Low Dimensional, Layered Transition Metal Dichalcogenides and Their Applications Toward Energy Applications - A Comprehensive Study on Buckled Materials
- Author(s): Chen, Yen-Chang
- Advisor(s): Tung, Vincent
- et al.
The research interests of two-dimensional (2D) materials have been explosively increased since the realization of unexpected single layer graphene in 2004. Leading by graphene, these decade-old materials, such as transition metal dichalcogenides (TMDs) have been extensively re-investigated due to their unique materials properties emanating from the 2D morphology. It is worth noting that all the exceptional intrinsic properties only present in the single-layered fashion, while the most significant challenge of scaling up these 2D materials in solution processing is aggregation that largely sacrifices the intrinsic properties.
In this dissertation, I will discuss the unique marriage between new physical understanding of the decades-old electrohydrodynamic (EHD) deposition and insights into exfoliation chemistry as the unprecedented and yet holistic approach to address long-standing challenges in synthesizing, and isolating graphene and molybdenum disulfide (MoS2) from bulk precursors into hierarchically structured functional composites with preserved and even enhanced materials properties, especially those relevant to energy applications, including surface area, conductivity, structural integrity and catalytic activity.
Here, the specialties of crumpled graphene nanostructures (CGN) were examined by integration into a photoelectrochemical cell. Compared to pristine TiO2 electrode, the combination of TiO2 and CGN electrode drastically boosted up the short circuit current by 200% and an improved fill factor of 15%, indicating that CGN could act as a current conductor. Another example was the crumpled molybdenum disulfide (c-MoS2) as a catalyst for hydrogen evolution reaction (HER). After dimensional transition, it not only preserved the intrinsic properties result from lithium interaction, such as metallic phase and abundant sulfur defects, but also generated permanent strain at the basal plane that leads to increasing number of active sites density, and thus leading to the superior metrics of both total electrode and intrinsic properties. Moreover, the processing temperature was at nearly room temperature compared to other processes, which required extreme processing conditions, and thus make it be beneficial to the large-scale production. This work represents a well-defined material science paradigm: the performance of a material depends on its properties, which in turn are a function of structures; furthermore, structures are determined by how the material was processed.