Reversible Redox Probes to Determine Band-Edge Locations and Dopant Concentrations of Nano-TiO2 Thin-Films: Settling the Mott-Schottky Conundrum

Abstract

Knowing the exact location of the semiconductor band-edges is key for mechanistic insights into their use for water and CO2 photo/electrocatalysis. In this regard, a reliable strategy for nano-semiconductors did not exist yet. We demonstrate the use of reversible redox probes on nano-semiconductor electrodes to determine their band-edge locations in aqueous solutions. Rectifying current-potential (i-U) characteristics with the high work function (i.e. more positive formal potential) Fe(CN)6 3-/Fe(CN)6 4- redox couple yielded the exact flatband potential at various pH whereas the reversible i-U characteristics with the low work function (i.e. more negative formal potential) Ru(NH3)6 3+/Ru(NH3)6 2+ redox couple provided the conduction band-edge location and dopant concentration for a 30 nm thin-film n-TiO2. The methodology can be extended to other nano-semiconductors and serves as an alternative to and goes beyond the capabilities of the Mott-Schottky procedure for bulk semiconductor electrodes.