Decoupling pH dependence of flat band potential in aqueous dye-sensitized electrodes

When a semiconductor is in contact with an electrolyte, its flat band potential (E_FB) is an important quantity for determining band edge positions in photoelectrochemistry. Oxide semiconductors generally have a E_FB shift of-59 mV in aqueous solutions when the pH is increased by 1 unit as a consequence of surface deprotonation. Many of the most desirable redox reactions, such as the reduction of CO₂ to HCOOH, or water to H₂, also show the same dependence on pH due to the involvement of protons. Therefore, pH cannot be used to tune the relative energy alignment between the electrode and the electrolytes. Here, we demonstrate via Mott-Schottky measurement that sensitized NiO with a membrane-inspired design of dye molecule (BH4) can decouple the pH dependence of E_FB. The E_FB of BH4-sensitized NiO films shows very little to zero change as a function of pH, whereas less hydrophobic dye (P1)-sensitized NiO and the bare NiO follow the Nernst shift with respect to pH.