Structure Control And Photocatalytic Performance Of Titania Thin Films

Over the past few decades, with the development of human society, growth of population and advancing of global industrialization, attendant problems such as environmental pollution and high energy consumption are becoming more and more prominent. Looking for an effective way to control pollution and achieving cleaner model of sustainable development has become the consensus of the modern society. Titania ?TiO₂? has been widely used in many fields such as photocatalytic environmental protection due to its high stability, low cost, environmental friendliness, wide application and excellent photocatalytic performance. TiO₂ nanopowders enjoy advantages such as having large specific surface area and high catalytic activity. But powders are easy to agglomerate and difficult to be completely recovered, which may result in secondary pollution. TiO₂ thin films are easy to be recycled and can be reused without secondary pollution. However, the preparation process of films is relatively complicated, and the photocatalytic performance of the films is not as high as the powders, thus limiting its large-scale industrial applications. Therefore, developing TiO₂ films with simple preparation process, good adhesion with substrates and excellent photocatalytic performance is of important scientific research significance and engineering application value.For TiO₂ thin films fabricated via in situ oxidation of metallic titanium substrates, a high interfacial strength can be achieved. A series of one/three-dimensional TiO₂ nanoarray thin films have been obtained through the in situ oxidation approach, including but not limited to nanotubes, nanowires, nanorods and nanoflowers. This dissertation mainly focused on the regulation of phase structure and nano-sized morphology of TiO₂ thin films prepared through direct oxidation of titanium substrates by using hydrogen peroxide solutions, as well as optimizing the photoelectrocatalytic performance of TiO₂ thin films by applying synergetic effects of phase junctions and one/three-dimensional nanostructures, so as to lay good foundations for successful utilization of TiO₂ photocatalysis in the field of environment protection and energy application.First of all, the working principle of photocatalysis was summarized; the research status of preparation process, doping methods, regulations of phase structure and nano-sized morphology along with related theories of TiO₂ as photocatalyst were reviewed. On this basis, the research background was put forward, and the main research ideas involved in this dissertation were elaborated.A successful fabrication of rutile TiO₂ monolayer nanoflowers was reported by utilizing as-prepared solutions of reactions between titanium and a specific hydrogen peroxide solution as precursor. The barrier layer which consisted of TiO₂ nanoparticles between the film and the substrate was eliminated. The formation of nanostructures including nanorods and nanoflowers of the TiO₂ monolayer can be explained by the oriented attachment theory. The specific morphology of the film was closely related to the pretreatment of titanium substrate. When utilizing dilute nitric acid instead of mixed acid for cleaning titanium substrates, the amount of corrosion pits resulted from the mixed acid pickling was reduced, which led to a lower nucleation rate and hence increased the possibility for the subsequent nucleation of TiO₂ on the defects of existing nanorods, thus resulting in the formation of nanoflowers instead of nanorod arrays. The effects of deposition temperature, duration, and subsequent heat treatment of the monolayer nanoflower films were investigated systematically. The results indicated that when utilized for photoelectrodegradation of rhodamine B in water, the nanoflower film deposited at 80? for 6 h followed by a heat treatment at 450? for 1 h showed the best photocatalytic activity which was 4.9 times that of a commercial rutile TiO₂ nanoparticle film, and 1.3 times that of commercial P25 nanoparticulate film. The improvement of photoelectric properties of the films can be attributed to the elimination of the intermediate barrier layer, the high specific surface area of the nanoflowers, and the high electron transfer rate induced by the single crystalline TiO₂ nanorods under the external bias voltage.TiO₂ nanowire films were successfully fabricated through in situ thermal oxidation of metallic titanium substrates in vacuum. The growth process and the structure parameters of the nanowire arrays were studied, and the growth mechanism of the nanowires was also discussed. The photocatalytic performance of the titanium plates after thermal oxidation for different durations was measured by photodegradation of rhodamine B in water. The structure characterization results showed that a large number of single crystalline rutile nanowires with the diameters ranging from 40 to 50 nm and the lengths ranging from 3.0 to 5.0 ?m can be derived after thermal oxidations of titanium substrates at 750? for 5 to 10 h under certain vacuum conditions. The growth of nanowires was significantly affected by the thin layer of porous amorphous TiO₂ obtained by the surface oxidation of titanium plate using H2O₂. The results of photocatalytic performance measurement showed that the rutile thin film with a flat sheet nanostructure fabricated through vacuum thermal oxidation for 1 h possessed the highest photocatalytic activity for the degradation of rhodamine B. The photocatalytic performance of rutile nanowire thin films increased when the vacuum thermal oxidation duration was prolonged from 5 h to 10 h.To increase the effective loading amount per unit area of TiO₂ films on titanium substrates, a sandwich-like TiO₂ structure was derived by a layer-by-layer deposition process in the order of an anatase/rutile composite nanorod layer, an anatase sol-gel layer and a rutile nanoflowers layer. The overall phase composition of the sandwich film is of 79% anatase and 21% rutile, which is close to commercial P25 TiO₂ nanoparticles, and the band gap of which is 3.0 eV. When utilized for photodegradation of rhodamine B under UV light, the pseudo-first-order reaction rate of the composite film was 12.3×10-3 min-1, which was significantly higher than that of the anatase/rutile composite nanorods ?3.9×10-3 min-1?, anatase sol-gel layer ?O.3×10-3 min-1? and rutile nanoflowers ?4.7×10-3 min-1?. The investigation results showed that a proper combination of different phase composition and nanostructure can effectively improve the photocatalytic activity of TiO₂ thin films.In order to make full use of the space between the nanowires in the nanowire film and increase the effective amount of TiO₂ in the thin films, the nanowire array thin film was immersed in a specific precursor solution to obtain a hierarchical nanostructured film. The surface of this thin film was composed of anatase TiO₂ nanowire trunks and rutile TiO₂ nanobranches. The nanobranches were of single crystalline structure, which gradually grew as the secondary deposition duration was prolonged. Due to its unique hierarchical morphology and phase composition, the film possessed a phase junction structure with appropriate energy level matching. The high specific surface area, unique branch morphology and mixed crystal effect of the film resulted in a significant increase in the light harvesting efficiency, and the photoelectrochemical response as well as photocatalytic performance for degradation of organics in aqueous solution was also effectively improved.With the purpose of further clarifying the role of phase junction structure in the branched TiO₂ nanofilms, anatase TiO₂ nanowire films were immersed into a titanium tetrachloride or titanium tetrafluoride solutions with certain concentrations, respectively, which resulted in hierarchical TiO₂ nanowire films with different phase structure. The film derived in the solution of titanium tetrachloride contained an anatase/rutile phase junction structure, while that derived in the solution of titanium tetrafluoride did not contain any phase junctions. The effects of phase junctions in the hierarchical TiO₂ thin films was confirmed by the precise control of the preparation process and in-depth analysis of the relationship among the loading amount of TiO₂, the phase composition, the nanostructure and the photocatalytic performance of the films. Besides, the effect of anion doping, "natural aging" phenomenon of composite films in photocatalytic applications, and the photocatalytic degradation characteristics of composite films on different organic compounds were also studied.