Synthesis and characterization of novel colloidal gold selenide nanostructures and their application as counter electrodes for dye-sensitized solar cells

Machogo, Lerato Florence Eugenia
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Energy demands have been on the rise lately due to the growing global population. Projection of global energy consumption has been estimated to increase by 44% between the periods 2006 to 2030. This puts tremendous pressure on most developing countries, who still today, generate majority of their electricity through burning of fossil fuels such as coal. These countries, like South Africa, therefore, need to upscale their energy production but also need to take into account the adverse environmental implications (i.e. increase in carbon emissions) that the “traditional” routes may have. Having this in mind and the perception of possible fossil fuel depletion, cleaner, renewable and more sustainable sources for energy generation need to be strongly considered. Harnessing of solar energy has over the years proven to effectively work through various types of photovoltaic/solar cell technologies. However, highly efficient devices in the market are commercially expensive, hence there is a need to reduce on these costs yet maintaining comparable power conversion efficiencies (PCEs). Dye-sensitized solar cells (DSSCs) are a cost-effective and environmentally friendly option, which has previously reported promising PCEs. However, the use of the corrosion-prone Pt metal counter electrode (CE) hinders the stability and reproducibly of the device long term. Hence the current study focused on synthesis of 2D-layered gold selenide (AuSe) nanomaterials for potential use as a CE in DSSCs. The colloidal synthesis using varied molar ratios of gold to selenium precursors yielded polymorphic samples with β-AuSe being the predominant phase over α-AuSe. Temperature studies showed that synthesis at lower temperatures gave high impurities of unreacted selenium where excess reduced gold was observed at high working temperatures. The ideal temperature was thus 200 °C which was used in subsequent experiments. Adding precursors in a different sequence to improve on the purity of AuSe instead produced intriguing morphology differences. XRD, TEM and SEM showed that the addition of gold at higher temperatures gave long flexible α-AuSe dominated nanobelts, contrary to the β-AuSe nanoplates observed when gold was added at lower temperatures. The α-AuSe nanobelts also had β-AuSe spherical deposits on their surfaces, which explained most of the polymorphism observed in all the as-synthesized samples. EDS mapping showcased the level of homogeneity of the AuSe nanostructures which presented each particle having both Au and Se at an appreciable amount. The use of different capping agents produced α-AuSe dominated materials, however, 1DDT had short rod-like nanostructures and OLA and OA depicted nanobelts. OLA was the preferred stabilizing agent in producing AuSe as a result of giving more of the desired product than the other two which gave more of Au and Se impurities. DFT studies confirmed the preference for nanobelt morphologies for OLA and OA capped particles. XPS not only depicted the presence of the Au+ ions, it also confirmed Au–Se bonding in the Se 3d high resolution spectra where a strong presence of the Au5p peak was observed. The presence of Au3+ ions in the AuSe system was confirmed by vibrational shifts associated with the square planar geometry in Raman spectroscopy. These results were true for both α-and β-AuSe. The electrocatalytic activity of AuSe was illustrated by using cyclic voltammetry. Alpha-AuSe CE had a lower ΔEpp value (532 mV) and a higher cathodic peak current of 6.1 mA while β-AuSe CE had 739 mV for ΔEpp and 4.2 mA for the cathodic peak current. AuSe therefore, showed good redox catalytic activity, stability and reproducibility towards the I-/I3-electrolyte, illustrating its potential as a counter electrode in dye-sensitised solar cells
A thesis submitted to the Faculty of Science, University of the Witwatersrand, in partial fulfilment for the degree Doctor of Philosophy, 2020