Synthesis, Characterization, and Application of Morphology-Controlled Semiconductor Nanostructures

Abstract

In this dissertation, synthesis, characterization, and applications of morphology-controlled semiconductor nanostructures are discussed from the point of views of the physical chemistry and materials chemistry. Hyung-Bae Kim and his supervisor Du-Jeon Jang have developed four unique synthetic approaches for the preparation of morphology-controlled nanostructures: (1) the co-precursor method, (2) the defect-driven method, (3) the co-solvent method, and (4) the additive assisted method. Two heating methods of microwave-assisted methods and hydrothermal methods are selectively employed for the preparation of morphology-controlled nanostructures. The formation mechanisms of all the presented methods are delicately proposed in terms of LaMer theory. In addition, the morphology-dependent photophysical, photocatalytic, and photovoltaic properties of the prepared nanostructures have been investigated for potential applications. Brief overviews of the Chapters 1.- 5 mentioned in this dissertation are given below. In Chapter 1, basic concepts of nanoscience especially on semiconductor nanostructures are introduced. The physical and chemical properties of nanosized anisotropic materials show the direction-dependent quantum confinement effects. The morphology-dependent properties of anisotropic nanostructures are summarized in three categories: sizes, shapes, and the surface conditions. The fundamental formation theories of nanostructures and the several factors that affect the growth kinetics of nanostructures are thoroughly explained. In Chapter 2, the unique phenomenon of precursor-dependent shape variations of CdSe nanostructures are discussed. Spherical and branched CdSe nanostructures have been fabricated by employing two intrinsically different cadmium precursors of CdCl2 and CdO, respectively. Whereas CdCl2 precursors are polyol-soluble, CdO precursors are polyol-insoluble. We have also selectively employed two different polyol solvents having significantly different boiling temperature and viscosity. Due to the differences in synthetic conditions, each precursor has proceeded through the different growth pathways, generating the morphology-controlled CdSe nanostructures. The mechanism of precursors-dependent morphological variation of CdSe nanostructures are discussed. Combining the advantage of respective precursors, we have successfully synthesized novel structures of sphere-decorated CdSe tetrapods by using co-precursors of CdCl2 and CdO. In Chapter 3, defect-driven synthesis of porous CdSe nanostructures having a near infrared emission at room temperature. It is worth emphasizing in that the defect-engineered preparation of nanostructures has been rarely reported. Porous CdSe nanorods have been prepared faciley via the hydrothermal treatment of CdSe∙(en)0.5 nanorods. During the hydrothermal process, various crystalline imperfections appear due to lattice mismatches between orthorhombic CdSe∙(en)0.5 and hexagonal wurzite porous CdSe nanorods and subsequently dissapear to release mismatch strains. In the self-healing defects process, point defects of atomic vacancies are heavily generated near the planar defects of twin boundaries in CdSe nanorods to produce volume defects of voids eventually. The emission of CdSe nanorods shifts to the red and decreases in intensity with the increase of surface states and selenium vacancies. The mean lifetime of emission increases with the increase of the hydrothermal-treatment time as the fractional amplitude of a surface-state-related component increases. Chapter 4 focuses on two subjects

(1) the strategies and formation mechanisms of morphological variation of anatase TiO2 nanostuctures (Chapter 4A) and (2) their photocatalytic and photovoltaic applications (Chapter 4B). The morphologies of anatase TiO2 crystals have been varied facilely from rod-like structures to various hedgehog-like hierarchical structures via forming titanium glycerolate precursors as sacrificial templates. The morphologies of the precursors have been controlled readily under microwave irradiation by adjusting the relative solvent volumes of isopropanol and glycerol. The variation of the relative volume fractions of isopropanol and glycerol having significantly different boiling points and viscosity values has changed the nucleation and growth kinetics to control the morphologies as well as the sizes of titanium glycerolate precursors. In this studies, it has been founded that hierarchical 3D nanostructures have 5.5 times higher photocatalytic activity than rod-like 1D nanostructures, and the DSSC efficiency of hierarchical 3D nanostructures becomes as high as 5.37%, which is 50% higher than the DSSC efficiency of 1D nanostructures. In Chapter 5, the ammonia-assisted synthesis of morphology-controlled ZnO microstructures under microwave irradiation is discussed. The morphologies and the surface conditions of ZnO microstructures have been controlled facilely via a one-pot synthetic route by varying the ammonia concentration of the reaction mixture. Ammonia affects the nucleation, growth, and hydrolysis kinetics of intermediate zinc glycerolate to induce shape variation from flower-like ZnO microstructures to various ZnO twin microstructures with the preferred exposure of ±(0001) polar planes. As-prepared ZnO microstructures are mesoporous with large specific surface areas and high specific pore volumes, which have resulted from the microwave-assisted fast hydrolysis of intermediate zinc glycerolate microstructures. Owing to the novel features of microwave, ZnO microstructures have numerous microcracks and wrinkles on their surfaces and show characteristic defect-driven orange emission, whose intensity increases with the specific surface area. The photocatalytic degradation rate constant of rhodamine B via our prepared ZnO microstructures have been founded to increase linearly with the specific surface area, the specific pore volume, and the polar-surface exposure. Our simple and rapid microwave-assisted synthetic method is considered to be beneficial to the development of morphology-controlled metal oxides that are applicable for eco-friendly waste-water treatment.