Phase, Morphology, and Particle Size Changes Associated with the Solid-Solid Electrochemical Interconversion of TCNQ and Semiconducting CuTCNQ (TCNQ = Tetracyanoquinodimethane)
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- 作者：Aaron K. Neufeld ; Ian Madsen ; Alan M. Bond ; and Conor F. Hogan
- 刊名：Chemistry of Materials
- 出版年：2003
- 出版时间：September 23, 2003
- 年：2003
- 卷：15
- 期：19
- 页码：3573 - 3585
- 全文大小：614K
- 年卷期：v.15,no.19(September 23, 2003)
- ISSN：1520-5002
文摘
The origins of extensive solid-solid-state interconversions that accompany the electrochemistry of microparticles of tetracyanoquinodimethane (TCNQ) and semiconductingCuTCNQ (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, infraredspectroscopy, and X-ray diffraction have been used to identify the phase changes that occurduring the course of potential cycling or bulk electrolysis experiments. All redox-basedtransformations require extensive density, volume, and morphology changes, and consequently they are accompanied by crystal fragmentation. The net result is that extensivepotential cycling ultimately leads to the thermodynamically favored TCNQ/CuTCNQ(phaseII) solid-solid interconversion occurring at the nanoparticle rather than micrometer sizelevel. 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) crystalshaving a density of 1.80 g cm-3 are predominately formed in the first stages of potentialcycling experiments that commence with micrometer-sized rhombic-shaped TCNQ crystalsof density 1.36 g cm-3. The rate of subsequent formation of thermodynamically stableCuTCNQ(phase II), which has an intermediate density of 1.66 g cm-3 and a crystal shapemore like that of TCNQ, is dependent on the number of potential cycles, the scan rate, andthe initial size of the adhered TCNQ crystals. Evidence obtained by cyclic voltametry anddouble 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 andreduction of 
ions (from the electrolyte). The reactions involved in the process arebelieved 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(phaseII) after a large number of potential cycles. The reverse oxidation process involving thetransformation of solid CuTCNQ(phases I and II) to TCNQ also involves a nucleation-growthmultistep process and significant crystal size and morphology changes. Finally, data led tothe postulation of a mechanism for the formation of CuTCNQ compounds via chemicalreaction pathways in which the existence of the electrochemically inferred transitional Cu2+
intermediate also is included.
+ C + 2e- CuTCNQ(S,GC)(phase I or phase II). Needle-shaped CuTCNQ(phase I) crystalshaving a density of 1.80 g cm-3 are predominately formed in the first stages of potentialcycling experiments that commence with micrometer-sized rhombic-shaped TCNQ crystalsof density 1.36 g cm-3. The rate of subsequent formation of thermodynamically stableCuTCNQ(phase II), which has an intermediate density of 1.66 g cm-3 and a crystal shapemore like that of TCNQ, is dependent on the number of potential cycles, the scan rate, andthe initial size of the adhered TCNQ crystals. Evidence obtained by cyclic voltametry anddouble 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 andreduction of ions (from the electrolyte). The reactions involved in the process arebelieved 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(phaseII) after a large number of potential cycles. The reverse oxidation process involving thetransformation of solid CuTCNQ(phases I and II) to TCNQ also involves a nucleation-growthmultistep process and significant crystal size and morphology changes. Finally, data led tothe postulation of a mechanism for the formation of CuTCNQ compounds via chemicalreaction pathways in which the existence of the electrochemically inferred transitional Cu2+ intermediate also is included.