Unraveling the effects of Fe-incorporation on high-performance water-splitting MIS photoanodes

Si photoanodes decorated with Ni nanoparticles (NPs) are known for their high performance in photoelectrochemical (PEC) solar water splitting and have been considered model systems for the mechanistic study of inhomogeneous metal-insulator-semiconductor (MIS) photoanodes. However, little attention has been paid to the effects of Fe impurities on the PEC properties despite Fe being recognized to significantly alter the electrocatalytic activity of Ni-based materials. Herein, we elucidate the crucial role of Fe in MIS photoanodes for water oxidation under light. Our results demonstrate that not only does the presence of residual Fe affect the electrocatalytic properties but also strongly influences the photoanodes’ junction energetics and photovoltaic properties. We show that the synergistic effects caused by Fe doping explain the high performance previously reported for these model photoanodes. The correlation between photovoltage, Fe content, and Ni NP size is rationalized by the pinch-off effect and the change in the effective barrier height of Ni(OH)2/NiOOH shell upon Fe incorporation. To achieve a high photovoltage (up to 500 mV), Fe incorporation into the outer shell of Ni NPs and the electrolyte is essential. Our results emphasize the importance of Fe in this PEC system, which has always been neglected in the past. These results are essential for the fundamental understanding of water-splitting photoelectrodes and will help improve the performance assessment of systems for converting solar energy into hydrogen.