Reversible Redox Probes to Determine the Band Edge Locations for Nano-TiO₂

Semiconductor (photo-)electrocatalysts show immense promise in answering the difficulties faced in CO 2 electrolysis in relation to the reduction product selectivity. We thus see materials like ZnO [1] , Cu 2-x Se [2] , MoS 2 [3] , TiO 2 [4] and their numerous composites being frequently investigated for their catalytic properties in CO 2 (photo-)electroreduction. These catalysts are fabricated via varying techniques and can assume very different thicknesses and morphologies. Although the nano-semiconductors may behave differently, often only the bulk properties are considered when proposing possible reaction pathways. Determining band edge locations of semiconductors in solutions is one such case, where Mott-Schottky (MS) measurements are a widely accepted strategy. The measurement of the potential dependence of the capacitance of the semiconductor space-charge layer under depletion indeed forms an elegant method to determine the flat-band potential and hence the band edge positions for bulk semiconductors in solution. However, the MS method cannot be used for nano-structured semiconductor electrodes in the form of thin films, nanorods or nanoparticles coated on conducting substrates, as their nano-dimensions are typically much smaller than that of a semiconductor depletion layer. In this work, we explored an alternate strategy to determine band edge positions of nano-semiconductors using reversible redox probes. A model thin film system comprised of 30nm anatase TiO 2 is used to demonstrate the feasibility of the strategy in aqueous electrolyte solutions. Fe, Ru and Cr based reversible redox species were used as electrochemical probes in cyclic voltammetry experiments with varying solution pH. Based on the presence/absence of the reversible behavior of these probes, we can deduce the TiO 2 band edge locations. The procedure also allows us to draw parallels between semiconductor-metal and semiconductor-solution interfaces to gain new insights into the CO 2 reduction mechanism on such semiconductor electrodes. Figure 1