The Transformation From Metastable Nanoparticles To Nanorods

ZnS has two kinds of structures,namely zinc blend crystal structure and wurtzite structure,which have wide band gaps of 3.72 eV and 3.77 eV,respectively. ZnS is typically found to have the zinc blend crystal structure at room temperature in its bulk form. Bulk ZnS can undergo a phase transformation from the cubic zinc blend structure to a hexagonal crystal structure known as the wurtzite structure at elevated temperatures. When there size are between 1～100nm,There will be fascinating and novel properties, as well as their new applications correlated with their specific nanostructures. Due to their morphology-characters,0 dimension nanoparticles and 1 dimension ZnS nanostructures have excellent light-emitting properties,low dispersion between visible and infrared range,high efficiency bases for transition elements and rare elements. ZnS also has eminent fluorescence,phosphors and IR transmission properties,hence it has been widely used in the new sensor, high-resolution displays and other electronic materials.ZnO, with a wide direct band gap (3.37 eV) and a large exciton binding energy (60 meV), is an important semiconducting material. It have wurtzite structure at room temperature , its applications in UV light-emitting are suitable for the room temperature or higher. Nanostructures ZnO has different properties from the bulk form,such as Surface effect,Small size effect,Quantum confinement effect and etal. ZnO nanostructures have recently attracted much attention due to their novel properties and potential applications and have been extensively used in manufacturing daily necessities, plastics productions,Solar cells ,Photocatalytic,electronic and optoelectric devices,etal.We introduce a slow chemical precipitation method by which controllable size of the nanocrystals can be prepared. The preparation process of this method meet to two conditions:the first ,large numbers of nucleus generate spontaneously at the beginning of the reaction,the nuclear size is close to the critical nucleus size and has a good monodispersion. Second,this method can maintain a slow growth after nucleation .In this paper ,we used two different diameter test tubes nested in different directions to prepare ZnS precipitation in accordance with the slow chemical method . Then Core-shell structure ZnS nanocrystals are prepared with the ions supplying from the tubes. TEM and SAED are used to analyst the structure and morphology of the samples. The morphologies of the samples change in response to the changing of concentration of reaction solutions. The reason is analyzed from the point of the balance of the Zn2+,S2- and ZnS monomer in reagent region as well as Ostwald ripening effect and others.A small space established via the contraposition of a Vial and a petri dish, So the ZnS nanoparticles grow up by the supplying of Zn2+ and S2- .ZnS nanoparticles and one-dimensional ZnO nanocrystals are obtained at last,as a result of O element participate in the reaction. XRD and TEM are used to analyst the structure and morphology of the samples. The mechanism of the morphologies in response to the changing of concentration of reaction solutions is also discussed.We prepared a sample in which main component is ZnO by two steps. ZnS metastable nanoparticles are prepared using two contraposed vials at first. Second,these nanoparticles are dispersed in the solution with some concentration, then are transfered into an autoclave and treated at the conditions of 150℃for 24 hours. Plentiful 1D nanorods and a spot of 0D nanoparticles are prepared in the end. The two structures are considered to be ZnO nanorods and ZnS nanoparticles analyzed by TEM, HRTEM and EDS measurements. TEM, SAED and EELS are used in analyzing the growth mechanism. By adjusting the concentration of reaction solution and reaction time in the first step, and the concentration in the second step, we get a series of morphologies. We obtained the better sample which is uniformed and has a high yield when the reaction solutions are both 0.06mol/l and the reaction time is 2.5h in the first step,the reaction solution is 0.01mol/l in the second step. TEM is used to monitor the changing morphologies due to the changing conditions. The effect of the reaction time in the first step is also investigated by UV spectroscopy, and further note that the size with homogeneous monodispersion is in favor of the growth of uniform nanorods.Comprehend the previous discussion, core-shell structure nanocrystals and 1D nanorod have been prepared via the slow chemical precipitation method. In order to verify the applicability of this method, we got nanowires using two contraposed vials in the condition of ammonia being introduced into the reaction solutions. This method also be used to make metastable nanoparticles, then these nanoparticles are treated in autoclave to obtain nanorods. Changing the reaction solution we still got 1D nanorods with uniform size. Note this method in the preparation of one-dimensional materials is effective.The slow chemical precipitation method is facile, controllable and low-cost; the two-step method making the metastable nanocrystals transform to 1D nanorod is effective. And in the process of all experiments organic solvents were not used, reducing the harm to the environment.