Orientation‐Dependent Surface Stoichiometry and Electronic Properties of ZnFe 2 O 4 Thin Film Photoanodes

We report ZnFe 2 O 4 (ZFO) thin film photoanodes with controlled (111), (311), and (110) orientations grown by pulsed laser deposition (PLD). Orientation control was achieved via growth of conductive Nb‐doped SnO 2 buffer layers on differently oriented sapphire single‐crystal substrates. Inductively coupled plasma optical emission spectroscopy (ICP‐OES) shows similar bulk cation compositions, whereas X‐ray photoelectron spectroscopy reveals pronounced orientation‐dependent surface iron enrichment, with the (111) surface being considerably more Fe‐rich than the (311) and (110) surfaces. Thus, out‐of‐plane orientation has little effect on the bulk composition but strongly influences the extent of surface zinc depletion during PLD growth. Conductive atomic force microscopy shows that this surface chemistry correlates with distinct electronic behavior: the (311) and (110) films conduct under both bias polarities, whereas the (111) film is strongly rectifying. Consistent with this, linear sweep voltammetry measurements under simulated solar illumination show that the (111) film, despite its more negative flat‐band potential, exhibits the most positive water‐oxidation onset, while the (311) and (110) films show onsets ≈70 mV more negative. These findings reveal a correlation between crystallographic orientation, surface stoichiometry, and interfacial electronic behavior in ZFO, highlighting surface cation composition as an important factor in complex‐oxide photoanode performance.