Preparation, Characterization And Application Of Metal Oxide (ZnO And SnO₂) Semiconducting Nanomaterials

As is well known,many metal oxides are functional materials with excellent physical and chemical properties,and widely applied in many fields including catalysts,microelectronic devices,energy storage and conversion.Among these metal oxides,ZnO and SnO₂ have been considered as the most promising functional materials due to highly sensitive gas sensing property and excellent photoelectrical property.Both of them are n-type wide direct band-gap semiconductors(Eg = 3.37 eV for ZnO and Eg = 3.65 eV for SnO₂,respectively).Recently,much effort has been devoted into the fabrication and their potential application in nanodevices based on various nanostructures of ZnO or SnO₂.Considering their potential importance in future functional materials,ZnO and SnO₂ are selected as research targets of my thesis.In this thesis,our research focus on three fields including:(1) fabrication and characterization of sole ZnO or SnO₂ nanostructures;(2) construction and application of single ZnO or SnO₂ nanowire-based nanodevices;(3) fabrication and characterization of core-shell ZnO/SnO₂ nanocomposites with epitaxial relation and related hollow SnO₂ nanostructures.Major results have been summarized as follows:Chapter 1.Briefly review the basic characteristic and recent development of ZnO and SnO₂ nanostructures and clarify my research significance and detailed plan.Chapter 2.By using tin alklane(SnH₄) as the gaseous precursor,high-yield SnO₂ nanowires are successfully synthesized on the Au-coated Si substrates by means of chemical vapor deposition.Electronic microscope characterization results demonstrate that the growth of these SnO₂ nanowires follows the vapor-liquid-solid (VLS) mechanism.In addition,a kind of flower-like SnO₂ nanorod assembly can be acquired via a self-catalytic VLS mechanism.Cathodoluminescence analysis indicates the luminescence properties of the SnO₂ nanowires are related with the concentration of oxygen vacancies in the nanocrystals.Chapter 3.Three typical morphologies of ZnO nanostructures including tetrapods, nanorods and nanowires are successfully fabricated by means of various synthetic strategies.Combining electronic microscope characterization results with intrinsic features of wurtzite-type crystal,possible growth mechanisms of these ZnO nanostructures are proposed and deep discussed.In addition,their optical properties are carefully investigated by means of photoluminescence and cathodoluminescence.Chapter 4.Using ZnO or SnO₂ nanowires synthesized in chapter 2 and 3 as building blocks,single metal oxide semiconducting nanowire-based devices are successfully fabricated on the silicon substrates via photolithography and focused ion beam deposition(FIB).Measurement results indicate that the contact behavior between metal oxide nanowires and metal electrodes are markedly improved by FIB deposition.And SnO₂ nanowires belong to NTC thermal-sensitive material while they don't suit thermal-sensitive resistor because its calculated thermal-sensitive constant B is only 433 K.In addition,it is found that the photoconductance of single ZnO nanowire is more sensitive to 365 nm UV light than that of single SnO2 nanowire due to the difference between their generating mechanisms.Finally,transconductance(g_m) and electron mobility(μ_e) of single SnO₂ nanowire are successfully acquired based on field-effect-transistor measurement.This research work laid a good foundation for further investigation of nanowire-based devices on the application in chemical nanosensors.Chapter 5.Single ZnO and SnO₂ nanowire-based devices are applied for the detection of relative humidity and toxic gas in the environment.Humidity-sensing results indicate that the resistance of a single SnO₂ nanowire has linear response to the relative humidity of atmospheres.Humidity-sensing properties of the nanowire should originate from competitive physical adsorption between water molecular and oxygen molecular on the surface of nanowires.At the same time,a simple gas sensor array constructed with a ZnO nanowire-based sensor and a SnO₂ nanowire-based sensor on the same substrate is successfully built up by double FIB process although this sensor array failed in the detection of CO and H₂S mixture gas.In addition,it is found that sensitivity and selectivity of single ZnO or SnO₂ nanowire-based gas sensor could be improved by surface functionalization.This research work will accelerate the practicability of metal oxide semiconductor nanodevices. Chapter 6.Using a ZnO/SnO₂ core-shell heterostructure as an example,we demonstrate the possibility of establishing a three-dimensional epitaxial interface between two materials with different crystal systems for the first time and show possible tailoring optical properties by building the heteroepitaxial crystal interface. The electron microscopy characterization results reveal that as-prepared ZnO/SnO₂ heterostructure has a tetrapod-like ZnO core and a SnO₂ shell with 15-30 nm,and their special epitaxial relation is(010)_SnO2|(01(?)0)_ZnO and[100]_SnO2|[0001]^ZnO. In addition,a strong green luminescence in the 450～600 nm is induced by epitaxial interface.This research work will break a new path to fabricate epitaxial hetero -structures and tailor the luminescence properties of metal oxide semiconductors.Chapter 7.Using ZnO hexagonal micro-prisms and micro-pyramids as two typical templates,epitaxial growth of SnO₂ on three typical ZnO crystal planes including {01(?)0},±(0001) and {10(?)1} is systematically investigated.Various nanostructures of the epitaxial SnO₂ from nanoparticles,to self-assembled nanowire arrays,and to continuous single-crystalline thick films,are controlled prepared on ZnO template under appropriate kinetics conditions.Structural analysis reveals that lattice-mismatch between two epitaxial planes plays a crucial role in the growth of self-assembled SnO₂ nanowire arrays on the ZnO surface.SnO₂ has a preferential growth direction along the minimum lattice-mismatch direction(i.e.[2(?)0]ZnO| [001]_SnO2),and the width of SnO₂ nanowires is strictly confined by the accumulated strain which is induced by large lattice-mismatch.Cathodoluminescence characterization indicates that as-prepared ZnO/SnO₂ composite nanostructures exhibit different optical properties from the original ZnO templates or SnO₂ hollow shells.A clear understanding of the role of lattice-mismatch-strain in ZnO / SnO₂ system will inspire great interest in exploring other epitaxial metal oxide heterostructures and their potential applications.Chapter 8.Using prefabricated ZnO nanorod arrays as sacrificial templates, various SnO₂ nanotube arrays are successfully synthesized via two-step process including SnO₂ coating and hydrochloric acid etching.Depending on the size of original ZnO templates and experimental parameters,these SnO₂ nanotubes are constructed with four possible types:nanoparticles,nanorods,nanorings and single-crystalline film.Epitaxial growth of SnO₂ on ZnO surface plays a key role on the formation of SnO₂ walls.These hollow SnO₂ nanostructures are believed to have potential application in high-sensitive gas sensors and photocatalysts.Chapter 9.A simple and efficient approach for coating multiwalled carbon nanotubes(MWCNTs) with size-controllable SnO₂ nanoparticles by chemical vapor deposition has been developed using SnH₄ as the source of SnO₂ at 550℃.The size and coverage of SnO₂ nanoparticles can be adjusted by simply controlling the deposition time and the flow rate of SnH₄/N₂ mixture gas during the CVD procedure. In addition,by using the MWCNTs as a sacrificial template,a kind of one-dimensional chain-like SnO₂ nanostructure has been synthesized by increasing the deposition temperature to 730℃.This technique may provide a good way to produce tunable SnO₂-MWCNT composites.Chapter 10.Crumpled carbon nanosheets with 3～8 nm thickness have been successfully synthesized via a catalyst-free solvothermal route at a very low temperature range(60℃～100℃),using tetrachloromethane(CCl₄) as carbon source and potassium as the reductant.The X-ray powder diffraction pattern and Raman spectrum indicates that the products are hexagonal graphite.The result of BET experiment shows carbon nanosheets have a large surface area(97.2 m²Â·g^-1).In addition,various hollow carbon nanostructures were prepared by means of similar method where C₆Cl₆ acted as carbon source and metal sodium acted as reductant. Such hollow carbon nanostructures should have potential application in many fields including catalysts supporter and drug delivery.