title: Synthesis and characterisation of nanoscale oxides for energy applications
Creators: name: Roberts, Joseph
Publisher: name: University of Liverpool Repository
Year: 2015

description: The motivation for this research is the synthesis and characterisation of nanostructured oxide materials for potential applications in renewable energy generation or storage. In particular, the research in this thesis addresses the development of nanostructured oxide materials which could be exploited as photovoltaics and transparent conductive oxides. The exploitation of anodised aluminium oxide (AAO) to fabricate nanostructured semiconductor pn-junctions as the basis photovoltaic devices is investigated. Various microstructures are modelled with the aim of identifying ones with high interface area and consequently high energy conversion efficiencies. Experiments reveal that AAO prepared using an oxalic acid electrolyte could be achieved with a growth rate of 5.7μm/h. The uniformity and size of the hexagonal cross-section pores is influenced by the first-step anodisation time. Atomic layer deposition (ALD) was used to coat commercially available AAO, to synthesise conformal nanotubes or wires. It was observed that the uniformity of the nanostructures breaks down towards the bulk of the AAO, without high exposures, due to the lack of precursor penetration. However, the proposed geometric mass-gain model to understand the process is supported by experimental mass gain measurements and how deposition occurs within the templates. A second materials system based on copper / copper oxide is also explored as a method of synthesising and controlling the formation of nanostructured oxide materials. The surface preparation of the starting copper sheet material was investigated and it was found to be pivotal in the nanoscale morphology achievable with subsequent heat treatment to grow nanowires and porous layers. Thermal growth of CuO and Cu2O on pre- treated copper foils at 500°C exhibited growth of high aspect ratio CuO nanowires. Further studies on the growth process disproved the vapour-liquid-solid and vapour-solid growth mechanisms for the nanowires and showed that tensile strain within the Cu substrate was the driving force behind the nanowire growth. The use of nanometre-scale ALD alumina barrier layers was employed to suppress spallation and preserve the nanowire surfaces. It was found that for samples with alumina layers between ~3 and 15nm the oxide spallation was significantly reduced and that for samples with ~20nm of alumina the diffusion of Cu atoms to the surface was hindered. Photovoltaic measurements were made on electrolytically synthesised Cu2O surfaces coated with ALD TiO2, ZnO and Ga-doped ZnO to form pn-junctions. The samples showed only weakly rectifying behaviour which was attributed to short-circuits between the n-type layers and the back contact. Several of the samples did show some difference in electrical response when under illumination indicating that, at least in some parts of the device, the pn-junction had formed. Lastly the growth and characterisation of ALD Ga-doped ZnO was investigated to determine the optimal doping levels for the growth of highly conductive and transparent oxides, to be used as a front contact for photovoltaic devices. It was found that Ga doping at around 1at% in ZnO produced film with the lowest resistivity. CdTe films were then grown onto substrates coated in 1at% Ga-doped ZnO and subjected to AM1.5 photovoltaic IV testing, yielding photovoltaic cells with conversion efficiencies ~10.8% and fill-factors of ~65%.