Synthesis and characterization of aluminum borate (Al₁₈B ₄O₃₃, Al₄B₂O₉) nanowires and nanotubes

Single crystal aluminum borate, including Al₁₈B ₄O₃₃ and Al₄B₂O₉, nanowires and nanotubes have been synthesized on sapphire substrates covered by NiB powder, Pt powder and Al films. For NiB powder covered sapphire substrates, Al₁₈B₄O₃₃ nanowires and nanotubes are formed by heating the substrates at 1100 °C under Ar gas flow. For Pt powder covered sapphire substrates, Al₁₈B₄O₃₃ nanowires and nanotubes are produced by a chemical vapor deposition (CVD) method using boron triiodide and boric acid (BI₃/H₃BO ₃) vapor with Ar gas as precursor at 1000 °C. The nanowires are several micrometers long with diameters ranging from 50 to 200 nm, while the diameters of the nanotubes are usually larger than 200 nm. For Al film covered sapphire substrates, Al₄B₂O₉ nanowires and nanotubes are synthesized by the CVD method using boron triiodide (BI ₃) vapor with Ar gas as precursor at 750-850 °C. The size of the nanostructures can be manipulated by the growth time, the growth temperature, the BI₃ vapor concentration, and the Al film thickness. The nanostructure diameter and length can be varied in the range of 10-500 nm and 300 nm-2 μm, respectively. All the nanostructures have been characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), transmission electron microscope (TEM), selected area electron diffraction (SAD), electron energy-loss spectrometry (EELS) and X-ray energy-dispersive spectrometry (XEDS). The growth mechanism of the aluminum borate nanowires and nanotubes is discussed.