Synthesis of very straight bismuth oxide nanowires by using thermal evaporation of bismuth powders

One-dimensional (1D) nanostructures of bismuth oxide (Bi₂O ₃) have been prepared by using various techniques, but there has been no report on the synthesis of Bi₂O₃ nanowires by using thermal evaporation yet. Very straight single-crystal Bi₂O ₃ nanowires with sizes of 50 - 300 nm in diameter and 50 - 100 um in length, depending on the process temperature and the N₂/Ar gas flow ratio were successfully synthesized on Au-coated c-plane sapphire (Al ₂O₃) substrates by using thermal evaporation with bismuth powders. The dependences of the morphology of the Bi₂O₃ nanowires on the process temperature, the oxygen partitial pressure and the substrate material were investigated. The straightness of the Bi ₂O₃ nanowire strongly depends on the oxygen partial pressure. The growth rate of Bi₂O₃ nanowires is much faster on (0001) Al₂O₃ than on (100) Si, despite the lattice mismatch (15.5%) between the c of β-Bi₂O₃ and the a of Al₂O₃ being much larger than that (3.6%) between the c of β-Bi₂O₃ and the a of Si (5.43 Å). The crystalline nature of the Bi₂O₃ nanowires was revealed by using high-resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED). X-ray diffraction (XRD) results indicate that the Bi₂O₃ nanowires are pure tetragonal β-Bi₂O₃ phase single crystals. Energy-dispersive X-ray spectroscopy (EDS) analysis confirms that the Bi₂O₃ nanowires grow via a vapor-liquid-solid (VLS) mechanism in thermal evaporation of Bi powders. The photoluminescence (PL) spectrum of the Bi₂O ₃ nanowires shows a broad blue emission peak centered at around 450 nm.