Study On The Preparation And Visible-light Photocatalytic Performance Of Loaded And Doped Nano-scale Titanium Dioxide

Titanium dioxide (TiO₂) has been extensively employed as heterogeneous photocatalysts for the degradation of many kinds of contaminants in water, because of its strong oxidizing power, low cost, chemical inertness and long-term photostability. However, there are two serious problems obstructing its large-scale commercial application: one is its low photocatalytic activity under visible light; the other is the aggregation among nanoparticles and tricky separation from reaction system. In this dissertation, with the aim of enhancing the photocatalytic activity of nano-TiO₂ under visible light and facilitating the separation of photocatalyst from reaction system, TiO₂-MWNTs nanocomposites and C-doped TiO₂ and Ag/C-TiO₂ composites with macroporous network had been prepared. The enhanced photocatalytic activity of as-prepared photocatalysts was probed by photodegradation reaction of methylene blue under visible-light irradiation.TiO₂-MWNTs composites were synthesized through nanocoating-hydrothermal process, using MWNTs treated by mixed acide as the immobilized support, and their characterization of TEM, HRTEM, FTIR and XPS confirmed that TiO₂ nanoparticles were immobilized on the surface of nanotubes with the covalent bonds, in which there is about 30 wt% TiO₂ components. The presence of MWNTs can efficiently inhibit the agglomeration of the TiO₂ nanoparticles and the formation of brookite TiO₂. Moreover, the difference of MB concentration between MWNTs and TiO₂, owing to high adsorption capacity of MWNTs, caused the surface diffusion of MB molecules from MWNTs to TiO₂. These resulted in higher photocatalytic activity of TiO₂-MWNTs composites than that of pure TiO₂ nanomaterials. The nanocoating-hydrothermal process may open a new and versatile way to prepare the nanocomposites like TiO₂-MWNTs under mild conditions.C-doped TiO₂ composites with macroporous network had been prepared by sol-gel method, using eggshell membrane as template.It was composed of interwoven and coalescing TiO₂ fibers which composed of nanopartices ranging in diameter from 10 to 15 nm. Compared with the eggshell membrane template, it has a considerable shrinkage; moreover, the porous C-TiO₂ materials shrank more considerably with the increase of the calcination time. It is referred that the doped carbon can form a mixed layer composed of active carbon and carbonate species on the surface of TiO₂ nanoparticles, which can enhance the adsorption of MB and facilitate the separation of charge-carriers, and had a strong absorption capability in the visible-light region, especially in the visible light of 400⁷00 nm. These resulted in higher photocatalytic activity of C-TiO₂ composites than that of Degussa P25.Ag/C-doped TiO₂ (Ag/C-TiO₂) nanomaterials has a similar structure with the EM template only with a little shrinkage, suggesting that the incorporation of Ag particles may suppress the shrinkage of TiO₂ during its crystallization process.XRD and XPS analysis confirmed that the metal nature of silver within this materials. It suggested that TiO₂ can accelerate the decomposition of eggshell membrane template, whereas the silver particles seem to act the opposite role by TG analysis during the remove of the organic template through calcination in air. Ag/C-TiO₂ can enhance the absorption capability in the visible-light region again, especially in the visible light of 700¹100 nm. Meanwhile, Ag/C-TiO₂ photocatalyst has a higher adsorption capability than pure TiO₂ materials due to the doped Ag nanoparticles and the carbon layer on the surface of TiO₂. Moreover, Ag nanoparticles can accept the photoinduced electrons from the surface of TiO₂ as the electronic traps; as a result, the recombination of photoinduced charge carriers can be effectively inhibited. Therefore, the cooperative organic carbon species and Ag nanoparticles lead to the significant enhancement of photocatalytic activity of Ag/C-TiO₂ materials under visible light irradiation.