Design And Optimization Of Wide- And Narrow-bandgap Semiconductor Photocatalysts For Environmental Purification

Photocatalytic reaction is one of advanced oxidation technologies, which offer the opportunities of using natural light directly. This reaction is governed by the photocatalytic capacity of semiconductor photocatalysts. The semiconductor photocatalysts can be divided into two categories: the wide-band gap photocatalysts that cannot directly use the visible light, and the narrow-band gap catalysts that can directly absorb visible light. A great challenge is to design and synthesize robust catalysts to enhance catalytic performance. In this dissertation, research efforts are focused on the two types of catalysts: titanium dioxide ?TiO₂? represents the wide-band gap photocatalysts, while bismuth sulfide ?Bi₂S₃? represents the narrow-band gap photocatalysts. Rational design, characterization and mechanism analysis of the catalysts are pursued. The main contents and achievements are as follows:1. A new approach for preparing doped TiO₂ single crystals was developed.Doped TiO₂ catalysts respectively with metals, nonmetals, and rare earth elements have shown a great potential in energy and environmental applications, but it is difficult to dope well-defined TiO₂ single crystals ?SCs? with {001} exposed facet due to their high crystallinity. A green and general approach to prepare the {001}-exposed TiO₂ SCs doped with various elements, on the basis of recycling the wasted ethylene glycol electrolyte from the anodic oxidation for TiO₂ nanotube preparation. All six representative elements could be successfully doped into the TiO₂ SCs without breaking their single-crystalline structure and exposed high-energy facet. The electronic properties of the doped TiO₂ SCs were significantly improved. All the doped TiO₂ SCs exhibited a superior photoactivity under visible-light irradiation for degrading Rhodamine B,a typical organic pollutant. The prepared doped TiO₂ SCs have a promising potential in environmental and energy applications.2. A new approach of preparing hexagonal TiO₂ mesocrystalline microrods was proposed. Generally, the exposed facets, crystallization and structural characteristics have significant influence on the photocatalytic performance of TiO₂. Novel TiO₂ mesocrystalline microrods with unique hexagonal structure were prepared via anodic oxidation and calcining. The hexagonal microrods were formed through self-directed self-assembly within intermediate scaffolds. The TiO₂ mesocrystalline microrods exhibit high photoactivity and good potential for electrocatalytic application. This work provides a useful reference for the design of some new structures of TiO₂.3. Improving the sunlight responses and photocatalytic efficiency of the special structure of Ag/TiO₂. As we known, the surface plasma resonance effect of metals will enhance the solar light response and photocatalytic performance of TiO₂. The hybrid materials of single silver nanoparticle selectively growing on special structures of TiO₂ have been synthesized via oil phase synthesis and light reduction reaciton. The reaction mechanism of the synthesis process follows the ripening rule of metal nanoparticles. The prepared materials exhibited good photocatalytic performance in producing hydrogen under simulated sunlight and reduction of methylene blue under visible light. The experimental results show that the catalytic activity of the metal/TiO₂ photocatalyst was improved by regulating the growth location of metal nanoparticles, the morphology and electronic capability of TiO₂. This work provides a new way for the design and synthesis of other TiO₂ heterostructures.4. Novel cocatalyst Pd₄S uniformly anchored on Bi₂S₃ nanorods hybrids were successfully synthesized. Pd₄S is one of the main poisoning products of metallic Pd catalysts, caused by hydrogen sulfide, and its catalytic applications have not been given sufficient attention. A novel photocatalyst hybrid, Bi₂S₃ nanorod uniformly anchored by Pd₄S nanoparticles was successfully synthesized using a facile method and used for efficient visible light photocatalysis. Pd₄S nanoparticles were formed in situ from the B12S3 nanorods through thermal reduction, and played an important role in the enhanced overall performance of this hybrid. The as-prepared Pd₄S/Bi₂S₃ heterostructure exhibited superior electrochemical and photochemical properties and had a higher photocatalytic activity in the degradation of atrazine,a typical herbicide found in the environment, compared to both Pd₄S and Bi₂S₃,individually. The uniformly dispersed Pd₄S nanoparticles, substantially facilitate charge separation and transfer on the Bi₂S₃ nanorods, and then serve as active sites, leading to excellent photocatalytic capacity. Through this study, the application of the new catalyst Pd₄S could be developed and provides a new perspective for design and synthesis of other related hybrid materials.5. Layer-controlled growth of MoS₂ on self-assembled flower-like Bi₂S₃ heterostructures was achieved. Metal sulfide semiconductors, such as molybdenum disulfide ?MoS₂? and bismuth trisulphide ?Bi₂S₃?, are of considerable interest because of their great application potentials in photocatalysis. However, the controllable synthesis of MoS₂/Bi₂S₃ hybrid nanostructures remains a challenge. A unique sacrificial templating strategy was adopted for preparing the layer-controlled MoS₂ on three-dimensional ?3D? Bi₂S₃ micro-flowers. In this approach, Bi₂S₃ was used as a sacrificial template to regulate the ion exchange, and the dosage of molybdenum was adjusted to tune the dynamic formation, thus converting the MoS₂ nanosheets on the Bi₂S₃ micro-flowers from monolayer to multilayer. Due to the increased mass transfer,robust light-harvesting capacity, improved charge separation, lower oxygen-activation barrier and enhanced active oxygen yield, this 3D flower-like hybrid nanostructure enables MoS₂/Bi₂S₃ to exhibit adsorption-promoted photocatalysis under visible light irradiation, especially for the excellent photodegradation of low-concentration organic pollutants. This study provides a new method for preparation some related heterogeneous photocatalysts.