Spectroscopic Evidence of Intraband Gap States in α‐SnWO₄ Photoanodes Introduced by Interface Oxidation

α ‐SnWO 4 is an emerging photoelectrode material for photoelectrochemical water splitting, with several promising properties such as the favorable bandgap of 1.9 eV and suitable positions of the valence and conduction band. However, a major challenge remains: unprotected α ‐SnWO 4 undergoes surface passivation that blocks further charge transfer, and α ‐SnWO 4 electrodes that are covered with a protection/catalytic overlayer (e.g., NiO x , CoO x ) show limited photovoltage. Earlier studies reveal that interfacial oxidation occurs due to the deposition of the overlayer. This negatively impacts the photovoltage that can be extracted, which is attributed to Fermi‐level pinning at the interface. The exact origin of this Fermi‐level pinning mechanism, however, remains unclear. In the present study, a combination of surface photovoltage analysis and hard X‐ray photoelectron spectroscopy is used to elucidate the electronic structure of the α ‐SnWO 4 /oxide interface. Both techniques offer compelling and consistent evidence for the presence of a defect state that is energetically located within the bandgap energy of α ‐SnWO 4 and is likely responsible for the Fermi‐level pinning.