Visible-light-responsive photocatalyst based on ZnO/CdS nanocomposite for photodegradation of reactive red azo dye and ofloxacin antibiotic

https://doi.org/10.1016/j.mssp.2020.105558Get rights and content

Highlights

  • ZnO/CdS photocatalyst was successfully prepared by a two-step method based on precipitation and hydrothermal route. .

  • Nearly 9°% of OFL antibiotic was photodegraded under solar light irradiation.

  • Photogenerated hole played the most important role in photodegradation of the pollutants.

  • The prepared photocatalyst exhibited good structural stability and reusability.

Abstract

Fabrication of ZnO/CdS nanocomposite with addition of sodium dodecyl sulphate (SDS) has been demonstrated. The ZnO photocatalyst was synthesized first by precipitation route and then the CdS photocatalyst was decorated on the surface of the ZnO via a hydrothermal technique. The PL intensity of the ZnO/CdS nanocomposite was lower than that obtained from ZnO. This indicates lower electron-hole recombination rate implying the improvement of photoactivity. The composite showed efficiency of 80% and 73% toward photodegradation of reactive red 141 (RR141) azo dye and ofloxacin (OFL) antibiotic, respectively. This is attributed to its high specific surface area together with suppression of charge carrier recombination. The efficiency of OFL degradation also reached 90% under solar light. The structural stability of the photocatalyst after three runs has been confirmed. The photogenerated hole played a crucial role in photodegradation of OFL antibiotic. The prepared ZnO/CdS photocatalyst shows a promising potential for environmental protection.

Introduction

Recently, it has been known that azo dyes were mainly found in industrial wastewater. The structure of the dye is generally stable. Incomplete degradation of the dye resulted in the existence of many harmful chemicals [1]. Therefore, removal of these dyes from natural water resources is urgently needed [2].

Fluoroquinolone-based antibiotics were mainly used for the treatment of diseases including bacterial infections for many years [3]. These antibiotics were discharged into the water after use. It was found that the antibiotics are not completely metabolized by living bodies. In addition, these drugs are non-biodegradable, and the residual part of the antibiotics can be accumulated in water. Therefore, the complete degradation of the pollutant is required [4]. In comparison to various conventional pollutant treatment methods, semiconductor photocatalysis have received considerable research interest due to its application for purification of water and air with the advantage of the clean technology [5]. To prepare photocatalyst nanostructures, agglomeration of the sample has been found if the crystal growth cannot be controlled. To solve the problem, addition of various capping agents during preparation step was introduced for controlling the morphology and photocatalytic property of the catalyst [6,7].

It is known that TiO2 with band gap energy of 3.2 eV shows only UV-light-responsive degradation of the pollutants [8]. Therefore, the photocatalytic performance of this photocatalyst is very low under natural solar light irradiation. ZnO photocatalyst, alternatively, shows advantages of good physico-chemical and photonic stability, low toxicity, and inexpensive [9]. Unfortunately, ZnO photocatalyst is active under UV light. This photocatalyst also shows fast electron-hole recombination rate implying low photocatalytic performance. Therefore, the preparation of visible-light-driven photocatalyst, which will be active under the natural sunlight (43% of visible light), is more promising [10,11]. Interestingly, preparation of composite photocatalysts based on either CdS photocatalyst [[12], [13], [14], [15]] or ZnO photocatalyst [[16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33]] have been reported.

Herein, we report the synthesis of ZnO/CdS photocatalyst with an excellent visible-light-harvesting capacity and a very high electron-hole separation efficiency. The prepared ZnO/CdS photocatalyst exhibited band gap energy of 3.41 eV and 2.24 eV corresponding to that of ZnO and CdS, respectively. The PL spectrum of the prepared ZnO/CdS photocatalyst showed lower peak intensity than that of the bare ZnO photocatalyst implying lower electron-hole recombination rate at the interface. Therefore, the expected photoactivity of the ZnO/CdS photocatalyst would be greater than that of individual CdS or ZnO photocatalyst. The prepared nanocomposite photocatalyst provided high efficiency of nearly 90% toward degradation of RR141 azo dye and OFL antibiotic. This photocatalyst can be used for three times demonstrating its promising photoactivity for removal of toxic dyes and antibiotics.

Section snippets

Materials

All chemical used were analytical (AR) grade.

ZnO photocatalyst

ZnO photocatalyst was prepared according to the previous work using a facile chemical precipitation method [33]. In detail, 0.4 M zinc nitrate hexahydrate [Zn(NO3)2·6H2O] solution (5.9732 g of zinc nitrate hexahydrate in 50 mL of water) was prepared first. Then, about 0.2889 g of sodium dodecyl sulphate (SDS) surfactant was added. After that, incorporation of 0.85 M NaOH solution (about 0.8529 g of sodium hydroxide in 25 mL of water) was performed.

Structural confirmation

The phase and crystalline structure of all samples were performed using XRD method. The XRD patterns in Fig. 1 showed that the individual ZnO and CdS nanoparticles were well matched with wurtzite structure (JCPDS File No. 10–0454 and No. 36–1451 for CdS and ZnO, respectively) [7,34]. Almost all peaks of the ZnO/CdS nanocomposite were well matched with the hexagonal phase of CdS photocatalyst. On examining ZnO photocatalyst, the peaks found at 2θ = 31.84°, 34.41°, 36.36°, 47.65°, 56.61°, 62.86°,

Conclusions

ZnO/CdS photocatalyst with excellent visible-light-harvesting capacity and high electron-hole separation efficiency was successfully prepared by using two-step method based on a chemical precipitation and a hydrothermal method. The prepared ZnO/CdS photocatalyst exhibited the band gap energy of 3.41 eV and 2.24 eV, due to that of ZnO and CdS, respectively. The PL spectrum of the prepared nanocomposite showed lower intensity than that of ZnO photocatalyst suggesting lower electron-hole

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We would like to thank Materials Chemistry Research Center (MCRC) and PERCH-CIC for financial support. T. Chankhanittha wishes to thank financial support from Science Achievement Scholarship of Thailand (SAST). T. Senasu would like to thank MCRC for the support. S. Nanan also would like to acknowledge partial fund from Research and Academic Affairs Promotion Fund (RAAPF), Faculty of Science, Khon kaen University, Fiscal year 2021.

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