Redox-Induced Solid−Solid Phase Transformation of TCNQ Microcrystals into Semiconducting Ni[TCNQ]₂(H₂O)₂ Nanowire (Flowerlike) Architectures:  A Combined Voltammetric, Spectroscopic, and Microscopic Study

Abstract

The facile solid-solid phase transformation of TCNQ microcrystals into semiconducting and magnetic Ni[TCNQ]2(H2O)2 nanowire (flowerlike) architectures is achieved by reduction of TCNQ-modified electrodes in the presence of Ni2+(aq)-containing electrolytes. Voltammetric probing revealed that the chemically reversible TCNQ/Ni[TCNQ]2(H2O)2 conversion process is essentially independent of electrode material and the identity of nickel counteranion but is significantly dependent on scan rate, Ni2+(aq) electrolyte concentration, and the method of solid TCNQ immobilization (drop casting or mechanical attachment). Data analyzed from cyclic voltammetric and double-potential step chronoamperometric experiments are consistent with formation of the Ni[TCNQ]2(H2O)2 complex via a rate-determining nucleation/growth process that involves incorporation of Ni2+(aq) ions into the reduced TCNQ crystal lattice at the triple phase TCNQ|electrode|electrolyte interface. The reoxidation process, which includes the conversion of solid Ni[TCNQ]2(H2O)2 back to TCNQ0 crystals, is also controlled by nucleation/growth kinetics. The overall redox process associated with this chemically reversible solid-solid transformation, therefore, is described by the equation: TCNQ0(S) + 2e- + Ni2+(aq)+ 2 H2O <==> {Ni[TCNQ]2(H2O)2}(S). SEM monitoring of the changes that accompany the TCNQ/Ni[TCNQ]2(H2O)2 transformation revealed that the morphology and crystal size of electrochemically generated Ni[TCNQ]2(H2O)2 are substantially different from those of parent TCNQ crystals. Importantly, the morphology of Ni[TCNQ]2(H2O)2 can be selectively manipulated to produce either 1-D/2-D nanowires or 3-D flowerlike architectures via careful control over the experimental parameters used to accomplish the solid-solid phase interconversion process.

Structural, Spectroscopic, and Electrochemical Characterization of Semi-Conducting, Solvated [Pt(NH3)4](TCNQ)2·(DMF)2 and Non-Solvated [Pt(NH3)4](TCNQ)2

“…As found for TCNQ/M nþ (TCNQ) n solid-solid interconversion reactions (M nþ ¼ Mn 2þ , Fe 2þ , Co 2þ , Ni 2þ , Cu þ or alkali metal cations), [36,[38][39][40][43][44][45]49] (aq) , presumably owing to increasing ohmic drop effects (iR u ). [36,[38][39][40][43][44][45] This effect is also apparent when the scan rate is increased from 5 to 50 mVs À1 (Fig. S6a, (aq) electrolyte concentration over the range of 5 mM to 0.50 M using TCNQmodified GC electrode (mechanical attachment) and a scan rate of 20 mV s À1 is relatively small, ,8 mV shift per decade change in concentration as shown in Fig.…”

“…electrolyte showed no faradaic current. However, on modification of the GC electrode with microparticles of TCNQ (mechanical attachment method), [38] symmetrical and reproducible voltammetric responses were obtained, as shown in Fig. 4.…”

“…and other related parameters are similar to those reported for electrochemically driven TCNQ/M-TCNQ solid-solid interconversion, where M is a group I (alkali metal), [39,40] group II (alkaline earth) [41,42] cation or first-row transition metal cation. [36,38,[43][44][45] On this basis, the observed voltammetry is attributed to the one-electron reduction of solid TCNQ 0 to TCNQ 1accompanied by rapid interaction with the [Pt(NH 3 ) 4 ] 2þ cation from the aqueous electrolyte at the TCNQ/TCNQ 1-(s) tGC (s) t[Pt(NH 3 ) 4 ](NO 3 ) 2(aq) triple-phase junction.…”

“…is 535 mV, as found for the reduction of the parent TCNQ under the same conditions. [38] (1) and [Pt(NH 3 ) 4 ](TCNQ) 2 (2) using a 10-mm Pt microelectrode (Fig. S4b, Supplementary Material) reveals that the acetonitrile solution contains ,95 % TCNQ 1-, which can be either oxidized to TCNQ 0 , giving rise to the positive (oxidative) current, or reduced to the dianion (TCNQ 22 ).…”

“…The procedures used for polishing these electrodes are described elsewhere. [35,36,[38][39][40] For studies in water, the reference electrode was an aqueous Ag/AgCl (3 M KCl, Bioanalytical Systems) and the counter-electrode was a platinum wire. A stream of N 2 gas was used to purge the aqueous solutions and a flow of this gas was maintained above the solution during the course of electrochemical experiments.…”