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 VRHE. 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.