Hierarchical Structure Control And Their Properties Of One-Dimensional Semiconductor Materials

Hierarchically structured materials is one of the most important areas in material chemistry, which is mainly focus on the development of new materials, the improvement of material performances and the extension of application fields. In nature, structural hierarchy is a prevailing feature observed in numerous biological materials from microscopic to macroscopic length scales. These materials hold excellent multiple functions resulting from their complex interplay between surface structure, morphology, and physical and chemical properties. With the demand of more functional materials,a great deal of work has been devoted to fabricating hierarchically structured materials for function integration through the biomimetic or bio-inspired approach. Advanced understanding of evolution process, formation mechanism, and the controllability of structural hierarchy is the basis of precision design for hierarchically structured materials. The current thesis mainly works on the hierarchical stucture control and their properties of one-dimensional semiconductor materials. Based on the hydrolysis and polycondensation of inorganic alkoxides and the phase separation, we successfully fabricated TiO₂-SiO₂ composite fibers with tunable interconnected porous hierarchy. In addition, by combined hydrothermal reaction and electrospinning, one-dimensional TiO₂ with surface nanoprotrusion and tunable interior structural hierarchy were also achieved. The evolution of structural hierarchy and possible formation mechanisms had been investigated in detail. The as-fabricated one-dimensional semiconductor materials hold exciting implications for the ?environment? and ?energy? applications, such as photocatalytic degradation of pollutants, water splitting and dye sensitized solar cell.Detailed research works are summarized as following:1. TiO₂-SiO₂ composite fibers with tunable interconnected porous hierarchy have been successfully achieved by single-spinneret electrospinning without using sacrificial templates and special heat treatment. Based on the hydrolysis and polycondensation of inorganic alkoxides, the porosity can be regulated from narrow mesopores, meso–macropores to micro–meso–macropores. The effect of the precursor ratios between TNB and TEOS, the acidity of electrospinning solutions and the surrounding humidity on the porous hierarchy have been investigated systematically. The as-fabricated TiO₂-SiO₂ composite fibers hold high specific surface area, good adsorption capacity for Rhodamine B, interconnected porosity and anatase–rutile heterojunction. Therefore, when used for the degradation of Rh B, mesoporous TiO₂–SiO₂ fibers exhibit excellent photocatalytic activity comparable to Degussa P25. The current work opens a new avenue to design of heterostructured inorganic fibers with desired porous hierarchy.2. Based on the phase separation of the electrospun precursors, meso-macroporous TiO₂-SiO₂ fibres were fabricated via a typical electrospinning procedure. The porous hierarchy of the as-fabricated products and possible formation mechanism have been investigated in detail. The results confirmed that the hydrolysis and polycondensation of inorganic alkoxides and solvent evaporation during the process of electrospinning may be responsible for the phase separation, leading to the formation of macropores inner the fibres. The obtained hierarchically macro-/mesoporous TiO₂-SiO₂ fibres exhibited amazing structure stability even at high temperature calcinations of 850 °C, and is supposed to be a highly efficient photocatalyst. The current work may guide the further design of functional inorganic fibres with controllable hierarchical porous structures.3. One-dimensional TiO₂ fibres with surface nanoprotrusion and tunable interior structures from hollow to porous cores were designed and fabricated by the hydrothermal alkaline etching of electrospun TiO₂ fibers and subsequent calcination. ?Titanate route? is a easy but effective method to fabricate TiO₂ materials. In current work, a mechanism combining alkaline etching and Ostwald ripening is proposed to account for the formation of hierarchical titanate. One-dimensional hierarchical anatase TiO₂ fibres that retain the structural hierarchy can be obtained using controlled calcination. When used as a photoanode material for DSSCs, the as-fabricated one-dimensional hierarchical TiO₂ exhibits about 1.3–1.5 times higher power conversion efficiency in comparison with that of commercial P25 TiO₂.4. One-dimensional hollow and solid porous titania were successfully fabricated via a strategy combined electrospinning method and hydrothermal reaction. Fluoride-mediated chemically induced transformation process was proposed to account for the formation of the hollow and solid porous structures. By controlling the parameters of the processes?i.e. porosity and diameter of the amorphous precursors, type of the mediated reactants?, titania with tunable wall thickness and inner structures could be selectively fabricated. Besides, the obtained one-dimensional hollow and solid porous TiO₂ exhibited good thermal stability. The organic residues and trace amounts of Na ions may act as stabilizers to raise the phase transformation temperature.