Original Research Paper
Zinc oxide quantum dots for textile dyes and real industrial wastewater treatment: Solar photocatalytic activity, photoluminescence properties and recycling process

https://doi.org/10.1016/j.apt.2020.04.017Get rights and content

Highlights

  • Zinc oxide quantum dots (ZnO QDs) were successfully prepared via a modified sol–gel method.

  • XRD and TEM proved the formation of high purity single phase of ZnO QDs prepared samples.

  • Quantum size has an interesting effect on photocatalytic activity processes by the fluorescent probe method.

  • The mineralization efficiency of real industrial wastewater carried out by sunlight for six months as a study case.

  • Recycle process for reusable of 8 times for ZnOQDs and nano ZnO prepared samples investigated and evaluated.

Abstract

Three samples of ZnO quantum dots (ZQDs) were synthesized by a modified sol–gel method at different temperatures for 3 h. The first sample (S1) was prepared at room temperature 27 °C, while second and third samples (S2) and (S3) were prepared by the calcination process at 500 and 900 °C, respectively. A study of XRD and TEM determines the purity, high crystallinity and the presence of elongated shape of the prepared catalysts. On using TEM, DRS and EBT analysis, the crystallite size values, bandgap energy, and active surface area were (7.1 nm, 3.49 eV, and 150.1 m2/g), (9.8 nm, 3.45 eV and 112.2 m2/g) and (13.5 nm, 3.39 eV and 78.94 m2/g) for S1, S2, and S3, respectively. The Photoluminescence properties showed that the fluorescence rate for S1 was doubled the observed one in the S2 sample. The photodegradation results of both methyl orange as an industrial raw material and real industrial wastewater of S1 sample showed the finest activity when compared with the rest samples. It was cleared from the collecting data that the photocatalytic performance decreases with the crystallite size increases The mineralization efficiency of the real industrial wastewater that exposed to sunlight for six months were evaluated according to the allowed COD limit for Egyptian Environmental Law, In addition the recycling process for reusable for ZnO prepared samples for 8 times investigated and evaluated.

Introduction

The environmental protection sector over the world is still having a disability for the problems that are generated from the huge quantities of wastewater discharged by the different industrial sites. One of the most harmful wastes is the effluent from the dye industries this is because it presumed to be a non-biodegradable material that have a high dangerous effect on either the ecosystem or human beings. Methyl orange (MO) is a colored dye, which is commonly used in the textile, photographic papers and printing industries [1], [2]. Also, it is considered as a model of dye that can be used for examining the photocatalytic performance of a photocatalyst [3], [4], [5], [6], [7].

Nano-heterogeneous photocatalysis using metal oxides as a base material, is considered as an effective trend in wastewater treatment depending on photocatalytic degradation of the organic pollutants. Where this process functionally is attributed to the wide bandgap, the environment-friendly nature, the low-cost, the biocompatibility of the semiconductor metal oxides. Also these compounds have the property to degrade the various types of pollutants under UV light [8], [9]. Among the nano-heterogeneous catalyst, nano zinc oxide particles has a wide bandgap located in the range of (3.37 eV) at room temperature, and this nano oxide compounds is considered as the most powerful photocatalyst in the fields of wastewater treatment, water splitting and photodegradation of organic dyes [10], [11].

Zinc oxide quantum dots (ZnO QDs) which their size is ≤ 10 nm have unique physicochemical and photoluminescent properties that are not observed for the micromolecules. Intensive interest directed to ZnO QDs preparation because of their 3D confinement structure that leads to the improvement of the optoelectronic properties such as exciton energy and radiative lifetime that depend mainly on the quantum size effect. Furthermore, according to “The particle-in-box” this quantization idea definitely describes the electronic structure changes with the particle size, where the holes and electrons are spatially confined and the energy levels become discrete, consequently, the absorption and the light emission undergoes pronounced frequency shifts as particle size decreases [12], [13], [14], [15], [16], [17], [18], [19].

ZnO QDs is used in different applications as in the bioscience cell labeling [20], antibacterial agent [21], and antifouling [22], [23] and also ZnO QDs used in the gene therapy as an emerging technique in cancers treatment [24], [25], [26], [27]. In more recent work, ZnO QDs used in some new applications for example in optoelectronic (light-emitting diodes as QD TV) [28], [29], photodetectors, UV lasers detectors and high-efficiency solar cells (expected reaches to 60%) [30], [31], [32], [33] and thin-film transistors [34]. In the present study, the structure, the surface and the optical properties were investigated for the prepared ZnO quantum dots (ZQDS) compounds. Where the work took a new steps in the direction of evaluating the tunable emission and measuring the photocatalytic activity of the prepared samples by observing and quantizing the photodegradation of methyl orange dye under UV–Visible (Xenon) irradiation. The work also never neglected the estimation of the photocatalytic efficiency and the recycling evaluation of the prepared samples in the quantum dot size in mineralizing real industrial wastewater and in dyeing factories that uses local commercial dyes under direct sunlight used as an irradiation source.

Section snippets

Materials

All chemicals used through the work, are analytical grade and used without further purification. Acetic acid, ammonia, propanol, Zinc acetate dehydrate [Zn(CH3COO)2·2H2O], lithium hydroxide monohydrate (LiOH·H2O), Ethanol (99.9%) and n-heptane purchased from Merck Company. Commercial Zinc oxide powder (99.9%) purchased from Fluka Company and the water used was de-ionized water.

Preparation of zinc oxide quantum dots samples

Zinc acetate dihydrate, lithium hydroxide monohydrate, ethanol used to synthesize ZnO QDs by sol–gel method. In the

XRDxxx

The characteristics of the crystal of the prepared quantum dot samples were investigated using X-ray diffraction analysis as shown in Fig. 1a. The XRD patterns were fully indexed to the wurzite structure of ZnO, JCPDS-ICDD index card No. 36-1451 [24], [35], [36]. The observed diffraction peaks at 2θ of 31.68, 34.4, 36.1, 47.58, 56.48, 62.61 and 67.92˚ identified as (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3) and (1 1 2) planes respectively. No other characteristic peaks of impurities

(b) Recycling of ZnO prepared samples during real industrial wastewater

The reusability of the photocatalysts (different ZnO prepared samples) investigated in Table 4 and Fig. 12. This recycling process is done for real wastewater during October 2019 for 6 hr/day (9 AM to 3 PM) which its COD value = 6312 ppm, and the evaluation recycling process after the repetition of using the photocatalysts for 8 times for recycling of real industrial wastewater treatment process.

From these results, It was noted that the mineralization efficiency by ZnO QDs (sample 1 = 7.1 nm)

Conclusion

Zinc oxide quantum dots (ZnO QDs) were successfully prepared via a simple sol–gel method at room temperature (27 °C) (7.1 nm) and annealed at different temperatures 500 °C (9.8 nm) and 900 °C (13.5 nm), respectively). XRD and TEM proved the formation of high purity single phase of all ZnO prepared samples. The average particle size of these samples was 7.1, 9.5 and 13.5 nm that increases with increasing the annealing temperature.

The photocatalytic efficiency of the prepared ZnO QDs against

Acknowledgements

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

References (45)

  • P.M. dos Reis et al.

    Photoluminescent and structural properties of ZnO containing Eu3+ using PEG as precursor

    J. Lumin.

    (2015)
  • J. Ma et al.

    Size-controlled nc-Si:H/a-SiC: H quantum dots superlattice and its application to hydrogenated amorphous silicon solar cells

    Sol. Energy Mater. Sol. Cells

    (2016)
  • F.Y. Fan et al.

    Synthesis and gas sensing properties to NO2 of ZnO nanoparticles

    Sensor Actuat. B-Chem.

    (2013)
  • W. Yang et al.

    Fast synthesize ZnO quantum dots via ultrasonic method

    Ultrason. Sonochem.

    (2016)
  • T. Hirai et al.

    Luminescence of excitons in mesoscopic ZnO particles

    J. Lumin.

    (2005)
  • K.L. Foo et al.

    Effect of different solvents on the structural and optical properties of zinc oxide thin films for optoelectronic applications

    Ceram. Int.

    (2014)
  • R. Wahab et al.

    Photocatalytic oxidation of acetaldehyde with ZnO-quantum dots

    Chem. Eng. J.

    (2013)
  • R. Wahab et al.

    Kaushik, Photocatalytic oxidation of acetaldehyde with ZnO-quantum dots

    Chem. Eng. J.

    (2013)
  • S. Singh et al.

    Applications of nanoparticles in wastewater treatment

  • D.M. EL-Mekkawi et al.

    Solar photocatalytic treatment of industrial wastewater utilizing recycled polymeric disposals as TiO2 supports

    J. Cleaner Prod.

    (2020)
  • A.A. Hanna et al.

    Synthesis, characterization and electrical properties of Zr doped ZnO nanoparticles and its effect on photodegradation of methyl orange

    Res. J. Pharm. Biol. Che.

    (2016)
  • E.F. Abo Zeid et al.

    The effect of CdO content on the crystal structure, surface morphology, optical properties and photocatalytic efficiency of p-NiO/n-CdO nanocomposite

    Res. Phys.

    (2019)
  • Cited by (0)

    1

    Analytical and Inorganic Chemistry Department – Al Azhar University, Cairo, Egypt.

    2

    Inorganic Chemistry Department – National Research Centre, Cairo, Egypt.

    3

    Photoschemistry and Nanomaterials Lab – Inorganic Chemistry Department – National Research Centre, Cairo, Egypt.

    4

    Photospectroscopy Lab – Inorganic Chemistry Department – National Research Centre, Cairo, Egypt.

    View full text