Uncovering Structure-Property Relationships with in-Situ Electrochemical Quantum Capacitance Spectroscopy

Abstract

Two-dimensional transition metal dichalcogenides (TMDs) have garnered significant attention due to their unique electronic properties and potential applications in various fields, including catalysis. Among these, monolayer molybdenum disulfide (MoS₂) has shown promise as a catalyst for the hydrogen evolution reaction (HER), which is crucial for sustainable hydrogen production. This thesis investigates the electrochemical performance and defect engineering of MoS₂ to enhance its catalytic efficiency. Chapter 1 provides a comprehensive introduction to two-dimensional materials with a focus on TMDs, emphasizing their electronic properties and potential as catalysts. It discusses the significance of hydrogen as a clean energy carrier and the role of electrochemical water splitting in hydrogen production. The chapter also reviews the use of platinum-based and TMD catalysts in HER, highlighting the need for alternative, cost-effective materials. Chapter 2 focuses on the measurement and manipulation of the density of states (DOS) and defect states in two-dimensional materials using a novel electrochemical quantum capacitance spectroscopy (EQCS) technique. This method enables the detection of defect states and band edges under ambient conditions, offering significant advantages over traditional techniques that require cryogenic temperatures and ultra-high vacuum. The EQCS method is demonstrated on monolayer MoS₂, revealing the influence of mechanical strain on the electronic structure and catalytic activity toward HER. Results show that small mechanical strains can significantly enhance HER performance, providing insights into the relationship between strain, electronic structure, and catalytic efficiency. Chapter 3 delves into the characterization of defect evolution in monolayer MoS₂ during prolonged electrochemical cycling. Cyclic voltammetry and the EQCS measurement are used to illuminate the functional properties and electronic structure change over time. Conductive atomic force microscopy (cAFM) and X-ray photoelectron spectroscopy (XPS) capture the formation and aggregation of sulfur vacancies, which are identified as key active sites for HER. Initially, the introduction of sulfur vacancies enhances catalytic activity by lowering the hydrogen adsorption energy. However, as cycling continues, vacancy clusters form, leading to a decline in catalytic performance. The chapter provides a detailed analysis of the defect states and their impact on the electronic structure, supported by computational modeling. The findings of this thesis underscore the critical role of defect engineering in optimizing the electrochemical properties of two-dimensional TMDs for catalytic applications. The development of the EQCS technique represents a significant advancement in the in-situ characterization of electronic structures, offering a powerful tool for future research. By elucidating the relationship between crystal structure, defects, electronic structure, and catalytic performance, this work paves the way for the rational design of high-performance functional materials.