Dissertation > Industrial Technology > General industrial technology > Materials science and engineering > Special structural materials

Design Synthesis and Property Studies of Complex Micro-nanostructures

Author GuoTaiBo
Tutor ChenYiQing
School Hefei University of Technology
Course Materials Physics and Chemistry
Keywords (quasi)1D nanostructure Homoepitaxial micro/nanostructures Photoluminescence DSSCs
Type PhD thesis
Year 2012
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This dissertation systemically investigates the synthesis of severalone-dimensional semiconductor nano-materials and the analysis ofrelated dye-sensitized solar cells (DSSCs) devices. Hierarchical In2O3nanostructure, Sn-catalytic ZnS nanorods,3D homoepitaxial flower-likeZnS nanostructure, and In (Indium)-doped ZnS nanowire bundles, ZnOnanocombs and urchin-like Zn/ZnO core-shell micro/nanostructurenanostructures were prepared by the design vapor phase synthesis;TiO2-decorated ZnO nanorod arrays directly grown on zinc foil arefabricated by a two-step approach combining hydrothermal oxidation anda sol–gel process; TiO2nanotube arrays were prepared by the anodicoxidation of pure titanium foil. Their formation mechanisms have beenanalyzed in this thesis. The morphologies and microstructures of theas-prepared materials have been investigated by using of X-raydiffraction (XRD), scanning electron microscopy (SEM), andtransmission electron microscopy (TEM). The room temperaturephotoluminescence of the products has been studied and analyzed. Inaddition, the as-synthesized ZnO nanocombs, TiO2-decorated ZnOnanorod, TiO2nanotube and urchin-like Zn/ZnO micro-nanostructureshave been used to fabricate DSSCs devices and the photoelectricconversion performance of them is investigated. The following is the main contents of this dissertation.(1) Large-scale hierarchical In2O3nanostuctures have beensynthesized using vapor transport and condensation method without anycatalyst, taking advantage of the self-assembly property and epitaxialvapor-solid (VS) growth mechanism. XRD, SEM, and TEM revealed thatthe In2O3nanorods are single crystals with body-centered cubic (bcc)structure, epitaxially grow along <100> and <111> directions.Homoepitaxial interconnections were observed at the branched junctions,and the growth process of the nanorods arrayed on the microcrystals wasdiscussed, which was proposed to be a combination of “secondarynucleation” and VS process. The room-temperature photoluminescencespectrum of In2O3nanostructures exhibited ultraviolet emission at383nm and blue-green emission at423and487nm, which can be ascribed tothe near-band-edge (NBE) emission and the possible recombination of aphoto-excited hole with an electron occupying the singly ionized oxygenvacancies, respectively.(2) Sn-catalytic ZnS nanorods followed VLS growth mechanism,3Dhomoepitaxial flower-like ZnS nanostructure, and self-assembled In(Indium)-doped ZnS nanowire bundles were synthesized via a thermalevaporation method without using any template. The morphology andstructure of these ZnS nanostructure were characterized by XRD andSEM. Vapor-solid homoepitaxial growth was found to be the key reason for the formation of close-packed nanowire bundles grown on the surfaceof microscale sphere-shaped ZnS crystal. XRD and TEM analysisdemonstrate that the In-doped ZnS nanowires have the cubic structure,and there are numerous stacking faults along the <111> direction.Photoluminescence (PL) spectrum shows that the spectrum mainlyincludes two parts: a weak violet emission band centering at about380nm and a strong green emission band centering at about510nm.(3) TiO2-decorated ZnO nanorod arrays directly grown on zinc foilare fabricated by a two-step approach combining hydrothermal oxidationand a sol–gel process for dye-sensitized solar cells (DSSCs) applications.Its dye absorption and light harvesting are increased by decoration with aTiO2particle layer, resulting in enhancement of the photocurrent density.In addition, the open-circuit voltage (VOC) of the DSSCs is improved bysuppressing interfacial carrier recombination. As a result, the conversionefficiency (η) of the TiO2-decorated ZnO photoanode is increased by afactor of1.78compared with that of the bare ZnO. The electrochemicalimpedance spectroscopy (EIS) analysis shows that depositing TiO2particles on the surface of the ZnO nanorod arrays can effectively extendelectron lifetime and decrease electron recombination rate.(4) Bulk of double-sided ZnO nanocombs has been successfullysynthesized on FTO glass via thermal evaporation process without anyother metal catalyst. The characterization results show that the nanocombs consist of the belt-like stem and rod-like teeth arrays orderlyaligned to the stem. The morphologies of nanocombs strongly depend onthe temperature of substrates. The PL spectrum of the nanocombs showsan ultraviolet emission and broad green emission, and raman scatteringmeasurement was also used to indicate the phase purity and vibrationalproperties of ZnO nanocombs. The thin films of as grown ZnOnanocombs were used as photoanode materials to fabricate thedye-sensitized solar cells (DSSC), which exhibited an overall light toelectricity conversion efficiency of0.37%with a fill factor of37%,short-circuit current of1.65mA/cm2, and open circuit voltage of0.613V.(5) Highly ordered TiO2nanotube arrays were prepared by theanodic oxidation of pure titanium foil. The effects of oxidation voltage onthe morphology of the nanotube arrays were studied. The mechanism offormation of multipodal nanotubes is that capillary forces strong enoughto bend the TiO2nanotubes during the imbibition of electrolyte into andout of the intertubular spaces between adjacent nanotubes. And,dye-sensitized solar cells (DSSC) were assembled with the prepared TiO2nanotube arrays electrodes. The light-to-electrical energy conversionefficiency of the DSCC is increased by13%after surface treatment byTiCl4.(6) In this work, we report on the synthesis of urchin-like Zn/ZnO micro/nanostructure via a simple two-step approach combining physicalvapor deposition and thermal oxidation. The Zn/ZnO micro/nanostructureis composed of Zn microspheres serve as core directly adhesion to thefluorine doped tin oxide (FTO) glass substrates, and a shell made up ofnumerous radially ZnO nanorods arrays coverd on the surface of Znmicrospheres. Finally, Zn/ZnO micro/nanostructures are used asphotoanode materials to fabricate dye-sensitized solar cells (DSSCs). Theperformance of Zn/ZnO-based DSSCs is shown to be dependent on thedensity of ZnO nanorods, which can absorb more dye molecules.

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