Microscopic observation of TiO₂ photocatalysis using scanning electrochemical microscopy

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 TiO₂-ITO (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.