| As a kind of II-IV important semiconductor material with direct band gap,ZnO has excellent properties such as non-toxicity,low cost,high efficiency,etc.Its application range includes electronics,photocatalysis,rubber fields and so on.This paper reviews the characteristics,hazards and treatment methods of phenolic wastewater,the crystal structure and properties of ZnO,chemical synthetic methods of ZnO and its photocatalytic degradation mechanism of phenol.Solvothermal/hydrothermal method was used to synthesize ZnO hierarchical structures with different morphologies by changing the reaction time,reaction temperature,dosage of additives and other parameters.At the same time,ZnO was characterized by XRD,SEM,UV-vis diffuse reflectance spectroscopy and PL emission spectroscopy.Furthermore,the synthesized ZnO hierarchical structures was used for photocatalytic degradation of phenol under UV-Visible light irradiation,and then the mechanism of photocatalytic degradation of phenol by ZnO was explored.The main research contents are as follows:Hierarchical ZnO microspheres were synthesized via solvothermal method using anhydrous zinc chloride as raw material,ethylene glycol as solvent and potassium acetate as additive,and the reaction conditions were optimized by adjusting the reaction time and reaction temperature.The result shows that as-synthesized hierarchical ZnO microspheres were assembled of nanoparticles with wurtzite crystal structure.In the investigation of photocatalytic degradation of phenol,compared with the samples synthesized at other temperatures,ZnO synthesized at 180 ℃ displays obviously better photocatlytic activity.In the case of using 0.10 g the ZnO sample in 100 m L5 mg/L phenol solution,the degradation ratio was 94.5% for 150 min under UV-Visible light irradiation.The enhanced photocatalytic performance of ZnO microspheres could be supposed to the following reasons: the narrowerband gap(3.08 e V),the hollow structure being benefit of the absorption of photons,the smaller paiticle size(25.38 nm),rough surface being in favor of sufficient contact between the phenol molecules and the surface of the catalyst,and a large number of crystal defect sites on the surface being to the benefit of effective separation of photogenerated electron-hole pairs.hexagonal wurtzite type ZnO hierarchical structures were synthesized via hydrothermal method with zinc acetate dihydrate as raw material and sodium carboxymethyl cellulose(CMC)as crystal structure directing agent.The result indicates that the morphology of as-synthesized ZnO samples could be controlled by adjusting the mole ratio of Zn: CMC.With the increase of the amount of CMC,the morphology of ZnO assembled of nanorods changed from flower-like to cage-like and to cauliflower-top-like structures.In the investigation of photocatalytic degradation of phenol,especially the cage-like ZnO shows the highest activity for phenol degradation due to its lower band gap energy(3.21 e V),cage-like hollow structure,surface-rich crystal defect sites and the higher separation efficiency of the photo-generated electron-hole pairs.In the case of using 0.05 g the cage-like ZnO in 100 m L 5 mg/L phenol solution,the degradation ratio was99.8% after irradiation for 150 min.After 6 times of photocatalytic cycling experiments,the degradation rate of phenol by cage-like ZnO was still94.4%.The mechanism of photocatalytic degradation of phenol by ZnO hierarchical structures was explored.The capture experiments confirms that:1)·OH is the main active radical and plays a major role in the degradation process of phenol,·O2-radical has activity to a certain extent and plays a certain role,h+ has no obvious effect in the photocatalytic degradation of phenol by hierarchical ZnO microspheres synthesized at 180 ℃,27 h;2)·OH and ·O2-are the main active radicals,and ·OH plays a greater role than ·O2-,In addition,h+ also has a certain activity and plays a certain role in the photocatalytic degradation of phenol by cage-like ZnO. |
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