20.02.21 – Spectroelectrochemical and Chemical Evidence of Surface Passivation is presented for Zinc Ferrite (ZnFe₂O₄) Photoanodes for Solar Water Oxidation.

In recent years, spinel zinc ferrite (ZnFe₂O₄, ZFO) has gained prominence among the emergent candidate materials for solar water splitting. However, the severe recombination at the reactive interface still remains as the main source of the poor onset potential. In a new report from Y. Liu et al. a sub-nanometer Al₂O₃ layer was conformally deposited onto nanostructured ZFO, leading to a 100 mV shift in the onset potential reaching 0.80 V vs RHE, and a 4-fold photocurrent increase at 1.0 V vs RHE. The passivation-only effect of Al₂O₃ was confirmed by the slowing down of the surface recombination detected by intensity-modulated photocurrent spectroscopy (IMPS) as well as by the transient photovoltage and photoluminescence quenching experiments. Further characterization of the chemical states at reactive interface revealed that the partial filling of the surface oxygen vacancies as well as the formation of a Zn²⁺ – Al³⁺ Lewis adduct, were 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.