The Study Of Microwave Hydrothermal Synthesis Of Doped ZnO Nanoparticles And Their Propertis

In recent years, the problem of environmental pollution is increasing, howto control the environmental pollution friendly has become a hot topic inscientific research filed nowadays. Zinc oxide (ZnO) is a semiconductorphotocatalysis with a wide direct band gap (3.2eV) and a high free-excitionbinding energy (60meV). It is known as one of the important photocatalysts andwidely used in photocatalysis filed. However, it also presents some drawbackslike fast recombination rate of the photogenerated electron-hole pair and a lowquantum yield in the photocatalytic reaction in aqueous solutions, highlyrestraining its photocatalytic activity under visible light. Recently, how toenhance the photocatalytic activity of ZnO has drawn much attention fromresearchers all over the world. It is accepted that the surface area and latticedefects play important roles in photocatalytic activities of metal oxidesemiconductors. Researchers also found that doping is an effective and facilemethod to improve the photocatalytic properties because the variation of thesurface area. The incorporation of dopant ions are able to generate lattice defectsand variation of band gap energy. Consequently, doping of transition metals,noble metals and non-metals is a very expedient way to improve thephotocatalytic activity.n the present work, we use a novel microwave hydrothermal approach toprepare Cr3+, Sr2+, K+doped ZnO crystallites and try to reveal theirphotocatalytic activity under visible-light irradiation. The phase andmicrostructure of the as-prepared crystallites were characterized by X-raydiffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Ultraviolet-visible spectrum (UV-vis) andphotochemical reaction instrument were used to analyze the photocatalyticproperties of the particles. The results show that:(1) ZnO nanoparticles with different Cr3+doping concentrations (2,5,8and10%) were synthesized by microwave hydrothermal method. Results showedthat the diffraction peaks of Zn1-xCrxO nanoparticles shifted slightly toward thehigher angle. When the doping concentration was5%, the impurities appeared.The morphology of these sheet-like Zn1-xCrxO nanoparticles were found totransform from regular to irregular morphologies. UV-vis tests showed that theproducts had a high absorption in UV region and the optical band gap Egdecreased slightly. Comparing with undoped ZnO exhibited that thephotocatalytic properties of the Cr-doped ZnO had been improved greatly. The2%Cr3+doped ZnO photocatalyst exhibits the highest photocatalyticdecolorization efficiency with RhB concentration reduced by as much as96.4%after150min irradiation.(2) ZnO nanoparticles with different Sr2+doping concentrations (0.1%,0.2%and0.3%) were successfully synthesized via a microwave hydrothermalmethod. Results show that the diffraction peaks of the as-prepared Zn1-xSrxOcrystallites shifted slightly toward lower2θ angle. When the dopingconcentration was0.3%, the impurities appeared. The pure ZnO crystallites withlamellar structure are found transforming to a hexagonal columnar morphologywith the increase of Sr2+doping concentration. UV-vis analysis shows that theparticles have a higher absorption in UV region with a slightly decreased ofoptical band (Eg) gap. The photocatalytic activity of Sr2+doped ZnO crystalliteswas evaluated by the Rhodamin B (RhB) degradation in aqueous solution undervisible-light irradiation. Compared with the pure ZnO particles, thephotocatalytic properties of the Sr2+doped ZnO crystallites are obviouslyimproved. The0.1%Sr2+doped ZnO photocatalyst exhibits the highestphotocatalytic decolorization efficiency with RhB concentration reduced by asmuch as92%after180min irradiation.(3) ZnO nanoparticles with different K+doping concentrations (0.1%,0.3%and0.5%) were successfully synthesized via a microwave hydrothermal method.Results show that the diffraction peaks of the K+doped ZnO crystallites shifted slightly toward higher angle when increasing K+doping concentration. Themorphology of these Zn1-xKxO crystallites transforms from lamellar structure tosmall granules with higher absorption in UV region and narrower band gaps (Eg).Finally, comparing the products to pure ZnO particles, exhibit obviouslyimproved photocatalytic properties of the K+doped ZnO crystallites. The0.3%K+doped ZnO photocatalyst exhibits the highest photocatalytic decolorizationefficiency with RhB concentration reduced by as much as93.7%after80minirradiation.