Defect-rich spinel ferrites with improved charge collection properties for efficient solar water splitting

Spinel zinc ferrite (ZnFe₂O₄, ZFO) is a potential photoanode material for photoelectrochemical (PEC) water splitting because of its ideal bandgap (1.9–2.1 eV) and superior chemical stability in aqueous solutions. However, the low charge collection efficiency significantly hinders the improvement in PEC activity. Herein, we report an ultrafast and effective flame activation route to enhance the charge collection properties of ZFO. First, high-temperature flame (> 1300 ℃) facilitated surface and grain boundary diffusions, increasing the grain size and connectivity of the ZFO nanoparticles. Second, the reducing atmosphere of the flame enabled the formation of surface defects (oxygen vacancy and Fe²⁺), thereby increasing the charge carrier density and surface adsorption sites. Significantly, these two factors promoted charge transport and transfer kinetics, resulting in a 10-fold increase in the photocurrent density over the unactivated ZFO. Furthermore, we deposited a thin Al₂O₃ overlayer to passivate the ZFO surface and then the NiFeO_x oxygen evolution catalyst (OEC) to expedite hole injection into the electrolyte. This surface passivation and OEC deposition led to a remarkable photocurrent density of ~1 mA/cm² at 1.23 V versus the reversible hydrogen electrode, which is the highest value among all reported ZFO photoanodes. Notably, the NiFeO_x/Al₂O₃/F-ZFO photoanode achieved excellent photocurrent stability over 55 h (96% retention) and superior faradaic efficiency (FE > 94%). Our flame activation method is also effective in improving the photocurrent densities of other spinel ferrites: CuFe₂O₄ (93 times), MgFe₂O₄ (16 times), and NiFe₂O₄ (12 times).