Synthesis and applications of chalcogenide nanomaterials [electronic resource]

Metal chalcogenides is a large family of functional materials with rich physical and chemical properties depending on their compositions, dimensions and phases. They have a broad range of applications in electronics, optoelectronics and energy conversion/ storage. The possibility to prepare these materials into low-dimensional nanostructures may allow the access to novel properties that are absent in their bulk counterparts. Such a development, however, requires a reliable approach to controllably synthesize chalcogenides into nanostructures. Here, we primarily employ a horizontal tube furnace reactor to grow such chalcogenide nanomaterials. These nanostructures serve as a material platform to expand the applications of this family of materials towards new areas. We first describe the opportunity to study topological insulator electronic properties in several chalcogenide materials. Topological insulators are narrow-gap semiconductors with metallic electronic states that are always present at their surfaces. The unique property associated with such surface carrier is that the spin direction is determined by its moving direction. The unusual spin texture gives rise to many unusual physical properties. Several chalcogenides are identified as topological insulators based on theoretical calculations and spectroscopic measurements. However, there are many challenges to make use of their exotic properties in electrical measurements. We propose topological insulator nanostructures, with large surface-to-volume ratio, enhance the surface properties in electron transport process. Our systematic transport measurements reveal the signature of the surface carriers on nanostructures of these chalcogenides. Combined with detailed structural and compositional analyses, our studies addressed some material challenges to probe and access the surface electronic properties on these chalcogenide topological insulators. Some chalcogenide nanomaterials also serve as attractive electrocatalysts for water splitting. With the interest to use hydrogen as a sustainable and carbon-free energy carrier, it is attractive to explore and optimize catalyst materials suitable for electrochemical water reduction. Certain layered dichalcogenides, such as MoS2 and WS2, are chemically active electrocatalysts for hydrogen evolution reaction (HER), with the promise to replace noble metal catalysts like Pt. Previous studies have identified the edges of the molecular layers are catalytically active, which opens up a rational pathway to boost the activity by increasing the exposed edge sites. We developed a synthesis technique to produce edge-rich thin films and nanostructures. The structural engineering and further chemical doping process allow us to access novel MoS2 catalyst with high activity for HER. In addition, we also identified a new group of first-row transition metal dichalcogenides (ME2, M = Fe, Co, Ni ; E = S, Se) catalytically active towards HER. These catalysts exhibit excellent activity and stablilty that expand the family of non-precious HER catalysts.

Keywords: Materials Science and Engineering.
Advisor: Cui Yi
Hwang Harold Yoonsung
Shen Zhi-Xun advisor. Thesis advisor