Facile Synthesis of Pure Boron Nanotubes and Nanofibers
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Facile Synthesis of Pure Boron Nanotubes and Nanofibers Jinwen Liu2 and Zafar Iqbal*1,2 1
Chemistry and Environmental Science, New Jersey Institute of Technology, Newark, New Jersey 07102, U.S.A 2 Materials Science and Engineering, New Jersey Institute of Technology, Newark, New Jersey 07102, U.S.A ABSTRACT A facile and scalable chemical vapor deposition (CVD) process in flowing argon using a solid instead of a reactive gaseous boron precursor has been carried out to synthesize crystalline boron nanostructures comprising of relatively straight boron nanotubes (BNTs) and nanofibers (BNFs). The synthesis involves the use of solid magnesium boride as the boron and magnesium catalyst precursor, nickel boride as co-catalyst, and MCM-41 zeolite as the growth template. The BNTs and BNFs produced have a narrow distribution of diameters between about 10 nm to 20 nm and lengths from about 500 nm to above 1 µm. Scanning and transmission electron microscope (SEM and TEM) imaging together with electron energy loss spectroscopy (EELS) and energy dispersive spectroscopy (EDS) have been conducted to characterize the structure, morphology and growth mechanism of these novel nanostructures. High resolution TEM imaging has been used to identify BNTs and BNFs in the nanostructures synthesized. INTRODUCTION Nanotubes and nanofibers have attracted a lot attention since the discovery of carbon nanotubes (CNTs) in 1991 [1]. CNTs, which can be considered to be seamlessly rolled up sheets of single or multiple layer graphene, show novel electronic properties and exceptionally high mechanical strength [2-4]. Boron-based nanotubes and nanofibers have recently attracted increasing attention because they have been predicted to have novel chemical, electrical and mechanical properties. For example, boron nitride nanotubes (BNNTs), first synthesized in 1995 [5], were observed to have similar mechanical properties as CNTs [6], but unlike CNTs they are electrically insulating. Pure boron nanotubes (BNTs) were first prepared by Ciuparu et al [7] by reaction of BCl3 with H2 over a Mg-MCM-41 zeolite template with parallel, uniform diameter cylindrical pores. Because boron is lighter than carbon, pure BNTs are lighter than CNTs, and are predicted to have metallic conductivities exceeding those of CNTs [8]. Boron-based nanostructures will have unusual three-center bonding [9-11], which have the potential for forming covalent intertubular bonds [9] by contrast to CNTs which primarily couple via van der Waals interactions [12]. In one study, theoretical calculations indicated that pure boron can form a very stable flat sheet like graphene which can roll up to boron nanotubes as in the corresponding carbon nanotubes, and can be either a metal or a semiconductor depending on diameter and chirality [13]. In another paper, Boustani and Quandt [14] using ab initio methods determined the structural and electronic properties of boron nanotubes and sheets. Here the synthesis of tubular and fiber-like nanostructures is reported using nickel boride (Ni2B) and Mg fr
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