Design And Control Of Ta-based Nanofibers And The Photocatalytic Activity For Hydrogen Production

In order to preserve the fossil energy and protect the environment, people are trying their best to seek clean, renewable and affordable alternative energy. Water splitting for hydrogen production by solar energy has become an ideal way to solve the question of energy and environment. Ta2O5and tantalates have the relatively higher conduction band and its photo-induced electrons have strong reducibility, so they are more suitable for water splitting for hydrogen production. However, the wider band gap limits its visible light absorption. The grain size and the specific surface area of photocatalysts are the most important factors influencing photocatalytic hydrogen production activities. In this thesis, we focus on controlling structures and compositions of Ta2O5, Ta3N5and BiTaO4nano fibers, and investigating their properties of water splitting for hydrogen production.(1) Ta2O5nanofibers were prepared by electrospining method. By changing the heating rate we got different structural characteristics samples. The grain size, surface area, mesoporous properties and photocatalytic hydrogen production activities were investigated as a function of heating rate. We found that with the heating rate increase, the grain size of samples decreases, the mesoporous and the specific surface area have a maximum value. This is mainly due to the decomposition of organic matter and crystallization of inorganic matter. The faster the heating rate was, the more the crystal nucleus generated in the fibers; Ta2O5nanoparticles on the fibers’surface crystallized firstly, and the chain scission and decomposition of PVP drived fiber directed contraction to the crystalline on the surface, then the decomposition of PVP left the holes in the fibers. But if the heating rate was too rapid, the combustion of PVP and the fast crystallization of Ta2O5hindered the connection of the particles to some extent, result in holes collapse and specific surface decrease. Photocatalytic hydrogen production rate of as-prepared samples depended on not only the specific surface area, but also the grain size.(2) Tantalum nitride porous nanofibers were prepared by the mothod of temperature programmed aminolysis. The influence of the precursor composition of Ta2O5nanofibers, nitridation time as well as nitridation temperature on optical absorption properties and the photocatalytic activities of the as-prepared samples were studied. We found that the amorphous Ta2O5was more easily nitride than the crystalline Ta2O5in the same way of nitridation, and had the smaller grain size and relatively smooth surface. But the later one was apt to obtain the nitride product with structural stability and porous structure. As nitriding temperature rising and time increasing, the products had a significant red shift of optical absorption band edge. The nitride products which used crystalline Ta2O5nanofibers as precursor and nitrided at800℃for4h showed the highest activity, and this was due to its well crystallization and porous structure as well as the strong absorption of visible light.(3) A convenient method for synthetizing multi-metal oxide nanofibers was developed. BiTaO4nanofibers were prepared by electrospinning, the influence of the annealing temperature on the structure, optical absorption and photocatalytic hydrogen production activities of the nanofibers was researched. We found that the products began to change from orthorhombic crystal to triclinic crystal at about700℃, and crystal transformation completed when annealing temprature rose to750℃. UV-Vis results showed that the annealing temperature had slight effect on the optical absorption band-edge of BiTaO4nanofibers, which were all about430nm. The sample annealed at750℃had the best hydrogen production rate of16.6μmol/h. This is related to its good crystallinity.