| Energy and environmental issues are two main topics of scientific research in the 21 century. Semiconductor photocatalytic materials exhibit great potentials in environmental protection and solar energy conversion, such as air and water purification, water splitting to generate hydrogen and dye-sensitized solar cells. However, owing to their relatively wide band gap, conventional photocatalysts (such as TiO2, ZnS and ZnO) can only absorb the UV light, which accounts for only 4% of the total sunlight, and thus greatly restricts its practical applications. On the contrary, visible light accounts for a large fraction (46%) of the solar spectrum, so it is highly desirable to develop photocatalysts with high activities under visible-light illumination. In this regard, metal sulfides have been intensively studied in photodegradation of organic pollutants and photocatalytic H2-production of water splitting due to their suitable band gap position and photocatalytic functions. In this dissertation, valuable explorations have been carried out on the synthesis, characterization and photocatalytic activity of the metal sulfides composite. Our main ideas are tuning the band gap structure of metal sulfides by doping, heterojunction interfacial coupling and forming solid solutions to enhance the visible-light photocatalytic performances.Firstly, monodisperse CuS/ZnS nanocomposite hollow spheres with diameters of about 255 nm and shells composed of nanoparticles have been successfully synthesized in high yield by an ion-exchange method using monodisperse ZnS solid spheres as a precursor. The prepared samples were characterized by X-ray diffraction, scanning and transmission electron microscopy, X-ray photoelectron spectroscopy, N2 adsorption-desorption isotherms, and UV-visible absorption spectroscopy. The photocatalytic activity was evaluated by the photocatalytic decolorization of Rhodamine B (RhB) aqueous solution under visible-light irradiation. The results indicate that the difference of solubility products (Ksp) of ZnS and CuS is the main driving force for the formation of CuS/ZnS hollow spheres. A sequential evolution process involving surface deposition, mutual diffusion, interior dissolution, and interfacial reaction is proposed to account for the formation of CuS/ZnS nanocomposite hollow spheres. The reaction time greatly influences the physical properties and photocatalytic activity of hollow spheres. With increasing reaction time, the BET surface areas decrease and, contrarily, the average pore size and pore volume increase. The prepared CuS/ZnS composite hollow spheres exhibit a higher visible-light photocatalytic activity than single ZnS solid or CuS hollow spheres for the photocatalytic decolorization of RhB aqueous solution. The photogenerated holes are thought to be the main active species responsible for the photocatalytic degradation of RhB. The ion-exchange method will provide new insight into the fabrication of composite hollow spheres and other new photocatalytic materials. We believe that the prepared CuS/ZnS hollow spheres are also of great interest in solar cells, catalysis, separation technology, biomedical engineering, and nanotechnology.Secondly, Bi-doped ZnS hollow spheres were successfully synthesized by a facile cation exchange reaction between preformed ZnS hollow spheres and Bi(NO3)3. The samples were characterized by X-ray diffraction, scanning and transmission electron microscopy, X-ray photoelectron spectroscopy, N2 adsorption-desorption isotherms, UV-vis absorption spectroscopy as well as transient photocurrent responses. The photocatalytic H2-production activity was investigated by using the Na2S and Na2SO3 as sacrificial reagents in water. Even without a Pt cocatalyst, the as-prepared Bi-doped ZnS hollow spheres exhibit significant visible-light and UV-light photocatalytic activity and good stability for H2-production. It is suggested that Bi doping into ZnS forms an isolated state originating from Bi 6s above the top of the valence band of ZnS, and the electron excitation from Bi 6s state to the conduction band occurs upon irradiation with visible light. Furthermore, the UV-light photocatalytic H2 evolution activity over Bi-doped ZnS hollow spheres is even higher than that of Pt/ZnS counterparts. This is ascribed to the fact that the Bi doping facilitates the separation of photogenerated electron-hole pairs and decrease in their recombination rate.Thirdly, CdS quantum dot (QDs)-sensitized Zn1-xCdxS solid solutions are successfully prepared by a simple cation exchange using hydrothermally synthesized ZnS nanoparticles (NPs) and Cd(NO3)2 as precursors. The prepared samples show especially high visible-light photocatalytic H2-production activity from aqueous solutions containing Na2SO3 and Na2S as sacrificial reagents even without a Pt co-catalyst. It is found that this high visible-light photocatalytic H2-production activity is attributed to the quantum size effect of CdS; the presence of CdS QDs alters the energy levels of the conduction and valence bands in the coupled semiconductor system, which favors the electron transfer and enhances photoactivity.Fourthly, a two-step method for fabricating NiS nanoparticles (NPs) modified CdS nanorods (NRs) composite photocatalyst is developed based on the hydrothermal reaction. It is found that NiS is an efficient cocatalyst for CdS NRs in the photocatalytic H2-production reactions and the activity over NiS/CdS NRs was even higher than that of Pt/CdS NRs under visible-light irradiation. The high activity of NiS NPs/CdS NRs composite photocatalyst can be attributed to the combined effects of several factors. Firstly, the specifically structure of CdS NRs is of great benefit to transport the electrons from CdS to the interface between NiS and CdS. Secondly, the intimate contact between NiS NPs and CdS NRs favors the formation of junctions and, as a result, improves the charge separation at the interface between the two components. Thirdly, the suitable band gap structure of CdS and NiS permit the transfer of electrons between CdS and NiS. Moreover, the unique web-like superstructure for NiS/CdS NRs composite is beneficial to the chemical reactions. This work presents not only a possibility for the use of NiS as a substitute for noble metals in the photocatalytic H2 produciton but also a new concept to design other composite materials with high photocatalytic activity.Fifthly, mesporous anatase-phase TiO2 hollow spheres with high photocatalytic activity were prepared by hydrothermal treatment and self-transformation of amorphous TiO2 solid spheres in an NH4F aqueous solution. The prepared samples were characterized by X-ray diffraction, scanning and transmission electron microscopy, X-ray photoelectron spectroscopy, N2 adsorption-desorption isotherms and UV-vis absorption spectroscopy. The photocatalytic activity was evaluated by photocatalytic oxidation decomposition of acetone in air under UV irradiation. It is found that F- plays an essential role in the formation of TiO2 hollow spheres. F- not only induces the hollowing of TiO2 solid spheres, but also promotes the crystallization of anatase TiO2 nanocrystals. A possible formation mechanism for the TiO2 hollow spheres by localized Ostwald ripening or chemically induced self-transformation is proposed based on the experimental observations. Furthermore, the molar ratios of NH4F to TiO2 (R) exhibit a significant influence on the morphology and photocatalytic activity of the TiO2 samples. The photocatalytic activity of the samples prepared in the presence of NH4F is higher than that of TiO2 sample prepared in pure water and commercial Degussa P25 powders. Especially, the TiO2 hollow spheres prepared at R=1 show the highest photocatalytic activity and the rate constant exceeds that of P25 by a factor of more than two.
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