Intrinsic photocatalytic water oxidation activity of Mn-doped ferroelectric BiFeO3

Abstract

The development of stable and efficient visible light-absorbing oxide-based semiconductor photocatalysts is a desirable task for solar water splitting applications. Recently, we proposed that the low photocurrent density in film-based BiFeO3 (BFO) is due to charge recombination at the interface of the domain walls, which could be largely reduced in particulate photocatalyst systems. To demonstrate this hypothesis, in this work we synthesized particulate BFO and Mn-doped BiFeO3 (Mn-BFO) by the sol-gel method. Photocatalytic water oxidation tests showed that pure BFO had an intrinsic photocatalytic oxygen evolution reaction (OER) activity of 70 μmol h-1 g-1, while BFO-2, with an optimum amount of Mn doping (0.05%), showed an OER activity of 255 μmol h-1 g-1 under visible light (λ ≥ 420 nm) irradiation. The bandgap of Mn-doped BFO could be reduced from 2.1 to 1.36 eV by varying the amount of Mn doping. Density functional theory (DFT) calculations suggested that surface Fe (rather than Mn) species serve as the active sites for water oxidation, because the overpotential for water oxidation on Fe species after Mn doping is 0.51 V, which is the lowest value measured for the different Fe and Mn species examined in this study. The improved photocatalytic water oxidation activity of Mn-BFO is ascribed to the synergistic effect of the bandgap narrowing, which increases the absorption of visible light, reduces the activation energy of water oxidation, and inhibits the recombination of photogenerated charges. This work demonstrates that Mn doping is an effective strategy to enhance the intrinsic photocatalytic water oxidation activity of particulate ferroelectric BFO photocatalysts.