ORIGINAL PAPER Synthesis and characterization of microstructured sheets of semiconducting Ca[TCNQ] 2 via redox-driven solid-solid phase transformation of TCNQ microcrystals Ayman Nafady & Nasser J. Al-Qahtani & Khalid A. Al-Farhan & Suresh Bhargava & Alan M. Bond Received: 30 December 2013 /Revised: 1 January 2014 /Accepted: 2 January 2014 /Published online: 4 February 2014 # Springer-Verlag Berlin Heidelberg 2014 Abstract 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, mechan- ically attached to an electrode surface, in the presence of an aqueous Ca 2+ (aq) electrolyte solution. Voltammetric probing of the electrochemically irreversible TCNQ/Ca[TCNQ] 2 inter- conversion revealed that it is highly dependent on scan rate and Ca 2+ (aq) electrolyte concentration. This voltammetric be- havior, 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 2+ (aq) cations into the TCNQ ·− crystal lattice at the triple phase TCNQ/TCNQ ·− (s) │GC (s) │Ca 2+ (aq) electrolyte junction. The overall redox process associated with this chemically reversible solid-solid transformation can be described by the equation: TCNQ 0 (S) +2e − +Ca 2+ (aq) ⇄ {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, and Cs). 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 conceptu- ally related group I cations and binary M[TCNQ] 2 (M 2+ =Mn, Fe, Co, and Ni)-based coordination polymers. Keywords TCNQ . Redox-based . Solid-solid transformation . Nucleation-growth . Microstructured sheets . Binary Ca[TCNQ] 2 Introduction Metal-tetracyanoquinodimethane (M-TCNQ) salts, charge- transfer complexes, and coordination polymers have attracted remarkable interest over the past four decades, owing to their intriguing structural, optical, conducting, and switching prop- erties [1–7]. Promising utilization of these materials in indus- trial and technological applications such as energy and data storage, optical and electrical media recording, sensors and catalysis, as well as transistors and magnetic devices has provoked many of the contemporary research activities in this area [8–19]. In particular, considerable efforts have been made to develop different synthetic approaches that enable control- lable growth of M-TCNQ microstructures/nanostructures, thereby providing facile access to tune their inherent chemical and physical properties [20–25]. In this context, synthetic routes such as vapor deposition of TCNQ on metal surfaces, reaction of TCNQ ·− monoanion with metal salts or precursors, A. Nafady (*) : K. A. Al-Farhan Department of Chemistry, College of Science, King Saud University, PO Box: 2455, Riyadh 11451, Saudi Arabia e-mail: anafady@ksu.edu.sa A. Nafady Chemistry Department, Faculty of Science, Sohag University, Sohag 82524, Egypt N. J. Al-Qahtani King Abdulaziz City for Science and Technology, Petrochemical Research Institute, Riyadh 11451, Saudi Arabia S. Bhargava Centre for Advanced Materials & Industrial Chemistry, College of Science, Engineering and Health, RMIT University, Melbourne, Victoria 3000, Australia A. M. Bond (*) School of Chemistry, Monash University, Clayton Campus, Victoria 3800, Australia e-mail: alan.bond@monash.edu.au J Solid State Electrochem (2014) 18:851–859 DOI 10.1007/s10008-014-2379-1