Atomically engineered semiconductor nanostructures for photoelectrochemical water oxidation

Greenhouse gas emission and global warming have become matters of serious concern in relation to the preservation of numerous global ecosystems. The drive to phase out fossil fuels has been well underway for a decade or so. Diversifying current approaches towards more efficient and cheaper renewable energy generation techniques is an ongoing effort. In the current renewable energy picture, certain sources such as solar photovoltaics, hydropower, wind and geothermal power can be categorised as truly environmentally benign. Hydrogen has also been an emerging contender within the environment-friendly energy generation drive. It boasts high efficiency with no detrimental combustion products as an energy source and harbour the capability of replacing currently used fossil fuel without completely changing the modern energy infrastructure. In this light, development of novel technologies to produce hydrogen utilising green approaches have become vital. Photocatalytic water splitting is an emerging technique which has shown promise as a viable method for hydrogen production. It relies on light-absorbing semiconductor materials able to catalyse the water splitting reaction. As such, careful synthesis of these semiconductors is paramount for achieving high reaction efficiencies. Successive Ionic Layer Adsorption and Reaction (SILAR) is a synthesis technique that enables to achieve high quality, conformal thin films on a variety of support substrates. Importantly, SILAR is currently an underexplored deposition technique for the fabrication of metal oxide thin films. Although there is a vast number of studies conducted on the deposition of chalcogenide material via SILAR, current knowledge on oxide deposition is significantly insufficient. The main objective of this PhD project was to assess the capability of the SILAR technique for the fabrication of semiconductor thin films of a selected few key metal oxides, ultimately targeting photoelectrochemical water splitting. Initially, the evolution and fundamentals of the technique were discussed in detail to provide the reader with a comprehensive understanding on workings and the current standing of the technique. A series of experimental parameters in relation to their effect on the final attributes of the finished thin films are also discussed. The state of the art in recent trials involving chalcogenides and oxides are elaboratively discussed drawing relationships between deposition parameters and electronic/photonic performance. Extensive studies on SILAR deposition of metal oxide films are almost non-existent. To address this knowledge gap, an elaborate study into the parameter space of SILAR deposition was conducted, selecting a highly photoactive material, bismuth vanadate (BiVO4) as the target oxide. The effect of the precursor concentration, the dip cycle number and the annealing temperature were thoroughly investigated in BiVO4 films fabricated by SILAR. BiVO4 films having high efficiencies were produced, achieving the photocurrent record for the SILAR-deposited films of BiVO4, demonstrating the capabilities of the technique in generating highly crystalline, and efficient photoanodes for photoelectrochemical water splitting. Results obtained during this study cement the standing and capability of SILAR technique as an efficient, versatile and a tuneable technique in deposition of thin film photoelectrodes for solar water splitting applications. One of the recognised limitations of SILAR-deposited BiVO4 thin films during the work outlined in this thesis was the instability caused by the annealing thermal tress with the increasing film thickness. It was identified as an obstacle in fabricating films with higher photoabsorption. In order to address this issue, attention was directed towards SILAR-based deposition on high-surface area substrates which could facilitate a greater loading of the photoactive material without significantly increasing the thickness of the active material. One-dimensional TiO2 and two-dimensional SnO2 nanostructures were hydrothermally developed, and SILAR technique was utilised to deposit photoactive material on them. It was demonstrated that SILAR technique is tuneable and is well capable of depositing photoactive materials on nanostructured high-surface area substrates without disrupting the underlying morphology and at the same time give rise to highly efficient heterostructures which could be used for solar-driven water splitting efficiently. These results were demonstrated to be applicable towards exceptionally photoactive BiVO4 and far more challenging Fe2O3 as well. Also in this thesis, the attention was directed towards investigating the possibility of depositing novel and emerging materials which possess promise as photoanode materials. SnWO4 is an emerging material with a low band gap (~1.9 eV) and band positions suitable for water oxidation. However, only a handful of studies have been conducted on this material. Here, an investigation was conducted for the first time into the SILAR deposition of highly crystalline SnWO4 films which could be used as efficient photoanodes. It was demonstrated that crystalline SnWO4 could be deposited successfully via the SILAR technique, and the photocurrent degradation due to the formation of SnO2 on the surface could be mediated at least in part by additional metal oxide coatings such as NiO and ZnO. Composite films were also tested as electrocatalysts for water oxidation, showing some promise. This thesis investigated the deposition of photocatalytic material via a convenient and scalable solution-based deposition technique: SILAR. The parameter space of the deposition technique was thoroughly examined in an attempt to understand the underlying relationships between various parameters, and the physical, chemical and photoelectrochemical properties of the developed films. The best SILAR-deposited samples fabricated during the work achieved exceptional performance compared to published literature. The techniques developed during this study have the potential to function as a platform in development of efficient photoelectrodes for solar-driven water splitting, and also for other semiconductor coatings.