Layer-by-layer Fabrication And Photocatalytic Properties Of Modified Tio₂ Films

Exploiting photocatalysts with visible light sensitivty, highly photocatalytic activity and long life time is the key for hydrogen production by photocatalytic method. TiO₂ with low cost, long-term stability against photocorrosion and chemical corrosion is highly desirable as photovoltaic and water-splitting devices. However, since TiO₂ nanomaterials are normally transparent in the visible light region and difficult for re-collecting, it is demanded to improve the optical sensitivity and activity of TiO₂ nanomaterials in the visible light region.In this paper, we focused on synthesis, properties, modification, and applications of modified TiO₂ films. The effects of pristine film number, anneal temperature, pre-treatment of substrate for structure and properties of TiO₂ films prepared on quartz slide by layer-by-layer assembly method (LBL) were studied in detail. Meanwhile, the technology of modified photocatalyst was optimized. The results showed that the optimal parameters of technology for modified TiO₂ films prepared by LBL were: the number of pristine film was lower than or equeal to 13, anneal temperature was 500℃.Two polyoxometalates (P₂Mo₁₈ and Mn₄W₃₀) with classical structure were selected as dopants for fabrication Mo-doped and Mn, W-codoped TiO₂ films. The films were characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scan electron microscopy (SEM), transmission electron microscopy (TEM), UV-vis spectrophotometry and fluorescence spectrophotometry. The results showed that layer-by-layer self-assembly was an efficient and often preferred process to build doped-TiO₂ nanoparticles into ordered macroscopic structures. The modification of optical properties of Mo-doped and Mn, W-codoped TiO₂ films fabricated by LBL were similar, the absorption edge for Mo-doped and Mn, W-codoped TiO₂ films red-shifted as well as a new absorption peak appeared in the visible region. The band structure was changed by doping. The photocatalytic activity of doped-TiO₂ films were evaluated by measuring hydrogen evolution from sacrificial reagents (sodium sulfide-sodium sulfite solution). The results of photocatalytic testes indicated that modified TiO₂ films, which presented in this paper, have long-term stability and higher photocatalytic activity.CdS/TiO₂ and ZnS/TiO₂ composite films with different Cd (Zn):Ti atomic ratio were prepared by a new straightforward particulate layer-by-layer method. Raman, XRD and XPS analysis of the composite films revealed that out layer TiO₂ formed from the titanium precursor retarded the crystal growth of CdS. Luminescence and UV-vis investigation showed that emission and absorption band of CdS/TiO₂ composite films blue shifted as crystal size of CdS in the films decreased. Furthermore, hydrogen formation curves reveal that hydroxyl of CdS/TiO₂ films plays an important role in water splitting reaction under both UV and visible light irradiations.In this work, we have studied the photochemical oxidation of sodium sulfide-sodium sulfite solution accompanied by hydrogen evolution under the irradiation of a high-pressure Hg lamp. The variations of UV-vis absorption spectrum for sacrificial reagent solutions versus reaction time as well as species and concentration for intermediate materials and final products in solutions were investigated by in-situ UV-vis absorption spectrum. A mechanism involving adsorption of bisulfide ion on the inner sleeve of the photoreactor is postulated. In sodium sulfide system, sulfide ions, principally bisulfide, could be photochemically oxidized under UV light to produce sulfur, complexed mainly as disulfide ion, while water was reduced to produce hydrogen in a complementary redox process. However, in sodium sulfide-sodium sulfite mixture solution, the results also indicated that sulfite ion in sacrificial reagents not only could react with intermediate materials, eg. S₀(ads), for S₂O₃^2- but also changed the reaction path of HS^- compared with that without sulfite, and then reduced hydrogen evolution rate for sacrificial reagent solutions system.By designing comparative testes, the effects for hydrogen evolution from sodium sulfide-sodium sulfite solutions and other factors were investigated. Mass balance analyses for a series of photolytic reactions and UV-vis absorption spectrum in-situ monitor were performed. Gas evolution rates changed very little as a function of pH, suggestion a pH-independent mechanism, at least in the range where HS^- is the predominate species. The results indicated that there was a cycle reaction between HS^- ions and intermediate material (S₄^2-) in mixture solutions with photocatalyst during the process of photocatalytic. Therefore, HS^- ions could be recovered from S₄^2- ions and hydrogen formation rate was kept.