For example, a resistance observed at surface potentials more positive than the HER onset can be attributed to the charge transfer resistance felt by photogenerated charges. As the onset potential is approached, this resistance decreases and ultimately becomes negligible, indicating that the potential at the surface is sufficient to drive charge transfer and the electrochemical reaction. Ambiguity in EIS analysis arises when there are two or more distinct semicircles in the Nyquist plot occurring at the same potential. In this case, there are multiple ECs that can be used to model the data. With such ambiguity, choosing the correct equivalent circuit for these systems goes hand in hand with understanding the physical mechanisms occurring within the device. Previously, it was reported that a-TiO2 protected photocathodes, specifically p-Si, Sb2Se3 and Cu2O, exhibit this ambiguity for potential ranges where the semiconductor is in depletion. In these cases, two semicircles are observed, where one can be attributed to the semiconductor through the depletion layer capacitance, regardless of the equivalent circuit used. Therefore, important insights into device operation were gained, although obtaining certainty regarding the correct equivalent circuit was outside the scope of that report. Nevertheless, it remains important to resolve this ambiguity due to the extent that semiconductor│a-TiO2 heterojunctions are used in PEC literature. a-TiO2 itself has many interesting properties that are behind its popularity as a protective layer. It has high chemical stability over a wide range of pH values, is a transparent window with respect to a large portion of the solar spectrum, and also serves as an anti-reflection layer. As it is deposited using atomic layer deposition (ALD), it forms conformal layers over structured semiconductors as well, which is useful for semiconductors that benefit from nanostructuring due to short minority carrier diffusion lengths. It has special significance for photoanodes, because a wide defect band (FWHM = 0.88 eV) facilitates hole transfer to the solution, which would otherwise be blocked by the position of the valence band. 2,21-23 However, it is worth noting that when a-TiO2 is used to protect photocathodes, this defect band is not expected to play a role during device operation, as photogenerated charges can pass through the conduction band of a-TiO2 to perform the HER.