Study On The Controlled Synthesis And Properties Of Mesoporous TiO₂ Composites

Mesoporous materials which belong to the category of nanomaterials have opened some interesting applications in the field of catalysis, separation, adsorption, and others due to their ordered mesopores, narrow pore size distribution, high porosity, and large surface area. Compared with Si-based mesoporous materials, ordered mesoporous nanocrystalline TiO₂ (meso-nc-TiO₂) will have vast potential applications in the field of photoelectricity and magnetism, due to their unique structural character. However, the stability of meso-nc-TiO₂ is usually low, so its crystallinity is poor, leading to its low separation efficiency of photo-generated electrons and holes. In order to extend their applications, it is essential to be able to synthesize mesoporous TiO₂ composite materials. In this dissertation, we designed and synthesized a series of TiO₂ composites. They were characterized in detail and their photoelectrical properties were also investigated. These convenient and low-cost strategies for fabricating other composites offer new ideas. The main contents are as follows:1. Mesoporous TiO₂/single walled carbon nanotubes (SWCNTs) thin films were successfully prepared through electrophoretic deposition method on large pore meso-nc-TiO₂ substrate. The dye-sensitized solar cells were assembled with the films and they were sensitized by TCPP and N719, respectively. The results showed that they exhibited enhanced power conversion efficiency. This may be attributed to the compact contact of mesoprous TiO₂ with SWCNTs and the excellent property of SWCNTs in favor of the transportation of photogenerated electrons.2. Multiwall carbon nanotubes (MWCNTs)-β-cyclodextrins (β-CD) composites have been successfully synthesized through combining polymer wrapping and layer-by-layer self-assembly techniques. The obtained MWCNTs-β-CD composites possessed good dispersibility both in ethanol and water media and the solution was found to be very stable for several weeks. Therefore, the problem of the insolubility of CNTs was solved. And then, wire-like TiO₂-β-CD-MWCNT composites were fabricated through solar-induced self-assembly process, combining the self-assembly behavior ofβ-CD, and its interaction with TiO₂ nanoparticles and MWCNTs. The wires were stable for more than several months both in water and under air in dry condition. Raman mapping results confirmed that monodisperse TiO₂ nanoparticles and MWCNTs distributed uniformly in this wire. Furthermore, the dye-sensitized solar cells were assembled with the sandwich structure electrode containing the TiO₂-β-CD-MWCNT composites wires, and they exhibited enhanced power conversion efficiency (8%). This maybe attribute to the compact composite structure of the wires and the unique electronic property of CNTs in favor of the transportation of photogenerated electrons.3. Bond-type TiO₂-SWCNTs composites were fabricated by hydrothermal method, combing with the interaction between the -COOH of SWCNTs and the -OH of TiO₂. The results confirmed that the heterogeneity between SWCNTs and TiO₂ was formed. The crystalline size of the obtained anatase was small and uniform. The composites exhibited better photocatalytic activity than Degussa P25 and TiO₂ nanoparticles. This may be attributed to the chemical bond, which was fabricated the effective transportation channel of electrons between TiO₂ and SWCNTs. Therefore, the effective separation of electrons and holes is achieved. Moreover, the reasonable photocatalytic mechanism of the bond-type TiO₂-SWCNTs composites was also presented.4. On the basis of the large pore meso-nc-TiO₂, a series of multi-modal mesoporous TiO₂-ZrO₂ composites were designed and synthesized for the first time through adding different amount of ZrOCl2 using one-step synthesis strategy. The results demonstrated that after the introduction of ZrO₂, the crystalline growth and aggregation, and the crystal phase transformation from anatase-to-rutile structure were strongly retarded. The obtained composites showed high thermal stability and the crystal phase of anatase was remained at a large scale of temperature. Compared with pure mesoporous TiO₂, the structure parameters of the composites, such as surface area, pore volume and porosity, was improved obviously. The photocatalytic activity of mesoporous TiO₂-ZrO₂ composites was superior to that of pure mesoporous TiO₂ and Degussa P25. This was attributed to the introduction of ZrO₂ enhanced the surface area, restrained the crystal phase transformation and crystalline growth. Therefore, the photo-generated electrons and holes were separated efficiently. Furthermore, the presented mesoporous composites thin films exhibit higher hydrophilicity than mesoporous TiO₂ in the absence of light irradiation. This was because the introduction of ZrO₂ changed the surface microstructure of the composites, enhancing the infiltration and wick effect. The reasonable mechanisms of photocatalysis and high hydrophilicity of our obtained multi-modal mesoporous TiO₂-ZrO₂ composites were also presented.5. Bifunctional mesoporous TiO₂/α-Fe2O3 composite were designed and synthesized in our laboratory for the first time by nanocasting method. The"Bifunctional"means that the composites possess of synergy of the photocatalytic ability of meso-TiO₂ for oxidation of As (III) to As (V) and the adsorption performance ofα-Fe2O3 for As (V). This will completely remove the arsenite from contaminated water by one time. According to the statement of arsenite in nature, the photocatalytic activity and adsorption performance of the composites under different pH values were investigated systemically. In comparison with pure mesoporous TiO₂ orα-Fe2O3, the obtained meso-TiO₂/α-Fe2O3 composites not only presented higher photocatalytic activity but also exhibited excellent adsorbed property. This showed that the composites improved the adsorption and photocatalytic activity obviously. Meanwhile, the photocatalytic oxidation mechanism of As (III) was presented. In addition, As (V) can be easily desorbed from the composites by heat treatment in alkali solution. After reusage for several times, the composites still present comparable catalysis and adsorption performance with that of first use, indicating the excellent stability of the bifunctional composites. The novel nanostructured bifunctional composites will have vast applications in contaminated water treatment. At the same time, this strategy for synthesizing other composites with special physical and chemical character offers new ideas.