Hydrothermal synthesis of ZnO photocatalyst for detoxification of anionic azo dyes and antibiotic
Graphical abstract
Introduction
It has been reported that wastewater containing organic pollutants including dyes and antibiotics may greatly influence the quality of aquatic organisms [[1], [2], [3]]. It is known that the anionic azo dyes including reactive red 141 (R141) and Congo red (CR), utilized in various industries, can be found in the wastewater as an important environmental contamination [4,5]. The presence of high dye concentrations contaminated in wastewater may cause the carcinogenesis and mutagenesis [[6], [7], [8], [9], [10]].
Ofloxacin (OFL) antibiotic has been utilized as one of the pharmaceuticals for treatment of the diseases and then discharged into the water [11]. OFL is toxic to aquatic organisms because it is the non-biodegradable antibiotic [[12], [13], [14]]. Some residual parts resulting from the incomplete degradation of OFL can be accumulated in water. Therefore, the complete removal of the pollutant is needed [15].
It has been reported that adsorption and coagulation methods are used for removal of various organic pollutants [16,17]. However, the main drawback of these method is the incomplete degradation of the pollutants. Additionally, the creation of the secondary hazardous pollution in the solid form is possibly detected [16,17]. Alternatively, photocatalytic route has been gained much attention due to its beneficial of eco-friendly property by utilization of natural solar light for removal of the organic pollutants [[18], [19], [20], [21], [22], [23], [24], [25], [26]].
Among various photocatalysts, ZnO has received considerable attention due to its advantages of excellent transport property, inexpensive, and versatile morphological structure. However, there are some limitations affecting the practical use of the ZnO photocatalyst including low visible-light photoactivity, low quantum efficiency and serious inherent photo-corrosion [27,28]. Zinc oxide (ZnO) has been fabricated by both physical and chemical methods [[29], [30], [31], [32], [33], [34]]. Among various techniques, hydrothermal technique is utilized for the synthesis of nanoscale photocatalysts because it is green, cheap, simple, and easy to control [[35], [36], [37], [38], [39], [40], [41], [42]]. The synthesis of nanomaterials in large scale has been performed by utilization of a continuous hydrothermal route. In addition, the hydrothermally grown photocatalyst also exhibits higher crystallinity than the photocatalysts which are prepared by the other solution-based methods [43,44].
In this study, the hexagonal ZnO photocatalysts with band gap (Eg) of 3.19 eV were prepared by a hydrothermal method. The prepared ZnO-1 photocatalyst showed the highest performance toward degradation of RR141, CR, and OFL assigning to its greatest electron-hole separation efficiency at the interface which was confirmed from the PL spectra and the electrochemical measurements. To the best of our knowledge, the novelty of this work is based on the suppression of electron-hole recombination rate which was perfectly obtained by using the optimal Zn2+/OH− mole ratio. The photodegradation reaction correlated well with the first-order reaction. High structural stability after three times of use was also confirmed. The ZnO photocatalyst retained its high performance even after the third cycle suggesting its excellent cycling ability. This work demonstrated a promising route for fabrication of highly efficient photocatalyst for environmental protection.
Section snippets
Chemicals
All analytical grade chemicals and the deionized water (DI, 18.2 MΩ cm) were used.
Synthesis of the ZnO photocatalyst
The ZnO microstructure was synthesized using a hydrothermal route. In the case of the ZnO-1 photocatalyst, the mole ratio of Zn2+/OH− was fixed at 1:5. In a typical procedure, 16.45 mmol of Zn(CH3CO2)2·2H2O and 82.19 mmol of NaOH were separately dissolved in 30 mL of DI water with stirring for 30 min. After that, the sodium hydroxide solution was carefully added dropwise into the zinc acetate solution, followed by
Characterization of the ZnO photocatalyst
The phase structures of the ZnO microstructures with different Zn2+/OH− mole ratios were examined by using powder X-ray technique (Fig. 1a). The XRD pattern of the ZnO-1 photocatalyst correlated well with the hexagonal phase (JCPDS No. 36–1451) [52]. The characteristic diffraction peaks at 2θ = 31.94°, 34.59°, 36.38°, 47.62°, 56.68°, 62.90°, 66.40°, 67.93°, 69.12°, 72.62° and 76.96° indicated the reflection from (100), (002), (101), (102), (110), (103), (200), (112), (201), (004) and (202)
Conclusions
The ZnO photocatalysts have been fabricated by a hydrothermal method using the Zn2+/OH− mole ratio of 1:5, 1:12, and 1:18 denoted as ZnO-1, ZnO2, and ZnO-3, respectively. All prepared photocatalysts showed the hexagonal phase with the band gap energy of about 3.19 eV and the specific surface area of about 10.6 m2g-1. These photocatalysts remained stable even heating up to 1000 °C. The prepared ZnO-1photocatalyst exhibited the lowest PL intensity implying its lowest electron-hole recombination
Author statement
Theepakorn Sansenya: Methodology, Writing - original draft. Nataporn Masri: Validation, investigation. Tammanoon Chankhanittha: Methodology, Investigation. Teeradech Senasu: Investigation, Jirayus Piriyanon: Investigation. Siriboon Mukdasai: Investigation. Suwat Nanan: Conceptualization, Writing - review & editing, Supervision.
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
Financial supports from Materials Chemistry Research Center and PERCH-CIC are gratefully acknowledged.
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