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Formation and Application of the Self-organized Structures

Author: TuYaFang
Tutor: SangJianPing
School: Wuhan University
Course: Condensed Matter Physics
Keywords: Self-organized structures Morphology Nanostructures Photocatalytic Photoluminescence
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Type: PhD thesis
Year: 2009
Downloads: 44
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Abstract

Self-organized structures are ordered structures formed by systems in a spontaneous way under certain inside or outside conditions. Morphologies formed by deposition or crystal growth are the most typical self-organized structure. The study on the formation mechanism of self-organized structures is not only theoretically meaningful, but also can guide the practical manufacturing and thus improve the quality and property of the materials. The porous anodic alumina with nanohole arrays formed by anodization of Al is also a typical self-organized structure, which can be used as template by gas or liquid reactions to fabricate nanoarrays of metal, semiconductor or other materials. The ZnO nanoarrays obtained from gas or liquid reaction also belong to this kind. These nanostructures formed by self-organized process have significant im-portance. On one hand, the nanostructures possess of unique properties, which may lead to the development of new functional materials and devices, one the other hand, the method of self-organization is low cost and simple, which is more practically significant. In this thesis, we carried out some research on the formation and application of self-organized structures, the main contents including the following aspects:(1) Alternating morphology transitions in crystallization of NH4Cl on agar platesTwo novel self-organized patterns formed by spontaneously alternating morphology transitions have been obtained in crystallization of NH4Cl on agar plates. One is the alternating morphology transitions between dense branching morphology and sparse branching morphology, and the other is the alternating morphology transitions between dense branching mor-phology and zigzag branching morphology. In these morphology transitions, the changes of branches happen simultaneously on an envelope, on the other hand, the transitions are continuously repeated, and do not just happen in the final stages of crystallization, so they are different from the previously mentioned morphology transitions. The investigation on the underlying mechanisms of these alternating morphology transitions would be helpful to understand the crystallization dynamics under a complex condition.The self-organized growing process of these alternating morphology transitions was investigated, it was found that the dense branching morphology grew faster than the sparse branching morphology and the zigzag morphology, and the growth rate suddenly changed during the transitions. To clarify the self-organized growth mechanism of the two novel al-ternating morphology transitions, overall investigations with various experimental conditions were carried out, and a morphological phase diagram was obtained. It was found out that the mass proportion of agar to NH4Cl and the relative humidity were the key factors to determine the morphologies of the NH4Cl aggregate. There exist eight typical patterns in this morphological phase diagram, and the patterns formed by al-ternating morphology transitions were located between some patterns. Further studies found that based on the macroscopic morphological features, these patterns could be divided into three types:DBM, fractal and cluster, while according to the characteristics of the microscopic surface morphology, these patterns could be divided into two types:rough and smooth surface. Thus we proposed a new partition of the previous morphological phase diagram, and found that the distribution of the three regions divided according to the macroscopic morphological features mainly depend on the value of Ca/CN, while the distribution of the two regions based on the characteristics of microscopic surface morphology mainly depend on the relative humidity. We considered that they were controlled by macroscopic solute transport dynamics and microscopic interfacial growth kinetics, respectively. The formation mechanisms of the different patterns were discussed from these two aspects. Based on this, the self-organized formation mechanisms of the alternating morphology transitions were suggested. We considered that both the two alternating morphology transitions result from the oscillation of solute concentration in front of the growing interface caused by the competition of crystal growth and solute transfer at a moderate mass proportion. Which one of them occurs depends on the relative humidity, which controls the supersaturation.(2) Effect of agar on the deposit morphology from thin-layer elec-trodeposition of ZnThe different morphologies of deposits from the thin-layer elec-trodeposition depend on the physicochemical environment in the vicinity of the growing interface. The effect of agar on the morphology of the Zn deposits from thin-layer electrodeposition was investigated. Under the chosen conditions and without agar, the macroscopic morphology of deposit is dendritic. As the concentration of agar increases, the regular dendritic morphology disappears progressively and gives place to the dense branch morphology. The microscopical grain texture and structural characteristics of the deposits have been investigated by scanning electron microscopy and X-ray diffraction. Microscopically, the den-dritic morphology displays an orderly alignment of grains with a preferred orientation, while the dense branch morphology has a randomly oriented grain texture and diminished grain size compared to that of the dendritic morphology. This macroscopical morphological transition can be asso-ciated to the impediment of preferential oriented growth of grains at the small scale, which is caused by the random perturbations on the electrocrystallization process introduced by agar gel. The refinement of grains caused by the gel can be attributed to two factors. Firstly, agar additives reduce the activation energy of nucleation, so the nucleation rate increases. Secondly, agar molecules adsorb at the growing interface and impede the electrocrystallization process, as a result, the growth rate decreases.(3) Formation and photocatalytic properties of Sn-doped and Fe-doped TiO2 nanotube arraysAmong various oxide semiconductor photocatalysts, TiO2 has been considered to be the most suitable materials for widespread environmental applications. Compared to other structures, TiO2 nanotube arrays show some advantages for practical applications, because of their high specific surface area and easiness in operation, recovery and recycling. We produced Sn-doped and Fe-doped TiO2 nanotube arrays by the template-based LPD method with the self—organized AAO membrane as the template. Their morphologies, structures and optical properties have been investigated by scanning electron microscope, transmission electron microscope, X-ray diffraction, UV-visible absorption spectroscopy. The photocatalytic properties of the Sn-doped nanotube arrays were evaluated with the degradation of methylene blue under UV irradiation. The result showed that doping an appropriate amount of Sn can effectively improve the photocatalytic activity of TiO2 nanotube arrays under UV irradiation, and the optimum dopant amount is found to be 5.6at% in our experiments. The UV-visible absorption spectra of the Fe-doped TiO2 nanotube arrays showed a red shift and an enhancement of the absorption in the visible region compared to the undoped sample. The photocatalytic properties of the Fe-doped TiO2 nanotube arrays were evaluated with the degradation of methylene blue under visible light. The result showed that the Fe-doped TiO2 nanotube arrays exhibited good photocatalytic activities under visible light irradiation, and the optimum dopant amount was found to be 5.9at% in our experiments. (4) Synthesis and photoluminescence properties of the ZnO@SnO2 core-shell nanorod arraysCoaxial one-dimensional nanostructures consisting of different materials have attracted considerable research attention because of their combined or unique properties and great potential for applications in electronic and optoelectronic nanodevices. We produced ZnO@SnO2 core-shell nanorod arrays by using a very simple and low cost method. In this method, self-organized ZnO nanorod arrays were first obtained by aqueous chemical growth method, and then SnO2 was deposited on the ZnO nanorod arrays by liquid phase deposition method. Scanning electron microscopy, transmission electron microscopy and X-ray diffraction were used to characterize the structure and morphologies of the products. Photoluminescence properties were also investigated. It was found that the ZnOiSnO2 nanorod arrays showed enhanced UV and green emissions compared with the bare ZnO nanorod arrays.

Formation and Application of the Self-organized Structures

Abstract

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