Nickel phosphonate MOF as efficient water splitting photocatalyst

A novel microporous two-dimensional (2D) Ni-based phosphonate metal-organic framework (MOF; denoted as IEF-13) has been successfully synthesized by a simple and green hydrothermal method and fully characterized using a combination of experimental and computational techniques. Structure resolution by single-crystal X-ray diffraction reveals that IEF-13 crystallizes in the triclinic space group P_i having bi-octahedra nickel nodes and a photo/electroactive tritopic phosphonate ligand. Remarkably, this material exhibits coordinatively unsaturated nickel(II) sites, free–PO₃H₂ and–PO₃H acidic groups, a CO₂ accessible microporosity, and an exceptional thermal and chemical stability. Further, its in-deep optoelectronic characterization evidences a photoresponse suitable for photocatalysis. In this sense, the photocatalytic activity for challenging H₂ generation and overall water splitting in absence of any co-catalyst using UV–Vis irradiation and simulated sunlight has been evaluated, constituting the first report for a phosphonate-MOF photocatalyst. IEF-13 is able to produce up to 2,200 µmol of H₂ per gram using methanol as sacrificial agent, exhibiting stability, maintaining its crystal structure and allowing its recycling. Even more, 170 µmol of H₂ per gram were produced using IEF-13 as photocatalyst in the absence of any co-catalyst for the overall water splitting, being this reaction limited by the O₂ reduction. The present work opens new avenues for further optimization of the photocatalytic activity in this type of multifunctional materials.