Solvent-Mediated Morphological Transition of Electrodeposited Cobalt-TCNQ Complexes

Divalent transition metal-tetracyanoquinodimethane complexes (CTCs) are of significant interest for their diverse properties. Electrocrystallization offers a facile route for CTC synthesis. Precise control of electrocrystallization is essential for tailoring CTC crystals for desired functionality. This study demonstrates solvent-mediated morphological control in Co(TCNQ)2 complex electrodeposition. In acetonitrile, randomly oriented, high-aspect-ratio nanorods are obtained. The addition of dimethyl sulfoxide (DMSO) results in crystals with progressively reduced aspect ratios. DMSO-to-Co2+ molar ratio exceeding 40 results in vertically oriented, hexagonal-shaped, plate-like crystals, independent of substrate. The transformation is suggested to arise from changes in precursor solvation, which likely influenced the nucleation and growth processes. DMSO stabilizes both TCNQ–, evidenced by an anodic shift in the reduction onset potential, and Co2+, increasing the desolvation energy barrier of the metal ions and shifting electrocrystallization to a more cathodic potential. Peak-force tunneling AFM shows that the vertically oriented crystals exhibit higher conductance than the nanorods lying parallel to the substrate, likely due to the perpendicular alignment of the π–π charge transport axis of the Co(TCNQ)2 crystal to the electrode. Electrochemical analysis in aqueous media further indicates improved charge transfer for solid-solid transformation in plate-like crystals. This work established solvent modulation as an effective strategy for morphology-controlled electrodeposition.