Synergistic Effects of Layering α‑Fe₂O₃ and NiO for Enhanced Photoelectrochemical Water Splitting

Manipulating the activity of transition-metal oxide (TMO) catalysts by combining two or more TMOs for their improved performance in photoelectrochemical water splitting is an effective yet challenging strategy. Here, an n–p type heterojunction (α-Fe₂O₃/NiO) photoelectrode is rationally designed and fabricated by successive deposition of NiO and α-Fe₂O₃ on a fluorine-doped tin oxide (FTO) substrate. The as-prepared α-Fe₂O₃/NiO@FTO catalyst exhibits substantially higher activity along with reduced overpotential for the PEC water splitting reaction as compared to pristine NiO or α-Fe₂O₃. Moreover, a 5-fold enhanced current density (75 μA/cm²) is obtained for the α-Fe₂O₃/NiO@FTO heterojunction thin film as compared to NiO/α-Fe₂O₃@FTO (16 μA/cm²) under solar simulated AM 1.5 G light illumination at an applied bias of 1.4 V_RHE. Thus, this makes the order of TMO layers (i.e., α-Fe₂O₃/NiO@FTO or NiO/α-Fe₂O₃@FTO) crucial in their PEC performance because of the variation in their electron–hole transport characteristics. Mott–Schottky plots and bandgap measurements confirm that the photogenerated electrons follow a path from α-Fe₂O₃ to NiO, whereas holes travel from NiO to α-Fe₂O₃ for the α-Fe₂O₃/NiO@FTO sample. The hybridization of transition metal–oxygen valence states further improves the charge conduction in the composite photoanode as compared to its constituent pure oxides. Thus, the synergistic merits of both semiconductors, viz., higher visible light absorption capability of α-Fe₂O₃ and better hole transport characteristics of NiO, manifest as improved catalytic activity of α-Fe₂O₃/NiO@FTO in PEC water splitting efficiency. Our study demonstrates a promising strategy to explore the charge transport regulation and overall activity improvement in layered TMO-based PEC catalysts.