Controllable Fabrication And Characterization Of Functional ZnO Nanomaterials And Their Photocatalytic Activities

Photocatalytic oxidation for organic wastewater treatment is a promising technology with low cost,easy operation and non-secondary pollution.However,it is limited to actual application due to the low photocatalytic efficiency.Therefore,the development of highly effective photocatalyst has become one of the most important research subjects.ZnO,as a well-known photocatalyst for its high efficiency,non-toxic nature and low cost,has been paid much attention in the degradation and complete mineralization of environmental pollutants. The morphologies of inorganic materials are demonstrated to have great effects on their widely varying properties and corresponding potential applications.It is well known that ZnO exhibits the richest range of morphologies among the wide band gap semiconductors. Therefore,the fabrication of ZnO nanomaterials with special morphologies could effectively improve their optical properties and photocatalytic activity.In the present work, nanostructured-ZnO powders and films with particular morphologies were prepared and characterized.Meanwhile,the optical properties and photocatalytic degradation activities were investigated.In this dissertation,our investigations are carried out as follows:(1) Nanostructured ZnO samples with different morphologies were successfully synthesized by simple low-temperature hydrothermal routes in the absence of surfactants. Systematic experiments were carried out to investigate the factors that affect the morphology. We demonstrate that ZnO with different morphologies such as flowers and rods can be controllably obtained by simply varying the basicity in the solution.The as-prepared samples were characterized by X-ray diffraction(XRD),scanning electron microscopy(SEM), transmission electron microscopy(TEM) and selected area electron diffraction(SAED).We found that the ZnO nanoflowers contain many radial nanorods and these ZnO single-crystalline nanorods with the wurtzite structure grow along the[0001]direction.The formation mechanisms of ZnO nanoflowers and nanorods were initially interpreted in terms of the general theory of the crystal nucleation and crystal growth direction.(2) Nanostructured ZnO samples with different morphologies exhibit interesting optical properties.They were characterized by UV-vis diffuse reflectance(DRS),Raman, Photoluminescence(PL),X-ray photoelectron spectroscopy(XPS) and surface photovoltage spectroscopy(SPS).The obtained samples with different morphologies have significant potentials in photocatalytic oxidation of dyes and organic pollutants.ZnO with flower-like morphology exhibited improved ability on the photocatalytic degradation of dye and phenolic compounds in aqueous solution under UV radiation compared with ZnO nanorods.The higher photocatalytic activity of the ZnO nanoflowers results from the larger content of oxygen vacancy on the surface of 1D nanomaterials as revealed by their Raman,PL,XPS and SPS spectra features.It is suggested that oxygen vacancy may act as the active centers of the catalyst,which could capture photo-induced electrons,whereas the recombination of photo-induced electrons and holes can be effectively inhibited.(3) Highly oriented and large-scale ZnO nanorod arrays film have been successfully synthesized on zinc foil by a simple,low temperature,solution-phase approach.Here,zinc foil was used as not only a substrate but also zinc-ion source for the growth of ZnO nanorod arrays film by direct oxidation of zinc foil in an aqueous solution of formamide.This is a simple,self-source and low-cost method without the need of pre-casting ZnO nanoparticles onto the substrates as a seed layer and adding materials such as Zn²⁺-contained salts.X-ray diffraction(XRD) analysis,high-resolution TEM(HRTEM) photographs and selected area electron diffraction(SAED) patterns indicated that the ZnO nanorod arrays on the zinc foil substrate was single crystalline and grown predominant crystal orientation along the c-axis direction with the wurtzite structure.In addition,the possible formation mechanism of ZnO nanorod arrays was discussed.(4) The band gap of ZnO nanorod arrays film is 3.24 eV estimated by DRS,which is red-shifted compared with bulk ZnO(3.37 eV).The PL spectrum of ZnO nanorod arrays film appeared a dominant UV emission at 383nm and two weak visible emissions at 450 nm and 468 nm.Raman spectrum indicates that the remarkable E_2H mode of ZnO nanorod arrays film is located at 437 cm^-1,which corresponds to the characteristic band of hexagonal wurtzite phase,and the weaker peaks appear at 332,379,415 and 580 cm^-1,which can be assigned to the 3E_2H-E_2L,A₁(TO),E₁(TO)and E₁(LO)mode of ZnO,respectively.Furthermore,the photocatalytic activity of ZnO nanorod arrays has been tested by degradation of 4-chlorophenol(4-CP) under UV light irradiation compared to that of the disordered ZnO nanorods film grown on the Ti substrate,indicating that the as-synthesized ZnO nanorod arrays exhibit excellent photocatalytic activity.