| Environmental pollutions and energy crisis are the two problems confronted with the world nowadays. Anxiety about depletion of fossil fuel reserves and pollution resulted from the combustion of fossil fuels make hydrogen an attractive alternative energy source. Recently, photocatalytic water-splitting has become a promising way for a clean, low-cost and environmentally friendly production of hydrogen by using solar energy. In comparison to other semiconductor photocatalysts, TiO2has been widely used because of its biological and chemical inertness, nontoxicity, low cost, availability and long-term stability against photo and chemical corrosion. However, the photocatalytic efficiency of TiO2for water-splitting is limited due to the high recombination rate of photogenerated electron-hole pairs. Furthermore, anatase TiO2is only effective under ultraviolet irradiation (λ<387nm) due to its large band gap (3.2eV). Usually, sunlight contains about4%ultraviolet light only. Therefore, it is highly desirable to develop a photocatalyst with enhanced photocatalytic activity for water splitting under both UV and visible-litht irradiation. Recently, anatase TiO2single-crystalline nanosheets with high percentage of reactive (001) facets have attracted lots of attention. Both theoretical and experimental studies indicate that higher surface energy of (001) facets are more effective for dissociative adsorption of reactant molecules compared with the thermodynamically more stable (101) facets. In this dissertation, valuable explorations have been carried out on the synthesis, modification (including noble metal Pt and/or CdS nanoparticles deposition), characterization and photocatalytic activity of the TiO2nanosheets with exposed (001) facets. Our main ideas are to improve the photocatalytic activity of TiO2for water-splitting under both UV and visible-litht irradiation by using anatase TiO2single-crystalline nanosheets with high percentage of reactive (001) facets as photocatalytst. The main points could be summarized as follows:1) Hydrogen production by photocatalytic water splitting over Pt/TiO2nanosheets with exposed (001) facets (Pt/TiO2NS). The photocatalytic activity of TiO2is strongly dependent on its morphology and structure. Compared with (101) facets,(001) facets are more effective for dissociative adsorption of reactant molemules. Furthermore, water molecules can chemically dissociate on the (001) surface but, contrarily, only physically adsorb on the (101) surface. Therefore, it is reasonable to infer that these (001) facets should be much more effective for water splitting than the (101) facets. Moreover, the single-crystalline structure of TiO2nanosheets with a low density of defects can reduce the recombination rate of photogenerated electron-hole pairs on grain boundaries and crystalline defects, thus improve the photocatalytic efficiency. So, we hope that TiO2nanosheets with exposed (001) facets will exhibit high photocatalytic activity for water splitting. In chapter2, Pt/TiO2NS were fabricated by a simple hydrothermal route in a Ti(OC4H9)4-HF-H2O mixed solution, followed by a photochemical reduction deposition of Pt nanoparticles on TiO2nanosheets under xenon lamp irradiation. The effects of Pt loading on the rates of photocatalytic hydrogen production of the as-prepared samples in ethanol aqueous solution were investigated and discussed. The results showed that the photocatalytic hydrogen production rates of TiO2nanosheets from the ethanol aqueous solutions were significantly enhanced by loaded Pt on the TiO2nanosheets, and the latter with a2wt%of deposited Pt exhibited the highest photocatalytic activity. All fluorinated TiO2nanosheets exhibited much higher photocatalytic activity than Degussa P25TiO2and pure TiO2nanoparticles prepared in pure water due to the synergistic effect of surface fluorination and exposed (001) facets. The surface-fluorinated Pt/TiO2NS are of great interest for solar cell, photonic and optoelectronic devices, sensors, catalysis, biomedical engineering and nanotechnology.2) Preparation and enhanced visible-light photocatalytic H2-production activity of CdS-sensibized Pt/TiO2NS. The photocatalytic activity of TiO2is limited by the high recombination rate of photogenerated electrons and holes. Moreover, TiO2is only effective under ultraviolet irradiation due to its large band gap. Semiconductor quantum dots sensitization is one of the methods to resolve this problem. So, we hope that the prepared CdS-sensitized Pt/TiO2NS can exhibit high photocatalytic activity for water splitting. In chapter3, CdS-sensitized Pt/TiO2NS were prepared by hydrothermal treatment of a Ti(OC4H9)4-HF-H2O mixed solution followed by photochemical reduction deposition of Pt nanoparticles on TiO2nanosheets and chemical bath deposition of CdS NPs on Pt/TiO2NS, successively. The UV and visible-light driven photocatalytic activity of the as-prepared samples was evaluated by photocatalytic H2production from lactic acid aqueous solution under UV and visible-light (λ>420nm) irradiation. It was shown that no photocatalytic H2-production activity was observed on the pure TiO2nanosheets under UV and/or visible-light irradiation. Deposition of CdS nanoparticles on Pt/TiO2NS caused a significant enhancement of the UV and visible-light photocatalytic H2-production rates. The morphology of TiO2particles had also a significant influence on the visible-light H2-production activity. Among TiO2nanosheets, P25and the nanoparticles studied, the CdS-sensitized Pt/TiO2NS show the highest photocatalytic activity (a13.9%apparent quantum efficiency obtained at420nm), exceeding that of CdS-sensitized Pt/P25by10.3%and that of Pt/NPs by1.21%, which can be attributed to the combined effect of several factors including the presence of exposed (001) facets, surface fluorination and high specific surface area. After many replication experiments of the photocatalytic hydrogen production in the presence of lactic acid, the CdS-sensitized Pt/TiO2NS did not show a great loss in the photocatalytic activity, confirming that the CdS/Pt/TiO2NS system is stable. The as-prepared CdS/Pt/TiO2NS are of great interest in solar cells, catalysis, photonic and optoelectronic devices, sensors, biomedical engineering and nanotechnology.3) From the aspect of practice application, the photocatalytic activity of TiO2needs to be further improved. The kinds and number of surface functional groups on the surface of TiO2have effects on the photocatalytic performance of TiO2. Ionic liquids have been widely used to control the morphology and modify the surface of inorganic materials. It is expected that the addition of ionic liquid affects the photocatalytic performance of TiO2. In chapter4, the effects of ionic liquid on the photocatalytic performance of TiO2were investigated by adding a water immiscible room temperature ionic liquid (1-butyl-3-methylimidazolium terafluoroborate) into the photocatalytic reaction of TiO2, and the photocatalysis mechanism of TiO2was also systematically investigated by designing different reactive radicals trapping experiments. The results show that photogenerated electrons are the main reactive species involved in the photocatalytic degradation of methyl organe, while·OH radicals and photogenerated holes play an important role in the photocatalytic degradation of rhodamine B. The addition of ionic liquid can enhance the photocatalytic degradation of methyl organe due to the enhanced separation of photogenerated electron-hole pairs through the trap and transfer of photogenerated electrons by ionic liquid, on the contrary, supress that of rhodamine B because of the competitive adsorption of ionic liquid and rhodamine B, which hinders the access of rhodamine B to the photogenerated holes and·OH radicals on the surface of TiO2. Ionic liquid exhibits ambilateral effects on the photocatalytic performance of TiO2and the effects are related to the kinds of contaminants and the different mechanisms in specific photocatalytic reaction. This report may provide new insight into the understanding of photocatalysis mechanism and the design of novel photocatalytic materials.
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