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.