Magnetron Sputtered Al Co-Doped with Zr-Fe[sub.2]O[sub.3] Photoanode with Fortuitous Al[sub.2]O[sub.3] Passivation Layer to Lower the Onset Potential for Photoelectrochemical Solar Water Splitting

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]O[sub.3] (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+] in the bulk of the Zr-doped Fe[sub.2]O[sub.3] photoanode, whilst an unintended Al[sub.2]O[sub.3] passivation layer forms on the surface. The optimized Al-Zr/HT photoelectrode achieved 0.945 mA/cm[sup.2] at 1.0 V[sub.RHE], which is 3-fold higher than that of the bare Zr/HT photoanode. The Al[sub.2]O[sub.3] 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]O[sub.3] layer ameliorated the surface charge transfer kinetics. The Al[sub.2]O[sub.3] 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.