Preparation And Sensing Properties Of TiO₂ And ZnO Nanostructured Films

The recent years have witnessed the rapid progresses for preparation and applications of nano-materials. Size effects from nanosize materials have impressively changed or improved the properties of original bulk materials, which have introduced many unprecedent terms into the traditional physical and chemical fields, among which the surface effect, small size effect and quantum tunneling effect are most concerned. It unwhelmingly bring forth the new era of nanophysics and nanodevices. So far, many newly emerged opto-electrical (or opto-chemical) devices based on oxide nanomartials such as ZnO and TiO₂, etc, have drawn much concerns worldwide, whose extensive applications include humidity sensor, optical sensor, new energy source and bio-medical science, and so on.Titannium dioxide (TiO₂) is a wide-band-gap semiconductor (Anatase: Eg=3.2eV, Rutile: Eg=3.0eV) , whose intrinsic light absorption band is in ultraviolet region with center wavelength of ³88 nm. It is beneficial to bring forth opto-electric effect or opto-chemical effect, which indicates TiO₂ is a promising material for fabrication of photovoltaics devices,photocatalysis devices,gas sensor. Morever, TiO₂ films with nanosize structure, show larger surface-volume ratio (aspect ratio) and superior adsorption ability, which prefigures the advantage of TiO₂ films with nanosize structure for application in abovementioned fields, in comparison with the bulk conuterpart. Nowadays, humidity sensor based on wide bandgap semiconductor film with nanostructure, such as SnO₂ and Al2O3, have been intensively studied, but this kind study is seldom carried on TiO₂. In our present view, humidity capacitive sensor based on TiO₂ film with nanostructure should show such outstanding advantages as small size, high sensitivity, rapid response, wide sensing region, direct electrical output, and convenient manipulation. Due to the larger aspect ratio and stronger photon adsorption capacity of TiO₂ films with nanosize structure, it is no doubt to see its promising application in opto-electronics as ultraviolet sensor, let alone its other advantages as simple fabrication procedure, easy doping, high sensitivity, high stability, etc.Znic oxide (ZnO) is another kind wide-band-gap semiconductor that has been greatly concerned. It shows many unique properties of direct bandgap of 3.37 eV, high exciton binding energy of 60 meV, high thermal conductance, and easy doping, wich makes it a promising candidate to be applied in fields of ultraviolet (UV) light emitters, gas sensors, transparent electronics, and surface acoustic wave devices. In adition, ZnO can be doped with transitional metal (Fe, Co, Ni, Mn), which is vital for fabrication of spin functional devices such as spin-polaried laser, which is also a intensively concerned study branch of ZnO. All in all, we should bear in mind that the preparation of ZnO with optimal microstructure is essential for realization of abovementioned devices.According to above consideratioin, this thesis is mainly organized on the preparation of TiO₂ and ZnO nanostructute films, followed by their application as humiditity sensor and ultraviolet sensor. The detailed works are spread out as follows:(1)TiO₂ nanotube films are fabricated by using electro-chemical method. As prepared nanotube films with high-aspect-ratio are well perpendicularly aligned on the Ti foil. The surface morphology, microstructure and chemical bonding states are characterized by means of SEM, TEM, and Raman spectroscopy. The formation mechanism of TiO₂ nanotube films is also studied.(2)Capacitive humidity sensor based on as-prepared TiO₂ nanotube films are fabricated, and its capacitance dependences on envirometal humidity and temperature are studied. Measuremnts are also performed on as-frmed sensor with different configuration of electrodes, namely dot electrod and comb-like electrod, which indicates humidity sensing properties are well enhanced (higher sensitivity and lower humidity hysterisis)using comb-like electrod.(3)Ultroviolet light sensor based on aminated TiO₂ nanotube films are fabricated, and the light sensing properties are studied at different experimental parameters of light wavelength, light strength, bias voltage. It is shown as-formed sensors show most optimal sensing properties at light excitation with wavelength at 365 nm. At simultaneous light excitation, the higher the biased voltage, and the higher sensor sensitivity.(4)The equivalent circuit of as-fabricated TiO₂ nanotube films humidity sensor is established based on our electrical measurement configuration, the humidity sensing mechanism at environmetal stimulation is analyzed. Especially the ultraviolet light sensing properties is analyzed by employing erengy band theory. (5)ZnO nanostructures are respectively formed by two different methods, namely the anodization method and the thermal growth method. It is shown by using the later method ZnO namostructure will be formed with advantages of easy fabrication procedure and large aspect ratio. By characterization of SEM, it is found the lower the mass ratio of Zn to C powder, the better the morphology of as-formed ZnO nanoneedles.