Synthesis And Photocatalytic Applications Of Crystalline Mesoporous Silicon Spheres

Utilizing semiconductor as photocatalyst to split water under sunlight is a sunstainable approach to solve the global energy crisis and environmental pollution. Efficient and low-cost photocatalysts from earth-abundant sources are keys to realize large-scale hydrogen production. Silicon, the most abundant semiconductor with a narrow band gap, is an ideal photocatalyst for hydrogen generation due to its high catalytic activity under sunlight.However, the application of Si in water splitting was restricted, because of the small difference between the conduction band edge of Si and the water reduction potential and the surface oxidation in aqueous solutions. Band gap can be enlarged by the decreasing of grain size due to quantum confinement; high crystallinity would reduce the defect of surface to relieve recombination of photogenerated carriers; and larger specific surface area can increase the catalytic surface. Thus, formation of mesoporous structure using crystalline Si nanoparticles is an effective method to improve catalytic activity of Si.In this dissertation, we use low melting eutectics salt as pore templates and colloidal silica as silicon source to prepare mesoporous silica spheres, and then reduce the silica to Si. SEM, TEM, XRD, N2 sorption isotherms measurements reveal that Si materials possess intact mesoporous spheres structure consisted of interconnected crystalline Si nanoparticles with a size around 20 nm. Raman and XPS spectra prove that less oxide of silicon and defect lies on the surface. UV-vis diffuse reflectance spectrum shows mesoporous Si spheres have a wider band gap about 1.73 eV, which is relatively well matched to the solar spectrum.The contrast of photocurrents of crystalline mesoporous Si spheres (CMSSs) and commercial Si nanoparticles (Si NPs) reveal the charge separation is more remarkable on CMSSs, so water can be reduced more effectively by electron. The results of visible photocatalytic water splitting show CMSSs have an optimized H2 generation rate of 1785 μmol H2 g-1h-1 Si; catalytic performance still keeps active even after 4 continuous cycles. Then a series of measurements are taken and no obvious changes are found in the morphology, phase and surface states of CMSSs, which prove that prepared CMSSs are stable and robust by combining aerosol synthesis and magnesiothermic reduction. Moreover, CMSSs exhibit a H2 evolution rate of 1167 μmol H2 g-1 h-1 Si under sunlight, which means CMSSs hold great potential for future application.In addition, the pore state of final product CMSSs can be adjusted by changing the dosages of pore templates salt in aerosol synthsis, so chemical and physical properties can be adjusted. Considering the unique properties of this Si materials, it should be promising for other applications such as drug delivery, optoelectronics, and Li-ion batteries.