Research Of Synthesis And Photoelectric Properties Of Titanium Oxide Clusters

Titanium oxides such as TiO₂ are an important class of transition-metal semiconductors. They are very useful in many applications such as photocatalysis, photovoltaic cells, photo/electrochromics, photonic crystals, surface coatings, and sensors. Polyoxotitanates-the molecular forms of titanate, are nanoscale molecules comprised of several titanium atoms bridged by O-atoms, and have potential applications in solar energy harvesting, and are nano-building blocks in the bottom-up assembly of organic-inorganic hybrid materials. As is known to all, the sol-gel method is the most fundamental and important strategy for synthesis of transition metal oxide nanomaterials. Polyoxotitanates are the prenucleation clusters and/or intermediates involved in TiO₂ nanocrystal formation, such that their isolation will greatly help to understand the mechanism of TiO₂ formation on the molecular level. In this thesis, the alcoholysis of Ti₄₊-the widely used method in the controllable synthesis of TiO₂ reported in the literature-has been adopted in the synthesis of polyoxotitanates. By controlling reaction time, we isolated a series of intermediates of TiO₂ nanocrystal formation, and successfully synthesized a few of novel polyoxotitanates, and remarkably, a unique host-guest Ti-22mer cluster encapsulating an iodide. We futher studied the photoelectric properties of these compounds. Specifically, the contents of this thesis are divided into four parts as follows:In Chapter I, the research progress of metal oxide clusters is reviewed and the synthetic methods of metal oxide clusters are introduced. Summary of the structure, properties and applications of polyoxotitanates is provided. The research progress of formation mechanism of TiO₂ is especially reviewed. Finally, the scientific significance of this paper is expounded.In Chapter II, we synthesised a novel titanium oxo-cluster [Ti₄O（O’Pr）₆（DTBC）₄] and studied its interfacial charge transfer mechanism. The compound can be viewed as molecular model of TiO₂ surface bonded with 3,5-di-tert-butylcatechol （DTBC）. All the four DTBC ligands adopt the bridging chelate binding mode, and the charge transfer from π electrons of benzene ring to the Ti3d orbitals occurs under visible light. The present precise structure and charge transfer mechanism contribute to understanding the mechanism of chromophore binding to semiconductor surfaces and charge transfer at the dye/TiO₂ interfaces.In Chapter Ⅲ, we adopted the method of alcoholysis of Ti⁴⁺, and during solvothermal reaction of a mixture of TTIP and TiX₄ （X=F, Cl, Br）, isolated three polyoxotitanates by modulating the experimental parameters such as time, temperature and reactant ratio. Their structures were determined by X-ray single-crystal diffraction. Spectral characterization and catalytic applications were subsequently researched.In Chapter Ⅳ, we adopted the method of alcoholysis of Ti⁴⁺, and using TiI₄ and TTIP as precursors, successively obtained three polyoxotitanates of different condensation degrees, especially a unique host-guest Ti-22mer cluster encapsulating an iodide. The structures, compositions, optical properties and photoelectric properties of these compounds were detailedly researched. By post-functionalization, we obtained another two functionalized host-guest clusters respectively attached with catecholate and carboxylate ligands.