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

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.

Electrodeposition of copper tetracyanoquinodimethane for bipolar resistive switching non-volatile memories

“…Our method for CuTCNQ electrodeposition was inspired by a prior work published by Shields (1985) and Neufeld et al (2003). The latter prepared CuTCNQ by the electrochemical reduction of (solid) TCNQ microparticles adherent to a glassy carbon electrode in contact with aqueous CuSO 4 electrolyte.…”

Fabrication and Field‐Emission Properties of Large‐Area Nanostructures of the Organic Charge‐Transfer Complex Cu‐TCNAQ

“…Tetracyanoquinodimethane (TCNQ)-based solidstate chemistry represents an area of considerable current interest in the field of materials science. [12][13][14] Although successful attempts to impart TCNQ-based CT complexes devices with high-quality performance, a sparsely investigate avenue for development is the exploration of materials based on other acceptors, [15][16][17] which may significantly enhanced the understanding of their intriguing physical and chemical, such as electronic, magnetic and optic, behavior. Tetracyanoanthraquinodimethane (TCNAQ; see Scheme S1 in the Supporting Information) shows more efficient electron-acceptor properties than TCNQ and has been extensively studied in conjugated polymers.…”

Ion‐Transfer‐Based Growth: A Mechanism for CuTCNQ Nanowire Formation

“…[1][2][3][4][5] Various forms of CuTCNQ including bulk crystals, solid films, and nanowires have been fabricated [6][7][8][9] by using techniques such as vapor deposition, electrochemical reduction, and spontaneous electrolysis. [9][10][11][12][13][14] All CuTCNQ fabrication methods reported so far are based on 1) a localized corrosion-crystallization process, wherein neutral TCNQ molecules in solution react with active metallic Cu species, as during the spontaneous electrolysis method; [15] and 2) the reduction of TCNQ in solution at an electrode surface in the presence of Cu þ (MeCN), as occurs during electrochemical processes. [16] These approaches usually involve the direct contact and mixing of TCNQ with a Cu-ion source.…”