Facile Fabrication Of Functional Hierarchical Transition Metal Oxides Nanofibers

The functional hierarchical transition metal oxides nanofibers were synthesized by a sol-gel combined electrospinning technique. The preparation of the fibers and the control of fiber morphology and hierarchical structures, and the photocatalytic properties, selective photocatalystic properties, and magnetic properties of the prepared fibers were studied.1. Long TiO₂ and SiO₂ hollow fibers with mesoporous wallLong TiO₂ hollow fibers with mesoporous walls and high surface areas were prepared via the sol-gel method combined with two-capillary spinneret electrospinning technique. The mesoporous fibers were as long as 30 cm with diameters of 0.1～4μm and wall thicknesses of 60～500 nm. The diameter and wall thickness of TiO₂ hollow fibers could be tuned by adjusting the N₂ pressure of the inner and outer capillaries. N₂ adsorption-desorption isotherms gave a BET surface area of 200～208 m²/g and average pore size of 6.7 nm. Most of the nanochannels in the walls are disordered, and some exhibited ordered hexagonal alignments. These TiO₂ hollow fibers with mesoporous walls showed higher photo-catalytic activities toward decomposition of methylene blue and gaseous formaldehyde than commercial TiO₂ nanoparticles (P25) and corresponding mesoporous TiO₂ powders, which show potential applications in photo-catalytic field.SiO₂ hollow nanofibers with mesoporous walls and high specific surface areas were prepared by the sol-gel related two-capillary spinneret electrospinning technique. The mesoporous nanofibers were as long as 10 cm with the diameter of 100～400 nm and 50～200 nm in shell thickness. N₂ adsorption-desorption experiment gave the BET surface area of 468.0 m²/g and the nanopore volume was 0.07 ml/g using the t-plot method, the nanopores hold 312.1 m²/g contributing much more than that of mesopores, whose average pore size was 6.6 nm. The shell of the as-prepared SiO₂ hollow nanofibers was constituted by the out-of-order mesopores and containing large numbers of nanopores in the mesoporous wall. These hierarchical SiO₂ hollow nanofibers may be used as nanodevices in the future.2. Preparation of mesoporous TiO₂ core/SiO₂ shell nanofibersMesoporous TiO₂ core/SiO₂ shell nanofibers with a length of 20～30 cm have been fabricated by co-electrospinning silicic and titanic sols. The nanofibers have outer diameters of 100～300 nm and shell thicknesses of 5～50 nm, which are adjustable by changing the electrospinning parameters. As calculated by the DFT method based on the N₂ absorption/desorption isotherm, the pore size of the TiO₂ core was 7 nm, that of SiO₂ shell was 3.5 nm and the SiO₂ layer also contained nanopores of 1.3 nm in size. The mesoporous composite nanofibers exhibited photo-catalytic selectivity for smaller molecules as evaluated by the oxidation decomposition of MB (AY) over DR dyes. In addition, the nanofibers could be conveniently fixed and reclaimed, and are good candidates for selective reactions in photocatalysis of waste water.3. Preparation of ferreous nanofibersLongα-Fe₂O₃ hollow fibers are prepared by sol-gel combined two-capillary spinneret co-electrospinning technique. The outer diameter of the as-prepared hollow fibers is 0.5～5μm with wall thickness of 200～800 nm. Hollowα-Fe fibers are also prepared by reducing the as-preparedα-Fe₂O₃ fibers under hydrogen flow at different temperatures andγ-Fe₂O₃ nanofibers were formed unpon reoxidation. The hollow ferric-based fibers have potential applications in catalytic and materials science in the future. Currently, detailed studies are in progress to prepare core/shell fibers as micro or nano devices. These results not only enrich the tubular inorganic compounds, but also provide facile route to nanostructured hollow fibers.ZnFe₂O₄ gel fibers have been obtained by sol-gel related electrospinning method, and these polycrystalline nanofibers were obtained via calcination of the xerogel fibers. The final ZnFe₂O₄ nanofibers composed of 20～30 nm nanocrystals were about 100 nm to several hundred nanometers in diameter. The ZnFe₂O₄ nanofibers would possess potential applications in maganetic materials area.