Synthesis And Characterization Of Boron And Boride One-dimensional Nanostructures

One-dimensional (1D) nanostructures such as wires, rods, belts, and tubes have become the focus of intensive research owing to their unique applications in mesoscopic physics and fabrication of nanoscale devices. Carbon nanotubes and semiconductor nanostructures of silicon, ZnO and CdS et al have been most researched until now. Among the most promising materials that may substitute for silicon within the nanodomain are single- and multiwalled carbon nanotubes. Additionally, researchers are looking for new fabrication methods and properties of 1D nanomaterials.The elemental boron and boride compounds occupy a unique place within chemistry and physics for the complexity of their uncommon structures associated with their unusual three-center electron-deficient bonds. Boron possesses a richness of chemistry second only to carbon. However, compared with other semiconductor nanostructures, boron nanostructures have been relatively less studied experimentally and their structures and properties have not been fully elucidated either. Even though during the past few years, boron and boride nanostructures such as nanowires and nanotubes have been synthesized successfully. Systematic investigation on the detailed growth conditions and growth mechanisms haven't been processed. Fabrication of aligned crystalline boron and boride nanowire arrays, the diameter and morphology controllable synthesis of boron nanowires, and large quantity synthesis of these one-dimensional nanomaterials by a simple method have been challenges so far.Based on reviewing of the recent progress of one-dimensional boron and boride nanostructures, we systematically studied the synthesis and characterization of amorphous boron nanowires, crystalline boron nanowires, MgB₂ nanowires and metal borate nanowires. Some innovative results were obtained:Aligned single a-tetragonal boron nanowires were synthesized by CVD in NCA with diborane diluted by argon, hydrogen, and nitrogen as the reaction gases. The diameter of the nanowires is 20-60nm. The nanowires are parallel to each other in NCA nanochannels and perpendicular to the substrates. The diameter of the nanowires could be adjusted by changing the diameter of the nanochannels.Amorphous boron nanowirse and crystalline boron nanorods on Si substrates were also synthesized by CVD. The influences of temperature, pressure, catalyst on the morphology and structure of the boron nanowires were investigated. The growth mechanism of boron nanowires were studied theoretically based on the thermal dynamics and kinetics.Crystalline boron nanowires were fabricated by PVD and ZnO/B hierarchical hetero-nanostructures were achieved by two-step thermal evaporation ZnO and boron. The source material used in PVD is nontoxic compared with diborane. The influences ofthe growth conditions including gas flow, souce temperature and collecting temperature were investigated: single crystal hexagonal and tetragonal boron nanowires were collected at high and low temperature zone when source temperature above 1200°C, respectivey;just amorphous boron nanowires were obtained when source temperature below 1200°C. Raman spectra of the different boron nanostructures were investigated. The morphology, structure and optical properties of boron nanowires and hierarchical hetero-nanostructures were investigated.Aligned single crystal superconducting MgB2 nanowires were synthesized by using NCA as substrates and aligned single crystal boron nanowires as precursors in a small vacuum quartz tube.The diameters of the MgB2 nanowires were in the range of 20-150 nm. The superconductivity of the nanowires was confirmed by magnetization measurement and it indicated that the nanowires had a superconducting transition temperature at -33 K. Additonally, Mg3B2O6 nanowires and MgO mico- and nanostructures including MgO polyhedral shells, nanotubes, cubes and nanowires were synthesized during the process when we tried to synthesize MgB2 nanowires by one-step, for the oxidization of Mg in the low vacuum of our system. The morphology of MgO mico- and nanostructures could be controlled by controlling gases and temperature. The growth mechanism was proposed based on the crystal structure and analysis of growth conditions.Metal borate nanowires including magnesium borate nanorods and aluminum borate nanowires were synthesized by a simple sol-gel process followed by calcining. Commercial metal nitrate and boric acid were mixed together in deionized water in different mol ratio, and citric acid was added to serve as ferment. The aspect ratio of the nanowires and nanorods could be adjusted by changing the mol ratio of metal nitrate and boric acid. The crystal structure of the nanowires and nanorods were determined by the calcining temperature. Boron oxide self-catalytic mechanism appeared to control the nanowires and nanorods growth. The large quantity of the metal borate nanowires and nanorods might find applications in enhancement of metal alloys and ceramics.