Amorphous to anatase transformation in atomic layer deposited titania thin films induced by hydrothermal treatment at 12
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Liang-Jen Fan National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan (Received 20 March 2008; accepted 2 June 2008)
Hydrothermal treatment has been applied successfully to convert amorphous titania films to crystalline anatase at 120 °C, a temperature compatible with most polymeric substrates. The amorphous films were deposited at 80 °C using atomic layer deposition (ALD). The crystallinity of the films was monitored by x-ray absorption near edge structure (XANES), and the film composition was determined by x-ray photoelectron spectroscopy (XPS). The effect of precursor chemistry and substrate material was investigated. It was found that titania films produced from Ti isopropoxide are easier to crystallize than those from Ti tetrachloride as the Ti precursor. The amorphous to crystalline transformation can be achieved more readily with films deposited on Si than polycarbonate substrates. The effect of a “seed” layer on the amorphous to crystalline transformation was also studied. Preformed anatase crystallites between the Si substrate and the amorphous film were shown to accelerate the crystallization process. The possible mechanisms responsible for the phase transformation are discussed.
I. INTRODUCTION
There has been a great deal of interest in the synthesis of TiO2 thin films in recent years due to its numerous industrial applications, including photocatalysts,1 dyesensitized solar cells,2,3 antireflective coatings,4 and electrochromic devices.5 Among the two main polymorphs of TiO2, anatase was reported to be more efficient than rutile in many of these applications. Although metastable anatase is difficult to produce in bulk, phasepure and crystalline anatase thin films can be grown using a wide variety of techniques. While most methods require high temperature for the formation or crystallization of anatase films, atomic layer deposition (ALD) is one of the few techniques that can produce anatase thin films at relatively low temperatures. For example, Aarik et al.6 reported that crystalline anatase films were grown at temperatures in the range of 150 to 350 °C using TiCl4 and H2O as precursors. These deposition temperatures, however, are still too high for temperature-sensitive substrates, such as polymers. At temperatures compatible with most polymeric sub-
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Address all correspondence to this author: email: [email protected] DOI: 10.1557/JMR.2008.0297 2472
http://journals.cambridge.org
J. Mater. Res., Vol. 23, No. 9, Sep 2008 Downloaded: 26 Mar 2015
strates (50%) was transformed to anatase after hydrothermal treatment for just 1 day (and complete transformation after 10 days). It was found that crystalline nuclei formed at either the film surface or the interface with the substrate and grew outward or inward, respectively, to form coarse anatase grains. Residual chlorine was completely expelled from the film upon crystallization, but haloes of densification voids formed at the crystallization front resulting in poor adhesion to the substrate. The study also demonstrated that
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