Fabrication of a photo-catalytic cell using polymer-based composite films and investigation of its performance in the degradation of methyl blue

Abstract

A facile method for the preparation of flexible semiconductor/polymer composite films with a CdS and ZnS particle layer on their surface has been developed. The as-prepared composite films were used as building blocks to fabricate a photo-catalytic cell. Methyl methacrylate (MMA) and butyl methacrylate (BMA) were copolymerized via free radical polymerization. BMA was used to tune the flexibility of the films. Allyl methyl sulfide (AMS) and N-allylmethylamine (AMA) were used as functional monomers to coordinate with Cd²⁺/Zn²⁺ ions. Cd²⁺/Zn²⁺ ions were chemically bonded to the PMMA-BMA-AMS (PMBS) or PMMA-BMA-AMA (PMBA) films. By addition of a thioacetamide (TAA) solution, S²⁻ ions were gradually released and combined with Cd²⁺/Zn²⁺ ions to form CdS/ZnS particles. Accordingly, CdS/PMBA, ZnS/PMBA, CdS/PMBS and ZnS/PMBS films were prepared. The composite films were characterized and used to fabricate a photo-catalytic cell. The well-designed cell showed excellent performance in the degradation of the pollutant methyl blue (MB). Also, this cell could be used repeatedly just by rinsing with water after use.

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1. Introduction

In recent years, cadmium sulfide (CdS) and zinc sulfide (ZnS) have been extensively studied. CdS and ZnS particles are used in many fields such as photo-catalysis, optics, photoelectricity,^1–3 light-emitting devices⁴ and other applications. ZnS can display various morphologies and nanostructures under different synthetic conditions. ZnS composite materials show good crystallinity and wide band-to-band transitions that lead to high quantum conversion efficiencies.^5,6 CdS and ZnS particles have high photosensitivity and thermodynamic potential under light irradiation, they can drive the water splitting reaction into hydrogen and oxygen.¹

Conducting/semiconducting polymers often have conjugate structures,⁷ such as polypyrrole and polythiophene. These structures guarantee that the electrons can move easily. However, polymers having conjugate chains are often rigid and are not easy to process.⁸ Some methods have been developed to prepare easily processed conducting polymers. Conducting particles such Ag and Cu are often doped in the polymer matrix. There are many methods for the preparation of conducting polymers, such as doping carbon nanotubes into the polymer⁹ or in situ polymerization with a conducting polymer on a host polymer.¹⁰ Considering the hole transporting characteristics, malleability, chemical stability and appropriate number of electrons of these organic materials, it is difficult to use organic polymers as optoelectronic devices.¹¹

As mentioned above, composite polymers have been studied by many scientists. Yuan et al.¹² have prepared liquid crystal (LC)/CdS nanocomposites by in situ thermal decomposition of cadmium xanthate on thermotropic LC small molecules. The photoluminescence and emission dynamics on different LC molecules in LC/CdS composites at different annealing temperatures were investigated. Both charge transfer from LCs to CdS and charge transportation through the CdS network were demonstrated. The in situ method affords the control of the LC/CdS nanostructures by the LCs. Ohara et al.¹³ have studied the light-emission behavior of CdS nanoparticles doped in PMMA films, which can be tuned by combining UV light irradiation and air. It was demonstrated that an external electric field can decrease the emission intensity. Wu et al.¹⁴ have prepared CdS/PMMA composites by a freeze drying method from two types of cadmium carboxylates. The size of the CdS nanoclusters in this method was smaller than the dispersed CdS particles in PMMA, and the CdS nanoparticles did not remain aggregated for more than 6 months. It might be used as a catalyst in gas-phase reactions, however, real applications¹⁵ still remain a challenge.

Nowadays, concerns about environmental pollution have become more and more serious. Organic dyes cause extensive harmful pollution in water and soil and the demand for safe water supplies is becoming urgent.¹⁶ The largest source of dye-contaminated water is the textile industry, since the wide use of organic dyes generates more and more wastewater. Many methods have been developed for the removal of organic dyes, such as physical methods,^17,18 chemical methods^19,20 and biological methods.^21,22 Semiconductor (CdS, ZnS, TiO₂ (ref. 23)) photocatalysis has been broadly used to solve this problem due to its easy operation.²⁴ In the published literature, CdS and/or ZnS particles are often loaded on the surface of particles²⁵ or embedded in the polymer matrix.²⁶ The polymer matrix can protect the embedded CdS/ZnS particles from the environment. However, when used as a catalyst, the activity of the CdS/ZnS particles is weaken by the polymer matrix because of the insufficient contact between the CdS/ZnS particles and the reactants.

In our previous work, we reported the preparation of monodispersed CdS/polystyrene and CdS/SiO₂ composite microspheres.²⁷ These products can degrade organic dyes pollutant to purify wastewater. Due to their large specific surface area, CdS nanoparticles can get in contact with the dye molecules, and catalyse the dye degradation efficiently. To recycle the composite particles, repeated centrifuge/wash steps are necessary. This process becomes thus time-consuming and tedious.

In this work, flexible and transparent functional polymer films have been synthesized by free radical polymerization. In the polymerization reaction, MMA and BMA were used as the main monomers, and AMS and AMA were used as functional monomers to functionalize the PMMA-BMA films. Due to the coordination ability of the N and S atoms in the functional monomers, CdS or ZnS particles were deposited on the surface of the PMMA films to obtain the final composite films. Because polybutyl methacrylate is soft, the PMMA-BMA films can be easily bent. The synthetic route is facile, and the films show good transmission of light. A photo-catalytic cell was fabricated with the as-prepared CdS/polymer films. This cell exhibited good performance for the degradation of methylene blue. Furthermore, the cell could be re-used just by washing with water.

2. Experimental section

2.1 Materials

Monomers for the copolymers, methyl methacrylate (MMA) and butyl methacrylate (BMA) were purchased from Aladdin Chemical Co., Ltd. 5% NaOH was used to remove the inhibitor before polymerization. Initiator azobisisobutyronitrile (AIBN) was recrystallized in ethanol. Functional monomers, allyl methyl sulfide (AMS) and N-allylmethylamine (AMA) were purchased from J&K China Chemical Ltd. and used as received. Zinc acetate [Zn(OOCH₃)₂], cadmium acetate [Cd(OOCH₃)₂], ethyl acetate (EA) and thioacetamide (TAA) were purchased from Aladdin Chemical Co., Ltd. Methyl blue (MB) was bought from Sigma-Aldrich and used as received. Ultrapure water (>17 MΩ cm⁻¹) from a Milli-Q water system was used throughout the experiments.

2.2 Preparation of PMMA-BMA-AMS (PMBS) and PMMA-BMA-AMA (PMBA) copolymers

In this work, a flexible transparent PMMA-PBMA copolymer was synthesized via free radical polymerization. To prepare the PMMA-PBMA solution, the polymerization was conducted in EA. The typical recipe is as follows: a weight ratio of about MMA : BMA = 1 : 3, 1%/3%/5% of functional monomers (to the total weight of all monomers) was used, the initiator content was 1.5 wt%.

MMA, BMA, AIBN and EA were placed in a dried 100 mL three-necked round-bottomed flask with a nitrogen inlet, condenser and a mechanical stirrer. The solution was bubbled with N₂ to deoxygenize it at room temperature for 30 min, and then heated to 65 °C under a stirring rate of 500 rpm. After 4 h, the AMS/AMA solution was added, stirred for another 6 h. The solution of PMMA-BMA-AMS (PMBS) was prepared. The other polymer PMMA-BMA-AMA (PMBA) was obtained using the same method.

2.3 Preparation of CdS/PMBS, ZnS/PMBS, CdS/PMBA and ZnS/PMBA films

The CdS/PMBS, ZnS/PMBS CdS/PMBA, and ZnS/PMBA films were prepared as follows: a PMBS/PMBA solution was casted on glass slide and dried for 24 h at room temperature. The dried film was peeled off and put into a 20 mL Cd²⁺/Zn²⁺ aqueous solution (1 wt%), and then kept at 20 °C. Before the temperature was raised to 30 °C, TAA (1 wt%) was added. After reacting for 7 min/15 min/30 min/600 min, the films were washed with high-pressure water, immersed in water for 12 h to remove the physically absorbed CdS particles, and then dried in a vacuum oven at 25 °C for 24 h. Transparent CdS/PMBS, ZnS/PMBS CdS/PMBA, and ZnS/PMBA films were thus obtained. The two faces of the films were covered with CdS/ZnS particles. If the polymer films were not peeled off the glass slide, and then immersed in the Cd²⁺/Zn²⁺ aqueous solution for the reaction with TAA, a composite film with a single face covered with CdS/ZnS particles would be obtained.

2.4 Characterization

¹³C-NMR spectra were measured in an AVANCE III 600 MHz (Bruker) spectrometer. X-ray photoelectron spectroscopy (XPS) analysis was conducted on a Thermo VG Multilab 2000 instrument. Gel Permeation Chromatography (GPC) was carried out in a Polymer Laboratories PL-GPC 50 Plus integrated GPC system (Wyatt Technolo U.S.A). Scanning electron microscopy (SEM) images were obtained in a JEOL JSM-7600F. X-ray powder diffraction (XRD) studies were carried out in a Rigaku Dmax 2200 PC 3Dmax X-ray powder diffraction instrument. Differential scanning calorimetry (DSC) was conducted on a SHIMADZU, DSC-60. The light transmittance was characterized by Double Beam UV spectroscopy, TU-1901. Fluorescent light analysis (FL) was carried out with a Hitachi, F-4600 FL Spectrophotometer. UV absorbance was measured with a UV-5900PC spectrophotometer (METASH). Thermogravimetric Analysis (TG) was conducted on a TG 209 F3 Tarsus (NETZSCH) instrument.

3. Results and discussion

3.1 Preparation of CdS/PMBS, ZnS/PMBS, CdS/PMBA, and ZnS/PMBA films

The preparation process for the CdS/PMBS, ZnS/PMBS, CdS/PMBA, and ZnS/PMBA films is shown in Scheme 1. In Scheme 1(a) and (b), AMS and AMA are used as the functional monomers, respectively. The S and N atoms on the surface of the films act as coordination sites to bond the Cd²⁺/Zn²⁺ ions. After a TAA solution is added, CdS and ZnS are formed on the as-absorbed Cd²⁺/Zn²⁺ ions. Accordingly, composite films with a CdS or ZnS layer on their surface are obtained. The preparation process is described in Scheme 1(c).

Scheme 1 Synthetic routes for (a) PMBS; (b) PMBA; and (c) the preparation process of CdS/PMBS, ZnS/PMBS, CdS/PMBA, and ZnS/PMBA films.

In this work, the functional monomers, AMS and AMA, are necessary. A control experiment was done just using the PMMA-BMA copolymer without AMS or AMA. Only a few particles were attached to the film. In order to prepare composite films with a CdS or ZnS particle layer on the surface, AMA and AMS were used in this work.

3.2 ¹³C-NMR and X-ray photoelectron spectroscopy (XPS) characterization

Solid PMBS/PMBA polymers were obtained by adding absolute ethanol to the PMBS and PMBA ethyl acetate solutions. Then, the precipitated polymers were centrifuged and washed with ethanol and ultrapure water repeatedly, and dried at 40 °C for 10 h. According to the ¹³C-NMR spectra (Fig. 1), the peak for the C=C double bond, which is in the region of 100–150 ppm,²⁸ is no longer observed, and the new peaks at 64.733 ppm and 64.712 ppm indicate that the AMA and AMS have been introduced into the polymers.

Fig. 1 The ¹³C-NMR spectra of (a) PMBA, (b) PMBS and (c) PMMA-BMA.

To further confirm the structures of PMBS, PMBA, CdS/PMBS, ZnS/PMBS, CdS/PMBA, and ZnS/PMBA film, XPS spectra were obtained. From the XPS spectra in Fig. 2a and b, peaks for nitrogen and sulfur at 399.75 eV and 163.1 eV, respectively, can be observed. The results indicate that the functional monomers copolymerized to PMMA-BMA successfully. In Fig. 2c and d, the peaks of nitrogen (at 395.85 eV and 399.94 eV), sulfur (at 164 eV), Cd (at 404 eV) and Zn (at 1023 eV) can be clearly seen. The shape of the N 1s peak changes after coordination to Cd²⁺ and Zn²⁺, because the electronic density of the nitrogen atom is changed after coordination with Cd²⁺ and Zn²⁺.²⁹ The appearance of the peak for S 2p is due to the presence of CdS and ZnS. It indicates that the nitrogen atom of PMBA is coordinated to the Cd²⁺/Zn²⁺ ions before the clusters of CdS and ZnS are loaded onto the surface of the PMBA films. In Fig. 2e and f, the peaks for Cd (at 405 eV), Zn (at 1023 eV) and S (at 160.2 eV and 164.27 eV) can also be observed in their expected positions. The differences observed in the peaks of S 2p are caused by the sulfur atoms in the polymer coordinated to the Cd/Zn ions and the S atoms in CdS/ZnS. Clearly, coordination between N/S atoms and Cd/Zn ions was achieved.

Fig. 2 XPS spectra of (a) PBMA powder, (b) PMBS powder, (c) CdS/PMBA film, (d) ZnS/PMBA film, (e) CdS/PMBS film, (f) ZnS/PMBS film.

3.3 Gel permeation chromatography (GPC) studies

The molecular weight and molecular weight distribution of PMBA and PMBS were measured by Gel Permeation Chromatography (GPC). In Table 1, it can be seen that the molecular weight of PMBA and PMBS is of the order of 10⁴. It can also be seen that the molecular weight of PMBA is larger than that of PMBS under the same polymerization conditions. The results might be explained by the sulfur atoms having a certain inhibition to free radical polymerization reaction.³⁰

Table 1 Molecular mass distribution of PMBA and PMBS

Polymer	Mw	Mn	PDI
PMBA	7.03 × 10^4	4.37 × 10^4	1.61
PMBS	4.41 × 10^4	2.77 × 10^4	1.59

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3.4 SEM characterization

Using the method mentioned above, PMMA–PBMA composites with CdS or ZnS particles on the surface can be obtained. Typical SEM images are shown in Fig. 3. Fig. 3a–d show CdS/ZnS particles loaded on the PMBS/PMBA films with an almost uniform CdS/ZnS particle size. Fig. 3e shows the cross-section of the CdS/PMBA composite film. The interface between the inorganic particles and the organic polymer can be clearly observed.

Fig. 3 SEM images of (a) the CdS/PMBS film, (b) the ZnS/PMBS film, (c) the CdS/PMBA film, (d) the ZnS/PMBA film, (e) the cross-section of the CdS/PMBA film.

3.5 X-ray powder diffraction (XRD) characterization

XRD measurements on the PMBS, PMBA, CdS/PMBS, CdS/PMBA, ZnS/PMBS and ZnS/PMBA films were conducted. All the powders were tested at a scanning rate of 10° min⁻¹ in the 2θ range of 3°–90°. For the pure polymer films, there were no crystal peaks. The crystalline forms of CdS/PMBS, CdS/PMBA, ZnS/PMBS and ZnS/PMBA were observed by XRD, as shown in Fig. 4. The main peaks shown in the XRD pattern (Fig. 4a) of ZnS/PMBS and ZnS/PMBA at estimated 2θ values of 26.23°, 43.83°, and 51.73° rightly fit the (111), (220), and (311) planes of the structure of ZnS (ICDD PDF 65-0309). For CdS/PMBS and CdS/PMBA powders, the peaks shown in the XRD pattern (Fig. 4b) at estimated 2θ values of 28.92°, 48.50°, and 57.66° closely resemble the (101), (103), (112) crystalline structures of hexagonal CdS (ICDD PDF 77-2306).

Fig. 4 XRD patterns of (a) ZnS/PMBA and ZnS/PMBS polymers and (b) CdS/PMBA and CdS/PMBS polymers.

3.6 Differential scanning calorimetry (DSC) studies

The glass transition temperatures (T_g) of PMBS, PMBA, CdS/PMBS, ZnS/PMBS, CdS/PMBA and ZnS/PMBA (the content of functional monomer was 3 wt%, particle formation reaction time was 30 min) were obtained from DSC (Table 2). The T_g of PMMA is decreased by the introduction of BMA into the polymer chains. Subsequently, the PMMA-BMA copolymer can be easily bent (see Fig. 5). With the deposition of CdS/ZnS particles on the surface of the PMBA and PMBS films, the T_g of the composite films barely changes. This can be explained as follows. The inorganic particles only bond on the surface of the polymers, and do not enter into the bulk of the polymer. Therefore, they do not have any effect on the movement of the polymer chains and, naturally, they do not have any influence on T_g.

Table 2 T_g of the CdS/PMBS, ZnS/PMBS, CdS/PMBA and ZnS/PMBA films

Designation	PMBA	CdS/PMBA	ZnS/PMBA	PMBS	CdS/PMBS	ZnS/PMBS
Tg (°C)	38	38	43	42	38	40

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Fig. 5 Images of (a) a flat CdS/PMBS film, (b) a flat ZnS/PMBA film, (c) a bent CdS/PMBS film, and (d) a bent ZnS/PMBA film.

To further test the flexibility of the polymeric films, composite films were bent. Fig. 5a and c show flat and bent CdS/PMBS films. Fig. 5b and d show flat and bent ZnS/PMBA films. It can be seen that the inorganic CdS/ZnS particles do not influence the flexibility of the PMBS/PMBA films.

3.7 Thermogravimetric analysis (TGA)

The amount of CdS/ZnS on the polymer films was determined by thermogravimetric analysis. The films were heated from 30 °C to 750 °C at a heating rate of 10 °C min⁻¹. The residuals after 450 °C can be attributed to CdS/ZnS. The results indicate that the amount of CdS/ZnS particles on the polymer films increases with the increasing reaction time for the Cd(CH₃COO)₂/Zn(CH₃COO)₂ and TAA solution. Fig. 6 shows the TG curves of the pristine polymer and the CdS(ZnS)/polymer samples obtained after different CdS/ZnS formation reaction times.

Fig. 6 TG curves of the (a) CdS/polymer and (b) ZnS/polymer films obtained after different reaction times.

3.8 Optical properties of CdS/PMBS, ZnS/PMBS, CdS/PMBA, and ZnS/PMBA films

The transmittance of the CdS/PMBA, ZnS/PMBA, CdS/PMBS, and ZnS/PMBS composite films obtained after different reaction times in the cadmium/zinc acetate aqueous solution is shown in Fig. 7a–d. It can be seen that, at longer reaction times, the transmittance of the CdS/PMBA and CdS/PMBS films in the visible region decreases from 90% to 70%, the transmittance of the ZnS/PMBA and ZnS/PMBS films in the visible region decreases from 90% to 88%, and an absorption peak is observed at 252 nm in the ultraviolet region. When the reaction time to form CdS/ZnS particles is prolonged, the thickness of the CdS/ZnS layer increases, and the transmittance accordingly decreases. The transmittance of the ZnS/polymer films is higher than that of the CdS/polymer films.

Fig. 7 Transmittance of the polymer films under different Cd/Zn reaction times: (a) the CdS/PMBA film (AMA 1 wt%), (b) the ZnS/PMBA film (AMA 1 wt%), (c) the CdS/PMBS film (AMS 3 wt%), and (d) the ZnS/PMBS film (AMS 5 wt%); and the transmittance of the polymer films with different amounts of functional monomers and the same reaction time: (e) the CdS/PMBA film, (f) the ZnS/PMBA film, (g) the CdS/PMBS film, and (h) the ZnS/PMBS film.

The effect of the amount of functional monomers on the transmittance was also investigated. The transmittance of CdS/PMBA, ZnS/PMBA, CdS/PMBS, and ZnS/PMBS films obtained under 1%, 3% and 5% functional monomer is shown in Fig. 7e–h. It can be seen that the light transmission of CdS/PMBA and CdS/PMBS in the visible region is quickly reduced (from 90% to 60%) when the proportion of functional monomers increases. This might be explained as follows: the larger amount of functional monomer used, the more CdS particles coordinated to the polymer films. When the proportion of functional monomer increases, the light transmission of ZnS/PMBA and ZnS/PMBS films is slightly reduced (from 90% decreased to 85%).

Fig. 8a and b show the fluorescence spectra of ZnS/PMBA and CdS/PMBA composite films. The spectrum of the ZnS/PMBA composite shows strong fluorescence at 444.7 nm (excited at 369 nm); while that of the CdS/PMBA composite, shows strong fluorescence at 442 nm (excited at 369 nm). Both composite films display a broad Stokes shift.

Fig. 8 Typical FL spectra of (a) ZnS/PMBA and (b) CdS/PMBA (c) CdS/PMBA films.

3.9 Photo-degradation of MB

The photo-catalytic performance in the degradation of methyl blue (MB) in aqueous solution using CdS/PMBS and ZnS/PMBS composites was studied. A photo-catalytic cell using CdS/PMBS and ZnS/PMBS composite films (6 cm × 2.5 cm) as building blocks was fabricated as shown in Fig. 9.

Fig. 9 Photo-degradation cell fabricated using a CdS/PMBS film as the building block and used for the degradation of a MB solution.

35 mL MB aqueous solution (10 mg L⁻¹) was added to the cell, and irradiated under UV light³¹ at 40 °C. One milliliter samples were taken every 15 min and promptly analyzed by UV-Vis spectrophotometry. It can be seen in Fig. 10a and b that the largest absorbtion peak was at 660 nm, and that the rate of degradation in the presence of CdS was about 77% in 150 minutes. However, the degradation of MB in the presence of the ZnS/PMBS film was very slow; it was only 19.5% after 150 minutes. In Fig. 10c, it is shown that with increasing reaction times (CdS/ZnS formation times of 7 min, 15 min, 2 h, and 10 h), the degradation efficiency increased from 25.7% to 62.6%. This might be attributed to a larger number of CdS/ZnS particles formed on the polymers. Furthermore, the reusability of the cell was investigated after rinsing with water for five cycles. Fig. 10d shows how the degradation efficiency decreased from 62.6% to 26% when the cell was reused for 5 times. It can also be seen that, under the same conditions, the degradation efficiency of the CdS/PMBS film is higher than that of the ZnS/PMBS film. In addition, the degradation efficiency of CdS/PMBA and ZnS/PMBA films was tested. The results indicate that there is not a notable influence caused by the polymer base. This can be explained by the polymers acting only as the carriers for the CdS and ZnS particles.

Fig. 10 UV-Vis spectra of the degradation of MB: (a) degradation with CdS, (b) degradation with ZnS, (c) degradation efficiency for CdS/PMBS after different reaction times (formation of CdS particles on the film), and (d) degradation efficiency of the CdS/PMBS film after five cycles.

Compared to CdS particles loaded on SiO₂ particles, the degradation efficiency of our composite is a little bit lower.² This might be due to the CdS/SiO₂ particles having a higher specific surface area, which leads to a higher degradation efficiency.

4. Conclusions

Functional PMMA-BMA copolymers were synthesized via free radical polymerization. Later on, CdS and ZnS particles were formed on the surface of the as-prepared functional polymers. Thus, CdS/PMBS, CdS/PMBA, ZnS/PMBS and ZnS/PMBA films were subsequently obtained. The composite films showed good flexibility, fluorescence and photo-catalytic performance. The cell fabricated with the composite films could be used repeatedly just by rinsing with water.

Acknowledgements

The authors would like to thank the financial support from NSFC (41240026), the Scientific Research Foundation for the new employees of Southwest University (SWU114110), and the Open-fund of Guangdong Provincial Key Laboratory of Fishery Ecology and Environment (LFE-2014-2).

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