Mechanistic Insight into Photoelectrochemical Water Oxidation by α-Fe₂O₃ Films Drastically Depending on Iron Sources and Acidity in Preparation by a Mixed Metal–Imidazole Casting Method

α-Fe2O3 films (denoted as Fe2O3-NO3, Fe2O3-Cl3, Fe2O3-Cl2, and Fe2O3-SO4, respectively) were prepared by a mixed metal–imidazole casting (MiMIC) method using different iron salts of Fe(NO)3, FeCl3, FeCl2, and FeSO4 used as iron sources in precursor solutions. The morphology, nanostructure, and crystalline nature of α-Fe2O3 films significantly depend on the iron salts used. α-Fe2O3 nanoparticles in films were more cohesively interconnected by acidity control with hydrochloric acid (HCl) addition into the precursor solution compared to the cases prepared without HCl. Fe2O3-NO3 and Fe2O3-Cl3 electrodes exhibited higher photocurrent (j1.23 = 0.36 and 0.79 mA cm–2) at 1.23 V vs reversible hydrogen electrode (RHE) under visible light irradiation (>420 nm, 100 mW cm–2) for photoelectrochemical (PEC) water oxidation by 1 order of magnitude compared to those (j1.23 = 0.03 and 0.05 mA cm–2) for Fe2O3-Cl2 and Fe2O3-SO4 electrodes, respectively. This is ascribed mainly to denser films (film thickness per Fe amount) with cohesive interparticle connections to provide larger numbers of active sites for Fe2O3-NO3 and Fe2O3-Cl3 electrodes. The j1.23 value (0.36 mA cm–2) for the Fe2O3-NO3 electrode increased by the addition of HCl to j1.23 of 1.00 mA cm–2. This is attributed to the more cohesively interconnected nanoparticles by HCl addition, which not only diminishes the inactive surface defect sites at the grain boundary but also consequently increases the interface area of the film surface/electrolyte solution to allow the channels for water access and O2 release to be enlarged.