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Dynamic Investigations and Applications of Multi-Nanomaterials Using Advanced Transmission Electron Microscopy Techniques
Heterogenous Catalysis is the mainspring of many chemical and energy transformation processes in several industries. The surface of a catalyst functions at the interface of the solid-gas phase or solid-liquid phase, paving in a pathway which is kinetically favorable. However, in certain reaction conditions, these catalysts tend to sinter which eventually deteriorates their catalytic activity. With the advent of Advanced Transmission Electron Microscopy (TEM) techniques, it has become possible to perform in-situ experiments at atomistic levels, thereby giving insights about the mechanism of sintering of catalytic materials in detail and also their wetting/de-wetting capabilities in virtue of the strong-metal-support-interaction (SMSI) and strong-metal-support-bonding (SMSB). Proper optimization and tuning of the aforementioned parameters and mechanisms could lead to the development of improved catalysts and functional nanostructures. In the first part of the study, in-situ laser heating with transmission electron microscopy (ILH-TEM) was utilized as a neoteric method to probe the thermal behavior and stability of metal (Pt, Au)-metal oxide (Fe3O4) heterodimer (HD) nanoparticles. The dynamic in-situ laser heating TEM experiments on the nanoparticles gave insights about their wetting and de-wetting capabilities. Au-Fe3O4 HD nanoparticles underwent a partial de-wetting procedure that can be used as a way to produce Janus particles, which was not observed in the Pt-Fe3O4 HD particles, providing new avenues towards dual nanoparticles development. The laser heating technique was shown to be a useful and novel method for such in-situ heating experiments to underpin future development of functional heterodimer nanoparticles.
The transmission electron microscopy techniques, coupled with dynamic impedimetric response analytical studies was extended towards a new family of two-dimensional materials - titanium carbide MXenes, Ti3C2Tx. They possess high surface area coupled with metallic conductivity and potential for functionalization, which make them especially attractive for the highly sensitive room-temperature electrochemical detection of gas analytes. However, they have not been thoroughly investigated for the detection of volatile organic compounds (VOCs), which hold high relevance for disease diagnostics and environmental protection. Furthermore, the insufficient interlayer spacing between MXene nanoflakes could limit their applicability and the use of heteroatoms as dopants could help overcome this challenge. The second part of the thesis will describe a method to synthesize the 2D material and prepare the sensors using silver electrodes to explore the response upon exposure to four selected volatile organic compounds comprising four different functional groups (ethanol, hexane, hexyl acetate and toluene). The Ti3C2Tx MXenes were synthesized and imaged using TEM and STEM to reveal 1nm thickness and open channels between each MXene flakes. Chemi-resistive detection results showed superior response of sulfur-doped titanium carbide MXenes to toluene compared to other VOCs’. Long term stability and cycling experiments indicated that the sensors were stable over a month and displayed a reproducible response upon 10 consecutive cycles. An extended branch of the study involving a more traditional hybrid sensors will also be discussed where a nanocomposite film composed of exfoliated MoS2, single-walled carbon nanotubes (SCNTs), is described for real-time detection of ethylene at levels as low as 100 ppb.
The third part of the thesis presents part of a collaborative work involving solid state battery materials where high resolution TEM and STEM, in conjunction with energy dispersive spectroscopy were used to explore and analyze the elemental composition of interphase of a hybrid material comprising perovskite electrolyte material Li0.33La0.57TiO3 (LLTO) and the spinel cathode material LiMn2O4 (LMO). The thesis highlights some of the many capabilities of advanced transmission electron microscopy, along with other state-of-the-art techniques in determining the nature and challenges of multiple nanomaterials with atomic sensitivity.