Component Design, Preparation, And Properties Of Photocatalytic Materials Of TiO₂-based Composite Nanofibers

In nowadays, the energy crisis and environmental pollution have become a global issue to be addressed urgently. Semiconductor materials gradually caused widespread concern due to their potential applications in environmental purification and solving the energy problem. Among numerous semiconductors, titanium dioxide (TiO2) has obvious advantages over other semiconductors due to its low cost, non-toxic, and stable performance. With the development of nanotechnology, scientists have found that the nanoscaled TiO2photocatalyst has a higher efficiency for degradation of pollutants than that of TiO2in the form of larger particles. TiO2nanofibers behave finer grain size, increased specific surface area as well as the contact area between the catalyst and the pollutants, and the superior recycling ability comparing with TiO2nanoparticles.In recent years, with the development of the electrospinning technique, a variety of inorganic oxides including TiO2have been prepared into nanofibrous structure. Electrospinning has become a conventional method for rapid preparation of continuous TiO2nanofibers with a uniform diameter. However, it is still a challenge to enhance the photocatalytic efficiency of these semiconductors to meet the practical application requirements because of the drawback of poor quantum yield caused by the fast recombination of photogenerated charge carriers (hole-electron pairs). TiO2is a wide band gap semiconductor only photoactivated under UV light irradiation. Although there is a lot of work to prepare modified TiO2to improve their catalytic activity, the development of new, efficient and practical photocatalytic materials with a simple preparation method is still the direction of our efforts. TiO2nanofibers were formed by the internal nanoparticles reunioning in a certain direction. Reducing size of the nanoparticles within the nanofibers, increasing the specific surface area and coupling with other semiconductor could further increase the UV/visible light catalytic ability of the nanofibers. Based on the above ideas, we optimize the performance of the nanofiber photocatalyst by the crystallization, doping and compositing of the nanoparticles within the nanofibers. Main contents and results are listed as follows:(1) Prepared optimized and modified TiO2nanoparticles, and then added them in the spinning solution for preparing TiO2nanofibers. By the microfluidic technology, the TiO2nanocrystals were prepared with the crystallite size of5.0nm and a surface area of314.5m2g-1. The TiO2nanocrystals were monodispersed in the PVP nanofibers with the potential application in self-cleaning materials, air purification, and filtration membranes.(2) The amorous TiO2nanoparticles/PVP composite nanofibers were treated by water vapor, and then the amorous TiO2in situ crystallized into anatase TiO2within the composite nanofibers. The effect of temperature on the crystallization was also studied. Dissolving the PVP by organic solution, the ultrafine TiO2nanoparticles (5-10nm) with their surfaces modified by organic function group could be added to other organic materials for application in catalysis.(3) W-doped TiO2nanofibers were prepared. They behave finer crystallite size, stronger visible light absorbance, and larger surface area comparing with pure TiO2nanofibers. The nanofiber structured morphology on the quartz tube promotes the reaction rates for the gas-phase photo-oxidation of toluene. The concentrations of the produced CO2keep steady during the photodegradation process, indicating the practicality and operability for the whole experiment. This research is conducive to the development of novel photocatalytic materials to efficiently mineralize toxic gas pollutants including toluene for practical application. (4) Heterostructured TiO2/SnO2nanofibers deposited with ultrafine Pt nanocrystals (Pt-TiO2/SnO2) were prepared by combining electrospinning and polyol reduced method. The samples have been characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflection spectroscopy (DRS), photoluminescence spectra (PL) and nitrogen adsorption-desorption isotherm analysis. The results indicated that heterojunctions formed between TiO2fibers and SnO2fibers in the side-by-side structure with Pt nanocrystals uniformly deposited on them. The photocatalytic activity of Pt-TiO2/SnO2for the degradation of methylene blue was much higher than that of bare TiO2and SnO2nanofibers, which could be ascribed to the formation of heterojunctions in the TiO2/SnO2nanofibers and the rapid transportation of electrons to the surface by Pt nanocrystals.(5) Hierarchically nanostructured TiO2/WO3photocatalyst was synthesized via the subsequent hydrothermal treatment of electrospun TiO2nanofibers in the presence of tungstic acid. The WO3nanorods with a diameter of about40nm and an average length of about150nm grew perpendicularly on the TiO2nanofibers. The TiO2/WO3heterostructures have larger surface area for adsorption of the pollutants, and WO3nanorods possess a single crystal structure, which facilitates the migration of the photogenerated electrons. Photocatalytic tests display that the TiO2/WO3heterostructures exhibit a remarkably higher degradation rate of organic pollutants than that of the bare TiO2nanofibers under visible light irradiation. The enhanced photocatalytic activity is attributed to the extended absorption in the visible light region and the effective charge separation derived from the coupling effect of TiO2and WO3nanocomposite.(6) Graphene is a good conductor of electrons and cheaper compared to the noble metals. C in graphene and Ti in titanium dioxide are easy to form C-Ti bond to broaden the spectral response range of TiO2. GO was prepared by modified Hummers method and dispersed in an organic solvent of dimethylformamide (DMF), and was added at different concentrations to the TiO2nanofibers using electrospinning method. The results show that adding GO broadened the spectral response range of TiO2to visible light region. With increasing the content of graphene, TiO2grain size bacame finer. The composite nanofibers with4.0wt%GO show the highest catalytic activity under visible light irradiation.Our study provides a feasible path to deal with the puzzles of the recombination of photogenerated electron-holes and the low visible photoactivity. Several new and efficient photocatalysts were developed in our work. Improving the performance of the nanofibers by optimization design of internal nanoparticles provides a reference for the use of electrospinning method to prepare other oxide nanofibers and expand its range of applications.