Controlling the morphological and redox properties of the CuTCNQ catalyst through solvent engineering

The solution-based synthesis of the coordination polymer, CuTCNQ (TCNQ = 7,7,8,8-tetracyanoquinodimethane), which exists in two phases, has predominately used acetonitrile (MeCN) as the solvent. However, our knowledge on the growth and properties of CuTCNQ in other solvents remains limited. In this work, the synthesis of CuTCNQ on Cu foil in two protic (MeOH, EtOH) as well as four aprotic (MeCN, DMSO, DMF, THF) solvents has allowed us to obtain new insights into the important role of the reaction medium in the spontaneous crystallization of CuTCNQ in discrete morphologies and phases. A new electrochemical method for phase identification also has been developed to support this study. Findings reveal that (i) the solvents with higher dielectric constants favor CuTCNQ crystallization; (ii) irrespective of the solvent, use of high temperature (60 °C vs. 25 °C in conventional synthesis) promotes CuTCNQ crystallization and facilitate conversion of phase I to phase II; (iii) phase I CuTCNQ possess enhanced redox catalysis (ferricyanide reduction by thiosulfate) performance over phase II CuTCNQ; and (iv) the amount of catalyst is not necessarily the most important factor for driving catalytic reactions, and other factors, such as, morphology, redox characteristics and solvent in which the CuTCNQ is synthesized may dictate the overall catalytic performance. These findings emphasize the importance of understanding the influence of parameters, such as, solvent and temperature in CuTCNQ synthesis as a means of providing materials with improved catalytic activity.