Hybrid Salt of Mo 6 Cluster and Keggin-Type Silicotungstate: Enhancing the Efficiency of Photocatalytic Hydrogen Evolution via S-Scheme Heterojunction Formation

In this study, we report the synthesis and characterization of a novel hybrid compound, [Mo₆I₈(H₂O)₆][α-SiW₁₂O₄₀]·12H₂O (denoted as [MoI][SiW]), formed by the electrostatic interactions between a cationic molybdenum aqua complex and an anionic Keggin-type polyoxometalate (POM). The composition and phase purity of the compound were determined using single-crystal and powder X-ray diffraction analyses, X-ray photoelectron spectroscopy, and elemental analysis. Diffuse reflectance spectroscopy revealed an optical energy gap (E_g) of 2.0 eV, suggesting potential photocatalytic activity under visible light. Flat-band potential (E_FB) measurements identified [MoI][SiW] as an n-type semiconductor with a conduction band (CB) potential of +0.4 V vs RHE and a valence band (VB) potential of +2.4 V vs RHE. While the VB potential is suitable for oxidation reactions, the CB potential is insufficient for hydrogen evolution, necessitating the formation of heterostructures with other photocatalysts. Photocatalytic experiments demonstrated that even a physical mixture of [MoI][SiW] with graphitic carbon nitride (g-C₃N₄) exhibits an 8-fold increase in activity for the hydrogen evolution reaction compared to pure g-C₃N₄. These results highlight the potential of hybrid materials combining molybdenum clusters and POMs for photocatalytic applications, particularly in the development of composites for efficient solar energy conversion.