The Preparation And Properties Of Metal Borides Nanostructures

Rare-earth metal hexaboredes (RB6) have excellent physical and chemical properties, such as low work function, high hardness, high melting point, high conductivity, high thermic stability, high chemical stability, respectively. It can be used as the field emission cathode materials for high stability and longer life equipment due to their excellent chemical stability. Therefore, the RB6materials have extensive applications in the civil and national defence industry. Theories and experiments results reveal that the metal borides have abundant structural and excellent physical and chemical properties. However, it is more difficult to synthesize the metal borides because of the high melting and boiling point of rare-earth metal and boron. Until now, the research on the inexpensively, safely and conveniently synthesis and properties of RB6nanostructural meterials with a large-scal, well-aligned, and smooth and dense on surface is not widely carried out. To our best knowledge, no results on the synthesis of AlB2one-dimentional nanometerials has been reported yet.In this desertion, the RB6nanotubes and nanowires and AIB2nanowires were synthesised by chemical vapor deposition method under the assistance of diffirent catalysts (such as Ni(NO3)2, Cu(NO3)2or Au). The structure, morphology and field-emission properties were characterized using X-ray diffractometer (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and field-emission measurement system. The growth mechanism of these nanotubes and nanowires was discussed.(1) Large-scale and uniform lanthanum hexaboredes (LaB6) nanotubes and nanowires have been successfully fabricated for the first time under the hydrogen and argon atmosphere, where lanthanum (La) powders and boron trichloride (BCl3) gas as the reactant and Ni(NO3)2or Au as the catalyst. The results of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high resolution transmission electron microscopy (HRTEM) indicate that the nanomaterials with smooth surfaces were single crystal lanthanum hexaboride with cubic structure. The preparation conditions of LaB6nanotubes and nanowires, such as reaction temperature, reaction time and the gas flow rate, were compared. The growth mechanism of LaB6nanotubes and nanowires was also discussed.(2)The column-like neodymium hexaboredes (NdB6) nanowires have been successfully fabricated for the first time using the Ni(NO3)2as the catalyst. The XRD patterns confirm that a single phase NdB6can be obtained under our experimental procedure. The result of scanning electron microscopy (SEM) show that the surfaces of nanowires under the assistance of Ni(NO3)2catalysts are smooth and the distribution of the diameters of nanowires are200nm,126nm,50nm, respectively. The results of the high resolution transmission electron microscopy (HRTEM) and selected area electronic diffraction (SAED) images clearly reveal that the neodymium hexaboride adopt the twin crystal. However, if Au was used as catalyst, NdB6nanowires obtained here have the morphologies of quadrangular and hexagonal prism.The Raman spectra and the field emission measurement were conducted for the first time with the neodymium hexaboride (NdB6) nanowires which has been fabricated using a catalysis-free method. The field emission of these one-dimensional NdB6nanowires shows that NdB6nanowires has the good performance for field emission.(3) A large-scale single-crystalline PrB6nanotubes and nanowires can be obtained using Pr and BCl3as starting materials and Ni(NO3)2, Cu(NO3)2or Au as the catalysts. The results of scanning electron microscopy (SEM) images show that PrB6nanowires obtained using Ni(NO3)2and Cu(NO3)2as the catalysts have the lenght of more20μm and the diameters about100nm. A large number of PrB6nanotubes can be obtained when using Au as the catalyst. The length of PrB6nanotubes obtained here can reach more than one micrometers, and the diameter is in the range of80～100nm..(4) Large-scale aluminum diboride (AIB2) nanowires have been successfully fabricated for the first time under the hydrogen and argon atmosphere, where aluminum (Al) powders and boron trichloride (BCl3) gas as the reactant and Ni(NO3)2or Au as the catalyst. The XRD patterns confirm that aluminum diboride obtained under our experimental procedure has hexagonal symmetry. The result of scanning electron microscopy (SEM) images show that the lenght of AlB2nanowires with smooth surface obtained using Ni(NO3)2as the catalyst can reach more than several micrometers and the diameters about200nm. The SEM images show that a large-scale, smooth and well-aligned AlB2nanowires were formed uniformly over the Si substrate when using Au as the catalyst. The AlB2nanowires have the length of several micronmeters with diameters in the range of500-600nm. The gold catalyst particles with same size as the diameters of AlB2nanowires can be observed on the top of each nanowire. HRTEM as well as the SAED resutls reveal that AlB2nanowires obtained in our experiments adopt single crystal structure, but it is unstable after exposing in air or under electron beam intensity. The growth mechanism dominated the AIB2nanowires growing process under this study was also discussed.