Abstract: In-situ observation of the charge transfer plays a key role in understanding the working mechanism of hematite for solar water oxidation. Here by using in-situ X-ray absorption spectroscopy (XAS), the electron injection from illuminated hematite (photon-excited electron) to the surface carbon layer can be clearly identified, which can facilitate the charge separation and then improve the performance. As a result, the carbon-coated and Sn-doped hematite photoanode (C-Sn-Fe₂O₃) shows a greatly enhanced photocurrent density of 2.3 mA/cm² at 1.23 V_RHE, which is 2.3 times that of the pristine hematite. The injected electron can modify the chemical state of surface groups in the carbon layer and be quickly transferred to the electrode due to the high conductivity of the carbon layer, leaving behind the high-valence Fe⁴⁺ with high oxidation capability to enhance the performance. By coupling with the FeNiOOH co-catalyst, the photoanode can finally achieve a high photocurrent density of 3.0 mA/cm² at 1.23 V_RHE with a low onset potential of 0.76 V_RHE. The understanding of the charge migration route by using in-situ XAS offers a novel way for the design of highly efficient solar water oxidation materials.

Key words: Hematite, Solar water oxidation, Carbon layer, In-situ XAS