Iron–cobalt bimetal oxide nanorods as efficient and robust water oxidation catalysts

Abstract

Cobalt-based oxides are considered as potential water oxidation catalysts for future artificial photosynthetic systems because of their high abundance, strong stability and efficient performance. Herein, a series of cobalt-based oxides, MnCo_3−nO₄ (M = Mn, Fe, Co) samples, were synthesized through changing the metal sources by a low-temperature coprecipitation method. These catalysts were investigated under photochemical and electrochemical water oxidation conditions. And they all exhibited efficient activity for water oxidation under alkaline, acidic and neutral conditions under visible light irradiation. An excellent O₂ yield of 90.4% for Fe–Co bimetal oxide (Fe_1.1Co_1.9O₄) nanorods was obtained under optimal conditions (photoirradiation at λ ≥ 420 nm, [Ru(bpy)₃](ClO₄)₂ as the photosensitizer, Na₂S₂O₈ as the oxidant in borate buffer at pH = 9.0, bpy = 2,2-bipyridine). Among MnCo_3−nO₄ samples, Fe_1.1Co_1.9O₄ nanorods were proved to be the optimal electrocatalytic water oxidation catalyst as well. Multiple experiments (SEM, FT-IR, XRD, XPS, Bulk electrolysis) were used to test the stability of Fe_1.1CO_1.9O₄ and these results indicate that Fe_1.1CO_1.9O₄ nanorods are highly stable. Furthermore, based on Mott–Schottky and cyclic voltammetry analysis, the best balanced flat-band potential of Fe_1.1CO_1.9O₄ nanorods is just located at the middle position between the oxidation potential of O₂/H₂O and the half-wave potential of [Ru(bpy)₃]^3+/2+, which was probably responsible for their superior photocatalytic water oxidation performance.