Photoelectrochemical evaluation of anatase TiO 2 polycrystalline aggregation layers with different crystalline orientati
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Jin Kawakitaa) Advanced Photovoltaics Center, NIMS, 1-2-1 Sengen, Tsukuba 305-0047, Japan; and World Premier International Research Center Initiative on Materials anoarchitectonics, NIMS, 1-2-1 Sengen, Tsukuba 305-0047, Japan
Yoshio Sakka Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba 305-8571, Japan; Nano Ceramics Center, NIMS, 1-2-1 Sengen, Tsukuba 305-0047, Japan; and World Premier International Research Center Initiative on Materials Nanoarchitectonics, NIMS, 1-2-1 Sengen, Tsukuba 305-0047, Japan
Tadashi Shinohara Materials Reliability Center, NIMS, 1-2-1 Sengen, Tsukuba 305-0047, Japan (Received 8 May 2009; accepted 22 July 2009)
In order to evaluate the characteristics of photocatalysts such as TiO2, it is important to separately estimate the oxidation and reduction reaction rates, since the overall reaction rate is limited by the rate-determining step. In this study, photoelectrochemical techniques were applied to thin films of crystalline oriented anatase TiO2 with polycrystalline aggregations deposited on the transparent conductive oxide (TCO) glass substrate, fabricated by the electrophoretic deposition (EPD) in a strong magnetic field. The influence of the plane orientation on the photocatalytic reaction rates was discovered for both oxidation and reduction with respect to current through the electrochemical measurements. The maximum photocurrent for the (001) plane orientation is three times higher than that for the (100) plane orientation, and is comparable with that of the random orientation. The rate of the anodic reaction determines the rate of the overall photocatalytic reaction, therefore affecting the photopotential. I. INTRODUCTION
In general, photocatalytic action is caused by the catalysis of a photochemical reaction at the solid surface of the semiconductor.1–3 TiO2 generates electron-hole pairs on the surface under UV irradiation. The oxidation reaction with the hole and the reduction reaction with the electron occur synchronously. The oxidizing reaction is a combination of oxidation of the existing redox species and/or resolution of the semiconductor itself. The reduction reaction is the reduction of oxygen to H2O2. The overall reaction rate, indicating the actual performance of the photocatalyst, is limited by the rate of either the oxidation or the reduction reaction. That is, the potential of the oxidation reaction (e.g., the photodecomposition of toxic organic molecules) may be underestimated in the case of rate-determining by the reduction reaction, even a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0002 J. Mater. Res., Vol. 25, No. 1, Jan 2010
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if the oxidation reaction rate were considerably higher than the reduction reaction rate. Therefore, it is important to separately evaluate the oxidation and reduction reaction rates and ascertain how to determine the overall reaction rate. It is known that the difference in surface ene
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