Research On Preparation And Growth Mechanism Of ZnO Nano-/Microstructure Through Aqueous Solution Method

ZnO is an important wide-bandgap semiconductor material. Because of its potential forphotocatalyst, piezoelectric and photoelectronic nanomaterials, ZnO becomes a research focusin recent years. Reports on controllable synthesis of ZnO has emerged in endlessly, and mostof them obtain the nanometer ZnO with different morphology by using some additives,template agents and guide agents, or change the structure and property of ZnO nanomaterialsthrough doping transition metals. However, these methods need a higher requirement for theequipment and include complex operations, long reaction time, etc, thus restrict a large-scaleproduction and wide application of ZnO. In this paper, ZnO nanoparticles with differentstructures and morphologies were prepared via an aqueous solution method at a lowtemperature in a short time, using zinc salts and sodium hydroxide as raw materials. Thismethod is a very simple and pollution-free process. The formation mechanisms of the ZnOnanostructures were further investigated in detail.The main content and results are summarized as follows:(1) Controllable synthesis and formation mechanism of the three-dimensional (3-D) ZnOmicro-/nanostructures.3-D wheatear-like ZnO nanostructures were successfully prepared via a simple one-stepsolution method, using Zn(Ac)2and NaOH as raw materials. The optimum synthetic conditionwas determined as: the initial Zn2+concentration is0.2M, the molar ratio of[Zn2+]:[OH-]=1:6~1:7, aging at85℃for30min. Under the same conditions,3-D flower-likeZnO microstructures were separately obtained by Zn(NO3)2, ZnCl2and ZnSO4. The optimumsynthetic condition was determined as: the initial Zn2+concentration is0.2M, the molar ratioof [Zn2+]:[OH-]=1:6~1:7, aging at85℃for1h.The formation mechanisms of flower-like and wheatear-like ZnO were respectivelystudied. In the formation of flower-like ZnO, an intermediate, plank-like Zn(OH)2, was firstlyproduced, and then a phase transformation from Zn(OH)2to flower-like ZnO occurred, whichfollowed the “dissolution-recrystallization” law. Due to the fast reaction, ZnO nuclei weregenerated in the solution explosively. Radial-like particles with polar structure formedthrough a secondary nucleation, which comes from the collision and aggregation of a mass of primary nuclei. The each individual primary nucleus of the radial-like particles (secondarynuclei) grows along the c-axis orientation, thus,3-D flower-like ZnO particles formed finally.In the formation of wheatear-like ZnO, the reaction speed was greatly accelerated because ofthe existence of CH3COO-, which resulted in a momentary present of the intermediateZn(OH)2. The crystal growth followed the law of “direct precipitation from supersaturatedsolution”. Similar to the formation of flower-like ZnO, radial-like particles with polarstructure firstly formed through the secondary nucleation of a mass of primary nuclei. Thenthe radial-like nuclei were connected end to end, and followed by an oriented aggregationgrowth, thus, the wheatear-like ZnO particles formed finally.(2) Controllable synthesis and formation mechanism of the one-dimensional (1-D) ZnOmicro-/nanostructures.1-D rod-like ZnO micro-/nanocrystals were prepared via a simple two-step solutionmethod, using Zn(Ac)2and NaOH as raw materials. The optimum synthetic condition wasdetermined as: the initial Zn2+concentration is0.2M, the molar ratio of [Zn2+]:[OH-]=1:5~1:7,aging at85℃for5h. Under the same conditions,1-D rod-like ZnO micro-/nanocrystalswere also obtained by Zn(NO3)2and ZnCl2, but ZnSO4.The formation of1-D rod-like ZnO was studied. An intermediate Zn(OH)2was producedafter the room temperature pre-stirring, then followed by the dissolution of Zn(OH)2and thenucleation of ZnO. The crystal forms of the intermediate Zn(OH)2depended on the solutionalkalinity, that was, the OH-concentration. When the intermediate was amorphous orplank-like Zn(OH)2, the phase transformation from Zn(OH)2to ZnO followed the“dissolution-recrystallization” law; and when the intermediate was octahedral ε-Zn(OH)2, thephase transformation from Zn(OH)2to ZnO followed the “in-situ crystallization” law, besidesthe “dissolution-recrystallization” law.