Synthesis And Optical Performance Of ZnO Nanostructures With Different Dimensions

Semiconductor nanomaterials with different dimensions have attracted much attention in recent years.Three-dimensional colloidal crystals or opals are generally applied to fabricate ordered porous materials.Porous materials based on colloidal crystal templating possess uniform pore size and highly ordered pore arrangement, which have many potential applications in separations,sensors,catalysis,bioscience and photonics.Using three-dimensional colloidal crystal template various types of ordered porous materials have been synthesized,including simple and ternary oxides, chalcogenides,non-metallic and metallic elements,and polymers.The wide-band-gap semiconductor zinc oxide is a unique material that exhibits semiconducting and piezoelectric dual properties.It has many potential applications in room-temperature ultraviolet laser,light-emitting diodes,sensors,transducers, biomedical sciences and photocatalysis.ZnO nanostructures with different dimensions have been extensively studied in synthesis,properties and applications. Ordered porous ZnO films are useful for the applications in sensors and semiconducting electrodes in virtue of the large internal surface area and improved mass transportation originated from size-controlled macropores.Titania as an attractive material has been investigated in recent years for its applications in photocatalysis,dye-sensitized solar cells and photonic crystals. Preparation of three-dimensionally ordered porous titania is very attractive especially in photonic crystals.The aim of this work is to prepare semiconductor ordered porous structures by colloidal crystal templating,and to prepare one dimensional ZnO nanostructures by thermal evaporation.The effects of the nanostructures on optical performance in these systems have been investigated.Our works are as follows:1.Monodisperse silica spheres were synthesized by a modified Stober method. Monodisperse polystyrene spheres were prepared using an emulsifier-free polymerization technique.The prepared silica and polystyrene spheres were assembled into three-dimensionally ordered colloidal crystals on the indium tin oxide (ITO)coated glass substrates by vertical deposition technique.The mechanism of the vertical deposition was discussed.2.We present a different method for the generation of two-dimensional ZnO ordered porous films by electrodeposition using three-dimensional opal templates. The hydrophilicity of the substrate and the hydrophobicity of the template are utilized to control the generated structures.The prepared 3D opal templates are hydrophobic, whereas the substrates are hydrophilic.During electrodeposition the aqueous solution can not wet the hydrophobic template,which blocks the electrolyte infiltrating the interstices of the opal template under high surface tension of the electrolyte.As a result,the substrate will be covered with only a thin layer of electrolyte within one-sphere thickness,in which the electrodeposition process is confined.This confined electrodeposition provides a precise control on the thickness uniformity of the porous film.The optical properties of the two-dimensional ZnO ordered porous films were investigated by photoluminescence and cathodoluminescence.3.Three-dimensional(3D)ordered macroporous ZnO films were prepared by electrochemical deposition in Zn(NO₃)₂ mixed ethanol-water solutions using polystyrene colloidal crystals as templates.Our prepared 3D ZnO inverse opals exhibited adjustable photonic band gap in the visible spectrum.It was found that the choice of the solvent influenced the surface morphologies of the ZnO porous films. The filling fraction of ZnO decreased with the increasing ethanol content in the mixed solution.The lower filling fraction of ZnO at higher ethanol content resulted in the decrease of the order degree of ZnO inverse opals.The excitation-power dependence of the near-band-edge emission in ZnO inverse opals and nanocrystal films has been studied.The dependence of the photoluminescence intensity I on the excitation power L can be described by a power law,i.e.,I～L^a,where a is an exponent that is often used to identify the origin of the near-band-edge emission from semiconductors in previous models.However,in this work,it was found that the values of a show a strong variation between ZnO inverse opals and nanocrystal films.According to the results of the photoluminescence excitation measurements,the ultraviolet emission of the ZnO inverse opals and nanocrystal films can be attributed to the first-longitudinal-optical phonon replica of the free exciton recombination.And our results show that the change of a is mainly caused by the laser heating effects.Therefore,the value of a could not be simply employed to unequivocally evaluate the origin of the near-band-edge emission in complex nanostructures.4.The fabrication of ordered porous titania films using polystyrene colloidal crystal templates by cathodic electrodeposition is reported.A new strategy using Ti(SO₄)₂ as Ti source for the potentiostatic electrodeposition of titania has been developed.In the electrodeposition the mixed ethanol-water solvent was employed to make the electrolyte wet the colloidal crystal template,thus the void space of the original template achieved a good filling resulting in the formation of a tough porous skeleton.After the electrodeposition,an improved post-treatment was utilized where the samples were dissolved in toluene before heat treating in order to obtain long-range ordered porous structure.The improved post-treatment of the electrodeposited samples made us to obtain long-range ordered porous films.5.Aligned ZnO nanorods and nanotubes were grown on the silicon substrates by thermal evaporation of zinc powders without any other metal catalyst.The growth process of the ZnO nanorods and nanotubes follows the vapor-liquid-solid mechanism. ZnO nanoneedle and nanoparticle films were synthesized by the same way,and their photocatalytic performances were tested for the degradation of organic dye methylene blue.The photocatalytic performance of the ZnO nanoneedle films was greatly higher than that of the ZnO nanoparticle films,TiO₂ films and flowerlike ZnO nano/microstructures.Repeated photocatalytic tests were executed to evaluate the recycled use of the ZnO nanoneedle films,which showed nearly undiminished photocatalytic activity after the repeated photocatalytic reactions.The decomposition kinetics of the organic pollutant was investigated by Langmuir-Hinshelwood model.