UC Davis

Global population increase has highlighted the scarcity of resources, specifically energy and potable water, and emphasized the need to develop new technologies to help meet these growing needs. Additionally, the impact of the historic use of carbon-intensive fuel sources to meet energy demand has become increasingly apparent through the environmental repercussions of anthropogenic climate change. Since energy and water security intrinsically consider cost and access to resources, disruptive technologies to improve energy and water security must exhibit translatability, scalability, and low-cost. Harnessing abundant renewable energy sources such as solar, hydro, and wind is a promising solution to carbon-intensive energy production, while also providing low-cost energy. The coupling of renewable energy resources with energy-intensive and expensive water remediation processes such as reverse osmosis, can lower their costs. However, the efficient storage of intermittent renewable resources during peaks hours of production for delivery of energy during times of peak demand has proven a challenge for decades. Furthermore, the development of robust membrane materials that drastically impact the cost of water remediation, especially considering increasing water pollutants from anthropogenic activity, have yet to be realized in recent years. As a result, research and development of more efficient energy storage and robust membrane technologies is vital for meeting society’s growing demand for energy and water remediation without increasing anthropogenic greenhouse gas production. A thorough understanding of both fundamental cationic-host relationships and gaseous precursor-substrate interactions are necessary for the intelligent design of next generation energy storage and membrane materials. This dissertation work focuses on the design of synthetic parameters for controlled material property engineering, the development of electrochemical methods elucidating cationic behavior within host materials, structural and electronic changes to host material as a function of cationic identity and amount, as well as the synthesis of a new compositional space. In this work, emphasis was placed upon identifying reaction conditions in a metal organic chemical vapor deposition system (MOCVD) that allows control over layer stacking and defect population in 2D metal chalcogenides. The MOCVD is a unique chemical vapor deposition system in which the concentrations of metal organic precursors in the gaseous phase can be finitely controlled to deliver highly specific precursor ratios for thin film synthesis. Additionally, the aqueous intercalation of cation(s) within 3D metal chalcogenides to observe cationic behavior as well as structurally and electronically induced changes to the host was investigated. These structural and electronic changes were investigated through a suite of electrochemical experiments to observe cationic behavior, X-ray absorption spectroscopy (XAS) for electronic changes, X-ray diffraction (XRD) for structural changes, and theoretical computational modeling to elucidate energetics of various scenarios. All these components aided in the development of an expanded fundamental understanding of material synthesis design and cationic behavior for remediation and energy storage applications, respectively. Chapter 1 presents an overview of the state-of-the-art in water desalination membrane and energy storage technology. There is also an introduction to the 2D and 3D molybdenum chalcogenide materials focused on in this work, inclusive of their structure, properties and historical applications. Chapter 1 also presents the current shortcomings of current state-of-the-art technologies and how the molybdenum chalcogenides presented have potential to overcome those barriers. Chapter 2 describes the development of experimental parameters for 2D transition metal dichalcogenides (e.g., MoS2 and MoSe2) in a MOCVD system. Major principles affecting crystal growth in the MOCVD system (thermodynamics, kinetics, and mass transport) and the need to deconvolute the “black box” of synthesis is also discussed. Focus is placed on using our understanding of the major principles affecting crystal growth to develop methodology in which wide-area coverage, layer-by-layer growth, and defect population are controlled. Lastly, we discuss major findings and current issues in the field, mainly the reproducibility of studies, and uniform reporting and open-access databases can mitigate that. Chapter 3 details the structural and electronic changes of the 3D molybdenum chalcogenide, Mo6S8, as a function of stoichiometric amount of Cu content. Electrochemical responses were correlated to structural changes and stoichiometry via ex-situ XRD analysis. Electronic changes to the host Mo6S8 material as a function of Cu content was observed via changes in character of the S K-pre-edge feature which was analyzed using X-ray Absorption Near-Edge Structure (XANES). We observed a depression of the S K-pre-edge feature as a function of increasing Cu content and posed using XANES pre-edge features as method to monitor metal content in other materials. Theory Density Functional Theory (DFT) and density of states (DOS) calculations were also employed by collaborators at CU Boulder to discuss the metastability of Cu4Mo6S8¬ at open-circuit voltage and metal-to-semiconductor bandgap transitions as a function of Cu content in the Mo6S8 system (i.e., CuxMo6S8; x=1-4). Chapter 4 outlines the dual intercalation of both Cu and Zn within the binary Mo6S8 system via electrochemical methods in aqueous electrolytes. Unique structural changes were identified via ex-situ XRD where apparently Cu and Zn had a similar structural effect on the host material due to their similarity in ionic radius, despite their difference in charge (Cu+1 vs Zn2+). Despite both Cu and Zn being present in the host material, confirmed via Energy-Dispersive X-ray Spectroscopy (EDX) and X-ray Photoelectron Spectroscopy (XPS), the resulting XRD pattern resembled that of pure-phase Cu2Mo6S8. Lastly, this work reports the synthesis of the CuxZnyMo6S8 composition for the first time.