Hot-Water-Induced Crystallization And Morpholoqy Transformation Of Amorphous Titania Nanowire Arrays

Titania thin films (TiO2) with ordered nanostructures find wide applications in the various environment and energy fields of photocatalytic wastewater treatments, air pollution remediation, photocatalytic water splitting, dye-sensitized solar cells, and lithium-ion batteries. The phase composition and the nanostructures affect directly the properties in service. The developments in fabrications of titania thin films with controlled nanostructures, which are appropriate for large scale productions, are therefore of great importance. In this thesis, the hot water induced crystallization and the morphology transformation of an amorphous titania nanowire array was studied, which led to the low temperature precipitation of titania thin films with controlled phase compositions and ordered nanostructures on titanium substrates. The amorphous titania nanowire array was synthesized by direct oxidation of metallic titanium substrates with hydrogen peroxide, into which trace melamine and nitric acid were added to guide the oriented growth of the nanowires under a low temperature. A subsequent treatment of the amorphous nanowires with distilled water resulted in titania arrays consisted of porous nanorods, which are a mixture of anatase and rutile. When HCl or H2SO4was utilized to adjust the pH value of the hot water, titania arrays of nanorods or nanoflowers, with controlled sizes, can be achieved. The morphology, structure, phase composition and photon-induced property of the achieved titania were investigated in detail with various techniques of field emission scanning electron microscope (FE-SEM), high-resolution transmission electron microscope (HR-TEM), X-ray diffraction (XRD). UV-Vis diffuse reflectance spectra (UV-Vis DRS), X-ray photoelectron spectra (XPS). photoluminescence spectra (PL), and Raman spectra. The photocatalytic activity was evaluated by photodegradation of rhodamine B in water.Follows are the main results obtained:1. Crystallized titania nanorod arrays achieved by treatments of amorphous nanowires in deionized water and HCl solutions.Crystallized titania nanorod arrays were achieved by immersing the amorphous titania nanowire arrays in deionized water at80℃. Increasing the amount of deionized water or prolonging the reaction time, the diameter of nanowires became larger till all of them transformed to rutile nanorods. Nanoparticles appeared on the top of nanowires/nanorods at the same time, and the quantity increased accordingly, leading to also an improved crystallinity. Nanorod arrays of phase pure rutile were achieved by immersing the amorphous titania nanowire arrays in diluted HCl solutions at80℃for25～72h. The nanorod size can be controlled via alteration of pH values or reaction time. The nanorod arrays obtained in HCl solutions possessed an activity to assist photodegradation of rhodamine B in water higher than that of the calcinated nanowires, as well as the P25counterpart.2. Crystallized titania nanoflower arrays achieved by treatments of amorphous nanowires in diluted H2SO4solutions.Crystallized titania nanoflower arrays were achieved by immersing the amorphous titania nanowire arrays in diluted H2SO4solutions at80℃for24～72h. The size of nanoflowers, as well as the phase composition, can be regulated by changing the pH value (concentration) of H2SO4. The size of nanorods constructing the nanoflowers decreased with increasing pH values from1.0to2.0. The weight percentage of anatase reduced from58%to29%. and the indirect energy gap decreased from3.13eV to3.0eV with the increasing pH value. A slightly improved photocatalytic activity to assist photodegradation of rhodamine B in water was achieved for the pH2.0-derived nanoflowers, when compared with that of the pH1.0-derived one.