GROWTH OF MOLYBDENUM DISULFIDE (MoS2) NANOSTRUCTURES AND THEIR APPLICATIONS

Abstract

7.1. Summary The objectives of the current thesis entitled “Growth of molybdenum disulfide (MoS2) nanostructures and their applications” disclosed the structural, morphological, electrical, compositional and optical properties of MoS2 nanostructures and discussed their utility for sensing and photocatalytic applications. The MoS2 has gained a great attention because of its structural similarity with graphene [1]. The layers tuned bandgap of MoS2 possesses magnificent optical and electronic properties which exhibit a wide range of applications, i.e., catalysis, transistors, batteries, photodetectors, gas sensing, and as well as optoelectronic devices [2-16]. When the layered MoS2 nanosheets are reduced to the zero-dimensional (0D) quantum dots (QDs) then the entirely different photo-physical properties of MoS2 arises because of its quantum confinement and edge effects [17–19]. MoS2 QDs have been extensively studied due to its low toxicity, high stability, abundance availability and excellent optical properties. The production of novel and advance MoS2 nanostructures, is the fundamental step towards their implications to the numerous applications in various divisions. This thesis holds synthesis of different MoS2 nanostructures by adopting the facile hydrothermal route. The hydrothermal/solvothermal method to synthesize MoS2 was opted, due to its ability to form nanostructures at high pressure and temperatures, ease in handling, uniform production of nanomaterials and high yield [20–22]. The MoS2 nanosheets and their quantum dots were prepared by altering the size of the particles and confining them in one or all axis to explore their properties comprehensively. The optical properties of MoS2 146 nanostructures (like absorption, photoluminescence and time-resolved photoluminescence) along with the different sensing and photocatalytic applications have also been discussed in detail. 7.2. Important findings of the research work The MoS2 nanostructures were hydrothermally synthesized and their constituent phase, phase purity and the crystallite size were analysed from the XRD and compared with the respective JCPDS files. The results were then confirmed by analysing HR-TEM images, for particle size and structure. The orderly patterned fringes confirmed the formation of crystalline structure of MoS2. The optical investigations through UV-visible absorption spectroscopy, PL spectroscopy and FTIR spectroscopy had been carried out for the prepared nanostructures to recorded their optical performances. The synthesized MoS2 QDs were then employed to detect the very explosive compound, 2,4,6-trinitrophenol (TNP), which has greater explosive strength than the very well-known explosive, trinitrotoluene (TNT). The TNP or picric acid is very acidic in nature and has been used in various industries, whose waste can cause severe health-related issues and destroy the soil and water resources. The primary detection of TNP of concentration as low as 5 μM, could be recorded through this sensing probe. The highest quantum yield (QY) of the prepared quantum dots i.e., 17% had been achieved among previously reported QDs through the same hydrothermal route. Then, the absorption and PL spectra were recorded of the MoS2 QDs and used to detect the lead metal ions (Pb2+) in water bodies. The Pb2+ ions can cause severe damage to the human brain and could be a preliminary cause of cancer. The detection of Pb2+ ions in water holds great significance for the sustainability of humanity. The detection of the Pb2+ ions was pursued the 147 greater sensitivity and larger selectivity among other metal ions in water. Therefore, certain experiments were carried out and the data has been represented wisely to demonstrate the MoS2 QDs as an efficient Pb2+ ion sensor. The detection of Pb2+ of concentration as low as 50 μM is possible through this probe. The synthesized nanosheets were then employed for the photodegradation of the organic pollutant, i.e., crystal violet (CV) dye in water. The photodegradation of CV dye was performed because of the bandgap of the nanosheets, which lies in the visible region and supports the environment by using the abundant solar energy for the decomposition of an organic pollutant. The presence of the CV dye in water is a severe threat to the environment and aquatic life. Therefore, its degradation holds great significance for the upliftment of a sustainable environment. The other transition metal dichalcogenides were also synthesized. The synthesis and characterization of tungsten disulfide (WS2) QDs were carried out. The bright blue fluorescence was observed when exposed to ultraviolet light (365 nm). The prepared QDs were then employed to detect the TNP compound in water. The sensitivity and selectivity of the WS2 QDs based sensor were calculated. The primary detection of TNP of concentration as low as 0.2 μM, could be recorded through this sensing probe.