Two-layer coating of F-doped SnO2 (SnO2:F) and TiO2 on the glass can be applied to a novel type of energy-conserving window, since inner-SnO2:F and outer-TiO2 layers could add low-emissivity (low-e) properties and photocatalytic activity, respectively [1]. The SnO2:F underlayer generally forms the rutile polymorph, which has a strong influence on not only the crystallization of upper TiO2 films but their photocatalytic activity. Meanwhile the TiO2 overlayer meets requirements for both a diminished thickness less than about fifty nm and the anatase phase-formation, in order to cope with both excellent optical properties and higher photocatalytic activity. As is reported earlier, we have successfully demonstrated the sputter deposition of a 40-nm-thick TiO2 film on the SnO2:F-coated glass substrate with a high performance of photodegradation of gaseous acetaldehyde under the UV light. By the use of reflection electron energy loss spectroscopy in the core electron excitation region, we have also characterized the electronic structure of TiO2 surfaces and confirmed the anatase phase formation in the near-surface region [2]. Furthermore, we have found that the formation of a thin Ta-doped TiO2 (TiO2:Ta) interlayer between outer-TiO2 and inner-SnO2:F layers induces a considerable improvement of the photocatalytic performance. From the analyses of Rutherford back-scattering spectroscopy, x-ray photoelectron spectroscopy and x-ray diffractmerty, the Ta atoms in the TiO2:Ta films are considered to segregate toward the grain boundaries. Furthermore, the accumulation of small Ta2O5 species may occur at the TiO2:Ta/SnO2:F interface, which contributes to suppressing the influence of the rutile SnO2:F underlayer on the crystallization of the TiO2 overlayer.
[1] M. Okada, et al., Thin Solid Films, vol.442, p.217 (2003).
[2] M. Okada, et al., Surf. Sci., vol.600, p.4385 (2006).
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