Hydrothermal Synthesis And Characterization Of Low-dimensional Inorganic Nanoscale Materials In Group IV

This thesis consists of two sections: First, the hydrothermal preparation and characterization, growth mechanism and photoluminescence properties of the self-assembled single-crystal germanium nano-onions and nanotubes, aligned germanium nanowires, silicon oxide nanowires, nano-junction and nanotubes have been studied in detail in the paper, which are synthesized by hydrothermal method from silicon monoxide powder, mixture of silicon monoxide and silicon powder, and germanium powder respectively. Second, an x-ray absorption fine structure (NEXAFS) spectrum also is used for the characterization of electronic structure, hybridization of carbon atoms, hydrogen storage and doping in carbon nanotubes and used for identifying the nanodiamond thin films and diamond thick films and studying the role of atomic hydrogen in the growth of diamond. The research is fundamental and pioneering, and is of great significance on theory and application potential.Firstly, silicon oxide nanowires and nano-junctions have been prepared from silicon monoxide by the hydrothermal method without metallic-catalysts and templates. The research results demonstrate that the silicon oxide nanowires are amorphous with a large slenderness ratio and a smooth surface, and display a straight morphology with the diameter ranging from 10 to 300nm. A water-assisted growth model is proposed to explain the formation and growth process of the amorphous silicon oxide nanowires. The FT-IR and NEXAFS spectra of the silicon oxide nanowires show that large -OH groups locate on the surface of the silicon oxide nanowires. The H⁺ and OH^- play an important controlling factor during the growth of silicon nanowires because the adsorption activity of the silicon oxide nanowires surface can be completely reduced after the defects of≡Si·and≡Si-O·blocked by OH^- and H⁺ respectively, and the H⁺ and OH- in the form of≡Si-OH can block the growth along the cross section and stabilize the structure. The silicon oxide nanowires sample exhibits a blue light emission at around 445nm. In addition, silicon oxide nanotubes with a quadrate cross section also have been synthesized from the mixture of silicon and silicon monoxide powder. The quadrate silicon oxide nanotubes named as nano-air brick by us show a hollow inner pores with a size of -300×500nm² and a small slenderness. The nanotubes is formed by the growth of the silicon oxide nanowires following screw with a quadrate cross section, but the really growth mechanism of the silicon oxide nanotubes still is not clearly.Secondly , self-assembled germanium nano-onions and nanotubes have been prepared from germanium powder by the hydrothermal method with a temperature (350℃,11MPa) below the critical temperature of water. The results of TEM and HRTEM show that the germanium nano-onions exhibits a perfect layered structure with a interplanar distance of 0.368nm which agrees well the separation between (111) planes of hexagonal Ge and a amorphous GeO₂ outer layer with the thickness of less than 1nm. The diameter of the germanium nano-onions is 10²5nm and the diameter of the inner pore is 2-5nm, and the diameter of the germanium nanotubes is 10²0nm and the diameter of the inner pore is 3-5nm. Straight single-crystallin aligned germanium nanowires with a uniform diameter distribution of 10-100nm and lengths up to several micrometers also have been synthesized from germanium powder in a high yield under supercritically hydrothermal conditions (450℃, 13MPa). The aligned germanium nanowires with thin amorphous GeO₂ outer layers have a smooth surface, uniform diameters, quadrate cross section, a cone-shaped or two incline growth tip, and a preferred growth direction of [211]. The self-assembled germanium nano-onions and nanotubes exhibit a blue-violet light emission at around 413nm while the germanium nanowires show Violet light emission at 360nm and 395nm.Thirdly, the hybridization behaviors in carbon nanotubes (CNTs) have been investigated using x-ray absorption fine structure spectrum before and after the annealing treatment and after doping. The results show that the hybridization in carbon nanotubes is a mixture between sp² and sp³ with the ratio of sp²/sp³>1, and indicate that the annealing treatment can enhance the ratio of sp² hybridizations while the doping can enhance the ratio of sp³-like hybridizations in CNTs.Fourthly, the orientation of carbon-based materials, such as high pure graphite, HOPG, nanodiamond powder, single and double-walled carbon nanotubes has been studied by the angle-dependant x-ray absorption fine structure spectrum. The results indicate that the intensity change of the C-CÏ€* resonance show an opposite trend with respect to the C-Cσ* resonance with the increase of the degree of long-rang order and the ordering of growth direction. The results indicate that the angle-dependant NEXAFS can be used for studying the orientation and properties of materials,especially for low dimensional materials.Fifthly, the germanium doping carbon nanotubes have been characterized using NEXAFS and extended x-ray absorption fine structure (EXAFS) spectrum. The results of C K-edge, Ge K-edge and O K-edge NEXAFS show that the wrappages on the surface of carbon are germanium oxide and the local electronic structure of Ge and C are changed with the increase of the ratio of germanium/carbon. EXAFS further show that the change of local electronic structure is induced by the Ge-C bonding at the interface between carbon and the germanium oxide.Sixth, the comparison study of chemisorption of hydrogen under normal temperature and pressure in various carbon nanotubes have been performed using NEXAFS spectra. The results show that under normal temperature and pressure conditions, some hydrogen still are absorbed in Multi-walled carbon nanotubes, while little hydrogen absorbed in single-walled and Double-walled carbon nanotubes. The further analysis indicates that the hydrogen was absorbed on the inner surface of the defects on the wall in Multi-walled carbon nanotubes in the forms of chemisorption.Finally,the nanodiamond thin films and the diamond thick films are identified using NEXAFS spectra. The NEXAFS spectrum of the nanodiamond thin films has a blueshif of 0.2eV with respect to that of the diamond thick films. This is a characteristic behavior of quantum confinement in nanosystems and indicates that the diamond thin films are nanodiamond thin films. The role of atomic hydrogen in the growth process of nanodiamond thin films and diamond thick films in hydrogen-rich system also studied in term of the intensity of the C-Hσ* resonance. The results suggest that the role of hydrogen is not uniform in the growth of two diamond films. In the nanodiamond, the H⁺ ions play a primary role because that the H⁺ ions can etch not only the graphite but also the grain of diamond and H⁺ ions etching much faster than atomic hydrogen, while in the diamond thick films, the atomic hydrogen is the most important element which can etch the sp2 nondiamond phase and stabilize the structure of the diamond.