The development of a rigorous nanocharacterization scheme for electrochemical systems

Abstract

This thesis reports on a methodology for the nanocharacterization of complex electrochemical systems. A series of powerful techniques have been adapted and applied to studies of two scientifically important electrochemical systems; namely polymer membrane solid-state ion-selective electrodes (ISEs) and electrochemically generated tetracyanoquinodimethane (TCNQ) charge-transfer materials. These studies have mainly encompassed the use of neutron reflectometry (NR), electrochemical impedance spectroscopy (EIS), secondary ion mass spectrometry (SIMS), small angle neutron scattering (SANS), synchrotron radiation / Fourier transform-infrared microspectroscopy (SR / FT-IRM), synchrotron radiation / X-ray photoelectron spectroscopy (SR / XPS) and synchrotron radiation / grazing incidence X-ray diffraction (SR / GIXRD). Significantly, an NR technique has been specially developed to enable simultaneous EIS measurements through the development and refinement of a novel electrochemical / reflectometry cell. Furthermore, the development of a versatile electrochemical cell that is capable of allowing SR / GIXRD measurements to be made in practically any conceivable electrochemical problem has also been of great significance.The investigation of polymer membrane solid-state ISEs focused on the problem of water layer formation at the buried polymer interface after prolonged exposure to an analyte. Initially, a rigorous surface and materials characterization scheme was developed and applied to plasticized poly(vinylchloride) (PVC) coated wire electrodes (CWEs) that are known to be adversely affected by water layer formation. It was determined that water and the associated ions from the sample analyte were transported through the PVC membrane. This resulted in the formation of a water layer (approximately 120 Å thick) at the substrate / ion-selective membrane interface. The results of the study suggested that this event occurred after 3 to 20 hours of constant exposure to solution. Moreover, the water layer at the buried interface was found to contain traces of plasticizer, whilst nanodroplets of water were also found in the membrane. The former is evidence for the exudation of plasticizer from the PVC membrane into the water layer at the buried interface.Further investigations on a solid-state ISE utilizing a hydrophobic poly(methylmethacrylate) / poly(decylmethacrylate) (PMMA / PDMA) copolymer as the ion-selective membrane revealed that water was transported through the membrane at a far slower rate than that of plasticized PVC ISEs. In fact, a regular ISE of this type severely restricted water accumulation at the buried interface, with such an event occurring after 460 hours. In addition, water was restricted to accumulation as droplets at the buried interface, as opposed to continuous water layers. A negligible amount of water was found in the bulk of this hydrophobic polymer membrane.Given CWEs are susceptible to forming water at the buried interface, it is customary to employ solid-contact (SC) underlayers. The primary function of the SC is to provide an appropriate mechanism for ion-to-electron transduction. Certain SCs are also theorized to discourage the formation of water layers. The results of this thesis revealed that a hydrophobic poly(3-octylthiophene-2,5-diyl) (POT) SC can prevent the formation of a water layer in SC ISEs altogether. This is not only achieved through the hydrophobic nature of POT, but also through the fact that the underlayer of POT is able to cover any imperfections at the buried interface, which water can use as a site for accumulation.By contrast, a hydrophilic polymer SC, known as poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS), was found to scavenge available traces of water at the buried interface. Instead of forming a well defined water layer or even water-droplets at the buried interface, the PEDOT:PSS SC system was found to soak up all traces of water transported through the ion-selective membrane to the buried interface. Water was detected in the PEDOT:PSS vii underlayer in a miscible state and not as a separate phase as observed with the CWE systems.The mechanism for ion-to-electron transduction in electroactive polymer SCs was also investigated. The study was performed in order to address the extent to which charger-transfer events occur throughout the underlying polymer SC. By studying the electrochemical doping of POT with [3,5-bis(triflouro-methyl)phenyl]borate (TFPBˉ) ions it was shown that the ion-to-electron transduction process is surface confined. This outcome demonstrates that the performance of various SCs does not depend on the thickness of the polymer film. In fact, it is proposed that the sparing use of the SC material may possibly achieve better charge-transfer performance. Such a hypothesis is based on the reduced electron path through the SC, hence reducing the probability that electrons are hindered by impurities and film imperfections. The suggestion of surface confined charge-transfer events also supports previous notions that the effectiveness of SCs is based on the capacitive nature of the material.The final part of the thesis deals with the characterization of the structure and morphology of TCNQ-based charge-transfer materials. Due to the lack of prior research on the electrochemical syntheses and structures of these materials, Cd(TCNQ)2 and Zn(TCNQ)2 were studied. By using SR / GIXRD together with synchrotron powder diffraction, the electrochemically synthesized Cd(TCNQ)2 was found to be crystallographically similar to the powder sample. Subtle differences between the two materials were evident; however, it was found that the major phase of non-hydrated Cd(TCNQ)2 phase was present in both samples. Notably, this phase was found to have a tetragonal unit cell, with cell parameters: a = 16.78Å and c = 8.83Å.Finally, a potential-dependant voltammetric study was carried out on a Zn(TCNQ)2 system. This was done in order to investigate the effects of electrodepositing Zn(TCNQ)2 under different electrochemical conditions. It was found that the material electrocrystallized prior to, or at, the peak potential for reduction of TCNQ to TCNQˉ comprised two layers. The upper layer was shown to consist of a densely packed and highly amorphous layer of Zn(TCNQ)2, while the lower layer was a crystalline phase of Zn(TCNQ)2. The material deposited at a potential after the peak suggested that only the crystalline phase of Zn(TCNQ)2 was present. This finding is significant for two reasons. First, in electrochemistry, it demonstrates that the in situ SR / GIXRD technique can be used to interrogate electrode reaction products under different voltammetric conditions. Next, it is important in the manufacture of electrocrystallized materials, where it demonstrates that complete control of the morphology and major phases is possible and that SR / GIXRD is a useful research tool to study the process.