Single-crystal TiO sub(2) nanowires by seed assisted thermal oxidation of Ti foil: synthesis and photocatalytic properties

TiO sub(2) nanowires (NWs) can improve the advantageous photocatalytic properties of TiO sub(2) by increasing the active surface area. Here we investigate the synthesis of TiO sub(2) NWs by thermal oxidation, studying the role of temperature, annealing time, and gas flow rates. The optimal thermal growth conditions were found to be 800 degree C for 4 h in a mixed gas flow of Ar and O sub(2). Morphological and structural characterizations, carried out by scanning and transmission electron microscopy, and X-ray diffraction, indicated a TiO sub(2) rutile monocrystalline structure of the nanowires. An in situ thermal growth analysis was performed by means of an environmental electron microscope, providing additional insights into the TiO sub(2) NWs growth dynamics. The photocatalytic properties were studied by using the degradation rate of methylene blue under UV light. TiO sub(2) NWs revealed a 70% improvement of the degradation rate compared to a reference TiO sub(2) bulk sample. Moreover, NWs were additionally annealed in forming gas (5% H sub(2) in N sub(2)) in order to promote the formation of oxygen vacancies and an increase of the carrier density. Indeed, the photocatalytic activity of the NWs treated in forming gas was 3 times higher than that of the reference TiO sub(2) bulk sample. The photonic efficiency and the quantum efficiency, also showed an increase versus bulk TiO sub(2) of about 20% in the presence of NWs, and of about 100% with the forming gas annealing. Moreover, this increase in the photocatalytic activity after the forming gas annealing also correlates with the disappearance of a deep level recombination center as observed by deep level transient spectroscopy.