Fabrication Of Mesoporous ZnO Nanofibers With Tailored Structures And Their Photocatalytic Activities

One-dimensional ZnO mesoporous materials have many outstanding features such as direct wide band gap, large exciton binding energy, chemical stability, as well as large surface area, which make them promising in photocatalysts, biomedicine, photodetectors, gas sensors and solar cells. So far, significant advances have been achieved in the preparation of efficient Zn O photocatalysts. However, it still remains as a grand challenge to develop a simple technique for producing 1D Zn O mesoporous materials with high purity and high surface area.In this work, we devote to produce the novel and efficient Zn O photocatalysts. The mesoporous Zn O nanofibers were synthesized via a foaming-assisted electrospinning strategy. By optimizing the main experimental parameters, the morphology of the fibers can be precisely manipulated, enabling the enhancement of their photocatalytic activities. The formation mechanism of mesoporous Zn O nanofibers were proposed, and the key factors affecting their photocatalytic H2-production activities were also investigated. Finally, the most suitable fiber structure of Zn O photocatalyst was clarified. The main progresses of this reseach have been made as below:(1) We report a simple strategy for the synthesis of large-pore mesoporous Zn O nanofibers through a foaming-assisted electrospinning process(FAEP), using diisopropyl azodicarboxylate(DIPA) as the foaming agent. The as-fabricated mesoporous nanofibers possess high surface area, enlarging the SBET of the Zn O nanofibers from 8.2 m2/g to 12.6 m2/g. The content of DIPA in the precursor solutions plays a crucial role on the growth of large-pore mesoporous Zn O nanofibers, and the optimized content is about 6 wt%. The resultant large-pore mesoporous nanofibers exhibit excellent photocatalytic activity of 791 μmolh-1g-1 and significant stability for hydrogen production compared to conventional solid nanofibers.(2) By changing the foamer of FAEP into tea saponin(TS), the specific surface area of the mesoporous Zn O nanofibers can be varied in a controlled manner. The high purity nanobelts which have increased the SBET of the Zn O fibers to 13.9 m2/g were prepared. The as-fabricated mesoporous nanobelts exhibit the much lower H2 generation rate(694 μmolg-1h-1) than that of large-pore mesoporous Zn O nanofibers with circular cross-sections(791 μmolg-1h-1) and even comparable to that of conventional solid nanofibers(644 μmolg-1h-1), disclosing that the specific surface area is not the only determinant factor for enhancing the photocatalytic performance of Zn O nanofibers.(3) Through regulating the H2 O content in the solvent, the mesoporous Zn O nanofibers can be tailored in the manner of shape structures, enabling the fabrication of elliptic cylindrical Zn O nanofibers, which have remarkly enhanced the SBET of the Zn O nanobers to 15.2 m2/g. The mesoporous Zn O fibers with elliptical cross-sections exhibit significantly enhanced H2 evolution rate of 930 μmolg-1h-1, which is nearly 1.5 times to that of currently fabricated solid fibers, and much more stable photocatalytic activites as compared to commercial Zn O nanopowders.(4) The above result suggests that not only the specific surface area but also the various stacking modes induced by the different cross-sections of the fibers can deeply effect the photocatalytic activities of the mesoporous Zn O fibers. The mesoporous Zn O fibers with elliptical cross-sections balance the two factors and exhibit the highest photocatalytic performance.