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

Abstract

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.

Identifying Reactive Sites and Surface Traps in Chalcopyrite Photocathodes

“…The analysis under illumination allowed the extraction of the density of surface states (DOSS), a proxy for the charge accumulated at the interface under operation, which appeared at the photocurrent onset (Figure 1 a). This electronic signature, although not previously reported for HER photocathodes, has often been detected in photoanodes under water oxidation conditions and was interpreted as the build‐up of intermediate species at the interface when the catalytic reaction takes place [17–20] . It is plausible to consider that this surface charging is linked to the adsorption of atomic hydrogen, which precedes the hydrogen evolution.…”

“…This electronic signature, although not previously reported for HER photocathodes, has often been detected in photoanodes under water oxidation conditions and was interpreted as the build‐up of intermediate species at the interface when the catalytic reaction takes place. [ 17 , 18 , 19 , 20 ] It is plausible to consider that this surface charging is linked to the adsorption of atomic hydrogen, which precedes the hydrogen evolution.…”

Recent Progress on Semiconductor Heterojunction‐Based Photoanodes for Photoelectrochemical Water Splitting

“…[ 63 ] A rapid decay certificates that there is a serious recombination of electrons and holes, and a high steady‐state photocurrent indicates excellent separation efficiency. [ 64 ] If a negative spike appears after the light illumination is removed, it suggests that there is back electron and hole recombination. [ 65 ]…”

“…[63] A rapid decay certificates that there is a serious recombination of electrons and holes, and a high steady-state photocurrent indicates excellent separation efficiency. [64] If a negative spike appears after the light illumination is removed, it suggests that there is back electron and hole recombination. [65] TPC/TPV can be used to study the kinetic information of photogenerated charge separation, including drift and diffusion processes during photogenerated charge separation.…”

Identifizierung von reaktiven Zentren und Oberflächenfallen in Chalkopyrit‐Photokathoden

“…Diese elektronische Signatur wurde zwar für HER-Photokathoden zuvor nicht beschrieben, wurde jedoch häufig in Photoanoden unter Wasseroxidationsbedingungen nachgewiesen und als Anreicherung von intermediären Spezies an der Grenzfläche während der katalytischen Reaktion interpretiert. [17][18][19][20] Es ist plausibel, dass diese Oberflächenladung mit der Adsorption von atomarem Wasserstoff verknüpft ist, die der Wasserstoffentwicklung vorausgeht.…”