Pyrolysis of an amorphous cobalt(II) cubane-like coordination polymer towards tunable structurally disordered materials

Abstract

Coordination polymers (CPs) can serve as versatile precursors for functional nanomaterials with tunable properties. The pyrolysis of an amorphous cobalt(II) cubane-like CP, [Co₄L₄(bdc)](bdc) (L = bisbenzimidazole, bdc = 1,4-benzenedicarboxylate), was examined to elucidate its structural evolution during heating. Analysis of the amorphous pyrolysis products with total scattering and pair distribution function analysis (TS/PDF) and X-ray absorption spectroscopy (XAS) reveals two regimes: ‘mild pyrolysis’ (heating to ≤ 500 °C) forms small cobalt clusters distorted relative to the pristine cobalt(II) cubane, with contracted nearest neighbor Co–O/N bonds and partial ligand loss, while ‘heavy pyrolysis’ (heating to 500 °C followed by isothermal heating, or heating to 600 °C) resulted in growth of disordered metallic cobalt and cobalt oxide nanoparticles. In situ XAS reveals an onset of a gradual breakdown of the cobalt(II) cubane at 350 °C. An electrochemical screening showed improved oxygen evolution reaction (OER) activity for a ‘mildly pyrolyzed’ sample, while extensive heating reduced performance, likely due to an agglomeration of cobalt centers. These results demonstrate how controlled pyrolysis can be used to tailor the structure and catalytic properties of amorphous CPs.