| TiO2 has a wide range of potential applications in areas of photocatalytic and photoelectrocatalytic degradation of organics, gas sensors, thin film solar cells, lithium-ion batteries, electrochromic and etc. The crystal structure, specific surface area and morphology of TiO2 are key factors that influence the phototcatalytic efficiency or photoelectric conversion efficiency. It is valuable in both theory and practice to fabricate well-crystallized TiO2 with large specific surface area at low temperatures. On the other hand, extending the optical response of TiO2 into visible light range is an effective way to enhance the quantum yield. Utilizing certain semiconductors with appropriate bandgap width to match that of TiO2 is an alternative to realize the visible light absorption.In this thesis, TiO2 with various nano/micro-structures of nanorods, nanosheets, nanobelts, microspheres and multi-faceted microballs were synthesized via low-temperature oxidation of Ti metal and their photocatalytic properties were studied. Composite thin films consisted of TiO2, but with different fabrication history and hence different morphologies and/or crystal structures, were prepared by embedding sol-gel nanoparticles into both porous TiO2 and gaps among TiO2 nanorods. TiO2 arrays of nanorods and nanotubes were also combined together to achieve a dual-layer structure. The photocatalytic and photochemical properties of such "homogeneous" composites were studied in detail. Pyrite FeS2 was used to composite with TiO2 nanotube arrays to enhance the visible light absorption. CdS was used to composite with TiO2 arrays of both nanorods and nanotubes, intending to investigate the influences of TiO2 nanostructures to the property of the resultant "heterogeneous" composites. The main contents are summarized as follows:(1) Low-temperature fabrications of TiO2 with various nano/micro-structures of nanorods, nanosheets, nanobelts, microspheres and multi-faceted microballs, and the corresponding growth mechanisms:One dimensional single-crystalline rutile TiO2 nanorods were derived by low-temperature reactions between Ti substrates and H2O2, which is believed to grow via an "oriented attachment" mechanism. Two dimensional TiO2 nanosheets and nanobelts were obtained through hydrothermal treatment of TiO2 nanorods in NaOH solutions under different temperatures, whose morphologies related closely to nucleation and growth of layered titanates. Three dimensional TiO2 microsphere were achived by chemically induced self-assembly when Ti(Ⅳ) precursor solutions derived from Ti-H2O2 interactions was hydrothermally treated in NaOH solution. Multi-faceted TiO2 microballs can be obtained by a "Kekendaer" diffusion mechanism when etching Ti substrates with HF solutions.(2) Degradation of selected target organics with various TiO2 nanostructures:Rhodamine B (RhB), methylene blue (MB) and methyl orange (MO) were chosen as target dyes for photodecompositions. Photocatalytic abilities of one dimensional TiO2 nanostructures were compared with TiO2 nanoparticls. All the TiO2 thin films exhibited a "natural aging" effect except decomposing MO, which is more reluctant to degradation when assisted by the present TiO2. MB decomposed much quickly than RB when assisted by titania with predominantly anatase; on the other hand, rutile favored the photodegradation of RB than that of MB. Titania thin films with top morphologies of quasi-aligned nanorods or nanotubes were found to possess advantageous turnover frequency and photonic efficiency over commercial P25 titania and sol-gel derived titania to assist photodegradation of RB and MB in water. When degradating RhB,600°C-calcined monodispersed TiO2 micropheres possessed higher photocatalytic efficiency than that of DP25, but lower efficiency when phenol was used as target molecules. The photocatalytic activity of multi-faceted TiO2 microballs was lower than that of DP25.(3) "Homogeneous" TiO2 composite films and the synergetic effect:Composite films were prepared by spin-coating a layer of sol-gel anatase TiO2 nanoparticles on porous anatase TiO2 substrate. Under UV irradiation, the photoelectrochemical response of the composite film was the most outstanding. The photocurrent density was more than twice that of the two single component films. The RhB degradation efficiency of the composite film was also two times that of the two separate components and 3.5 times that of the sol-gel film with the same quantity coated on Ti substrates. This indicates that, an appropriate configuration of two titania layers derived by distinct routes, even if the titania is of the same crystal structure such as anatase, is possible to improve significantly the photoelectrochemical response as well as the photocatalytic activity, providing that the band gap of each titania is aligned properly so that the charge separation efficiency can be enhanced significantly.Another kind of composite film was fabricated by filling the sol-gel anatase nanoparticles into gaps among the nanorod arrays. Under UV light illumination, the photocurrent density of the composite film was more than twice that of the two separate components. Anatase nanoparticles have better optical and chemical properties, and single-crystal rutile nanorods have higher electron transfer ability. The combination of the two components in an appropriate way enhanced significantly the charge separation effect arising from the anatase/rutile couple, which hence combined efficiently the "mixedcrystal effect" and the "mixed morphological effect".(4) "Heterogeneous" TiO2 composite films and the synergetic effect:Pyrite FeS2 was successfully sensitized onto TiO2 nanotubes to fabricate composite films with ideal visible light absorption ability. The UV-Vis spectra showed that the absorption edge of the composite films was expanded from the original 380 nm to 425 nm. Under simulated solar light illumination, the composite film exhibited increasing photocurrent and decreasing open circuit voltage when compared with the TiO2 nanotube array. When used to decompose RhB under visible light, the composite thin film has much higher photocatlytic efficiency than that of TiO2 nanotubes.Composite thin films of CdS/TiO2 were fabricated by an electrodeposition appraoch. CdS nanoparticls were deposited on TiO2 nanorods and nanotubes and their photo-induced properties were studied. It indicated that, single-crystalline rutile nanorods were more advantageous to improve the photoelectrical performance of the composite films than the polycrystalline nanotubes.(5) Photoelectrocatalytic decoposition of RhB assited with low-temperature derived anatase TiO2 thin films:A comparative study of photocatalytic (PC) and photoelectrocatalytic (PEC) degradation of RhB in water assisted with low-temperature derived crystalline TiO2 thin films with different thickness and calcined at different temperatures were carried out. It was found that the PEC efficiency can be improved with an applied potential of 0.4 V, independence of the film thickness and the calcination temperature. The PC degradation rate increased with increasing film thickness and reached a stable value; whilst the PEC efficiency was the highest corresponding with an optimal film thickness. This is because the photogenerated electrons have to migrate through an increasing length within the TiO2 film to the conductive Ti substrate, which inhibited the positive contribution of the applied external bias potential. With increasing calcination temperatures, the crystallinity improved, the conductivity increased and the crystal size grew larger. The interactions of the three factors lead to different optimal heat treatment temperatures of TiO2 thin film for the PC and PEC procedures.
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