Electrochemical redox probes to map light-modulated band edge shifts of nano-TiO2 thin film electrodes in aqueous solutions

Abstract

Nano-semiconductor electrodes are employed for light harvesting applications in (photo-)electrochemical conversion reactions and dye-sensitized solar cells. Accurate determination of their band edge energetic positions is central to their efficacy for isoenergetic charge transfer to the electrolyte. Yet, conventional methods fail for nano-dimensional semiconductor electrodes due to full depletion and extremely small potential-independent space charge widths. In this regard, a new approach was demonstrated on thin film nano-TiO2 electrodes where the current-potential (i-U) characteristics in high and low work function redox probes were interpreted to infer band edge positions in darkness. The methodology is now extended for nano-TiO2 band edge interpretation to illuminated interfaces. Using compact 30 nm TiO2 thin films of both anatase and amorphous phases, we analyze open-circuit potentials and onset potentials in darkness and illumination across varying pH conditions to infer band edge shifts. We also examine temperature rise due to solution heating under illumination from the diffusion-limited currents in the i-U characteristics. Additionally, signatures from surface states in the i-U characteristics are identified, their densities estimated, and their implications discussed. The findings are consolidated through energy band diagrams, offering a comprehensive view of the fully depleted nano-semiconductor/electrolyte interface under operational conditions.