Titania Nanowire Arrays Bull. Korean Chem. Soc. 2004, Vol. 25, No. 9 1341 Synthesis and Characterization of Highly Crystalline Anatase Nanowire Arrays Yongnan Zhao： U-Hwang Lee, Myungkoo Suh, and Young-Uk Kwon* ‘Department ofMaterials Science, School ofMateri s Science and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300160, P.R. China Department of Chemistry and BK21 School ofMolecular Science, Sungkyunkwan University, Suwon 440-746, Korea Received May 30, 2004 We developed a novel synthesis strategy of titania nanowire arrays by employing simple hydrothermal reaction and ion-exchange reaction techniques. Hydrothermal reactions of metallic titanium powder with H2O2 in a 10 M NaOH solution produced a new sodium titanate compound, Na2Ti6Oi3，xH2。(x〜4.2), as arrays of nanowires of lengths up to 1 mm. Acid-treatment followed by calcination of this material produced arrays of highly crystalline anatase nanowires as evidenced by x-ray diffraction, Raman spectroscopy, and transmission electron microscopy studies. In both cases of sodium titanate and anatase, the nanowires have exceptionally large aspect ratios of 10,000 or higher, and they form arrays over a large area of 1.5 x 3 cm2. Observations on the reaction products with varied conditions indicate that the array formation requires simultaneously controlled formation and crystal growth rates of the Na2Ti6O13，xH2。phase. Key Words : Nanowire array, Anatase, Titanate, Hydrothermal synthesis Introduction Arrays of one-dimensional (1D) materials are a recent research focus for their potentials in nano-scale electronic and photonic devices.1-22 However, although a large number of 1D nanomaterials have been synthesized,23 fabrication of well-aligned nanowire arrays still is a paramount challenge. Various methods have been reported for this end including vapor-liquid-solid or vapor-solid growth techniques1-8 and methods employing templates such as porous aluminum oxides9-18 or lyotropic liquid crystals. 19 The vapor techniques generally need special equipments and high temperatures, and the template method often encounters difficulties of pre- fabrication and post-removal of the templates and usually results in impurities. The nanowire arrays by above- mentioned methods are usually in the lengths of several to tens of microns and aligned over a small area supported by substrates or templates, for which aspects further develop- ment of synthetic methods is required. Substantial recent studies have indicated that hydrothermal route is a powerful and promising strategy for preparing 1-D nanomaterials, such as nanowires and nanotubes,24-31 but mostly without any orientational order. Because of its many advantages such as simple procedure and low cost, it would be desirable to use hydrothermal reactions for the synthesis of arrayed nanowires if ever possible. Recently, this technique has been successfully applied to the synthesis of ZnO nanorod arrays that exhibit an interesting optical property.22 Unfortunately, ZnO appears to be the only example of arrayed nanowires or nanorods from hydrothermal reactions.32 In this study, we have synthesized titania (anatase) nano- wire arrays for the first time employing the hydrothermal ^Corresponding Author. Tel: +82-31-290-7070; Fax: +82-31-290- 7075; e-mail: ywkwon@skku.edu technique. Titania is a wide band gap semiconductor having important applications in environmental protection, photo- catalysis, chemical sensing and solar energy conversion. Titania nanotubes and nanowires have been prepared by sol- gel or electro-deposition methods within the tunnels of porous alumina or polycarbonate.21,33-42 The typical products of these methods are mixtures of titania phases in poly- crystalline forms. On the contrary, our strategy is to combine the well-known anisotropic morphologies of alkali titanate compounds and their facile conversion into titania by ion- exchange and thermal treatments.43 As a result, we have obtained freestanding arrays of nearly single crystalline anatase nanowires of very high aspect ratios up to 20,000. Experiment지 Section Synthesis of arrayed nanowires of NazTROi^xHzO (x〜4.2). The precursor for titania nanowires was prepared by a hydrothermal treatment of metallic Ti powder in an aqueous sodium hydroxide medium using H2O2 as the oxidant. In a typical procedure, 0.2 g Ti powder (100 mesh, Aldrich) was added in a 20 mL Teflon vessel followed by the addition of a 15 mL 10 M NaOH solution without stirring. Then 1mL H2O2 (35 wt%) was injected. The hydrothermal reaction was carried out statically at 220 oC for 48 hours. The reaction produced a new sodium titanate compound (Na2Ti6O13，xH2。，x〜4.2) as precipitated white masses and floccules suspended in the solution. The precipitates were isolated by decanting the solution, washed with water for several times, and dried at room temperature. Analyses on this material showed that it was composed of sodium titanate nanowires aligned almost parallel to one another. Ion-exchange and therm지 conversion of Na2Ti6Oi3- xH2O (x〜4.2) into anatase The arrayed sodium titanate