Photocatalytic reactions were monitored on a macroscopic model system, cont
aining millimeter scale regions for oxidation and reduction, for a microsco
pic photocatalytic particle containing both oxidizing and reducing sites, w
ith the use of the scanning electrochemical microscopy (SECM) technique. We
employed a TiO2-ITO (indium-tin oxide) composite film: half of a macroscop
ic 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 K4
Fe(CN)(6) and 0.1 M K2SO4. When the microelectrode was placed at a relative
ly 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, th
e oxidation current at the microelectrode increased significantly after tur
ning on the UV light, and the oxidation current increase observed after tur
ning on the UV light became even larger when the exposed ITO portion was co
vered by epoxy resin. This current increase is due to positive feedback; i.
e., ferricyanide produced electrochemically at the microelectrode is reredu
ced at the illuminated TiO2 surface by photogenerated electrons. We propose
that both oxidation and reduction reactions can occur simultaneously on th
e illuminated unbiased TiO2 photocatalyst film. These results indicate the
utility of the SECM method for clarifying the mechanisms of photocatalytic
reactions on TiO2 surfaces.