Facile And Green Synthesis Of ZnO Nanocomposites With Photocatalytic Degradation For 4-Nitrophenol

ZnO composites have been widely utilized on photocatalytic degradation of organic wastewater, which are considered to be an ideal photocatalytic material. However, the traditional preparation processes of ZnO composites were limited by its high energy consumption, long reaction cycle and poisonous additives. In addition, photocatalytic activity and solid-liquid separation performance of the synthesized ZnO composites were needed to be improved. To effectively counter the above problems, Fe3O4@SiO2@ZnO was synthesized by DEG additive and microwave assisted with Fe3O4 who was protected by the SiO2 layers as the carrier and ZnO to be the multifunctional shell. To research the effects of the amount of magnet and microwave processing on material synthesis and analyzed the optimized synthesis of materials by a variety of characterization means. The difficult biodegradation aromatic compounds of 4-nitrophenol was regarded as target pollutants to explore the optimal reaction conditions on photocatalytic performance of Fe3O4@SiO2@ZnO. And the reusability of catalysts, the mechanism of photocatalytic reaction and the model-fitting of catalytic processes were also investigated. The main conclusions were as follows:(1) The Fe3O4@SiO2@ZnO composites were facilely synthesized via combining the solvent thermal method and microwave-assisted process. The analysis of characterization and the results of photocatalytic experiment exhibited that the photocatalytic and magnetic response performance of Fe3O4@SiO2@ZnO were improved with increasing the amount of magnet. The overall performance of the synthesized materials achieved optimum when the amount of magnet is 0.2 g·g-1. Comparisons between different microwave processing showed that the materials with heated for 15 min possessed regular shape, excellent dispersion and photocatalytic activity. Optimizing synthetic Fe3O4@SiO2@ZnO as a composite material which owned core-shell structure with specific surface area of 27.52 m2·g-1, sizes distribution in 80-150 nm, a band gap of 3.21 eV, outstanding magnetic response performance, eximious crystallinity and dispersion.(2) The photocatalytic degradation tests of 4-nitrophenol demonstrated that Fe3O4@SiO2@ZnO responded to ultraviolet light efficiently and degradation rate rose up at first then gradually reached equilibrium by increasing of the catalyst dosage. High concentration of 4-nitrophenol molecules would hinder the contact of photon and catalysts causing the degradation efficiencies decreased gradually. Fe3O4@SiO2@ZnO could adapt to a broad pH range and the longer illumination time, the better degradation efficiencies. Low concentration of humic acid (HA) could effectively promote the photocatalytic activity of the catalyst. In comparison with TiO2, Fe3O4@SiO2@ZnO owned broader application range, better photocatalytic performance and stability. Furthermore, outstanding magnetic separation performance also helped to control the cost and reduce the harm of secondary pollution. Degradation efficiencies did not decline sharply even after eight successive runs and it could still reached 82.3% at the eighth cycle test. XRD patterns also did not change significantly before and after experiments. It could draw a conclusion that the obtained materials had strong stability and photocatalytic activity.(3) The photocatalytic degradation processes of 4-nitrophenol by Fe3O4@SiO2@ZnO and TiO2 were fllowed with pseudo-first order kinetic model which was indicated that the catalysts possessed weak surface adsorption performance. The respective reaction rate constant of Fe3O4@SiO2@ZnO and TiO2were 0.024 and 0.018 min-1 exhibiting that the photocatalytic degradation process of Fe3O4@SiO2@ZnO for 4-nitrophenol was faster. And the photocatalytic mechanism demonstrated that specific surface area was the main reason to improve the photocatalytic activity. In addition, Fe3+ could capture photogenerated electrons then suppress the rapid recombination of electronic and holes. Stability of carbon atoms on aromatic ring of 4-nitrophenol played a significant role to determine the path on catalytic degradation.