Atomic-Scale Insights into the Conversion Chemistry of Metal Chalcogenides using Advanced Electron Microscopy for Structure-Property Correlation

Abstract

Conversion chemistry is a pivotal technique for transforming pre-synthesized nanocrystals into new materials with precise control over their properties. The investigation of phase transformations and conversions in 2D metal chalcogenides is of importance from both a fundamental and application perspective, as distinct electronic, optical, and structural properties emerge in different phases. The conversion of one phase to another with control of morphology and composition is challenging and requires tremendous optimization. In this thesis, various means to control and change the phase of 2D metal chalcogenides to form lateral/vertical heterostructures are discussed along with a detailed investigation of growth mechanism and morphology evolution during conversion reaction using advanced electron microscopy. Furthermore, synthesized heterostructures are evaluated for various applications in gas sensing and electrocatalysis. The firstchapter explores the phase transformations in mixed metal oxides, Mo0.5W0.5O3, by ex-situ heating in air at varying temperatures (RT-500°C). Atomic resolution HAADF-STEM imaging reveals that on increasing temperature, the initial orthorhombic phase of mixed metal oxides is first transformed into a partial hexagonal phase followed by a monoclinic phase with [100] as the growth direction. As-obtained phases are correlated with their gas-sensing activity. In the second chapter, the conversion chemistry of Mo₀.₅W₀.₅O₃ is demonstrated to produce a variety of 2D layered materials (MX₂ (M = Mo/W, X = S/Se)) by either sulfurization or selenization reactions. Partial conversions lead to the formation of a unique heterostructure comprising WO₃-MoS₂ (inaccessible through direct synthetic routes), which are important candidates for bifunctional electrocatalysts for water-splitting reactions. The third chapter illustrates the anion-exchange conversion in 2D layered material, SnS₂ to SnSe₂. Electron microscopy investigation reveals a sideways transformation, with the formation of SnS₂-SnSe₂ lateral heterostructure as an intermediate. The final product, SnSe₂, SnSe₂,exhibits a unique hexagonal nanoring morphology with an increase in lateral dimension compared to the template, SnS₂. These SnSe₂ nanorings exhibit excellent room-temperature selective NO₂ sensing. In all the above-mentioned methods of conversion, we utilized a two-step approach for the synthesis of heterostructures. However, novel heterostructures of 2D layered materials can also be generated in a single-step method by careful optimization of reaction thermodynamics and kinetics. In the fourth chapter, we have explored the synthesis of Bi₂Te₃-Sb₂Te₃ lateral heterostructures in a single-step wet chemical method and extended our synthesis for the generation of double-shell Bi₂Te₃-Sb₂Te₃-Bi₂Te₃ lateral heterostructures. Furthermore, the screw-dislocation-drive growth mechanism of these heterostructures is evaluated by employing cross-section STEM analysis. In the fifth chapter, we have developed techniques to explore the strain relaxation mechanism in the synthesized Bi₂Te₃-Sb₂Te₃ nanosheets. The strain-relaxed, uniformly bent nature of these nanosheets has been explored using conventional TEM diffraction techniques, and further, a highly defocused probe in HAADF-STEM is utilized to enhance channeling contrast in these nanosheets. 4D-STEM simulations provided conclusive evidence for the enhanced contrast and bent nature of the nanosheet. Overall, this thesis provides insights into the conversion reactions in metal chalcogenides and detailed investigations of interfaces and defects using electron microscopy.