Synthesis and characterization of microstructured sheets of semiconducting Ca[TCNQ]2 via redox-driven solid-solid phase transformation of TCNQ microcrystals

Microstructured sheets of semiconducting Ca[TCNQ]2 (TCNQ = 7,7,8,8-tetracyanoquinodimethane) have been synthesized via electrochemically driven (TCNQ)/Ca[TCNQ]2 solid-solid phase transformation that occurs upon one-electron reduction of solid TCNQ, mechanically attached to an electrode surface, in the presence of an aqueous Ca2+ (aq) electrolyte solution. Voltammetric probing of the electrochemically irreversible TCNQ/Ca[TCNQ]2 interconversion revealed that it is highly dependent on scan rate and Ca2+ (aq) electrolyte concentration. This voltammetric behavior, supported by double potential-step chronoamperometric evidence, clearly attests that formation of Ca[TCNQ]2 takes place via a rate-determining nucleation/growth process, which involves ingress of Ca2+ (aq) cations into the TCNQ·- crystal lattice at the triple phase TCNQ/TCNQ·- (s)│GC(s)│Ca2+ (aq) electrolyte junction. The overall redox process associated with this chemically reversible solid-solid transformation can be described by the equation: TCNQ0 (S)+2e-+Ca2+ (aq) ⇄ {Ca[TCNQ]2}(S). SEM characterization of the morphology of the generated Ca[TCNQ]2 material showed the formation of microstructured sheets, which are substantially different from those of parent TCNQ crystals and the needle-shaped crystals of group I cations (M+=Li,Na, K, Rb, andCs). The kinetic and thermodynamic implications of the ΔEp and Em values as a function of scan rate are discussed in terms of nucleation- growth and their relevance to those reported for the conceptually related group I cations and binary M[TCNQ]2 (M2+=Mn, Fe, Co, and Ni)-based coordination polymers.