Synthesis Of Tungstate Photocatalyst And Their Photoelectric Properties

Photocatalytic degradation of organic pollutants by semiconductor photocatalysthas the potential to be a beneficial technology for environmental purification. TiO₂has been dominantly used because of its high activity, long-term stability, low price,and availability. Unfortunately, TiO₂could only be activated by UV light because ofits wide band gap (3.2eV). In recently years, tungstate is one of promisingphotocatalyst, which showed high photocatalytic activity on decontamination. On theone hand, the morphology and structure of tungstate samples are key factors forimproving photocatalytic activity of sample. Many efforts have been made, such ascrystallinity, hierarchical architectures and porous structure, etc. On the other hand, itis now commonly recognized that the key to enhancing the photocatalytic activity liesin effectively combining the photon absorption, bulk diffusion, and surface transfer ofphoto-induced charge-carriers in the photocatalyst, and understanding detailedinformation of the behavior of photo-induced charge carriers in the system, includingthe generation, separation and recombination of the photogenerated charges. It is verymeaningful for us to understand and design new photocatalyst. Therefore, there is aneed to investigate the photoinduced charge transfer properties of tungstate.In this thesis, we focused on the morphology control of ZnWO₄and Bi₂WO₆nano/micro structure, and tried to design the heterojunction based on tungstate, investigated the optimal synthetic method and conditions. The surface photovoltagespectrum (SPS) and its phase spectrum, field induced surface photovoltage spectrum(FISPS), and the transient photovoltage technology (TPV) were used for the researchof photo-induced charge transfer properties of different samples. The relationshipbetween photocatalytic activity and behavior of photo-induced charge transferproperties (including separation, transport and recombination) is discussed.Specific work includes the following aspects:1. Different sizes of ZnWO₄photocatalysts were synthesized by a hydrothermalmethod. SPS, FI-SPS and TPV are used to investigate the detailed photoinducedcharge transfer behavior. The as-prepared sample shows highly efficientphotocatalytic activity for the degradation of RhB under UV irradiation, which varieswith the increase of the hydrothermal temperatures. Results indicate that the ZnWO₄synthesized at413K possesses the largest BET surface area and the abundant donorsurface states which are assumed to inhibit the recombination of the photogeneratedelectron–hole pairs, and thus a significant enhancement in the reaction rate isobserved.2. Bi₂WO₆/ZnWO₄composite photocatalysts have been successfully synthesizedby a facile hydrothermal process. The catalysts were characterized by powder X-raydiffraction (XRD), UV-vis diffuse reflectance spectrum (DRS),and transmissionelectron microcopy (TEM). The results show that Bi₂WO₆nanoparticles grow on theprimary ZnWO₄nanorods. The Bi₂WO₆/ZnWO₄composites have better UV lightphotocatalytic activities compared to single ZnWO₄nanorods. Furthermore, thephotoinduced charge transfer properties of Bi₂WO₆/ZnWO₄composites wereinvestigated by means of TPV in detail. The interconnected interface ofBi₂WO₆/ZnWO₄composites led to the low recombination ratios of photoinducedelectron-hole pairs and enhanced photocatalytic activities.3. Three-dimensional (3D) hierarchical clew-like microspheres with small size ofBi₂WO₆were successfully synthesized by a simple hydrothermal route at a propervolume ratio (1:1) of H2O/EG mixed solution. It has been found that ethylene glycol(EG) plays important roles in the formation of the clew-like structure. Bi₂WO₆with different morphologies could be obtained by simply adjusting the volume ratio of theH2O/EG in the mixed solution. The kinetic features of photo-induced charges transferproperties of samples have been discussed by SPS and TPV techniques in detail. TheBi₂WO₆with hierarchical clew-like structure possesses the highest efficiency ofelectron-hole separation and the lowest recombination rate of photoinduced charges,resulting in the highest photocatalytic activity for the degradation of Orange-II underthe visible light (>420nm) illumination. The information provided here is thoughtto be beneficial for engineering high efficient photocatalysts.4. The Pt/Bi₂WO₆composites were prepared by a simple photodepositionmethod. XRD shows that the (111) peak position of Pt/Bi₂WO₆with differentplatinum contents shifts slightly toward lower angles than pure Bi₂WO₆. Thisphenomenon indicates that partial Pt is possibly doped in the lattice of Bi₂WO₆. XPSindicates that the Pt species are in the metallic state. TEM shows that Pt nanoparticleswith diameter of about5-8nm are well deposited on surfaces of Bi₂WO₆nanoplates.SPS and TPV can prove that platinum deposited on the surface of Bi₂WO₆acts aselectrons acceptors. The presence of Pt enhances efficiently the photocatalytic avticityof Bi₂WO₆through promoting the separation and restraining the recombination of thephoto-induced electron-hole pairs in Pt/Bi₂WO₆.