Synthesis and characterization of microstructured sheets of semiconducting Ca[TCNQ]_2 via redox-driven solid-solid phase transformation of TCNQ microcrystals

摘要

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 Ca_((aq))~(2+) electrolyte solution. Voltammetric probing of the electrochemically irreversible TCNQ/Ca[TCNQ]_2 interconversion revealed that it is highly dependent on scan rate and Ca_((aq))~(2+) 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 Ca_((aq))~(2+) cations into the TCNQ~(?-) crystal lattice at the triple phase TCNQ/TCNQ_((s))~(?-)│GC_((s))│Ca_((aq))~(2+) electrolyte junction. The overall redox process associated with this chemically reversible solid-solid transformation can be described by the equation: TCNQ_((S))~0+2e~-+Ca_((aq))~(2+) ? {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 ΔE_p and E_m 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 (M~(2+)=Mn, Fe, Co, and Ni)-based coordination polymers.