anchoring bare TiO2 particles onto a polymer support (natural rubber latex) has been reported by Sriwong et al.27 However, the photocatalyst developed by them was nonbuoyant and effective only under UV light. The advantages of buoyant photocatalysts are well-known and are summarized in the literature.4,15,28
1. INTRODUCTION Titania (TiO2) is an efficient and the most investigated photocatalyst.1 It finds use in a wide range of applications such as air purification, photoinduced hydrophilic coating and selfcleaning devices, self-sterilization, wastewater treatment, and production of hydrogen fuel.1 This is attributed to its unique properties like low cost, nontoxicity, relatively high photocatalytic activity (PCA) in ultraviolet (UV) light, high chemical stability, strong oxidizing power, and ready availability in the market.1−4 However, this potential to be fully utilized for commercial applications like water and wastewater treatment is greatly hindered by the costly and time-consuming posttreatment recovery of the fine particles.5,6 To overcome this problem, extensive research is being carried out to immobilize TiO2 on various substrates such as hollow glass spheres, reactor walls,7 inorganic carbon fabrics,8 glass mats,9 synthetic fabrics,10 natural fabrics,11 and polymers.12 Out of the various supports reported in the literature, it has been found that polymeric substrates are of great interest to researchers. This could be attributed to their properties such as chemical inertness, mechanical stability, high durability, ease of availability, thermosoftening, and hydrophobic nature. Most of them also have a low density.1,4,7,13−15 The methods for fixing titania particles on the chosen substrate may vary from simple dipcoating or sol−gel techniques16 to more complex, specialized, and expensive methods like electrophoretic deposition,17 chemical vapor deposition,18 thermal treatment,7,19 hydrothermal methods,20 and flame synthesis.21 Another major limitation of the TiO2 photocatalyst is its activity only in the UV region of light. Using a UV source for illumination becomes quite uneconomical because it attracts high investment and operational costs. 22 Therefore, for large-scale practical applications of TiO2-mediated photocatalysis, dedicated efforts are being put forth to modify TiO2 in order to make it effective in the solar light that is “freely and abundantly available”. Some of the various approaches for making TiO2 effective in the visible light are doping, capping, dye sensitization, surface modification by noble metals, and coupling.1 It has been © XXXX American Chemical Society
Received: July 21, 2014 Revised: September 27, 2014 Accepted: September 30, 2014
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2.3.2. Preparation of an Impregnated Ag+-TiO2/PS Photocatalyst. The buoyant PS-supported Ag+-doped TiO2 photocatalysts were prepared at room temperature (25 ± 2 °C) by using the solvent-cast method. PS beads (5.0 g) were completely dissolved in 20 mL of xylene. To the required amount of doped titania powder (as calculated with respect to a particular weight percent based on the amount of polymer used) was added another 20 mL of xylene, and the resulting solution was stirred for 20 min. Thereafter, the PS solution was carefully and slowly added to the Ag+-TiO2 suspension and further stirred at high speed for 30 min. Aliquots (10 mL) of the Ag+-TiO2/PS/xylene suspension were cast in glass Petri dishes (85 mm diameter), and these dishes were placed in a fume hood at room temperature in the dark to allow slow evaporation of the solvent. The dried polymer films were carefully removed from the Petri dishes, flipped, and further dried at room temperature for 24 h. Thereafter, the polymer sheets were washed with triple-distilled water, dried, and stored in the dark until use. In this work, 10 wt % has been chosen, and the polymer sheets thus obtained were designated as PSPC(Ag+-I-10). An illustration of the developed photocatalyst is shown in Figure1a.
PS has been chosen for anchoring titania particles because it thermosoftens, is readily available, inert, and inexpensive, has low density, and is nontoxic.29 Furthermore, being a hydrophobic material, it promotes the preconcentration of organic pollutants on its surface, thereby increasing the efficiency of adsorption and subsequent oxidation of the contaminants.15 From an environmental point of view, PS, because of its variety of applications, has the potential to become a waste material in the future, thus causing “white pollution” because of its nonbiodegradability.30 Therefore, the motivation of the present work is to find ways for judiciously utilizing the waste PS as a substrate for developing TiO2 photocatalysts to degrade or completely remove organic or inorganic pollutants present in the environment. PS has been used by researchers to anchor titania particles either for bringing about solid-phase degradation of the polymer itself29,31−33 or for photodiscoloring organic contaminants such as methylene blue (MB).7,28,30 The PCA of the prepared photocatalysts was investigated by studying the photocatalytic discoloration of MB dye, a model contaminant, in aqueous solutions under both UV and solar light without any additional pH adjustment employing nonstirred and nonanchored conditions. The hazards of MB (C.I. 52015) are summarized in Sahoo et al.34 The reusability of the developed photocatalysts was also investigated.
2. EXPERIMENTAL DETAILS 2.1. Materials. TiO2 (Degussa P25) obtained from Evonik Degussa AG, Germany, was used as a photocatalyst. PS beads purchased from Sigma-Aldrich, USA, were used as substrates. Silver nitrate (AgNO3, 99.8% pure) from SD Fine-Chem Ltd., India, was used for Ag+ doping. Xylene (GR) purchased from Merck India Ltd. was used as a solvent. Methylene blue (MB) from Merck India Ltd. was used without further purification as the organic probe molecule. HNO3 and NaOH, obtained from Merck India Ltd. were used for pH adjustment of the reaction medium. Triple-distilled water was used throughout the study. 2.2. Instruments. The instruments used for the study were as follows: ELICO SL159 UV−vis spectrophotometer, PANalytical X’pert PRO X-ray diffractometer, JEOL model JSM-6390LV scanning electron microscope (SEM), JEOL model JED-2300 energy-dispersive X-ray spectroscope (EDS/ EDX), Thermo Nicolet-Avatar 370 Fourier transform infrared (FTIR) spectrometer, Nikon Eclipse E600 POL optical microscope, muffle furnace (Apex industrial electronics), Philips UVC lamp (5 × 20 W, 254 nm), Remi magnetic stirrer, Remi R-8C laboratory centrifuge, digital hot air oven, Systronics digital pH meter, and Sartorius BSA423S-CW electronic precision balance. 2.3. Preparation of Buoyant Ag+-TiO2/PS Photocatalysts. 2.3.1. Preparation of Ag+-TiO2 Nanoparticles. Using the technique of liquid impregnation, Ag+-doped TiO2 (1 mol %) nanoparticles were prepared. AgNO3 (0.01 mol) was dissolved in 100 mL of triple-distilled water taken in an amber conical flask. Then 0.99 mol of TiO2 was added into the solution, and the solution was stirred with the help of a magnetic stirrer for 45 min. Thereafter, the suspension was left undisturbed for 24 h. Subsequently, to evaporate water, the suspension was kept in an oven maintained at 100 °C for 12 h. The dried solid material was ground in a porcelain mortar and then calcined in a muffle furnace at 400 °C for 6 h. The resulting Ag+-doped TiO2 P25 powder was stored in dark bottles for further use.
Figure 1. Illustrations of (a) impregnated and (b) strewn doped TiO2 photocatalysts.
2.3.3. Preparation of a Strewn Ag+-TiO2/PS Photocatalyst. With a method similar to that described in section 2.3.2, strewn PS-supported Ag+-doped TiO2 photocatalysts were developed. Aliquots (10 mL) of a PS/xylene solution were cast in glass Petri dishes (85 mm diameter) and placed in a fume hood at room temperature (25 ± 2 °C) in the dark for about 2.5 h until gel formation. Thereafter, the required amount of Ag+-TiO2 P25 (as calculated with respect to a particular weight percent based on the amount of polymer used) was randomly strewn onto the surface of the polymer by using a conventional tea strainer fitted with a nylon 32 mesh sieve (mesh opening size = 0.5 mm). It may be noted that, at this stage, the polymer is still in gel form, thereby allowing for the possibility of the highly dense doped TiO2 P25 particles to sink inside the polymer. Subsequently, the dishes containing TiO2-strewn PS sheets were placed in a fume hood at room temperature in the dark to allow slow evaporation of the solvent. After about 36 h, the dried strewn polymer films were carefully removed from the Petri dishes and further dried in an oven maintained at 100 ± 5 °C for 1 h. Thereafter, the polymer sheets were gently washed B
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During the experimental runs, samples were analyzed with the help of a UV−vis spectrophotometer at a λmax (wavelength for maximum absorbance) value of 664 nm. For determination of the MB concentration, a calibration graph (R2 = 0.9966) was used. The samples were returned to the Petri dishes after each analysis in order to maintain the volume of the MB solution. 2.5.3. Experiments under Solar Light. The PCA of the prepared photocatalyst sheets was also investigated by photocatalytic discoloration of MB in aqueous solution under solar light at the dye’s natural pH. The reactions were carried out in the city of Guna (India) between 10:30 am and 4:30 pm. For this, the complete Ag+-TiO2/PS sheets were placed in separate Petri dishes, containing 50 mL of a MB solution having a concentration of 5 mg L−1. The dishes were covered by a thin sheet of transparent glass in order to minimize evaporation loss. After 6 h of solar-light illumination, the absorbance of the sample was measured. It may be noted that two controlled experiments in UV light as well as solar light were also conducted: one with only a MB solution and the other with a pristine PS sheet placed in the MB solution.
with triple-distilled water to remove any nonadhered titania particles, dried, and stored in the dark until use. The obtained PS sheets corresponding to 10 wt % were designated as PSPC(Ag+-S-10). An illustration of the developed photocatalyst is shown in Figure1b. It may be noted that the selection of 10 wt % was based on the experiments conducted for optimization of the amount of bare titania for the preparation of undoped PS-supported TiO2 sheets. 2.4. Photocatalyst Characterization. X-ray diffraction (XRD) is a widely used characterization technique especially for crystalline materials.35 In order to investigate any change in the crystal structure of TiO2 that might have occurred due to Ag+ doping, XRD analyses of TiO2 Degussa P25, Ag+-TiO2 P25, PSPC(Ag+-I-10), and PSPC(Ag+-S-10) photocatalysts with an X-ray diffractometer using Cu Kα radiation (λ = 0.154056 nm) were performed. To identify the crystalline phases, the peak positions were compared with the standard files.36 The lattice parameters were determined by employing a Rietveld refinement method using the Fullprof program. Using the well-known Scherrer equation,37 the average anatase crystallite sizes of undoped and doped TiO2 powders were determined from the broadening of the anatase (101) peak (2θ = 25.331° and 25.313°, respectively). In order to establish the buoyancy of the developed photocatalysts, density measurements were also done. The surface morphologies of the TiO2 Degussa P25, Ag+TiO2 P25, PSPC(Ag+-I-10), and PSPC(Ag+-S-10) sheets were observed by SEM. To identify the elements present in the Ag+doped titania powder and to determinine its elemental composition, EDS was done. To investigate the possibility of any surface deformation of the developed photocatalysts during UV-light exposure, FTIR analyses of the pure PS sheet as well as the developed photocatalysts before and after UV exposure were carried out. Finally, to test the stability of the strewn photocatalyst sheet [PSPC(Ag+-S-10)], i.e., attachment of Ag+doped titania nanoparticles on the polymer matrix, optical microscopy was used. 2.5. Investigation of the PCA. 2.5.1. Effect of the pH. The pH of the reaction mixture is a vital parameter that affects the process of photocatalytic discoloration. The prepared doped sheets were subjected to testing under varying pH conditions from 3 to 11. HNO3 (0.1 M) and NaOH (0.1 M) solutions were used to alter the initial pH of the dye solution. 2.5.2. Experiments under UV Light. The experiments for investigating the PCA of the as-prepared sheets were carried out in a batch reactor that consisted of a glass Petri dish (100 mm diameter) in which the entire Ag+-TiO2/PS photocatalyst was placed. A MB solution (50 mL) with a concentration of 5 mg L−1 was poured into the dishes. In order to facilitate the use of floating photocatalysts, neither were the solutions stirred nor were the photocatalysts anchored. Photocatalytic discoloration of MB in aqueous solution was carried out under UVC light (254 nm) at the dye’s natural pH of 7.68 (concentration = 1.56 × 10−5 M). Prior to irradiation, the solutions were kept in the dark for 2 h to reach an adsorption−desorption equilibrium. Thereafter, the solutions were irradiated with UV light of 100 W for 5 h. The distance from the liquid surface in the Petri dish to the UV-light source was approximately 14 cm. For comparison, photocatalytic discoloration by 0.05 g (loading amount = 1 g L−1) of TiO2 P25 and Ag+-TiO2 P25 in slurry form was also investigated.
3. RESULTS AND DISCUSSION 3.1. Photocatalyst Characterization. The XRD patterns of TiO2 Degussa P25, Ag+-doped TiO2, PSPC(Ag+-I-10), and PSPC(Ag+-S-10) photocatalysts are shown in Figure 2. The
Figure 2. XRD patterns of the TiO2 P25, Ag+-TiO2 P25, PSPC(Ag+-I10), and PSPC(Ag+-S-10) photocatalysts.
peaks obtained at 2θ of 25.30°, 48.03°, 53.89°, 55.06°, and 62.69° in the case of TiO2 Degussa P25 powder, 25.43°, 48.21°, 54.05°, 55.21°, and 62.92° in the case of Ag+-doped TiO2 P25, 25.17°, 47.99°, 54.05°, 55.03°, and 62.75° for a PSPC(Ag+-I10) sheet, and 25.15°, 47.87°, 53.81°, 54.99°, and 62.57° for a PSPC(Ag+-S-10) sheet illustrate that all of the photocatalysts have anatase crystal structures (JCPDS card nos. 84-1285 and 83-2243 for the powder and sheet, respectively). It can also be observed that, except for the changes in the intensity of the peaks, the 2θ peak positions of the major diffraction patterns in all of the Ag+-doped titania photocatalysts possess almost the same value as that of the pure TiO2 powder. This may be attributed to the fact that doping titania with Ag+ might influence the crystallite size but not the crystal structure of Ag+TiO2.38 In fact, the anatase crystallite size of doped titania (∼19 nm) was found to be smaller than that of pure titania (∼21 nm), as calculated by the Scherrer’s equation. These results are in accordance with that reported by Chao et al.39 and suggest that the Ag+ dopant reduces the anatase grain size of TiO2 powder, thereby having an inhibiting effect on the anatase grain growth. C
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Figure 3. Rietveld refinement of XRD patterns: (a) pure TiO2 P25 powder; (b) Ag+-TiO2 P25 powder.
Table 1. Lattice Parameters, Crystal Phase, Crystal Volume, and Crystallite Size of Pure and Doped TiO2 Particles lattice parameters
crystal phase % 3
photocatalyst
a (Å)
b (Å)
c (Å)
unit cell volume V (Å )
anatase
rutile
crystallite size DA (nm)
TiO2 P25 Ag+-TiO2P25
3.7817 3.7840
3.7817 3.7840
9.4943 9.4973
135.778 135.987
91.85 90.22
8.15 9.78
21.24 19.14
Figure 4. SEM images of (a) TiO2 P25, (b) Ag+-TiO2 P25, (c) PSPC(Ag+-I-10) (before UV), (d) PSPC(Ag+-S-10) (before UV), (e) PSPC(Ag+-I10) (after UV), and (f) PSPC(Ag+-S-10) (after UV) photocatalysts.
The Rietveld fittings for pure and doped titania powders are shown in Figure 3. The values obtained for the lattice parameters, crystallite volume, crystallite phase, and crystallite
size for the undoped and doped TiO2 powders are summarized in Table 1. The XRD patterns of the Ag+-TiO2 powder and the synthesized doped sheets do not show any diffraction peak D
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Figure 5. EDX spectrum of Ag+-TiO2 P25 powder.
Figure 6. Investigation of the PCA of (a) PSPC(Ag+-I-10) and (b) PSPC(Ag+-S-10) sheets under various pH conditions.
SEM image of PSPC(Ag+-I-10) (Figure 4c) suggests the dispersion of Ag+-doped TiO2 P25 particles into the polymer matrix. Furthermore, it is clear that the TiO2 particles are embedded in the PS matrix. On the other hand, the SEM micrograph of PSPC(Ag+-S-10) (Figure 4d) shows that aggregates of doped titania particles cover most of the surface of the PS matrix and impart roughness to the surface. Small pores are also observed that could possibly be ascribed to evaporation of the solvent from the polymer surface. These cavities could also be responsible for lowering the bulk densities of the prepared strewn photocatalysts. The SEM micrographs of photoirradiated impregnated and strewn photocatalysts are shown in Figure 4e,f. From these images, it is clear that the titania particles remain embedded in the polymer and are present in the form of aggregates on the PS matrix in the impregnated and strewn photocatalysts, respectively, even after exposure to UV light. Thus, no significant change in the morphology of the developed photocatalysts is observed after the photoirradiation process. The EDS spectrum for Ag+-doped TiO2 (Figure 5) indicates the presence of O, Ti, and Ag at binding energies of 0.525, 4.508, and 2.984 keV, respectively. The atomic percentages of O, Ti, and Ag are 15.56, 52.38, and 0.97%, respectively. Although the peaks obtained for Ag are quite insignificant because of its very low concentration in the TiO2 matrix, the presence of Ag in the prepared doped TiO2 powder is clearly indicated by them.
characteristic for the doped element, Ag, and/or its oxides (Ag2O, AgO). This could possibly be attributed to very low Ag content (1 mol %) in the developed photocatalysts, which was lower than the X-ray detection limit.40 There is a high possibility that Ag+ did not incorporate into the crystal lattice of TiO2 because of its higher ionic size (1.26 Å) compared to Ti (0.605 Å)41,42 Furthermore, it was observed that the lattice parameters a and c remained more or less the same, around 3.78 and 9.49 Å before and after Ag doping, respectively. No significant changes in the lattice parameters and the cell volumes of the undoped and doped titania powders suggest that Ag probably did not dope into the TiO2 lattice.43 Thus, the XRD patterns clearly suggest that no significant change in the crystal structure of TiO2 was produced during the preparation of the photocatalyst. A large broad peak near 2θ = 20° is observed in the diffraction patterns of both impregnated and strewn polymer sheets (Figure 2). This can be attributed to the scattering of the X-ray by the polymer. The densities of all of the prepared TiO2/PS photocatalysts were found to be in the range of 0.8−1 g cm−3, and thus all of the developed doped sheets were buoyant in nature under the experimental conditions employed. SEM analyses of pure TiO2 and Ag+-doped TiO2 powders are shown in Figure 4a,b. These images illustrate a porous, spongelike structure having a high degree of roughness. Such a structure is probably responsible for imparting a high surface area to these photocatalysts and thereby a higher PCA. The E
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3.2. Comments on the Robustness of the Strewn Ag+TiO2/PS Photocatalyst. The solidified PS sheet acts as a substrate on which doped TiO2 nanoparticles are immobilized. It is quite possible that the relatively denser Ag+-doped titania particles got trapped in the pores that could possibly be generated by slow evaporation of the solvent. Upon further drying of the photocatalyst, these pores may have decreased in size, which led to strong adherance of the doped TiO2 on the PS matrix. The subsequent thermal treatment of the photocatalyst softened the polymer sheet, leading to a subtle melting at its surface, which further strengthened the attachment of the doped titania particles. In order to test the adherance of the doped TiO2 particles on the PS surface, the prepared PSPC(Ag+-S-10) samples were subjected to vigorous stirring in triple-distilled water for 24 h. For this, the doped strewn sheet was cut into smaller pieces. The surface of the strewn sheet and the water in which the doped sheet was stirred were observed before and after stirring with the help of an optical microscope. The photocatalyst surface showed no significant change before and after the process. Also, no unadhered or loose titania particles were observed in the water in which the doped strewn sheets were stirred. Moreover, to rule out the presence of Ag+ in the dye solution, a 1 N NaCl solution was added to the dye solution after the reaction runs. No white precipitate pertaining to AgCl was observed, which indicated the absence of Ag+ in the solution after exposure to UV and solar irradiation. The above observations clearly suggest that the doped titania particles are strongly attached on the PS matrix and, hence, attest to the robustness of the prepared strewn doped photocatalysts. 3.3. PCA. 3.3.1. Effect of the pH. From Figure 6, it is observed that dye discoloration for both impregnated and strewn doped photocatalysts is favored in alkaline conditions and inhibited in acidic conditions. This is because, in an alkaline solution, the pH is greater than the point of zero charge (pzc) of Ag+- doped TiO2 (pzc = 6.6).44,45 Consequently, the surface of titania gets negatively charged, which promotes adsorption of the cationic MB dye parent fragment. Vice versa happens in the case of an acidic medium, and hence the PCA is retarded. The color removal percentages of MB were found to be almost similar for the dye at its slightly alkaline natural pH (7.68) and at a higher pH value of 9. It was observed that, at pH 11, the photocatalyst showed a decline in the PCA. This might be attributed to the blocking of active sites of titania particles due to excessive adsorption of MB molecules, which reduces the formation of reactive radical species. Consequently, a reduction in the PCA of the prepared photocatalysts occurs. On the basis of the above results, the natural pH of the dye solution was chosen for investigating the PCA of the developed doped photocatalysts because it eliminates any need for additional pH alterations. 3.3.2. Photocatalytic Discoloration of MB Dye under UV Light. The percentage of photodiscoloration of MB from its aqueous solution by the PSPC(Ag+-I-10) and PSPC(Ag+-S-10) photocatalysts under UV irradiation is shown in Figure 7. It must be noted that the percentage of color removal of MB signifies its removal by both adsorption and photocatalytic discoloration. It was observed that the prepared PSPC(Ag+-I10) and PSPC(Ag+-S-10) photocatalysts could remove around 86% and 94% of MB, respectively, from its aqueous solution in 5 h. It was also observed that the bare TiO2 P25 and Ag+-doped TiO2 in slurry forms completely decolorized MB in solution within 1 h of UVC illumination. A slight discoloration of MB
Figure 7. Efficiency of MB discoloration by PSPC(Ag+-I-10) and PSPC(Ag+-S-10) samples during three consecutive reaction runs after 5 h of UV irradiation and 6 h of sunlight.
was also observed in the blank experiments conducted, which could be attributed to the high intensity of light irradiation employed and/or adsorption of the dye onto the PS sheet. 3.3.3. Photocatalytic Discoloration of MB Dye under Sunlight. The percentage of discoloration of MB achieved by the developed PSPC(Ag+-I-10) and PSPC(Ag+-S-10) samples under solar light is shown in Figure 7. The maximum discoloration achieved was around 68% and 83% by the impregnated and strewn doped sheets, respectively, after 6 h of solar illumination. TiO2 P25 and Ag+-doped TiO2 in the slurry form (0.05 g) completely decolorized the MB in solution within 1 h of solar illumination. The above results suggest that the developed doped sheets are effective in decolorizing MB from its aqueous solution in both UV light and sunlight. From Figure 7, it is also observed that the strewn doped photocatalyst exhibits a higher PCA under UV light as well as sunlight compared to the impregnated sheet. The difference in the PCA of the impregnated and strewn doped sheets could be ascribed to the location of titania particles into and onto the polymer matrix, respectively. The particles being adhered externally onto the surface of PS in the case of the strewn doped sample have a greater probability of interacting with the MB molecules compared to when they are impregnated in the PS matrix and thus possess a higher rate of adsorption and subsequently a higher rate of photocatalytic discoloration of the dye. The enhancement in the PCA by doping TiO2 with Ag+ may be attributed to the ability of Ag+ to trap the photoelectrons present in the conduction band of titania, thereby reducing the recombination of charges and facilitating oxidation of the contaminants, i.e., Ag + + e− → Ag
The above reaction is substantiated by the slight darkening of the doped photocatalysts during the irradiation process.45,46 3.3.4. Investigating Reusability of the Developed Photocatalysts. To explore the reusability of the developed polymersupported doped titania sheets under both UV and solar light, the doped sheets were easily recovered by filtration and then, without any intermittent cleaning, were used for discoloration of fresh MB solutions for two more consecutive reaction runs. The results of the reusability of the PSPC(Ag+-I-10) and PSPC(Ag+-S-10) sheets under UV and solar illumination are shown in Figure 7. One noticeable observation is that the PCA of the developed doped photocatalysts in UV light as well as sunlight in the first reaction run is lower compared to those of subsequent runs. This could be attributed to the following two F
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C−H stretching vibrations, respectively. The band at 1601 cm−1 corresponds to the aromatic C−C (in plane) stretching vibration. The absorption band at 1493 cm−1 is assigned to the C−H stretching vibration of the ring in plane, whereas the bands at 1068 and 1028 cm−1 correspond to the C−H bending vibrations of the ring in plane. The band at 1455 cm−1 corresponds to the C−H deformation of CH2. From the FTIR analyses, it can be observed that the spectrum (absorption peaks) obtained for the pure PS sheet dipped in water and exposed to UV for 8 and 24 h reflects changes in the intensity of the absorption peaks. This indicates the possibility of PS undergoing structural changes upon exposure to prolonged and highly intense UVC irradiation (=100 W) due to photolysis (degradation and cross-linking) and/or photooxidation (photoinduced oxidation).47−52 From the FTIR spectra of the impregnated and strewn photocatalysts before and after UV exposure, not many changes can be observed. This suggests that the TiO2 particles immobilized on the PS matrix probably act as a barrier between the UV rays and the polymer, thereby significantly protecting the PS substrate from the damaging effect of UV light. The role of TiO2 as a UV blocker is well-known and is reported in the literature.53−55 It may be noted that the developed photocatalysts are effective in solar light, which contains a very small fraction of UV light (
