Vapour-phase Hydrothermal Synthesis of Functional Nanomaterials for Energy Conversion Applications

Abstract

There has been an increasing demand of energy for socioeconomic activities and global population growth. Such gigantic amount of energy has been dominantly supplied with the consumption of fossil fuels that cannot be reliably generated in near future. This energy crisis has recently raised as a global issue that has attracted widespread attention. Abundant, renewable and environmentally friendly solar energy is one of the alternative energy sources to reduce the current dependence on traditional fossil fuels and help alleviate the environmental pollution. As such, photovoltaic devices, or solar cells, that convert solar energy into electricity have been explored intensively by academia and industries. Dye sensitized solar cells (DSSCs) have been proven to be one of the more economic, versatile and robust photovoltaic devices. However, the use of expensive, scarce novel metal, normally platinum or its alloys, as the counter electrode material has to a large extent been limiting the practical application of DSSCs. Thus there are urgent needs to develop low cost, earth abundant, high performance, chemical stable materials as the electrocatalysts for DSSCs. Metal oxides are one of the most abundant and stable substances on earth however they do not demonstrate high electrocatalytical activities as counter electrode materials. The aim of this thesis is to develop a generic approach to convert the wildly available metal oxide into high performance electrocatalyst as counter electrode materials for DSSCs. The thesis has been structured as followings to achieve the goals: in chapter 1, an overview on the photovoltaic devices and DSSCs has been presented. In particular, the research background on the counter electrode materials of DSSCs has been provided with a mini review of the electrode materials developed to date. In chapter 2, Co3O4 nanowires films have been fabricated as the starting materials and subsequently converted into sulphur doped (S-doped) Co3O4 using a vapour-phase hydrothermal (VPH) approach. The structure of Co3O4 film and VPH parameters have been optimized to achieve the best electrocatalytic performance that demonstrates significant improvement compare with the pristine Co3O4 film. In chapter 3, thin Co3O4 nanosheets film has been fabricated and converted in to S-Co3O4 film using VPH method. It has been demonstrated that an unprecedentedly high surface S content (>47%) has been achieved and the photovoltaic conversion efficiency of the DSSCs with such S-Co3O4 film was as superior as the one with benchmark Pt electrocatalysts. In chapter 4, the applicability of VPH method as a generic approach to convert metal oxides into electrocatalytic active materials has been investigated. Metal oxide films such as NiO and Fe2O3 have been successfully fabricated and modified with VPH treatment. The DSSCs equipped with such films as counter electrode have exhibited significantly improved solar to electricity conversion efficiency compared to those with equipped with pristine metal oxide film. Based on the findings in thesis, a conclusion has been provided in chapter 5 followed by a perspective on future research.