Thermal Oxidation Preparation, Growth Mechanism And Functional Properties Of Metal Oxide Nanostructures

Metal oxide nanomaterials with large surface area and quantum confinement effectexhibit novel mechanical, electrical, optical, magnetic, chemical and biological properties.They can be directly used in the sensors, electronic and optoelectronic nano devices with highoxidation resistance, low power consumption, and quick response. For commercialapplications, development of new growth technologies for mass production of nanostructuresis needed. These new synthesis techniques should be efficient, controllable, low coat,non catalyst, suitable for various materials, and without use of catalyst. Furthermore, thenanostructures should be able to formed at low temperatures and are easy to assembly ontovarious substrates for further characterization and applications.In this work, large area α Fe2O3, CuO and Co₃O₄nanostructures and MoO₃micro/nanostructures were synthesized by a very simple thermal oxidation method using ahotplate technique at relatively low temperatures in air condition. The morphologies andcrystal structures of these nanostructures were characterized via SEM, TEM, XRD, XPS,Raman spectrum, etc. The relationships between the structure and growth process wereinvestigated in detail. The growth mechanism of the thermal oxidation method was alsodiscussed deeply. In addition, the gas sensing, field emission （FE）, photoluminescence （PL）and magnetic properties of as prepared product were extensively studied. The detailed resultsobtained are as follows.1. α Fe2O3nanowire, CuO nanowire and Co₃O₄nanoflakes with large area weresynthesized on Fe, Cu, and Co substrates, respectively, by thermal oxidation method. MoO₃micro/nano plates with layered structure were also prepared on various substrates by thermalevaporation.2. The effects of processing parameters, including thermal oxidation temperature,duration time and metal source on the morphology and crystal structure of the metal oxidenanostructures were investigated in detail.（1） There is a temperature “window” for α Fe2O3nanostructure growth. The nanowiresor nanobelts with a sharp tip were obtained at a low temperature. The structure changes intonanoflakes when the temperature increased. All the α Fe2O3nanowires/nanobelts are singlecrystal structure and grew along the [110] direction.（2） CuO nanowires with an average diameter of30250nm and length of115m grewperpendicularly on the surface of the Cu sheets or powders. The diameter depend on the heating temperature, and the growth length follows a Parabolic law with increasing oxidationduration.（3） Co₃O₄nanoflakes were obtained by thermal oxidation Co thin film at280450°C.These nanostructures had a cubic structure and a thickness of2580nm and length up to1m.（4） Lamellar α MoO₃micro/nanostructures were synthesized by thermal vaporizationand deposition on different substrates. All of the orthorhombic α MoO₃micro/nanoplatesgrew along [001] direction, with width of about100nm to1μm and length up to100m.And the thickness of the lamella was about80nm.3. A “short circuit diffusion” mechanism was proposed to explain the1D metal oxidenanostructure growth via thermal oxidation. In general, the thermal oxidation process includesthree steps: Metal oxide layers were formed firstly, and then metal oxide nanostructuresnucleated on the grain defect of the outside metal oxide layer. Finally, multi layered structures,Cu/Cu2O/CuO/CuO nanowires or Fe/Fe3O4/α Fe₂O₃/α Fe₂O₃nanostructures, were formed.The nucleation and growth of1D metal oxide nanostructures are related to surface energy anddifferent growth rates along different crystal directions and are controlled by the diffusion ofthe metal and oxygen irons. The growth dynamic behavior of metal oxide nanostructuresshow that the growth process follows a parabolic growth law. However, the grow mechanismfor the MoO₃micro/nanoplates is the common VS based on the vapor deposition.4. The gas sensing, FE, magnetic and PL properties of the α Fe₂O₃nanowires/belts, CuOnanowires, Co₃O₄nanowalls and MoO₃micro/nanoplates were investigated in detail.（1） The sintering and thin film type gas sensors were constructed based on theas prepared α Fe₂O₃nanowires/belts and CuO nanowires, respectively. The optimum workingtemperature of the sensors based on α Fe₂O₃nanowires was about383°C in50ppm ethanol,with the sensitivity of2.4, response and recovery time of13s and6s, respectively. Theoptimum working temperature of the sensors based on CuO nanowires was about260°C in100ppm ethanol, with the sensitivity of1.64. Both the response and recovery time are lessthan3s.（2） Field emission properties of the α Fe₂O₃nanobelts prepared on different Fe substratesshowed that the turn on field （Eon） was14.5V/m, and the maximum emission currentdensity （Jmax） was17.4A/cm2for the α Fe₂O₃grown on the Fe foil directly. The fieldemission enhancement factor β was about1529, which determined on the tip shape and anappropriate density of the emitters.（3） The Morin transition temperature （TM） and Néel transition temperature （TN） of the as prepared α Fe₂O₃nanobelts were about113K and814K, respectively, which were muchlower than that of their bulk counterpart. The coercivity Hcdecreased sharply with increasingtemperature above TMdue to the temperature dependent magnetic anisotropy.（4） The PL spectra of the α Fe₂O₃nanostructures showed that a broad and intenseemission band centered at⁷90nm. There was an intense emission peak near the infrared areaat493nm, which shifted to the low energy side with the temperature increasing for the CuOnanowires. And the PL spectra of MoO₃micro/nanobelts excited by a250nm laser at roomtemperature showed a purple emission peak, a strong blue emission peak and a flat greenemission band and the violet emission band blue shift at the mean time. The green emissionintensity increased with the increasing deposition temperature. PL phenomenon of thenanostructures are different from their bulk counterparts mainly due to thequantum confinement effect, surface defects and impurities in the metal oxide nanostructure,or phase transition during the nanostructure growth.The current work shows that the thermal oxidation technique is a low cost approach forsemiconducting metal oxide nanostructure synthesis. The growth of the nanostructure can becontrolled by metal substrate selection and growing process parameters modification. Theas prepared nanomaterials have shown important potential in the applications of electrical,magnetic, optical and gas sensing devices.