Microwave Assisted Preparation And Photocatalytic Degradation For Dyes Of NaNO-ZnO

ZnO had been widely used in photocatalytic degradation of pollutants. However, it took a long time for ZnO preparation in the present study. In addition, the ZnO samples that prepared by using traditional solvents such as water, ethanol and ethylene glycol could not achieve effective photocatalytic degradation of pollutants. To countered the problems above, the preparation method of nano-ZnO was optimized by using microwave assisted coprecipitation method with polyethylene glycol 400 (PEG 400) as solvent. Triphenylmethane dye Crystal violet (CV) and azo dye Congo red (CR) were regarded as target pollutants to test the photoctalytic degradation performance of nano-ZnO. Type of heating, zinc sources, precipitants, ratio of zinc salt and alkali, dosage of PEG 400, microwave heating temperature and calcination temperature were investigated to make clear the relationship between preparation conditions and morphological structure & photocatalytic performance of nano-ZnO. Then the effects of light source, dosage of catalyst, initial concentration of dyes, pH value and illumination time on CV and CR photocatalytic degradation were also studied in this paper. The concrete conclusions are as follows:(1) Main influencing factors of nano-ZnO preparation:Different precipitating agents resulted in different morphologies of nano ZnO. ZnO nanorods with better photocatalytic performance could be acquired by using NaOH as precipitating agent. Alkaline zinc ratio and the amount of PEG 400 have an important influence on particle size which is smallest when the alkali zinc ratio is 1:1, dosage of PEG 400 is 15 mL. When calcination temperature which effects the crystallization of nano particles is 350℃, nano-ZnO has better crystallinity and photocatalytic performance.(2) The influences of light source, dosage of catalyst, initial concentration of dyes, pH value and illumination time on dyes degradation were studied. The experimental results show that the photocatalytic degradation of dyes by nano-ZnO under UV light is preferable. ZnO could be used in a wide range of pH. With the increase of the catalyst dosage, dyes degradation increases at first and then keeps constant. The degradation rate is inversely proportional to the initial concentration of dyes because the color of dyes could absorb light. With the extension of illumination time, the degradation efficiency increases rapidly and then maintain steady.(3) The catalytic kinetics shows that the photocatalytic degradation of dyes by ZnO follows the pseudo-first order kinetic model. The photocatalytic mechanism of nano-ZnO is as follows:when ZnO is illuminated by UV light with wavelength equal to or less than 387 nm, electron-hole pairs generated by the energy of photon participate in the redox reaction after migrating to the surface of catalyst. A great deal of hydroxyl radicals are generated by the reaction between electron-hole pair and hydroxyl ions, water & dissolved oxygen. Organic pollutants are eventually degraded by hydroxyl radicals.