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Morphosynthesis and Structural Construction of Semiconducting Nanomaterials for Enhancing Efficiency of Their Photoelectrochemical Properties

Van, Thanh Khue (Sogang university, Graduate School)

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Subject I: Morphology Controlling Synthesis: A Study on the Crystallographic Growth of Metal Oxide Nanomaterials Chapter 1 An Insight into Crystal Growth of Hematite Nanocrystals with Unconventional Truncated Hexagonal Bipyramid Shape and Its Optical and Magnetic Properties The capping agent play...
Subject I: Morphology Controlling Synthesis: A Study on the Crystallographic Growth of Metal Oxide Nanomaterials Chapter 1 An Insight into Crystal Growth of Hematite Nanocrystals with Unconventional Truncated Hexagonal Bipyramid Shape and Its Optical and Magnetic Properties The capping agent plays a critical function for the anisotropic crystal growth to induce polyhedral morphology of a nanocrystal. Uniform and single-crystalline α-Fe2O3 polyhedral nanoparticles in the hexagonal single crystal system named truncated hexagonal bipyramid for the first time was successful synthesized by a facile one step hydrothermal method with the aid of carboxymethyl cellulose and hydrazine molecules. The appearance and crystal structure of these iron oxide nanoparticles were characterized in detail by physicochemical methods. The results show that the as-synthesized α-Fe2O3 particles are bound by twelve same side-crystalline facets {101} and two other same facets {001} at tops. These obtained iron oxide particles belong to a pure hematite phase and the particle size is around 400 nm. The optical property of the as-synthesized product was analyzed and the determined indirect band gap value Eg is 2.08 eV. The magnetic property studies of truncated hexagonal bipyramid hematite particles have shown that this kind of α-Fe2O3 possess a weak ferromagnetism under the TM and the saturation points do not reach up to the maximum applied magnetic field. Role of the reactants was discussed and investigated systematically in the work. Furthermore, a schematic illustration for the probable formation of this α-Fe2O3 morphology in whole of the synthetic process was also proposed. Chapter 2 Controlling Crystal Growth Orientation and Crystallinity of Cadmium Sulfide Nanocrystals in Aqueous Phase by Using Cationic Surfactant. Strategies for Enhancing the Applicable Potentials to Produce Methanol Production Homogeneously dispersed CdS single crystal nanoparticles were obtained via a hydrothermal method by using cetyltrimethylammonium bromide (CTAB) as a surfactant. The strong interaction between the cationic head groups of CTAB molecules and the (001) planes of CdS hexagonal crystals results in a single-crystalline crystallographic growth along the [100] direction. Subject II: Architecting Construction of Metal Oxide Semiconducting Nanomaterials for Enhancing the Efficiency of Photocatalytic Activity Chapter 3 Manually Assemble Metal Oxide Single Crystal Nanoparticles onto TCO as Crystal Facet-Oriented Monolayered Films Homogeneous TiO2 single crystals with high exposure of {100} reactive facets were constructed as a seed monolayer on transparent conductive substrates with the desired orientation of reactive facets. A secondary growth process was subsequently carried out on the monolayer seed film to form an axis-oriented continuous reactive film. Performing secondary growth with different precursors led to optimized conditions for high-performance photoelectrochemical activity of anatase TiO2 films. Experimental techniques such as UV/Vis absorption spectroscopy, X-ray diffraction, high-resolution SEM, and photoelectrochemistry were used to characterize the structural, optical, and photoelectrochemical properties of the as-synthesized films. As a photoanode in a photoelectrochemical cell, the axis-oriented reactive film shows a maximum photocurrent density of 0.3 mA.cm-2, as opposed to 0.075 mA.cm-2 for non-axis-oriented (randomly oriented) TiO2 film. Chapter 4 Formation of a CdO Layer on CdS/ZnO Nanorod Arrays to Enhance Their Photoelectrochemical Performance The performance and photocatalytic activity of the well-known CdS/ZnO nanorod array system were improved significantly by the layer-by-layer heterojunction structure fabrication of a transparent conductive oxide (TCO) CdO layer on the CdS/ZnO nanorods. Accordingly, a CdO layer with a thickness of approximately 5?10 nm can be formed that surrounds the CdS/ZnO nanorod arrays after annealing at 500 oC under air. At an external potential of 0.0 V vs. Ag/AgCl, the CdO/CdS/ZnO nanorod array electrodes exhibit an increased incident photon to conversion efficiency, which is significantly higher than that of the CdS/ZnO nanorod array electrodes. The high charge separation between the electrons and holes at the interfaces of the heterojunction structure results from the specific band energy structure of the photoanode materials, and the unique high conductivity of the CdO layer is attributed to the suppression of electron?hole recombination; this suppression enhances the photocurrent density of the CdO/CdS/ZnO nanorod arrays. The photoresponse of the electrodes in an electrolytic solution without sacrificial agents indicated that the CdO layer also has the ability to suppress the well-known photocorrosive behavior of CdS/ZnO nanorods.