Pollutant degradation of different organic dyes using the photocatalytic activity of ZnO@ZnS nanocomposite materials

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Abstract

The current work focuses mainly to overcome the low surface properties drawbacks of Zinc oxide (ZnO) by spreading of ZnO nanoparticles on the surface of Zinc sulfide (ZnS) synthesized by sol-gel route using Cetyltrimethylammonium-Bromide (CTAB) as capping agent to form ZnO@ZnS nanocomposites (NCs) materials with new properties gathering between the two nanoparticles materials. ZnO@ZnS is an active semiconductor with high efficiency for removal of many organic dyes and pollutant degradation. Different techniques like XRD, BET, FESEM, PL, Uv-vis spectroscopy as well as photocatalytic activity of ZnO, ZnS and ZnO@ZnS results were investigated to confirm the nanostructure and homogenous distribution of particles inside the matrix. In addition, the scavenger study indicates that all charge carriers and reactive radicals contribute by roughly close extent, which explains the observed increase in the rate of degradation. Catalytic reactivity of ZnO@ZnS NCs exerts the same extent of higher efficiency for removal of methylene blue (MB) as well as Eosin dyes under UV lights. Furthermore, the Chemical Oxygen Demand [COD] study indicates that the majority of dyes were removed. The COD of the dye solution before and after the photo catalytic process was calculated, where it is found that COD reduces from 25.4 ppm for pure methylene blue dye to 11.1 ppm after 90 min and 2.4 ppm after 120 min irradiation by ultra violet applied on ZnO@ZnS NCs. Moreover, the recycled experiments for the photodegredation of eosin dye under ultra violet lights were performed to evaluate the stability of ZnO@ZnS nanocomposites after 5 successive cycles.

Introduction

Many challenges related to organic dye stuffs like resist biodegradation, type of dyes, dye concentration and rate of degradation have been widely studied. Different types of textile dyes and solvents, which may be carcinogenic, are being used worldwide. Workers are therefore, in continuous exposure to these industrial dyes, solvents and various other toxic chemicals. Data were collected using different sources and number of studies reported the association of textile industry and different types of cancer including lung, bladder, colorectal and breast cancer [1]. Organic wastes which founded in water such as organic dyes becomes one of the greatest problems that represents the biggest challenges against our universe lately, they contain various kinds of function groups that are mutagenic and very toxic [2], [3]. Enormous researches are prevalent to find a solution for removing these organic pollutants and degrade organic dyes by photo-catalytic methods under solar radiation or ultraviolet rays to produce less harmful products. Recently, researches which concern with photo-degradation of dyes have got a lot of attention. Observed results which indicate the increase in efficiency of this modified treatment route resulted from the non-selectivity of the photo catalyst nanoparticles used in the removal reaction; and often lead to the complete removal of dyes [4], [5]. Photocatalytic process, which is one of the advanced oxidation processes involves photo-catalyst excitation by ultraviolet irradiation or solar light with energy value higher than or on a par with its band gap energy (Eg) causing excitation of e from valence band (V.B.) level to the conduction band (C.B.) level leading to the existence of electron hole pairs [6], [7]. Various studies in this purpose done not only by using pure photo-catalysts materials, but also by improving catalytic activity of these materials via doping, such as doping of TiO2 with metals [8], [9], [10], [11], [12], [13]. Also, many researches which focused on ZnO as a photocatalyst confirmed that it has higher photo activity for the removal of several scavenger comparing to other photocatalysts [14], [15], [16], [17]. The photo-catalytic activity studies of other photocatalysts such as tin-oxide (SnO2), cadmium sulfide (CdS), tungsten tri oxide (WO3), indium oxide (In2O3), zirconium oxide (ZrO2) have been introduced to confirm that this way is the best in removing organic pollutants [18], [19], [20], [21], [22]. In this article, we present a study for synthesis, characterization and the photocatalytic activity of zinc oxide (ZnO), zinc sulfide (ZnS) nanoparticles (NPs) and how to improve the catalytic activity by doping ZnO in the surface of ZnS to form ZnO@ZnS nanocomposites (NCs) under the Ultraviolet light (UV) irradiation. The reason beyond using ZnS in this work is to use very cheap semiconductor materials with low concentrations, low temperature, high chemical stability with no binder additives, and potential for formation of electron-hole pairs to prepare our nanocomposites with mechanical binding methods comparing to other workers who used expensive doped materials for the nanocomposite preparation [23], [24], [25]. In addition, the aim of the present study was to investigate the removal of methylene blue as cationic model and eosin and anionic model which have organic structure in presence of UV lamp source and ZnO@ZnS as a photocatalyst where methylene blue and eosin give absorption peak at wavelength equal to 664 and 500 nm, respectively. The reason behind the selection of the organic compounds/pollutants (cationic and anionic models) in the current study is that they are industrially and commercially of relatively minor importance. Subsequently, the detection of the photocatalytic activity was done by UV–vis spectrometry. Several techniques like XRD, BET, FESEM, PL, UV-VS spectroscopy as well as photocatalytic activity were investigated for characterization of ZnO, ZnS and ZnO@ZnS samples. These techniques were used to provide information about the structure of pure ZnO, ZnS as well as the prepared ZnO@ZnS nanocomposites samples.

Section snippets

Materials

All the materials used in this work were highly pure and bought from Sigma Aldrich; including Cetyl trimethyl ammonium bromide (CTAB), Zinc acetate, sodium sulfide, hydrochloric acid, absolute ethanol, Ammonium hydroxide, Absolute ethanol. In addition, Methylene blue (MB) as well as eosin used as cationic and anionic models.

Synthesis of ZnO nanoparticles

ZnO nanoparticles were prepared by sol-gel route. Firstly, for hydrolyzing step, in a beaker, we mix 35 gm of zinc acetate (ZnAC2·2H2O), as Zn precursor source, in a mixture

Results and discussions

The X-ray diffraction (XRD) peaks obtained for the prepared ZnO nanoparticles (NPs) samples were clearly, refer to wurtzite structure and the sharp peaks indicate the high crystallinity of ZnO. Wurtzite ZnO reveals to the space group of P63 mc number 186, with cell constants of a = 3.11 Å and c = 5.28 Å hexagonal structure, all the observed peaks matched with the structure of Zinc Oxide standard card as shown in Fig. 1b. Furthermore, XRD of ZnS NPs shows 3 diffraction peaks appearing at 2θ=

Conclusion

In this study, pure ZnO and ZnS nanoparticles as well as ZnO@ZnS nanocomposites were synthesized by sol-gel method using CTAB as capping agent. The prepared nanoparticles were characterized by various developed techniques as XRD, N2-adsorption-desorption isotherm, Uv-vis, PL, FESEM, and HRTEM analysis. The doping of ZnO nanoparticles on ZnS surface was associated with a change in sample texture, creating supermicropores and decreasing the surface area. Absorbance and PL analysis reveal a

Author contributions

M.F.S. and A.E.S. gave an equal sharing to this work by pursuing the experimental work as well as writing the manuscript.

Notes

No contending financial interest proclaim by the authors.

Acknowledgments

British University in Egypt is appreciated by the authors for their assistance to follow up this study. Furthermore, the technical service units of Central Laboratory, Ain Shams University as well as CMRDI are also gratefully gratitude. Moreover, authors want to thank Ms. Esraa Samy, undergraduate student at Faculty of Science, Cairo University for her valuable discussion.

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