Fabrication of ZnWO4-CdS heterostructure photocatalysts for visible light induced degradation of ciprofloxacin antibiotics

doi:10.1016/j.jiec.2016.03.043

Journal of Industrial and Engineering Chemistry

Volume 37, 25 May 2016, Pages 340-346

https://doi.org/10.1016/j.jiec.2016.03.043 Get rights and content

Highlights

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ZnWO₄-CdS heterostructure photocatalyst has been successfully prepared by hydrothermal method.
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The as-prepared photocatalyst reduces recombination of photo-generated electrons and holes.
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ZnWO₄-CdS composite photocatalyst shows remarkable photocatalytic activity and stability.

Abstract

ZnWO₄-CdS heterostructure photocatalysts have been successfully synthesized by hydrothermal method with assembling CdS on the surface of ZnWO₄. The obtained composite photocatalysts were characterized by X-ray diffraction, transmission electron microscopy, Raman, thermo-gravimetric analysis, UV–vis diffuse reflectance spectroscopy, photoluminescence and photocurrent measurement. The results show that the interface between ZnWO₄ and CdS is well formed, and the CdS nanoparticles are uniformly distributed onto ZnWO₄ nanorods which could facilitate charge transfer and reduce recombination of photo-generated electrons and holes. Compared with pure ZnWO₄ and CdS, the as-prepared heterostructure photocatalysts exhibit excellent photostability and photodegradation ability of ciprofloxacin (CIP) under visible light irradiation.

Graphical abstract

Introduction

In order to effectively utilize solar light and reduce the environmental pollution, exploration of photocatalytic materials is a hot and challenging work [1], [2]. A variety of mixed metal oxides photocatalysts, such as transition metal tungstates and ferrites, which are widely applied in gas sensors, optical fibers, humidity sensors, pigments and catalytic reactions, have been extensively studied [3], [4], [5]. Among these, as an important kind of tungstate, ZnWO₄ has been studied in the past several decades due to its unique properties and applications, for instance, as an X-ray photoanodes and solid-state laser host, searches for dark matter, double beta decay [6], [7], [8], [9], [10], [11], [12], [13], [14]. Moreover, in recent years, some reports have shown that broader light absorption range and higher electron mobility contribute to the high photocatalytic activity for water splitting and degradation of pollutants under UV irradiation. However, the wide application of ZnWO₄ is hampered because the low photocatalytic activity of pure ZnWO₄ is not enough for the requirements of practical application [5], [15], [16], [17].

With this demand, different modification methods such as doped ions, controllable morphologies, and coupled with other semiconductors have been considered to improve the photocatalytic performance of ZnWO₄. Huang et al. have synthesized the F-doped ZnWO₄ catalyst, and the strength of absorption in the UV region and transfer rate of photogenerated electrons to the surface increased after doping with fluorine. As for the case of F-doped ZnWO₄, it has been shown that the inclusion of this element in the oxide matrix improves the photocatalytic performance of the bare oxide in the degradation of RhB using UV light [18]. Moreover, because the morphology definitely influences the performance of the obtained materials, ZnWO₄ products with different morphologies have been synthesized and the photocatalytic performance has been investigated. Lin's group has investigated the effects on the photocatalytic performance by the controllable synthesis of ZnWO₄ nanostructure. The morphology and crystallinity of ZnWO₄ photocatalyst exhibit the significant effect on the photocatalytic degradation of Rhodamine B and gaseous formaldehyde. The ZnWO₄ nanorods showed higher photocatalytic performance than that of the nanoparticles. Distinctively, the photocatalytic performance of ZnWO₄ with different morphologies is mainly caused by their crystalline and surface area [16]. A novel Ag-AgBr/ZnWO₄ nanorod heterostructure has been synthesized by Li et al. The visible-light-responsive composite heterostructure photocatalyst has shown the efficient activity for the photocatalytic degradation of dye AR18 [19]. Also, Hojamberdiev's group has synthesized the ZnWO₄/Bi₂WO₆ composite photocatalyst by one-step hydrothermal method. The photocatalytic performance of the ZnWO₄/Bi₂WO₆ composite photocatalyst has been evaluated by degradation of gaseous acetaldehyde (AcH) under UV light irradiation [20]. These above mentioned studies provided new ways for improving the photocatalytic performance of ZnWO₄, and the real challenge is to find some semiconductors with high performance for modified ZnWO₄.

CdS, as one of the most important II–VI semiconductors, possesses the direct band gap of 2.42 eV at room temperature, and its unique structure could lead to novel properties in nano-electronic and photocatalytic field [21], [22], [23], [24]. However, the low separation of photo-generated electron–hole pairs and easy corrosion of CdS limit the wide photocatalytic applications. In recent years, many efforts have been devoted to the fabricate CdS nanomaterials by different methods with the desired size, structure and other semiconductors through different methods [25], [26]. By engineering the photo-stable materials that exhibit long-living excitons at elevated free energies, efficient photocatalysts should be achievable. Thus, it is highly desirable to develop ZnWO₄-based photocatalysts that can be used in visible light with improved photocatalytic efficiency and significant speciality. Many methodologies, including conventional solid-state reactions, coprecipitation and hydrothermal methods by microwave assisted synthesis have been extensively studied to prepare ZnWO₄-based nanocrystals that, although attractive, still face issues in efficient control of the crystal size, morphology, and chemical composition that are crucial for tunable electronic structures and physical properties. In addition, crystal structure, specific surface area, particle size, defective site and electron–hole recombination rate are also important for the development of highly active photocatalysts for solving environmental pollution problems.

Herein, we have been successfully synthesized the ZnWO₄-CdS composite photocatalysts by the facile hydrothermal method with assembling CdS on the ZnWO₄ surface. ZnWO₄-CdS heterostructure composites take advantages of both components and acquire more benefits than pure CdS, such as the uniform distribution of CdS nanoparticles on ZnWO₄ nanorods to facilitate charge transfer and reduce recombination rate of photo-generated electrons and holes, easy recovery of this composite due to the large size of ZnWO₄, the excellent contact between ZnWO₄ and CdS to prevent CdS photocorrosion. The as-prepared composites were examined as the photocatalyst for the degradation of ciprofloxacin (CIP) under visible-light irradiation. In addition, the synergic effect of CdS and ZnWO₄ have been investigated, and the possible mechanism was also proposed.

Section snippets

Materials

Cadmium chloride hemipentahydrate (CdCl₂·2.5H₂O), zinc acetate (C₄H₆O₄Zn·2H₂O), sodium tungstate dehydrate (Na₂WO₄·2H₂O), sodium sulfide (Na₂S·9H₂O), were purchased from Aladdin Chemistry Co. Ltd (No.196, Xinjinqiao Road, Pudong New District, Shanghai). Ciprofloxacin were purchased from Shanghai Shunbo Biological Engineering Co. Ltd. (No.36, Jingtai Road, Chongwen District, Beijing). In addition, all the materials used in the experiments were of analytical grade and directly used without any

XRD

Fig. 1 shows XRD patterns of photocatalyst samples prepared in different hydrothermal treatment times. From pure ZnWO₄, no peaks belonging to foreign phases were observed, which indicates the formation of single-phase monoclinic wolframite structure and structural parameters are in good agreement with the literature values (JCPDS file no.15-0774) for ZnWO₄ [27], [28]. And the pure CdS is attributed to the cubic phase of CdS (JCPDS no. 41-1049) [29]. From Fig. 1, the diffraction peaks of ZnWO₄

Conclusion

In summary, we have developed a two-step method to synthesize the ZnWO₄-CdS heterostructures as a photocatalyst for degradation antibiotics waste water-based solution under the visible-light irradiation. The heterostructure of ZnWO₄-CdS has displayed the significant role on photodegradation processes. Owing to the heterostructure, it not only enhances the photocatalytic activity of ZnWO₄, but also prevents pure CdS from photocorrosion. The heterostructure promoted the direct shift of electron

Acknowledgments

We gratefully acknowledge the financial support of the National Natural Science Foundation of China (No. 21576125), the Natural Science Foundation of Jiangsu Province (BK20131259, BK20130489, BK20140532), and the Research Foundation of Jiangsu University, China (No. 11JDG107).

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Fabrication of ZnWO4-CdS heterostructure photocatalysts for visible light induced degradation of ciprofloxacin antibiotics

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials

XRD

Conclusion

Acknowledgments

J. Alloys Compd.

J. Cryst. Growth

Mater. Chem. Phys.

Powder Technol.

Ceram. Int.

Appl. Surf. Sci.

Appl. Catal. A-Gen.

J. Power Sources

Sens. Actuators B-Chem.

Sep. Purif. Technol.

J. Catal.

Chem. Eng. J.

Appl. Surf. Sci.

Solid State Sci.

J. Mol. Catal. A: Chem.

J. Ind. Eng. Chem.

Mater. Res. Bull.

J. Am. Chem. Soc.

ACS Appl. Mater. Interfaces

Inorg. Chem.

ACS Appl. Mater. Interfaces

Cryst. Growth Des.

Fabrication of ZnWO₄-CdS heterostructure photocatalysts for visible light induced degradation of ciprofloxacin antibiotics