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Study on Self-assembly and Property of 1D ZnO Nanostructures

Author: HeYing
Tutor: SangWenBin
School: Shanghai University
Course: Materials Science
Keywords: one-dimensional ZnO nanostructure self-assembling polymer complexation photoluminescence field emission antimicrobial photocatalytic degradation
CLC: TB383.1
Type: PhD thesis
Year: 2006
Downloads: 1318
Quote: 4
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Abstract


One-dimensional (1D) zinc oxide (ZnO) nanomaterials have received intensive interests due to their novel physical, chemical, and biological properties as well as the potential applications in nanodevices. Large-scale, cost-effective, simple and practical synthesis and assembly of one-dimensional ZnO nanomaterials are of importance for the fundamental research and application. In this thesis, the research progress in the preparation, property and applications of one-dimensional nanomaterials of ZnO was comprehensively reviewed. The self-assembling preparation, characterization, formation mechanism, kinetics and application attempt of 1D ZnO nanostructures were throughly studied. Moreover, their unique photoelectricity and photo-catalysis have been discussed. A series of new results are obtained:1. A novel process was reported for preparing oriented ZnO nanowires on various planes of silicon substrates by using polar polymer (such as PVA) as self-assembling complex medium. Well-distributed and uniform-diameter ZnO nanowires on a (111) or (100) planes of Si substrate have been successfully self-assembled and an ion complex transformation mechanism and polymer grid backbone localization model have been proposed. The self-assembling growth behavior of ZnO nanowires was well explained by polymer-controlled nucleation and crystal growth based on polymer grid backbone localization model. The key of the method allows us to synthesize highly regular stable nanoscale quantum dots because of PVA containing well-distributed ligand radicals on chain and the polymer chain with winding mesh structure in PVA subconcentrated solution, which limits impacting and growing scale of ZnO nanowires core. The well-oriented ZnO nanowires self-assembling grew on Si substrates by itself polar growth peculiarity and the polymer localized ZnO growth model. This technique has the advantages over the other reported methods because it solved the lattice mismatch problem of the ZnO to silicon, and was more economically feasible.2. Based on the ion complex transformation mechanism and polymer grid backbone localization model, well-distributed and uniform-diameter ZnO nanowires/nanorods on a Si substrate have been successfully self-assembled by using polymer as the medium via polymer complexation-vapour growth, polymer complexation-liquid growth and polymer complexation-sintering respectively. The ZnO nanowires/nanorods, with diameter in range of from 20 to 150 nm and the length from 0.5 to 6μm, evenly distribute on Si substrate and possess a hexagonal wurtzite structure which is consistent with the better results reported in the literatures.3. The influence of preparation condition on the growth behavior of ZnO nanostructures has been systematically investigated in detail. The investigation on the self-assembling formation mechanism of ZnO nanostructures revealed that the growth of ZnO nanostructures was in well accordance with the polar growth behavior and polymer grid backbone localization model. The effects of process conditions on the ZnO nanostructures were achieved by controlling the ratio of ZnO nuclei to growth units of [Zn(OH)4]2-, as well as the sizes of ZnO nuclei in the precursor solution. The ZnO nanostructures can be tailored by using different complexing materials. The regular wire-like, flower-like, flake-like and club-like ZnO nanostructures were obtained by using polymer (such as PVA and PAM), ammonia, trisodium citrate (TSC) and hexamethylene tetraamine (HMTA) as complexing media, respectively. Polymer kind and concentration are the key factors controlling the size and spacing of the 1D ZnO nanostructures. The weak base of the complexing solution at suitable pH value promoted the [0001] oriented growth of ZnO in the self-assembling process thus leading to the ZnO nanorods/nanowires. And the morphology of the ZnO nanocrystal prepared at different experimental conditions is different. It is found that only in polymer complexation-sintering and polymer complexation-liquid growth process, especially the polymer complexation-sintering process, the top of the ZnO nanowires appears smooth and the wires show hexagon columnar structure. Apparently, these nanowires show no layered structure compared to the others produced by polymer complexation-vapour growth process. The formation of the layered structure ZnO nanorods was explained by crystal growth theory.4. The optical properties of the 1D ZnO nanostructures were studied by photoluminescence spectroscopy and UV absorption spectroscopy at room temperature. The room temperature PL spectra of the ZnO nanomaterials prepared by polymer complexation process, excited with a wavelength of 325 nm, exhibit a strong UV emission of about 383 nm related to the band to band transition, and a weak blue emission of around 445 nm or a weak green emission of around 506 nm. The near band-edge UV emission might be attributed to a well-known recombination of free excitons, and the blue/green emission might result from the oxygen vacancies and zinc interstitial defects in the ZnO nanomaterials. The bands recombination is dominant in our samples, which indicates the integrity of the ZnO nanocrystal. The optical properties of our samples are consistent with the better results reported in the literatures.The UV-visible absorption spectra at room temperature show a well-defined exciton band at approximately 360 nm, which are blue-shifted about 13 nm than the absorption band at 373 nm for bulk ZnO. And with decreasing the nanowires diameter, a blue shift of absorbtion peak appears because of the quantum size effect of nano materials.And these nanomaterials have a photocatalyzed degradation of an organic dye, methyl red, under sunlight irradiation. The obtained results demonstrated that after about 120 min of irradiation, there is a nearly complete degradation of methyl red at the presence of ZnO, and the degradation percentage is about 100%. It suggested that by using 1D ZnO nanomaterials as photocatalyst, the environment can be purified with the degradation of the poisonous organic waste under sunlight or common light irradiation.5. The kinetics of the crystallization of ZnO nanowires obtained by using polymer complexation approach and the thermaldecomposition of the complex PAM-Zn2+ were investigated by means of differential scanning calorimetry (DSC) and thermogravimetry (TG), and the kinetics equations were 1-Xt =exp(-7.475×10-2t1.9) and, respectively. The relationship between the reaction rate and time, concentration and temperature has been realized. The calculated results are in accordance with the experiments.6. The application in photoelectric nanodevices of these 1D nanomaterials was investigated experimentally. The field emission of the ZnO nanomaterials obtained by using polymer complexation process has been investigated. It was found that the ZnO nanowires have a good electron field emission property. The turn-on field of the ZnO nanowires is around 2.2 V·μm-1, which is consistent with the better results reported in the literatures. These results also exhibit the potential application of 1D ZnO nanomaterials as field emission nanodevices in the future.7. By using ZnO nanowires/nanorods as additive, the modification behavior of the benzene-acrylic emulsion was studied in the first time. It is found that the properties of the waterproofing, alkaliproofing, washingfastness and rigidity of the benzene-acrylic coating have been improved by adding the 1D ZnO nanomaterials, especially the antimicrobial, antimildew and dielectric behavior increased significantly. The optimal properties of the benzene-acrylic/nano-ZnO hybrid emulsion coating were obtained when 0.06 % nano-ZnO was added to benzene-acrylic emulsion.

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