The onset potential is an important parameter that affects the water oxidation performance of photoanodes. Herein, we investigated the behavior of the photocurrent onset potential of hematite (α-Fe₂O₃) photoanodes by incorporating Sn⁴⁺ cations via external (surface overlayer) or self (underlying FTO substrate) doping. The α-Fe₂O₃/FTO photoanodes fabricated at both low (550 °C) and high (800 °C) temperatures were chosen for surface Sn⁴⁺ doping (0–10 mM SnCl₄). At the lower temperature, Sn⁴⁺ doping enriched the conductivity of α-Fe₂O₃/FTO, thereby improving the photocurrent response at higher applied potentials. In addition, the surface incorporation of Sn⁴⁺ shifted the onset of the water oxidation reaction in the positive direction. In the case of high temperature-annealed photoanodes, Sn leaching (resulting from FTO deformation) also affected the water oxidation performance of the photoanodes. This was caused by the loss of FTO conductivity as well as by the unfavourable surface properties due to the excessive incorporation of Sn ions (SnO_x) into the hematite matrix. The anodic shift of the onset potential in both cases was due to the decreased surface state capacitance, as revealed by electrochemical impedance spectroscopy (EIS). The different annealing conditions, where lattice distortion and deformation-directed Sn diffusion-doping occur, were also found to affect the surface states associated with hematite and its water oxidation onset potential. Crystallographic analyses made by synchrotron XRD further support the results obtained from the EIS study. Sn doping was found to be concurrent with the respective changes in the (104) and (110) planes of hematite, which are associated with the onset potential-driving surface states and the photocurrent-boosting electron mobility, respectively.