Preparation of Zinc Oxide Nanorods Array and Its Application in Photocatalytic Degradation of Phenol Wastewater

ZnO nanorod arrays were prepared by the hydrothermal synthesis. SEM images were used to observe the morphologies of ZnO nanorod arrays. The phase structures of ZnO nanorod arrays were characterized by means of XRD. The phenol wastewater was degraded with ZnO nanorod as catalyst. The effects of UV light application time and ZnO nanorod catalyst quantity on phenol removal efficiency were investigated. The results show that the phenol removal efficiency is as high as 70.12% when 5.7 mg ZnO nanorod catalyst is added, pH is 7.3, and UV light application time is 2 h. The ZnO nanorod arrays prepared under the action of pulsed electromagnetic field (PEMF) have greater order than that prepared by conversional hydrothermal synthesis. The catalytic performance of ZnO nanorod arrays prepared with PEMF is better accordingly. Introduction Phenol is a kind of extremely nerve poison and causticity chemical substance, which is harmful to all life-form, especially to aquatic life. The characters of phenol wastewater are non-biodegradable, wide source, and seriously hazardous [1-4] . It is necessary for phenol wastewater treatment to find an effective technology. The treatment methods of phenol wastewater include traditional technology and advanced oxidation processes. However, traditional methods such as physical adsorption, coagulation, chemical oxidation and biological treatment are likely to cause secondary pollution [5,6] . Photocatalysis technique as an important advanced oxidation technology is widely researched in organic wastewater treatment. Owing to the small size effect, high surface effect and quantum size effect, nano particles exhibit excellent characters in catalyst fields [7-12] . Nanometer materials such as titanium dioxide and zinc oxide are usually used as photocatalysts to degrade organic pollutants [7,8,10-12] . Previous research has shown that the degradation effects of phenolic wastewater are satisfied using ZnO nanoparticles or ZnO thin as photocatalysts [12-16] . Especially supported nanometer ZnO is characterized by not easy to run off, easy to recover and separate, no secondary pollution in the process of wastewater treatment. In this study, ZnO nanorod arrays are prepared by the hydrothermal synthesis, and the ZnO nanorod samples are used as catalysts to treat simulated wastewater of phenol. Experimental Main chemicals. C6H6O (Tianjin Tianda Chemical Experimental Plant), ZnCl2 (Yanggu Zhongtian Zinc Industry Co., Ltd.), Zn(Ac)2 (Shanghai Xinbao Fine Chemical Plant), C2H5ONH4 (Tianjin Damao Chemical Reagent Factory), C11H13N3O (Tianjin Jinbei Fine Chemical Co., Ltd.) and K3Fe(CN)6 (Tianjin Tianyi Chemical Reagent Factory) were of analytical grade and used as received without further purification. Preparation of ZnO nanorod array. When reaction time was 4 h, Zinc chloride precursor solution concentration was 0.12 mol/L and reaction temperature was 90°C, ZnO nanorod array sample 1 was synthesized via a hydrothermal method [17] . The ZnO nanorod array sample 2 was prepared under similar conditions as those of sample 1, but with the presence of the external electromagnetic field(parameters: pulsed voltage 400 V, pulsed frequency 5 Hz, treatment time 60 s) which was used before the reactor was put into the constant temperature drying oven(see Figure 1). 7 Figure 1. The preparation scheme of ZnO nanorod arrays. Characterization of ZnO nanorod. X-ray diffraction was applied to determining the phase structure, performed on a D/max-Rigaku XRD diffraction spectrometer with 2θ ranging from 20° to 80°, using Cu Kα radiation(λ=0.154178 nm). Morphologies of the samples were studied by scanning electron microscopy (SEM) (HITACHI S-3000 N). Photocatalytic activity tests. The photocatalytic activities of ZnO nanorod samples were evaluated by the degradation of simulated phenol wastewater under UV light irradiation. The distance between ultraviolet lamp and wastewater liquid level was 8 cm. The reaction temperature was 20°C. In each experiment, the sample was put into phenol wastewater (100 mL) containing phenol (5 mg). According to Water quality—Determination of volatile phenolic compounds—4-AAP spectrophotometric method(HJ 503-2009), the volatile phenolic compounds remaining in the solution after the photodegradation process was measured using a 722N visible spectrophotometer. Results and Discussions Characterization of ZnO nanorod arrays. The ZnO nanorod sample 1 and sample 2 were characterized with respect to morphology by SEM. The SEM images of sample 1 and sample 2 are depicted in Figure 2 and Figure 3, respectively. The effects of PEMF on the morphology and size of the ZnO nanorod can be demonstrated with these SEM micrographs. The average diameter, average length and aspect ratios of samples are shown in Table 1. Table 1. The average diameter, average length and aspect ratios of samples. sample average diameter(nm) average length(nm) aspect ratio sample 1 30