J. Phys. Chem. B 1999, 103, 3213-3217
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Microscopic Observation of TiO2 Photocatalysis Using Scanning Electrochemical Microscopy Hiroyuki Maeda,† Katsuyoshi Ikeda,‡ Kazuhito Hashimoto,‡ Katsuhiro Ajito,§ Masao Morita,§ and Akira Fujishima*,† Department of Applied Chemistry, School of Engineering, The UniVersity of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan; Research Center for AdVanced Science and Technology, The UniVersity of Tokyo, Komaba 4-6-1, Meguro-ku, Tokyo 153-8904, Japan; NTT Basic Research Laboratories, Nippon Telegraph and Telephone Corporation, Atsugi, Kanagawa 243-0124, Japan ReceiVed: August 24, 1998; In Final Form: February 23, 1999
Photocatalytic reactions were monitored on a macroscopic model system, containing millimeter scale regions for oxidation and reduction, for a microscopic photocatalytic particle containing both oxidizing and reducing sites, with the use of the scanning electrochemical microscopy (SECM) technique. We employed a TiO2ITO (indium-tin oxide) composite film: half of a macroscopic ITO glass substrate was coated with a TiO2 film, leaving the ITO exposed on the other half of the sample, in an aqueous solution containing 5 mM K4Fe(CN)6 and 0.1 M K2SO4. When the microelectrode was placed at a relatively large distance above the TiO2 portion of the illuminated surface, there was a small effect: ferrocyanide was photooxidized, thereby decreasing the amount that could be oxidized at the microelectrode. In contrast, when the microelectrode was placed very close to the TiO2 portion of the surface, the oxidation current at the microelectrode increased significantly after turning on the UV light, and the oxidation current increase observed after turning on the UV light became even larger when the exposed ITO portion was covered by epoxy resin. This current increase is due to positive feedback; i.e., ferricyanide produced electrochemically at the microelectrode is rereduced at the illuminated TiO2 surface by photogenerated electrons. We propose that both oxidation and reduction reactions can occur simultaneously on the illuminated unbiased TiO2 photocatalyst film. These results indicate the utility of the SECM method for clarifying the mechanisms of photocatalytic reactions on TiO2 surfaces.
Introduction Photocatalytic and photosynthetic reactions occurring at TiO2 semiconductor electrodes, particles, and thin films have been the subject of numerous studies because of the strong oxidative power of TiO2.1-8 It is well-known that the modification of the TiO2 surface by the deposition of a catalytic metal is effective in increasing the quantum efficiency of photocatalytic reactions.9-13 For instance, the photoassisted water splitting reaction is significantly accelerated on a Pt-loaded TiO2 surface compared with the unmodified surface, because Pt acts as a good catalyst for H2 production.11 Wang et al. reported that modification of the TiO2 surface with palladium increases the efficiency of photoassisted oxidation on photocatalyst slurries.12 For these catalysts, it is considered that the TiO2 surface acts mainly as an oxidation site, and the deposited metal acts as a reduction site, increasing the charge separation efficiency and thus expediting the transport of photogenerated electrons in the conduction band of TiO2 to an external chemical system. In fact, we have reported the separate monitoring of both oxidation and reduction reactions above TiO2 sites and metal sites, respectively, using a microelectrode technique.14-17 However, both oxidation and reduction reactions can occur simultaneously at TiO2 sites when the consumption of photoexcited * To whom correspondence should be addressed. Fax: 81-3-3812-6227. E-mail:
[email protected]. † Department of Applied Chemistry, The University of Tokyo. ‡ Research Center for Advanced Science and Technology, The University of Tokyo. § Nippon Telegraph and Telephone Corporation.
electrons is slow on the reduction sites. In addition, the use of nonmetallized TiO2 photocatalysts is well-known, in which case the reduction and oxidation processes must take place on essentially the same surface at the same rate. Our long-range objective is to understand the factors controlling the oxidation and reduction processes at the microscopic level on TiO2 surfaces. In the present work, we have employed a TiO2-ITO (indium-tin oxide) composite film formed by coating polycrystalline TiO2 onto half of a macroscopic ITO glass substrate, leaving the ITO exposed on half of the sample, upon which the bulk of the reduction process occurs, as a simple macroscopic model for the metal-deposited photocatalyst. We then monitored the electrochemically assisted and/or photoassisted reactions on the TiO2 surfaces using scanning electrochemical microscopy (SECM). In recent years, progress in SECM has been rapid, and numerous applications in electrochemical measurement, imaging, and microfabrication have been reported.18,19 For instance, SECM has been used to monitor electroassisted or photoassisted reactions on semiconductor surfaces.20-24 By use of the SECM technique, we will show clear evidence that, in addition to oxidation reactions, reduction reactions are also simultaneously occurring on the photoirradiated TiO2 portion of the TiO2ITO composite film. Experimental Section Preparation of TiO2-ITO Composite Film and TiO2 Electrode. TiO2 film (microcrystalline anatase) was prepared
10.1021/jp983464g CCC: $18.00 © 1999 American Chemical Society Published on Web 04/02/1999
3214 J. Phys. Chem. B, Vol. 103, No. 16, 1999
Maeda et al. into heat at the irradiated surface influenced the delivery of electroactive species to the microelectrode. We checked for the possible influence of thermal gradients due to the UV illumination as follows: when the TiO2 thin film sample was immersed in an aqueous solution, the change in the temperature of the TiO2 surface and solution on the TiO2 surface was monitored using a thermometer (∼1 mm diameter). No change (
