Schottky junction/ohmic contact behavior of a nanoporous TiO2 thin film photoanode in contact with redox electrolyte solutions

The nature and photoelectrochemical reactivity of nanoporous semiconductor electrodes have attracted a great deal of attention. Nanostructured materials have promising capabilities applicable for the construction of various photonic and electronic devices. In this paper, a mesoporous TiO 2 thin film photoanode was soaked in an aqueous methanol solution using an O 2 -reducing Pt-based cathode in contact with atmospheric air on the back side. It was shown from distinct photocurrents in the cyclic voltammogram (CV) that the nanosurface of the mesoporous n-TiO 2 film forms a Schottky junction with water containing a strong electron donor such as methanol. Formation of a Schottky junction (liquid junction) was also proved by Mott–Schottky plots at the mesoporous TiO 2 thin film photoanode, and the thickness of the space charge layer was estimated to be very thin, i.e., only 3.1 nm at −0.1 V vs Ag/AgCl. On the other hand, the presence of [Fe(CN) 6 ] 4− and the absence of methanol brought about ohmic contact behavior on the TiO 2 film and exhibited reversible redox waves in the dark due to the [Fe(CN) 6 ] 4−/3− couple. Further studies showed that multiple Schottky junctions/ohmic contact behavior inducing simultaneously both photocurrent and overlapped reversible redox waves was found in the CV of a nanoporous TiO 2 photoanode soaked in an aqueous redox electrolyte solution containing methanol and [Fe(CN) 6 ] 4− . That is, the TiO 2 nanosurface responds to [Fe(CN) 6 ] 4− to give ohmic redox waves overlapped simultaneously with photocurrents due to the Schottky junction. Additionally, a second step photocurrent generation was observed in the presence of both MeOH and [Fe(CN) 6 ] 4− around the redox potential of the iron complex. It was suggested that the iron complex forms a second Schottky junction for which the flat band potential ( E fb ) lies near the redox potential of the iron complex.