Construction Of Core/Shell Structured Photocatalysts Via Vapor Phase Hydrolysis And Their Properties

Core/shell structured photocatalysts have received a great deal of attentations for their potential in a wide range of fields, such as energy and environment, etc. Although the synthesis technologies and characterizations of the core/shell structured photocatalysts are improved rapidly, there still exist many unresolved issues. For example, how to improve the fabrication method and enhance the properties of the core/shell structured photocatalysts? How to deeply understand the formation mechanism of the core/shell structured photocatalysts? How to find a general strategy which can be globally used for preparing any kind of core/shell structured photocatalysts? Traditionally, the chemical vapor deposition (CVD) could obtain the well core/shell structured materials, but the shell is always consisted with monocrystal which owns low specific surface area. And the initial investment of CVD equipment is high. Layer-by-Layer assembly is a time-consuming and inefficient way to fabricate the core/shell structured photocatalysts. and the polyelectrolyte employed in the Layer-by-Layer process may be detrimental to their properties. The special precursor and solvent should be used in the sol-gel method. In this dissertation, the vapor phase hydrolysis is selected as a novel method to recover the above shortcomings and used to prepare the core/shell structured photocatalysts, such as the light PS/TiO2, Fe3O4/C/TiO2magnetic photocatalyst, rutile/anatase TiO2heterojunction and diatomite/Ta3N5visible-light driven photocatalyst. The vapor phase hydrolysis as a simple and general method can be used to prepare most kinds of core/shell structured photocatalysts. (1) Magnetic separation technology as a cost-saving and energy-saving method has inspired scientists for their potential in separation of the nano-photo catalyst from the liquid. The core/shell structured Fe3O4/TiO2magnetic photocatalyst is fabricated via vapor phase hydrolysis method. Anatase TiO2is coated onto the surface of the un-modified Fe3O4/C microspheres uniformly and the thickness of TiO2shell can be tuned from monolayer to several hundred nanometers by adjusting the ratio of the titanium precursor and Fe3O4/C microspheres. The size and crystallinity of anatase TiO2crystallite in the shell could be tuned by temperature and duration, the anatase TiO2is crystallized perfectly without calcination. Fe3O4/C/TiO2photocatalyst shows high magnetic sensitivity and the magnetic saturation value of Fe3O4decreases with increasing the quantity and thickness of the non-magnetic shell. The photocatalytic activity of Fe3O4C/TiO2photocatalyst has relatively higher activity than commercial anatase TiO2in degradation of methylene blue (MB). However, the recycling property and stability are improved largely. The intermediate carbon layer can avoid the electron interaction or photodissolution of Fe3O4probably occurring at the point of contact interface effectively. The recycle degradation experiment indicates the Fe3O4/TiO2hybrid spheres are well likely to be promising catalyst in the future.(2) The nano-TiO2owns high reactivity due to its high specific surface area and small particle size, which can effectively shorten the immigration distance of photo-induced electrons and holes from the internal to the surface of the catalyst, inhibitting their combination. However, the surface free energy and surface binding energy increase rapidly with the specific surface area increasing. Nano-TiO2substantially aggregates, lowers the light utilization efficiency and photocatalytic activity significantly. The core/shell structured PS/anatase TiO2photocatalyst has been prepared by a vapor phase hydrolysis process, the crystalline anatase TiO2has been coated onto the surface of polystyrene microspheres without calcination which can decompose the PS core easily. The size and crystallinity of anatase TiO2crystallite in the shell could be tuned by temperature and duration. The thickness of TiO2shell can be tuned from monolayer to several hundred nanometers by adjusting the ratio of the titanium precursor and the PS microspheres. Although PS/TiO2has a higher activity than bare TiO2due to the crystalline nature of TiO2shell, better floating ability and improved light harvesting property. The PS core has consumed some active radicals, which are generally used in the photocatalytic oxidation of MB, phenol or their intermediate derivatives. After introducing the insulating SiO2coating between the PS core and photocatalytic TiO2shell, it can retain its high photocatalytic efficiency and the long-term stability of PS/TiO2composite is also enhanced.(3) The rutile/anatase TiO2heterojunction is a most promising photocatalyst due to its high activity and ’magic’ effects on carrier transfer in photocatalysis application. But it is still a challenge to find a facile and less energy consumption method to prepare rutile/anatase composite with tightly interfacial contact and high photocatalytic activity. The core/shell structured rutile/anatase photocatalyst is synthesized via the vapor phase hydrolysis process. The crystallite size of the anatase TiO2increases from7.1nm to15.6nm accompanied with the VPH temperature elevating from100℃to200℃. The thickness of TiO2shell can be tuned from monolayer to several hundred nanometers by adjusting the ratio of the titanium precursor and the rutile particles. The core/shell structured rutile/anatase photocatalyst has a higher activity than pure rutile or anatase alone in methylene blue (MB) and phenol due to the synergistic effect of rutile and anatase.(4) Nano-Ta3N5visible-light photocatalyst owns low band energy and good visible-light response. However, it also owns weak adsorption properties, and the separation of nanosized Ta3N5from wastewater is high-cost and energy-consumptive. So it is very important to immobilize it onto the large-scale diatomite carrier in order to improving its adsorption and recycling properties. The core/shell structured diatomite/Ta3N5visible-light driven photocatalyst is prepared combining the vapor phase hydrolysis and the nitridation process. The amorphous Ta2O5is coated onto the surface of diatomite uniformly, then the Ta3N5shell is indeed formed and the absorption edge of the Ta2O5is blue shifted to the visible light region after the nitridation process. The absorption edge of the samples shifts to600nm with elevating the nitridation temperature.700℃is the lowest temperature which the nitridation reaction occurs. The Ta3N5shell retained the ordered sub-micron pore structures of the diatomite. The core/shell structured diatomite/Ta3N5nitrided at700℃for5h shows the excellent photocatalytic activity. The concentration of MB is degraded by95.8%after visible-light irradiating for15min, and the core/shell structured diatomite/Ta3N5could be recycled easily.