Synthesis Of Upconversion/TiO₂Composite Nanomaterials And Their Use As Photocalysts

Lanthanide doped fluorescent rare earth nanocrystals have gained much popularity in material research communities because of their low phonon energies, excellent thermal stability, high refractive indexes and efficiency in up-converting near-infrared (NIR) to visible light. For such notable properties, they have been widely applied in a variety of fields ranging from short wavelength solid-state lasers, display devices, to optical communication. Among various upconversion (UC) materials, YF3is an important host matrix for lanthanides doping to give both strong down-and up-conversion luminescence with high quantum efficiency. Controlled synthesis of YF3nanocrystals is an effective route to obtain various types of YF3nanocrystals satisfying for different application purposes and finely controlling their fluorescent properties.Here we present a facile approach to selectively synthesize different YF3nanocrystals using ethylene glycol (EG) as a solvent. Five distinct morphologies can be conveniently obtained through changing the fluoride sources and chelating ligands in the EG solution. The prepared products were characterized and their formation mechanisms were proposed. After doped with different lanthanides, these samples can give both down-and up-conversion luminescence, suggesting that they are good nanoparticulate hosts. A comparison of fluorescent intensities between different products has been made, offering us an opportunity to study the relationship between fluorescence intensities and morphologies of YF3nanostructures. Our results suggest that the formation of assemblies is a feasible way to enhance the fluorescence of these lanthanide nanocrystals in addition to the size control.In order to utilize these YF3nanocrystal for photocatalytic applications, we synthesized flat and rhombic YF3architectures and characterized the samples in detail. Interestingly, the YF3nanostructures are formulated through the hierarchical assembly of YF3nanocrystals along a specific crystalline orientation. Investigations on the formation process suggest that an assembly disassembly process is responsible for the construction of this novel structure. Enabled by doping with different lanthanides ions, the products can exhibit various down-or up-conversion luminescences, showing their potentials in serving as versatile host matrixes. The tunable luminescent properties allow designing effective upconversion photocatalysts when the doped YF3nanostructures are coated with a TiO2shell on their surface.Through conventional sol-gel process, a layer of TiO2could be facilely coated on the YF3nanostructures. In the upconversion/TiO2core-shell structures, the core can efficiently upconvert the NIR light to be visible one which is subsequently absorbed by the shell and produce electron-hole pair for photocatalysis. The YF3@TiO2hybrid structures have a porous morphology that is partially inherited from the YF3architectures, whose high surface-to-volume ratio facilitates their use as photocatalysts. In this thesis, we have demonstrated that the YF3:Yb,Tm@TiO2structures exhibit satisfactory photocatalytic activities under the irradiation of both UV and near IR light. As compared with the conventional TiO2catalysts, the hybrid structures here offer better performance in photocatalysis in the full solar spectrum. It is anticipated that this work provides a new approach to design photocatalysts with responses to a broader spectral range.