Controlled growth of high-quality TiO₂ nanowires on sapphire and silica

The vapour-liquid-solid (VLS) growth of TiO₂ nanowires (NWs) was performed using a thermally evaporated Ti source and sputter-deposited Au catalysts under an O₂ gas flow. High-density single-crystalline TiO₂ NWs having the rutile structure were successfully grown on sapphire (single-crystal α-Al₂O₃) and quartz (amorphous SiO₂) substrates. Ti buffer layers, deposited on the substrates to prevent undesirable reactions between the Ti vapour and substrates, were identified to promote the TiO₂ NW growth by providing supplementary Ti vapour to the Au catalysts. Crystallinity of TiO ₂ NWs was investigated by x-ray diffraction (XRD) and their morphological features were characterized by field emission scanning electron microscopy (FESEM). High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) analyses reveal growth of the rutile NWs having twofold twin structures along the growth direction on Ti/sapphire and the defect-free single-crystalline rutile NWs on Ti/quartz substrates. TiO ₂ NWs grown on Ti/quartz showed a short-wavelength (∼402 nm) and high-intensity photoluminescence (PL) emission compared to those grown on Ti/sapphire substrates. By introducing a Ti buffer layer and using quartz substrates, the crystallinity and PL properties were successfully improved for VLS-grown TiO₂ NWs.