Experimental and Theoretical Investigation of a Reductant-Activated Methodology for Covalent Functionalization of 1T' Transition Metal Dichalcogenides MoS₂ and WS₂

Citation

Yan, Ellen (2021) Experimental and Theoretical Investigation of a Reductant-Activated Methodology for Covalent Functionalization of 1T' Transition Metal Dichalcogenides MoS₂ and WS₂. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/gcjw-bb78. https://resolver.caltech.edu/CaltechTHESIS:05242021-164534392

Abstract

Chemically exfoliated MoS₂ (ceMoS₂) is a two-dimensional layered transition metal dichalcogenide in the 1T' phase that can be synthesized by intercalating and exfoliating MoS₂ of the thermodynamically stable and relatively inert 2H phase. Several functionalization techniques have emerged in the past decade to functionalize both the 2H and 1T' phases, with growing interest given the array of applications for MoS₂ in optoelectronics, catalysis, sensing, bioimaging, drug delivery, and photothermal treatment. To aid in this effort, we expanded upon a recently reported covalent functionalization method by developing a reduction-activated methodology to functionalize ceMoS₂ using one-electron metallocenes and showed that the coverage of a functional group increases as the reduction potential increases, allowing for greater control of the coverage. Using density functional theory (DFT), we found that the coverage of the smallest functional group, a methyl, is expected to be limited to ~64% per MoS₂ due to the steric hinderance associated with the methylation of sulfur sites that are adjacent to more than one methyl group. We also found that a similar coverage trend can be observed when applying reduction-activated functionalization to ceWS₂, albeit with a lower coverage at every potential that can be explained using DFT calculations as a difference in the thermodynamic favorability of the reaction. Reductant-activated functionalization provides a driving force that enables ceMoS₂ and 2H-MoS₂ to be functionalized when it is otherwise unreactive with electrophiles. Conceptualizing the work herein as part of a redox-activated functionalization method, there is an abundance of opportunity to explore oxidant- and reductant-activated functionalization on other chalcogenides, pnictides, and materials in the carbon and boron groups using both solution oxidants and reductants, as well as electrode-based electrochemical methods. Further exploration of redox-activated techniques expands the functionalization toolbox and enables researchers to develop application-specific functional materials.

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