Metal oxide nanomaterials and their application in solar photoelectrolysis of water

Solar generated hydrogen as an energy source is green, sustainable, with a high energy density. One day the majority of current fossil fuel based technology could be replaced with hydrogen technology reducing CO2 emission drastically. The goal in this research is to explore hybrid metal oxide photocatalysts in the pursuit of achieving highly efficient photoanodes for use in photoelectrochemical cells (PEC). Achieving high efficiencies of hydrogen production in photoelectrochemical cells is the key challenge for the commercialisation of PEC technology as a viable, sustainable, hydrogen source; limited only by the lifetime of the sun and the resources of the metal oxide materials. In this research TiO2, Fe-Ti-O, ZnO, and Zn2TiO4 are the photocatalysts explored. Alloys of Ti-Fe-O showed improvement over TiO2, whilst a hybrid heterostructure of ZnO/Zn2TiO4/TiO2 enhanced photocurrent densities significantly. A barrier layer in the photoanode achieved localised exciton separation and reduction of recombination rates by inhibiting back flow of electrons after injection into the TiO2 layer. Nanotubes are created by the simple electrochemical process of anodisation. The nanotube composition depends on the anode material. To control the composition ofthe anode, iron and titanium are co-deposited onto a substrate using electron beam evaporation. The introduction of iron into titania nanotubes engineered the band gap, lowering the band gap energy to that of iron oxide whilst the positions of the conduction and valence bands with respect to the oxidation and reduction potentials of water remained favourable. Fe-Ti-O nanotubes showed remarkable photocurrent density improvement compared to TiO2 nanotubes. ZnO nanostructures deposited by vapour transport mechanisms showed variability in the morphology of the structures, as governed by the growth dynamics. Herein, it is shown that an electronically favourable situation arises by the formation of a ZnO-Zn2TiO4-TiO2 heterostructure and a high photocatalytic activity is reported. The structure is composed of a large surface area ZnO nanorod photoabsorber formed on a Ti foil which forms a Zn2TiO4 barrier layer between ZnO and TiO2. The Zn2TiO4 layer inhibits electron transport toward the surface of the photoanode whilst encouraging charge transport to the hydrogenation electrode. The heterostructure interfacial surface area is extended through the utilisation of TiO2 nanotubes, which demonstrated a 20.22 % photoelectrochemical efficiency under UV illumination. Surface modification of ZnO nanorods with aerosol assisted chemical vapour deposited TiO2 nanoparticles enhanced photocurrent densities of the ZnO rods, improving charge separation of excitons created within the TiO2 nanoparticles. ZnO nanotubes formed via a novel route using chemical bath deposition of ZnO is investigated, an annulus ZnO seed layer facilitated the site specific growth of ZnO nanotubes whilst a uniform seed layer formed ZnO nanorods.