Magnetic Fe3O4-encapsulated VAN@MIL-101(Fe) with mixed-valence sites and mesoporous structures as efficient bifunctional water splitting photocatalysts

Abstract

Fe₃O₄/VAN@MIL-101(Fe) with both mesoporous and mixed-valence Fe³⁺/Fe²⁺ structures was controllably synthesized in the synthesis of MIL-101(Fe), and it was used as a bifunctional photocatalyst in both oxygen evolution reactions (OERs) and hydrogen evolution reactions (HERs) of photocatalytic water splitting. By the reduction of auxiliary ligand vanillin (VAN) and the introduction of Fe₃O₄, the mixed-valence Fe³⁺/Fe²⁺ structure in Fe₃O₄/VAN@MIL-101(Fe) was obtained, which improves the band gap of the Fe³⁺ reactive active center and increases the separation efficiency of photogenerated carriers. Owing to the partial difference in the structure between VAN and ligand terephthalic acid (H₂BDC), hierarchical porous and vacant structures were effectively improved in Fe₃O₄/VAN@MIL-101(Fe), which can induce more active sites to adsorb more water molecules and shorten the electron–hole migration distance to improve the transfer efficiency of photogenerated carriers. Therefore, Fe₃O₄/VAN@MIL-101(Fe) presents excellent photocatalytic activities for improving the O₂ and H₂ production rate up to 360 000 μmol g⁻¹ h⁻¹ and 584 μmol g⁻¹ h⁻¹, respectively. Meanwhile, Fe₃O₄/VAN@MIL-101(Fe) maintains the excellent catalytic activity in OERs and HERs after recycling for 5 times. Moreover, the introduction of magnetic Fe₃O₄ nanoplates into Fe₃O₄/VAN@MIL-101(Fe) can make it easily recyclable by magnetic separation, which can maximize its performance.