A low-temperature and low-cost method to produce high-quality epitaxial anatase TiO 2 thin films
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Epitaxial anatase TiO2 thin films were successfully grown on lattice-matched SrTiO3 (001) substrates by a novel hydrothermal method at very low temperatures (120– 200 °C). This method is extremely simple and inexpensive in that the SrTiO3 substrate itself provides the source material for the TiO2 film. X-ray diffraction confirmed the high crystallinity and phase purity of the anatase films. The epitaxial relationship between the film and the substrate was determined as (001)[100]anatase // (001)[100]SrTiO3. Atomic force microscopy revealed the average size of the anatase crystallites as approximately 50 to 200 nm.
TiO2 is one of the most widely studied oxide materials due largely to its great technological importance. Among the two main polymorphs of TiO2, anatase is the preferred crystallographic phase over rutile for many applications, including catalysis,1 photocatalysis,2,3 dyesensitized solar cells,4,5 and lithium ion batteries.6 Anatase was also discovered recently to be a room temperature ferromagnet when doped with Co.7 Although metastable anatase is difficult to produce in bulk, phase-pure and crystalline anatase thin films can be grown using a variety of techniques.8–19 A common method is to produce epitaxial anatase films on latticematched substrates.8–15 In most cases, however, high temperature (T 艌 500 °C) is necessary for the formation or crystallization of anatase films. Here, we report a novel low-temperature (as low as 120 °C) method, which is extremely simple and inexpensive, to grow highquality (001) oriented anatase on SrTiO3 (001) substrates with an in-plane orientation of [100]anatase // [100]SrTiO3. Dissolution of perovskite (CaTiO 3 , SrTiO 3 , and BaTiO3) in an aqueous environment is a well-known phenomenon.20,21 The reaction of SrTiO3 with water under hydrothermal conditions can be summarized as SrTiO3 + H2O → Sr2+ (aq兲 + TiO2 共anatase兲 + 2OH −共aq兲 . We have used this reaction to grow anatase crystals from SrTiO3 (001) substrates at a temperature ranging from 120 to 200 °C. Each SrTiO3 (001) substrate (purchased from ESCETE B.V., the Netherlands) was first a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0036 292
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 2, Feb 2005 Downloaded: 28 Aug 2014
mechanically polished to 0.25 m finish with diamond paste, and then put in a Teflon-lined Parr Acid Digestion Bomb of 45 ml in capacity (Parr Instrument Company, Illinois), which was half filled with deionized water. The sealed digestion bomb was left in an oven maintained at the reaction temperature for a period ranging from several days to several weeks. The digestion bomb was then taken out of the oven and allowed to cool down to room temperature before the sample was taken out and dried in air. The formation of TiO2 on SrTiO3 involves the congruent dissolution21 of Sr and Ti from the SrTiO3 substrate, followed by the removal of Sr from the alteration sites as aqueous species and the precipitation of TiO2 crystallites on the surface. I
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