Photoelectrochemical properties and crystalline structure change of Sb-doped TiO 2 thin films prepared by the sol-gel me
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Ti1−XSbXO2 samples were obtained from dip-coating sol-gel method and a subsequent anneal at 450 °C. They had an average crystallite size of 13.3–20 nm. Cyclic voltammograms taken under ultraviolet (UV) and Xe lamp illumination in a 0.5 M Na2SO4 electrolyte showed that the Sb-doped samples had greater photocurrent densities than pure titania electrode, with an optimal Sb concentration of 0.2%. Oxidative peaks were observed in the cyclic voltammograms obtained in the dark after certain exposure duration to UV light. X-ray diffraction patterns and Raman spectra show a phase transformation from brookite to anatase in the samples with Sb concentration up to 0.2%. Ti4+ ions were substituted by Sb to form the anatase structure of Sb–O–Ti, improving the crystallization efficiency. The Sb–Sb bonds were formed due to the introduction of excessive Sb atoms.
I. INTRODUCTION
A critical problem of TiO2 thin film is how to improve the photo-to-electron conversion efficiency. In earlier works, Wang et al.1 reported a highly hydrophilic TiO2 surface that was obtained under ultraviolet (UV) for a couple of hours. However, it could only absorb 5% of the incident solar light. Gratzel and co-workers2 developed a photoelectrochemically stable TiO2 electrode with an overall photo-to-electron conversion yield of approximately 12% in diffuse daylight. They pointed out that2 the photo-to-electron conversion is not only related to the light harvesting but also to the surface and bulk recombination losses. Therefore, two methods are widely used to enhance the efficiency of photo-to-electron conversion: surface modification by dye sensitization3,4 and metal dopants introduction.5–9 The mechanism underlying is to produce intermediate bands that lie within the bandgap of titania and facilitate the charge transfer to minimize the charge carrier recombination. However, there are still many factors limiting the conversion efficiency.10 First, Schottky barriers form between the host surface and the dopants, retarding the electron transfer; secondly, the dopants serve as the recombination centers of the photoinduced electron-hole pairs. With reference to the TiO2−XNX composite reported by Asahi and coworkers,11 we consider it an effective way to introduce
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0412 J. Mater. Res., Vol. 19, No. 11, Nov 2004
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antimony into the matrix of anatase or rutile TiO2 by sol-gel method and a high-temperature anneal.12,13 The resultant Ti1−XSbXO2 composite shows higher photocurrent generation with a proper amount doping of Sb in this work. Another controversial problem about TiO2 is the mechanism that contributes to the formation of a highly hydrophilic surface. Wang et al.1 explained that the Ti3+ ions were closely associated with the hydrophilicity of titania. Therefore, a number of experiments have been performed to prove the existence of Ti3+ ions on the surface of TiO2 under UV illumination.14–17 It has been rec
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