Aaron K. Neufeld

Voltammetric, EQCM, Spectroscopic, and Microscopic Studies on the Electrocrystallization of Semiconducting, Phase I, CuTCNQ on Carbon, Gold, and Platinum Electrodes by a Nucleation-Growth Process

Cu 7,7,8,8-tetracyanoquinodimethane (CuTCNQ) may be chemically synthesized in two phases, one of which is significantly more conductive (phase I) than the other (phase II). Because CuTCNQ is sparingly soluble in acetonitrile, reduction of TCNQ to TCNQ- in the presence of CU(MeCN)+ under conditions where the solubility is exceeded in this solvent allows CuTCNQ nucleation-growth processes to occur at defect sites on carbon, gold, and platinum macro- and microdisk electrode surfaces. Rapid growth of large branched needle-shaped phase I crystals occurs on the time scale of cyclic voltammetry at semiconducting CuTCNQ nucleation sites. Infrared spectra and the crystal morphology detected by electron microscopy of electrocrystallized solid, are all consistent with growth of purely phase I CuTCNQ solid. The smaller crystals formed on the electrode surface, but not larger ones, may be stripped from the electrode surface by application of positive potentials. Mechanistic aspects of the electrocrystallization and stripping processes are considered.

Pitting Corrosion of Zn and Zn-Al Coated Steels in pH 2 to 12 NaCl Solutions

Corrosion behavior of hard zinc, Zn-5% Al (Galfan), and Zn-55% Al alloy (Zincalume or Galvalume) coated steels was studied using cyclic voltammetry and potential scan/hold technique in the presence of 0.10-0.90 mol L -1 NaCl over a pH range of 2 to 12. Influence of chloride concentration and electrolyte pH on pitting potential values of the above three types of specimens were examined. On the basis of the electrochemical, scanning electron microscopy with energy dispersive X-ray spectroscopy, and chemical equilibria data obtained from hard zinc, a pitting corrosion mechanism involving a series of processes from anion competitive adsorption to penetration to nucleation and growth was proposed.

Characterisation of two distinctly different processes associated with the electrocrystallization of microcrystals of phase I CuTCNQ (TCNQ = 7,7,8,8-tetracyanoquinodimethane)

Semi-conducting phase I CuTCNQ (TCNQ = 7,7,8,8-tetracyanoquinodimethane), which is of considerable interest as a switching device for memory storage materials, can be electrocrystallized from CH3CN via two distinctly different pathways when TCNQ is reduced to TCNQ˙− in the presence of [Cu(MeCN)4]+. The first pathway, identified in earlier studies, occurs at potentials where TCNQ is reduced to TCNQ˙− and involves a nucleation–growth mechanism at preferred sites on the electrode to produce arrays of well separated large branched needle-shaped phase I CuTCNQ crystals. The second pathway, now identified at more negative potentials, generates much smaller needle-shaped phase I CuTCNQ crystals. These electrocrystallize on parts of the surface not occupied in the initial process and give rise to film-like characteristics. This process is attributed to the reduction of Cu+[(TCNQ˙−)(TCNQ)] or a stabilised film of TCNQ via a solid–solid conversion process, which also involves ingress of Cu+via a nucleation–growth mechanism. The CuTCNQ surface area coverage is extensive since it occurs at all areas of the electrode and not just at defect sites that dominate the crystal formation sites for the first pathway. Infrared spectra, X-ray diffraction, surface plasmon resonance, quartz crystal microbalance, scanning electron microscopy and optical image data all confirm that two distinctly different pathways are available to produce the kinetically-favoured and more highly conducting phase I CuTCNQ solid, rather than the phase II material.

Monitoring Cuprous Ion Transport by Scanning Electrochemical Microscopy During the Course of Copper Electrodeposition

Scanning electrochemical microscopy (SECM), in the substrate generation–tip collection (SG-TC) mode, has been used to detect the cuprous ion intermediate formed during the course of electrodeposition of Cu metal from aqueous solution. Addition of chloride is confirmed to strongly stabilize the ion in aqueous solution and enhance the rate of Cu electrodeposition. This SECM method in the SG-TC mode offers an alternative to the rotating ring disk electrode (RRDE) technique for in situ studies on the effect of plating bath additives in metal electrodeposition. An attractive feature of the SECM relative to the RRDE method is that it allows qualitative aspects of the electrodeposition process to be studied in close proximity to the substrate in a simple and direct fashion using an inexpensive probe, and without the need for forced convection.

Electronic Properties of Modified Surfaces Using Contact and Non-Contact Scanning Probe Microscopy Techniques and SECM.

Electrochemical experiments with tetracyanoquinodimethane (TCNQ) modified electrodes in contact with aqueous copper containing electrolytes leads to the incorporation and expulsion of copper ions. This process occurs concomitantly with nucleation and growth processes and significant crystal fragmentation to produce particles with dimensions of the order of 10s of nanometres. During reduction of TCNQ and intercalation of copper ions, different phases of the semiconducting compound CuTCNQ are formed.[1,2] The preparation of both conducting and insulating substrates coated with electroactive TCNQ and CuTCNQ particles of variable size have been made by dip and spin coating procedures. Results suggest that the phase and hence electronic properties of CuTCNQ is dependent on the size of particles that decorate the electrode surface. Combining atomic force microscope (AFM) based methods that interrogate the morphological and electronic properties of nanometre sized particles with use of a scanning electrochemical microscope (SECM) is a new advance in materials characterisation that has proved highly valuable in understanding the highly complex behaviour of these semi-conducting particles.