Phase, Morphology, and Particle Size Changes Associated with the Solid−Solid Electrochemical Interconversion of TCNQ and Semiconducting CuTCNQ (TCNQ = Tetracyanoquinodimethane)

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Abstract

The origins of extensive solid−solid-state interconversions that accompany the electrochemistry of microparticles of tetracyanoquinodimethane (TCNQ) and semiconducting CuTCNQ (phases I and II) adhered to glassy carbon (GC) electrodes, in contact with CuSO4(aq) electrolyte, have been identified. Ex situ analyses with electron microscopy, infrared spectroscopy, and X-ray diffraction have been used to identify the phase changes that occur during the course of potential cycling or bulk electrolysis experiments. All redox-based transformations require extensive density, volume, and morphology changes, and consequently they are accompanied by crystal fragmentation. The net result is that extensive potential cycling ultimately leads to the thermodynamically favored TCNQ/CuTCNQ(phase II) solid−solid interconversion occurring at the nanoparticle rather than micrometer size level. The overall chemically reversible process is described by the reaction TCN + C + 2e- ⇌ CuTCNQ(S,GC)(phase I or phase II). Needle-shaped CuTCNQ(phase I) crystals having a density of 1.80 g cm-3 are predominately formed in the first stages of potential cycling experiments that commence with micrometer-sized rhombic-shaped TCNQ crystals of density 1.36 g cm-3. The rate of subsequent formation of thermodynamically stable CuTCNQ(phase II), which has an intermediate density of 1.66 g cm-3 and a crystal shape more like that of TCNQ, is dependent on the number of potential cycles, the scan rate, and the initial size of the adhered TCNQ crystals. Evidence obtained by cyclic voltametry and double potential step techniques indicate that the formation of CuTCNQ(phase I and II) involves a rate-determining nucleation and growth process, combined with the ingress and reduction of ions (from the electrolyte). The reactions involved in the process are believed to be TCN + e- + ⇌ [Cu2+][TCNQ-]S,GC and [Cu2+][TCNQ-]S,GC + e- ⇌ [Cu+][TCNQ-]S,GC in which CuTCNQ(phase I) is formed initially and then CuTCNQ(phase II) after a large number of potential cycles. The reverse oxidation process involving the transformation of solid CuTCNQ(phases I and II) to TCNQ also involves a nucleation−growth multistep process and significant crystal size and morphology changes. Finally, data led to the postulation of a mechanism for the formation of CuTCNQ compounds via chemical reaction pathways in which the existence of the electrochemically inferred transitional Cu2+ intermediate also is included.

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