Voltammetric, Spectroscopic, and Microscopic Investigations of Electrocrystallized Forms of Semiconducting AgTCNQ (TCNQ = 7,7,8,8-Tetracyanoquinodimethane) Exhibiting Different Morphologies and Colors
journal contributionposted on 13.11.2007, 00:00 by Alexander R. Harris, Ayman Nafady, Anthony P. O'Mullan, Alan M. Bond
Chemically synthesized AgTCNQ exists in two forms that differ in their morphologies (needles and microcrystals) and colors (red and blue). It is now shown that both forms exhibit essentially indistinguishable X-ray diffraction, spectroscopic, and thermochemical data, implying that they are not separate phases, as implied in some literature. Electrochemical reduction of TCNQ(MeCN) in the presence of Ag+(MeCN) generates both red and blue AgTCNQ. On glassy carbon, platinum, or indium tin oxide electrodes and at relatively positive deposition potentials, slow growth of high aspect ratio, red needle AgTCNQ crystals occurs. After longer times and at more negative deposition potentials, blue microcrystalline AgTCNQ thin films are favored. Blue AgTCNQ is postulated to be generated via reduction of a Ag+[(TCNQ•-)(TCNQ)](MeCN) intermediate. At even more negative potentials, Ag(metal) formation inhibits further growth of AgTCNQ. On a gold electrode, Ag(metal) deposition occurs at more positive potentials than on the other electrode materials examined. However, surface plasmon resonance data indicate that a small potential region is available between the stripping of Ag(metal) and the oxidation of TCNQ•-(MeCN) back to TCNQ(MeCN) where AgTCNQ may form. AgTCNQ in both the red and blue forms also can be prepared electrochemically on a TCNQ(s) modified electrode in 0.1 M AgNO3(aq) where deposition of Ag(metal) onto the TCNQ(s) crystals allows a charge transfer process to occur. However, the morphology formed in this solid−solid phase transformation is more difficult to control.