Electrochemical characterisation of metal oxide nanoparticles, nanofilms and membranes

The work in this thesis focuses on the electrochemical effects of metal oxide nanoparticles at electrode surfaces. Hydrous iron oxide, which is readily prepared in the form of a nanoparticulate sol, is shown to interact only very weakly with electrodes. Direct electron transfer is observed, but much clearer electrochemical responses are observed for hydrous iron oxide adsorbed onto inert electrode surfaces. Monolayers of hydrous iron oxide and of hydrous ruthenium oxide readily form on tin-doped indium oxide (ITO) or on boron-doped diamond electrodes. The inert substrate material allows a wide potential window to be explored and an essentially insulating oxide materials to be investigated electrochemically.

Nanoparticles readily form mono-layer (and not multi-layer) films due to interparticle repulsion effects. However, when a small binder molecule such as the negatively charged phytate for the positively charged oxide particles is introduced, a second layer and further layers of oxide particles can be deposited. In this way multilayer structures are assembled with characteristic effects on the electrochemical properties.

The layer-by-layer assembly strategy is then applied for electrochemically inert particles of TiC>2 (anatase). Mesoporous TiC>2 phytate films are formed at ITO electrode surfaces and their properties are explored. Although not electrochemically active within the potential range studied here, the Ti02 films are shown to promote electron transport within the film. A range of binder molecules are tested and the calcination of the resulting Ti02 films is shown to lead to purely inorganic mesoporous structures.

The negatively charged phytate binder molecules dominate the Ti02 phytate membrane properties and lead to adsorption of positively charged biomolecules such as horse heart cytochrome c. The cytochrome c is shown to penetrate the mesoporous films three dimensionally and surprisingly fast electron transport in the film allows the redox protein to be coupled to the electrode even for thick films. Parameters for transport and electron transfer are explored.

Finally, biphasic redox systems are considered. In order for an electrode to be in contact to two liquid phases simultaneously (the condition for biphasic voltammetry) a finely structured interface is required. Two types of electrodes are investigated with (i) a simple mesoporous oxide film on an ITO electrode and (ii) a gold sputter-coated mesoporous oxide structure. The latter form of electrode is shown to be more efficient in optimising the triple phase boundary reaction zone. In addition to electrodes prepared with the mesoporous TiC>2 multi-layer film, sol-gel silica structures are investigated.