Single-crystal TiO2 nanowires by seed assisted thermal oxidation of Ti foil: synthesis and photocatalytic properties

Abstract

TiO₂ nanowires (NWs) can improve the advantageous photocatalytic properties of TiO₂ by increasing the active surface area. Here we investigate the synthesis of TiO₂ 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 °C for 4 h in a mixed gas flow of Ar and O₂. Morphological and structural characterizations, carried out by scanning and transmission electron microscopy, and X-ray diffraction, indicated a TiO₂ 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 TiO₂ NWs growth dynamics. The photocatalytic properties were studied by using the degradation rate of methylene blue under UV light. TiO₂ NWs revealed a 70% improvement of the degradation rate compared to a reference TiO₂ bulk sample. Moreover, NWs were additionally annealed in forming gas (5% H₂ in N₂) 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 TiO₂ bulk sample. The photonic efficiency and the quantum efficiency, also showed an increase versus bulk TiO₂ 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.