Colloidal synthesis of WS2 nanostructures for application as electrode materials

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2019

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Mphahlele, Lerato Sharon

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Abstract

WS2 is a material that has a two-dimensional, layered structure. The properties of WS2 have made it an attractive material to be used in various applications such as sensors, photovoltaic devices and even as electrocatalysts. Its properties include a band gap that is comparable to that of silicon, electrocatalytic properties and good electronic properties. These properties have made WS2 a candidate for application as a counter electrode in dye-sensitized solar cells. The counter electrode catalyses the redox reaction of the iodine electrolyte used within the cell. The synthesis of WS2 nanostructures using the colloidal synthesis is reported in this project. Colloidal synthesis, unlike readily used methods such as exfoliation and chemical vapour deposition, has advantages such as mild synthetic conditions, reaction parameters that can be varied and the possibility of scaling up the reaction. Therefore, the effects of the capping agents, tungsten precursors as well as the sulphur precursors on the properties of WS2 were investigated. The use of oleylamine as the sole precursor produced WS2 with a flower-like morphology. When oleic acid was added in different ratios to oleylamine, the morphology changed to a stacked, petal-like morphology and flattened out as the oleic acid ratio was increased. When oleyl alcohol was used as a co-capping agent, the lower concentration of oleyl alcohol produced flower-like WS2 and evolved into flatter, folded WS2 nanosheets as the oleyl alcohol concentration was increased. Octadecene as a co-capping agent produced WS2 with a flower-like morphology. All the WS2 produced from these capping agents were semiconducting with visible light absorption. The metal precursors had a significant effect on the structure of the WS2. Ammonium tungstate ((NH4)10(H2W12O42)) produced highly crystalline, flat, disc-like materials while tungsten hexacarbonyl (W(CO)6) and tungsten hexachloride (WCl6) resulted in the formation of less crystalline material that lacked the optical characteristics of WS2. WCl6 also produced a mixture of WS2 and WO3, while W(CO)6 formed very small flakes. Sulfur precursors varied from thiourea to thioacetamide to diphenyl thiourea. Semiconducting WS2 was obtained from all these precursors. The morphologies were flower-like for thiourea and diphenyl thiourea, while thioacetamide formed WS2 that had a crumpled sheet-like morphology. Electrocatalytic measurements through cyclic voltammetry revealed that the WS2 produced in some of these reactions are promising as they were able to oxidize and reduce the iodine electrolyte. However, some showed an inefficient reduction of the electrolyte. Reversibility was observed as a significant problem as the reduction of the electrolyte was not as efficient as the oxidation.

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A dissertation submitted to the Faculty of Science, University of the Witwatersrand in partial fulfillment of the requirements for the degree of Masters of Science in Chemistry

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