Preparation of ZnO nanocrystal loaded on PVDF nanofiber mat
Solutions of inorganic salt are usually not suitable for electrospinning of nanofiber because they do not have appropriate concentration, viscosity and surface tension that are required by the technique for continuous long-range fibers. Instead, a PVDF solution pre-dissolved with zinc acetate (Zn(Ac)2) was successfully used for electrospinning, giving a PVDF/( Zn(Ac)2) nanofiber mat. Simple thermal treatment of the as-prepared nanofiber mat leads to nanocrystalline ZnO-loaded PVDF nanofiber. The influence of thermal treatment temperature on the morphology of ZnO was investigated from 80°C to 200°C. It was found that at 80°C, only few particulate ZnO was obtained, whereas from 100°C to 140°C, both the number and size of particulate ZnO increased with temperature. The fiber morphology is also well-maintained at 140°C, but starts to have cracks at 160°C where the fibrous structure starts to fail, and finally collapses at 200°C due to the melting of the polymer14,15. The influence of duration time of thermal treatment on the morphology of ZnO nanocrystal was also investigated. The most number and largest size of crystals are obtained after 12 h of treatment. Further increase of thermal treatment time does not give any increase in the number and size of the crystals.
Preparation of ZnO nanorods loaded on PVDF nanofiber mat
The ZnO nanorods were prepared by hydrothermal synthesis of the ZnO nanocrystal-loaded PVDF nanofiber mat, thermally treated at 140°C, in a growth-promoting solution consisting ZnCl2, hexamethylenetetramine (HMTA) and ammonia. The growth of ZnO nanorods is a complex chemical reaction among ZnCl2 (or ZnNO3 and Zn(Ac)2), where OH−, released by hydrolysis of HMTA and/or ionization of ammonia, first complexes with Zn2+ to give [Zn(NH3)4]2+ or [Zn(OH)4]2−, followed hydrothermal dehydration to give rise to the ZnO nanorods on the surface of nanofiber.
The concentration of the growth-promoting solution, proportion of ammonia, time and temperature of thermal synthesis, temperature of thermal treatment all have an influence on the growth of ZnO nanorods. The results show that the best orientation of the ZnO nanorods were obtained at a growth-promoting solution concentration of 0.1 M with ZnCl2:HMTA molar ratio of 1:1 and 5 mL of ammonia. ZnO nanorods with the smallest diameter are obtained at a hydrothermal temperature of 100°C, while well-dispersed and uniformly structured ZnO nanorods are achieved when the time of hydrothermal synthesis is 3 h. The nanofiber mat of PVDF/ Zn(Ac)2 without pre-thermal treatment mainly consists interstacked ZnO crystals of film shape. In comparison, the one thermally treated at 140°C has well-defined nanorods that are aligned along the c axis.
Research on photocatalytic performance
To investigate the photocatalytic property of the ZnO nanorods, an organic dye, rhodamine B was selected as the target material. The ZnO nanorods synthesized by the polymer nanofiber mat template shows clearly better catalytic effect than the pristine ZnO nanorods powder, as evidenced by the larger photocatalytic degradation of rhodamine B under the catalysis of the ZnO nanorods-loaded nanofiber mat. The catalytic efficiency increases with the increase of ZnO nanorods amount. This may be because that the pristine ZnO nanorods powder may aggregates at the bottom of the solution, whereas the one formed on the nanofiber mat has much larger area that facilitates the light-absorbing efficiency. Besides, larger amount of ZnO nanorods also leads to larger area of nanofiber mat, which effectively increases the irradiation area, resulting in elevated photocatalytic degradation efficiency. The time duration of hydrothermal synthesis also affects the photocatalytic efficiency. The ZnO-doped nanofiber prepared with 3 h of hydrothermal synthesis exhibits the highest efficiency in the photocatalytic degradation of Rhodamine B, while shorter or longer hydrothermal treatment leads to lower efficiency. This is because that the nano ZnO hydrothermally prepared for 1 h only gives short nanorods with irregular structure that closely aligned at barely the surface of the fiber, and the one prepared for 3 h is too condensed to be exposed to light. Both products cannot provide optimal capturing surface that are essentially important for effective absorption of light.