Morphology Regulation Of ZnO Micro-/Nano-structures By Anionic Surfactants

As a new multifunctional semiconductor materials, ZnO has a broad application prospect in gas sensors, piezoelectric materials, photocatalysts, solar cells, and so on. The properties of ZnO micro-/nanostructures closely depend on morphology and size of the samples. Surfactant is an assistant additive, which plays an important role in the morphology regulation and particle dispersion in the solution through steric hindrance and electrostatic stabilization mechanism. Anionic surfactants can release negative ions that tightly absorb on(002) crystal face containing a large number of Zn2+ and have a significant effect on the morphology by hindering the growth of the crystal along c axis. In this paper, hierarchical ZnO micro-/nanopowders were prepared via a simple one-step hydrothermal method using zinc salts and sodium hydroxide as raw materials with the assistant of three anionic surfactants. The growth mechanisms of ZnO crystals were investigated. The photocatalytic properties of ZnO samples with novel and special morphologies were explored. The main content are summarized as follows:(1) Flower-like ZnO were synthesized via a simple one-step hydrothermal method using ZnCl2 and NaOH as raw materials. The structure and morphology of the samples were characterized by XRD and FESEM. The growth of the crystal is suggested as direct nucleation from supersaturated solution based on the research of changes in crystal phase and morphology of the particle in the process. Heating the Zn(OH)42- precursors up 85 ℃, Zn2+ ions were released from the hydrolysis of Zn(OH)42-. The Zn2+ concentration reached the supersaturation of ZnO instantly and a large number of ZnO nuclei formed. To reduce surface energy, ZnO nuclei aggregated directionally to produce large particles. The assembled particles grew along c axis and finally formed the flower-like structures.(2) Novel wheatear-like ZnO were synthesized via a simple one-step hydrothermal method using Zn(Ac)2 and NaOH as raw materials. The structure and morphology of the samples were characterized by XRD and FESEM. The growth mechanism was also studied. The growth of the crystal followed the “dissolution-recrystallization” mechanism. The intermediated of plank-like Zn(OH)2 was firstly produced in the reaction. Then Zn(OH)2 dissolved, followed by the nucleation of ZnO. Because the samll nuclei had very high surface energy and the reaction rate was very fast, the nuclei had no time to diffuse. Instead, they connected end to end along the plank direction to reduce the surface energy. Later the particles further grew along its c axis and wheatear-like ZnO formed.(3) The regulation of morphology of flower-like ZnO by three anionic surfactants(sodium dodecyl sulfate(SDS), trisodium citrate and citric acid) were investigated. The addition of SDS lead to the changes of petals of ZnO from individual rods to clusters, which consisted of numerous vertically packed nanorods. With higher concentration of SDS, the nanorods became more compactly and formed peach-blossom-like petals. Trisodium citrate had weak effect on the morphology of ZnO. The petals of flower-like ZnO changed nanorods with sharp ends to hexagonal prisms with regular hexagonal plane of the top ends. Citric acid had an outstanding effect on the morphology of ZnO. The morphology of ZnO underwent “chrysanthemum sheet-like microspheres solid microspheres”.(4) The regulation of morphology of wheatear-like ZnO by three anionic surfactants(SDS, trisodium citrate and citric acid) were investigated. With the addition of surfactants, the wheatear-like structure was gradually destoryed. In the presence of SDS, perfect peony-like ZnO were obtained. In the presence of trisodium citrate, mono dispersed chrysanthemum-like ZnO were obtained. The modifying effect of citric acid on wheatear-like ZnO was same as that on flower-like ZnO.(5) The photocatalytic behaviors of six ZnO samples with special morphologies were studied by the photodegradation of methylene blue(MB). The result indicated that the sheet-like microsphere ZnO showed the most excellent photocatalytic activity. The degradation efficiency for MB with sheet-like microsphere ZnO was about 90.15% after 180 minutes irradiation under Hg lamp. The degradation efficiency of MB with peony-like and wheatear-like ZnO were about 80%. The flower-like ZnO was 58.57%. The wintersweet-like and solid microspheres ZnO had almost no photocatalytic behaviors.