Abstract
In this work, the idea of fabricating ionic diode or rectifier devices using a materials approach was pursued, based on embedding nanostructured ion conducting materials and membranes on insulating poly (ethylene terephthalate) (PET) substrates with laser drilled microholes. The ion conducting membranes provided ion selectivity, whilst the microhole substrate allowed current rectification and ion concentration polarisation phenomena to be possible. Current rectification in ionic diode devices fabricated in this fashion were shown to result in the similar “ion pump” effect reported for cell membranes in living organisms, but with a much higher ionic current flow. If properly understood and optimised, this behaviour may be valuable in sensing and in desalination amongst many other suggested applications. The first part of the doctoral thesis dealt with developing a method for joining two ready made films together; one an ion-exchange membrane with semi-permeability for cations (Fumasep FKS-30), and the other a PET substrate with a microhole, to allow current rectification. Hot-pressing, was chosen as a suitable method to achieve this, offering stable and better-defined ionic diode geometries. Confocal microscopy was used to define symmetric and asymmetric diode geometries, and to measure the thickness of the films after hot-pressing. Cation rectification phenomena are shown to be dependent on microhole diameter, electrolyte concentration, and on the nature of the electrolyte. Significant competition of cation transport with proton transport was observed. In the second milestone, surface oxidised carbon nanofibers were used in ionic diode fabrication. The carbon nanofibers were synthesised using chemical vapour deposition, and hydrogen peroxide as oxidising agent, to induce negative surface charges, which when embedded asymmetrically on a microhole in PET, allow cation selectivity and cationic diode behaviour. The carbon nanofibers were characterized with transmission electron microscopy (TEM), scanning electron microscopy (SEM), elemental analysis, and by zeta potential measurements. And the effects of pH, ionic strength, and nature of electrolyte, on ionic diode behaviour were investigated...
Ph.D. (Chemistry)