| Summary: | In this paper, we investigate the magnetron sputtering deposition of an Al-layer on Zr-doped FeOOH (Zr-FeOOH) samples to fabricate a Zr/Al co-doped Fe<sub>2</sub>O<sub>3</sub> (Al-Zr/HT) photoanode. An Al-layer is deposited onto Zr-FeOOH through magnetron sputtering and the thickness of the Al deposition is regulated by differing the sputtering time. Electrochemical impedance spectroscopy, intensity-modulated photocurrent spectroscopy, Mott-Schottky and time-resolved photoluminescence spectra analyses were used to study, in depth, the correlations between sputtered Al-layer thicknesses and PEC characteristics. High-temperature quenching (800 °C) assists in diffusing the Al<sup>3+</sup> in the bulk of the Zr-doped Fe<sub>2</sub>O<sub>3</sub> photoanode, whilst an unintended Al<sub>2</sub>O<sub>3</sub> passivation layer forms on the surface. The optimized Al-Zr/HT photoelectrode achieved 0.945 mA/cm<sup>2</sup> at 1.0 V<sub>RHE</sub>, which is 3-fold higher than that of the bare Zr/HT photoanode. The Al<sub>2</sub>O<sub>3</sub> passivation layer causes a 100 mV cathodic shift in the onset potential. Al co-doping improved the donor density, thus reducing the electron transit time. In addition, the passivation effect of the Al<sub>2</sub>O<sub>3</sub> layer ameliorated the surface charge transfer kinetics. The Al<sub>2</sub>O<sub>3</sub> passivation layer suppressed the surface charge transfer resistance, consequently expediting the hole migration from photoanode to electrolyte. We believe that the thickness-controlled Al-layer sputtering approach could be applicable for various metal oxide photoanodes to lower the onset potential.
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