Spectroelectrochemical and Chemical Evidence of Surface Passivation at Zinc Ferrite (ZnFe2O4) Photoanodes for Solar Water Oxidation

Recent advances in low-cost manufacturing as well as in bulk/interface engineering have positioned zinc ferrite (ZnFe₂O₄, ZFO) in the spotlight as a candidate material for solar water oxidation. However, the severe recombination at the reactive interface remains as the main source of the poor onset potential. Although catalytic overlayers have shown to override, at least partially, the surface recombination, passivating-only coatings are barely explored despite holding the key to specifically suppress the recombination. Here, a sub-nanometer Al₂O₃ layer is conformally deposited onto nanostructured ZFO, leading to a 100 mV shift in the onset potential reaching 0.80 V versus reversible hydrogen electrode (RHE) and a fourfold photocurrent increase at 1.0 V versus RHE. The passivation-only effect of Al₂O₃ is confirmed by the slowing down of the surface recombination detected by intensity-modulated photocurrent spectroscopy and by the transient photovoltage and photoluminescence experiments. Further characterization of the chemical states at the reactive interface reveals that the partial filling of the surface oxygen vacancies and the formation of a Zn²⁺–Al³⁺ Lewis adduct are potentially involved in the surface passivation. This study not only demonstrates that Al₂O₃ improves ZFO's onset potential but also sheds light on the up until now unknown surface passivation mechanism.