X-Ray Diffraction Analysis of Residual Stress in Thin Polycrystalline Anatase Films and Elastic Anisotropy of Anatase
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TANIUM DIOXIDE (TiO2), a material that is used in most products in daily life, is still attracting research interest not only because of its low cost, nontoxicity, good chemical stability, mechanical hardness, and optical transmittance with high refractive index, but, recently, mainly due to its photo-induced properties. It is a unique material that connects two distinct photo-induced phenomena, photocatalytic activity[1,2] and photo-induced superhydrophilicity,[3–5] after UV light illumination. Therefore, this material offers various applications: (a) in the photocatalytic decomposition of environmentally unfriendly compounds,[5,6] (b) as an antibacterial and self-cleaning material,[7,8] (c) as an antifogging coating,[9] (d) in solar cells,[10] or (e) possibly in clean hydrogen generation from water and sunlight.[11] Perspective enhancements of the photoinduced properties of TiO2 based materials in the visible light spectra have also been demonstrated in the literature (for example, in References 12 and 13). Photocatalytic behavior of bulk TiO2 products and thin films is influenced by the crystallinity (amorphous, crystalline, nanocrystalline), phase composition (anatase, Z. MATEˇJ and L. NICHTOVA´, Postdoctoral Students, and R. KUZˇEL, Associate Professor and Head of the Department, are with the Department of Condensed Matter Physics, Faculty of Mathematics and Physics, Charles University in Prague, 121 16 Praha 2, Czech Republic. Contact e-mail: [email protected]ff.cuni.cz Manuscript submitted March 16, 2010. Article published online December 14, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
rutile), or their particular microstructure (crystallites size, density of structural defects).[14–16] Material microstructure can be strongly affected by processing conditions. TiO2 thin films prepared on various substrates (silicon, glass, sapphire) exhibit different residual stress.[17,18] This is related to the deposition process itself, but often it is caused by different thermal expansion coefficients of the coating and the substrate or by their lattice parameter mismatch. In general, the residual stress affects the mechanical properties, crack resistibility, and durability of the material. For anatase thin films, it was also shown recently[17,18] that the introduction of a large tensile stress in the film can enhance its photo-induced hydrophilic behavior significantly. The residual stress in thin films is most commonly determined from the substrate curvature,[19–21] X-ray diffraction (XRD),[22–25] or Raman spectroscopy.[26–30] These methods were explicitly applied for TiO2 in the literature: References 21 and 30; 17 and 18; and 17, 18, and 30, respectively. XRD became almost a routine method for residual stress measurements in the crystalline phases, but there is still not much comprehensive information in the field of the photocatalytic TiO2 films research concerning this method. XRD does not directly measure the stress, but, rather, the strain in the material, and strain values must be converted into the stress via material elastic c
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