Morphology Changes and Mechanistic Aspects of the Electrochemically-Induced Reversible Solid−Solid Transformation of Microcrystalline TCNQ into Co[TCNQ]₂-Based Materials (TCNQ = 7,7,8,8-Tetracyanoquinodimethane)

The chemically reversible solid−solid phase transformation of a TCNQ-modified glassy carbon, indium tin oxide, or metal electrode into Co[TCNQ] 2 (H 2 O) 2 material in the presence of Co 2+ (aq) containing electrolytes has been induced and monitored electrochemically. Voltammetric data reveal that the TCNQ/Co[TCNQ] 2 (H 2 O) 2 interconversion process is independent of electrode material and identity of cobalt electrolyte anion. However, a marked dependence on electrolyte concentration, scan rate, and method of electrode modification (drop casting or mechanical attachment) is found. Cyclic voltammetric and double potential step chronoamperometric measurements confirm that formation of Co[TCNQ] 2 (H 2 O) 2 occurs through a rate-determining nucleation and growth process that initially involves incorporation of Co 2+ (aq) ions into the reduced TCNQ crystal lattice at the TCNQ|electrode|electrolyte interface. Similarly, the reverse (oxidation) process, which involves transformation of solid Co[TCNQ] 2 (H 2 O) 2 back to parent TCNQ crystals, also is controlled by nucleation−growth kinetics. The overall chemically reversible process that represents this transformation is described by the reaction: 2TCNQ 0 (s) + 2e - + Co 2+ (aq) + 2H 2 O ⇌ [Co(TCNQ) 2 (H 2 O) 2 ] (s) . Ex situ SEM images illustrated that this reversible TCNQ/Co[TCNQ] 2 (H 2 O) 2 conversion process is accompanied by drastic size and morphology changes in the parent solid TCNQ. In addition, different sizes of needle-shaped nanorod/nanowire crystals of Co[TCNQ] 2 (H 2 O) 2 are formed depending on the method of surface immobilization.