Synthesis of nitrogen-doped titanium oxide nanostructures via a surfactant-free hydrothermal route
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Soo Hyun Chung Korea Institute of Energy Research, Daejon 305-343, Republic of Korea (Received 21 September 2004; accepted 27 July 2005)
Nitrogen-doped titanium oxides nanostructures were synthesized by a new method proposed here from titanium oxysulfate precursor in a NH4OH solution under hydrothermal conditions without any extra templates as structure driving agents. The material synthesized with NH4OH was an ammonium titanate and showed curled nanosheets, nanofibers or nanorods morphologies depending on the molar ratio of NH4OH to titanium precursor and the hydrothermal temperature. The nanofibrous titanates had a high surface area over 500 m2 g−1 and a pore volume of 0.72 cm3 g−1. The calcination of as-synthesized material at 673 K produced a titanium oxynitride TiO2−xNx with anatase phase, which absorbed visible light. Ion exchange of ammonium ion of the titanate with sodium Na2Ti3O7−xNx enhanced the thermal stability of the titanate phase.
I. INTRODUCTION
Recent efforts in the synthesis of nanostructured materials have been focused on fabricating one-dimensional (1-D) nanowires, fibers, rods, or tubes. Those 1-D nanostructured materials have attracted interest because of their unique electronic, optical, mechanical properties and wide-ranging applications in diverse areas.1,2 Also, synthesis of materials with a well-defined nanoscale cavity is currently under intensive investigation. The reported materials in 1-D nanostructured form include carbon,1 NbS2,3 TaS2,3 MoS2,4 SiO2,5 Al2O3,6,7 V2O5,8 MoO3,9 WO3,10 ZrO2,11 and TiO2. Titanium oxide is one of the most extensively researched materials because of its high photocatalytic activity,12 which could be exploited for such applications as environmental purification, water splitting to produce hydrogen, gas and humidity sensors,13 self-cleaning devices13 and photovoltaic cells.14 The material is also widely used as a chemical catalyst and a catalyst carrier.15 Particularly titanium oxynitride with anatase phase TiO2−xNx shows a photocatalytic activity under visible light.16 The performance of titania in its various applications depends on its crystalline phase, dimensions and morphologies.17 Many attempts have been made to control the structure and shape of titanium oxides. The
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0376 J. Mater. Res., Vol. 20, No. 11, Nov 2005
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methods to prepare 1-D nanostructured titania include using an anodic aluminum membrane as structuredirecting template,18 organo-gels as templates via a solgel process19 and solid-state synthesis by carving from bulk titania crystals.20 However, these materials generally have large diameters over several hundreds of nanometers, and their walls are composed of nanoparticles with a crystalline phase. Recently, Kasuga et al.21,22 produced high-quality TiOx nanotubes with uniform diameters of around 10 nm via simple hydrothermal treatment of crystalline TiO2 particles in concentrated NaOH aque
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