Preparation of ZnO Photocatalyst for the Efficient and Rapid Photocatalytic Degradation of Azo Dyes
© The Author(s). 2017
Received: 31 December 2016
Accepted: 6 February 2017
Published: 21 February 2017
Zinc oxide (ZnO) photocatalysts were synthesized by sol–gel method using zinc acetate as precursor for degradation of azo dyes under UV irradiation. The resultant samples were characterized by different techniques, such as XRD, SEM, and EDX. The influence of preparation conditions such as calcination temperature and composite ratio on the degradation of methyl orange (MO) was investigated. ZnO prepared with a composite ratio of 4:1 and calcination temperature of 400 °C exhibited 99.70% removal rate for MO. The effect of operation parameters on the degradation was also studied. Results showed that the removal rate of azo dyes increased with the increased dosage of catalyst and decreased initial concentration of azo dyes and the acidic condition is favorable for degradation. Furthermore, the kinetics and scavengers of the reactive species during the degradation were also investigated. It was found that the degradation of azo dyes fitted the first-order kinetics and superoxide ions were the main species. The proposed photocatalyst can efficiently and rapidly degrade azo dyes; thus, this economical and environment-friendly photocatalyst can be applied to the treatment of wastewater contaminated with synthetic dyes.
Synthetic organic dyes are used in the textile, paper, plastic leather, food, and other industries. About half of these dyes are azo compounds, such as methyl orange (MO), Congo red (CR), and direct black 38 (DB38), which contain chromophore (–N=N–) in their molecular structures . However, effluents containing azo dyes are discharged into lakes, rivers, or ground waters during the dyeing process and contain many health hazards such as mutagenic and carcinogenic . These dyes can lead to very serious environmental problems, due to their good stability under ambient conditions. Therefore, scholars have focused on eliminating azo dyes from wastewater to satisfy stringent environmental regulations. Up to now, various treatment methods such as physical methods and chemical methods have been investigated to remove azo dyes [3–7]. However, these methods cannot completely destroy contaminants and only transfer dyes from the solution to the adsorbent; as such, the dyes are transformed into their carcinogenic, mutagenic, or toxic intermediates, which cause secondary pollution. Thus, inexpensive and environment-friendly processes for the complete conversion of pollutants must be developed.
Recently, photocatalysis can be conveniently applied for their degradation of dye pollutants because it can mineralize organic dyes completely into H2O, CO2, and mineral acids without bringing secondary pollution. Metal semiconductor materials, such as TiO2 , ZnO , Fe2O3 , CdS , and ZnS , are used as photocatalyst. These cost-efficient, effective, and environment-friendly materials can be used to alleviate environmental problems. It is reported that among various semiconductors, zinc oxide (ZnO) exhibits higher efficiency in the photocatalytic degradation of some organic dyes than TiO2 [13, 14]. Therefore, it is extremely possible that ZnO will become another photocatalyst after TiO2, which is widely applied to treatment of contaminants.
ZnO is a representative n-type semiconductor, with a wide band gap of 3.37 eV and a high excitation binding energy of 60 meV , and produces electron–hole pairs under UV light or visible light irradiation. The electron and hole can interact with the O2 adsorbed on the surface of the photocatalyst and H2O to generate ·O2 − and ·OH, respectively, which can reduce and oxidize the organic contaminants completely into their respective end products (CO2 and H2O, respectively) [16, 17].
ZnO nanoparticles are synthesized through various techniques, such as hydrothermal synthesis , homogeneous precipitation , and sol–gel method . Hydrothermal synthesis has many drawbacks, such as expensive equipment, large investment, large particle size, and poor dispersion . However, the sol–gel method exhibits wide application potential not only due to simple operation and mild conditions but also because of the narrow size distribution and excellent crystalline structure of particles synthesized by sol–gel . In recent years, research on ZnO has paid more attention on emphasizing the degradation of a separate azo dye over ZnO [1, 23, 24]. However, the degradation of ZnO for azo dyes containing different azo bonds has not been reported yet. Furthermore, some degradation conditions affecting the degradation of ZnO for different azo bonds dyes are worthy of discussion and analysis.
In this work, ZnO nanoparticles were prepared using the sol–gel method with zinc acetate as precursor for the degradation MO, CR, and DB38. The crystal structure and chemical properties of the samples were characterized using X-ray diffraction (XRD), scanning electron microscope (SEM), and energy-dispersive X-ray spectroscopy (EDX) analyses. Moreover, the photocatalytic activity of ZnO was evaluated using the degradation of azo dyes. The preparation conditions (calcination temperature and composite ratio) and degradation conditions (initial concentration of azo dye, dosage of ZnO, and initial pH) were also explored to analyze their effect on the degradation. The current study provides a basis for the application of ZnO as a photocatalyst to alleviate azo dye pollution.
Preparation of ZnO
ZnO was synthesized by the conventional sol–gel method. In a typical experiment, 2.196 g (0.01 mol) of zinc acetate was dissolved in 60 mL of EtOH and stirred at 60 °C for 30 min to obtain solution A. Solution B was prepared by dissolving 2.520 g (0.02 mol) of oxalic acid dehydrate in 80 mL of EtOH and stirred at 50 °C for 30 min. Solution B was added to the warm solution A dropwise and continuously stirred for 1 h. A white sol was obtained and aged to form a gel, which was dried at 80 °C for 24 h. Finally, ZnO was obtained by thermal treatment at different calcination temperatures of 300, 400, 500, and 600 °C. Solutions with different composite ratios (molar ratio of oxalic acid to zinc acetate), ranging from 2 to 5, were prepared while keeping the ratio of zinc acetate at 0.01 mol.
XRD patterns of all photocatalysts were collected in the region 2θ = 10°–80° using a Rigaku GiegerFlex D/Max B diffractometer with Cu–Kα radiation. The surface morphology of the samples was examined using SEM (JSM-6490LV, Japan) analysis at accelerating voltages of 20 kV. Elemental analysis of the sample was carried out using energy-dispersive X-ray spectroscope (EDX) (EDAX, GENESIS).
MO, CR, and DB38 were initially dissolved in water to prepare the 200 mg/L stock solution. The concentrations of various degradation solutions were measured by a UV–vis spectrophotometer (UV-5100). The concentrations of MO, CR, and DB38 were calculated based on the following calibration equations, respectively: (1) at 466 nm, (2) at 500 nm, and (3) at 595 nm. C = 0.0350A466 (1), and R 2 was equal to 0.9993. C = 0.0252A500 (2), and R 2 was equal to 0.9994. C = 0.0048A595 (3), and R 2 was equal to 0.9990.
Experiment of Radical Scavenger
To further study the photocatalytic mechanism of photocatalyst, main reactive species (radicals and holes) were detected through radical scavenging experiments in the photocatalytic process. The holes (h+), hydroxyl radical (·OH), and superoxide radical (·O2 −) are trapped by adding ammonium oxalate (AO) (h+ scavenger), tert-butanol (t-BuOH) (·OH scavenger), and p-benzoquinone (p-BQ) (·O2 − scavenger) into the reaction solution, respectively, during the process of photocatalytic degradation. Typically, 10 mg of ZnO and 10 mM of radical scavengers were placed into 50 mL of 30 mg/L dye solution; then, the suspension was irradiated using the UV lamp for the same time. Finally, the removal rate (η) of the dye can be calculated to determine the main role of active species.
Results and Discussion
Effect of Preparation Conditions of ZnO on MO Degradation
Crystallite size of ZnO nanoparticles under different conditions
Calcination temperature (°C)
Crystallite size (nm)
Weight% and atomic% results of ZnO under the optimal conditions
Effect of Operating Parameters on the Photodegradation of Azo Dyes
Dosage of ZnO
Initial pH of Solution
First-order kinetic constants and relative coefficients for photocatalytic degradation of azo dye over the photocatalysts
Stability of Photocatalyst ZnO
Mechanism of Photodegradation
Meanwhile, through the scavenging radicals, the main degradation pathway of ZnO is the decomposition of MO, CR, and DB38 by ·O2 −, which indicates that the mechanism of ZnO for the degradation of MO, CR, and DB38 is the same.
The photocatalyst ZnO prepared by sol–gel method exhibits simple operation, flexibility, and high photocatalytic efficiency. The photocatalyst ZnO prepared with the composite ratio of 4:1 and calcination temperature of 400 °C presents satisfactory photocatalytic properties under UV irradiation. Based on the XRD and SEM results, the ZnO contains hexagonal wurtzite and the size of ZnO was 20–50 nm. The removal rate of azo dyes increased, with increased dosage of the photocatalyst and decreased initial concentration of the azo dye. The acidic condition is more favorable for degradation than alkaline condition. The degradation of azo dyes on ZnO was fitted by the first-order kinetics. Moreover, cycle experiment and radical scavenging tests on the degradation indicated that ZnO still remains at high photocatalytic activity and stability for a long time and superoxide ions are the main reactive species indicating that the azo dyes have the same degradation mechanism.
This work was supported financially by funding from the International Scientific and Technological Cooperation Project of Xinjiang Bingtuan (2013BC002) and International Science and Technology Cooperation Program of Shihezi University (GJHZ201701).
XC was involved in the design, development of material and photochemical properties measurements, and manuscript writing; ZW supervised the whole work and helped in manuscript writing; and DL revised the manuscript. ZG carried out photocatalytic activity measurements of ZnO. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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