Atmospheric-Water-Induced Reversible Structural Transformation of a Two-Dimensional Ni(II)-Based Ferromagnetic MOF: A Highly Efficient Water Oxidation Electrocatalyst and Colorimetric Water Sensor

A two-dimensional metal–organic framework, formulated as [Ni3(nic)2(μ-Cl)4(DMF)4]·2DMF (MOF1), was self-assembled from nickel(II) ion and 4-pyridinecarboxylic acid. The MOF1 has a 6c-connected uninoidal net topology and can absorb atmospheric moisture, causing the structural transformation of MOF1 into MOF2. The transformation is reversible, as MOF2 can be returned quickly to its initial structure MOF1 upon heating. The reversible conversion is associated with a color change between green (MOF1) and blue (MOF2), and it can be used as a colorimetric water sensor with a limit of detection (LOD) of 0.246 μM (inorganic solvent) and 0.224 μM (organic solvent). The water-capturing ability also leads to an electrocatalytic oxygen evolution reaction (OER). MOF2 is the first Ni-containing OER electrocatalyst that needs only 180 mV overpotential in 0.001 M KOH to drive 10 mA cm–2 current density. The MOF shows an immense turnover number (TON = 3.8 × 105) and frequency (TOF = 10.6 S–1) and Faradaic efficiency (93.7%) and excellent robustness. The structural change (MOF1 ↔ MOF2) and OER process are additionally supported by a density functional theoretical (DFT) study. The structural transformation in the solid phase further changes the magnetic properties, with both MOF1 and MOF2 showing ferromagnetic interactions. However, the Néel temperature (TN) for MOF1 of <5 K changes to ∼16 K for MOF2.