Synthesis, characterization and application of 2d semiconducting layered inorganic nanostructures of In2S3 and WS2 in gas sensing

Gqoba, Siziwe Sylvia
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2D semiconductor nanostructure based chemical sensors hold the promise of portable, fast, low power, simple and low cost technologies for the detection of volatile organic compounds (VOCs) and gases. Detection, monitoring and quantification of these analytes are important for the improvement of the quality of human life, safety and the surrounding environment. Their electrical conductance is extremely sensitive to changes in the local chemical environment and can be chemically modified to increase selectivity. Nanostructures have tunable band gaps and exhibit new and improved properties at the quantum confinement limit. The band gap is affected by the size, shape, dimensionality and chemical composition of the nanostructures. So, precise control of these factors is achieved by manipulating reaction parameters like time, temperature, choice of precursors and choice of capping agent as well as concentration. In this study, we unravel the effect of reaction parameters on the structural, optical and morphological properties of In2S3 and WS2 nanostructures during colloidal synthesis. The reaction parameters under investigation were time, concentration, solvent or capping agent and precursor. For instance, different shapes of ß-In2S3 were obtained for mono- and bi-ligand systems, reaction time, concentration of the precursor and solvent. Capping agents influence the growth kinetics and the shape of nanostructures. Well defined hexagonal ß-In2S3 nanostructures of the tetragonal phase were obtained as a function of time with elemental S and indium chloride reacted at 1:1 mole ratio in oleylamine (OLA) medium. OLA, an alkylamine played the triple role of the reducing agent, solvent and capping agent. An increased amount of S to In3+ proved to be an unfavourable condition for the formation of the hexagonal shape as seen with the 1:2 mole ratio. Hexadecylamaine (HDA) and octadecylamine (ODA), alkylamines like OLA were also used in separate experiments. The hexagonal shape like with OLA evolved with time for ODA while it never materialized for HDA. Dodecanethiol (1-DDT), a thiol ligand produced microspheres as a function of time. These alkylamines were then each used in bi-ligand systems with a controlled amount of 1DDT at a 1:1 mole ratio of In3+ and S. For OLA/1-DDT, the hexagonal morphology was favoured and retained regardless of the duration of the reaction time. However, the hexagonal shape transformed into nanorods with prolonged reaction time for the ODA/1-DDT combination. The morphology was rather elusive for HDA/1-DDT system even at extended reaction time. HDA and ODA yielded the cubic phase of ß-In2S3 in both the mono- and biligand systems. An increased amount of 1-DDT to OLA resulted in mixed morphologies regardless of the reaction time, once again proving the importance of concentration. It is interesting to note that hexagonal nanostructures were retained when 1-DDT was used as a source of S (1:1) with OLA serving a triple role. In the case of WS2, only OLA was used as a capping agent and the variation of reaction of time yielded nanoflowers, nanoflake-like and nanorod-like structures. The nanostructures of these semiconductors were used as components in chemi-resistive sensors for the detection and identification of NH3 gas and selected VOCs. Unlike their oxide counterparts, their gas sensing potential has been largely overlooked despite their capability of operating at room temperature. Preliminary studies on ß-In2S3 sensors based on the 330 min nanostructures showed gas sensing potential towards formaldehyde vapour. In the case of WS2 nanostructures, all the sensors regardless of the reaction time exhibited gas sensing potential. However, the percentage of that response was based on the morphology which was associated with the reaction time. For instance, the microflower morphology obtained at 15 and 45 min displayed the best response compared to 60, 180 and 240 min. However, 45 min had a higher response than 15 min because the ‘petals’which make up the microflowers had opened up. This meant that the reaction not only took place on the surface of the microflower but also in between the ‘petals’. It is well known that humidity is an interferant and can either reduce or improve a sensor’s performance. The sensor’s performances towards NH3 varied depending on the relative humidity they were operating under. Annealing of the sensors showed improved performance at lower temperatures while higher temperatures led to reduced performance. OLA, a long chain ligand renders the semiconductor an insulator thereby reducing its performance. Effect of replacement of OLA with shorter chain ligands on the gas response was also investigated. Mercaptoethanol (ME) and ethanedithiol (1, 2EDT) were used as the short chain ligands and showed improved response towards a lower concentration of NH3. Application of the OLA/WS2 sensors in a tristimulus analysis proved that they can be used in chemical sensor arrays despite the fact that they are made of the same chemical composition. The various morphologies obtained at different time intervals provided the distinguishing factor between the nanostructures.
A thesis submitted to the Faculty of Science at the University of the Witwatersrand, Johannesburg in fulfilment of the requirements for the degree of Doctor of Philosophy in Chemistry, Johannesburg 2018
Gqoba, Siziwe (2018) Synthesis, characterization and application of 2D layered inorganic nanostructures of In2S3 and WS2 in gas sensing, University of the Witwatersrand, Johannesburg,