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Design of Low-Dimensional Materials for Energy Storage, Computing, and Catalysis Based on Electronic Structure Considerations
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The known crystal structures of solids often correspond to the most thermodynamically stable arrangements of atoms. Yet, oftentimes there exist a richly diverse set of alternative structural arrangements that lie at only slightly higher energies and can be stabilized under specific constraints (temperature, pressure, alloying, point defects). Such metastable phase space holds tremendous opportunities for accessing nonequilibrium structural motifs and distinctive chemical bonding, and ultimately for the realization of novel function. In the first section of this dissertation, we explore challenges with the prediction, stabilization, and utilization of metastable polymorphs of Vv2Ov5. Specifically, we highlight the tunability of electronic structure, the potential richness of bonding motifs, and their implications for functional properties with a focus on their ability to serve as intercalation hosts for monovalent and multivalent cations. We have examined five metastable phases (ζ-, γ'-, ρ'-, ε'-, δ') of Vv2Ov5, which offer some specific advantages with respect to thermodynamically stable α-Vv2Ov5 in terms of a higher chemical potential difference (giving rise to a larger open-circuit voltage), providing access to “frustrated” oxygen coordination environments (underpinning low barriers for ion migration), and mitigating polaronic diffusion. We demonstrate that a wide diversity of cation diffusion pathways can be accessed within 1D tunnel-structured and 2D layered metastable polymorphs by alteration of the cross-linking of layers, stacking sequence of layers, and the layer thickness. The ζ-Vv2Ov5 polymorph predicted to be an excellent insertion host for multivalent cations has indeed been experimentally realized and is this time the best-in class cathode material for Mg batteries in terms of showing an unparalleled combination of high voltage (1.65 V vs. Mg), high capacity (100 mAh/g), and excellent cyclability (>100 cycles). In the second section of this dissertation, we examine a 1D tunnel-structured β′- CuvxVv2Ov5 phase, which exhibits a metal—insulator transition that is strongly dependent on the copper stoichiometry, x. Density functional theory (DFT) calculations, photoemission spectroscopy, and single crystal diffraction have been used to probe the mechanistic underpinnings of the metal-insulator transition. We demonstrate the melting of this polaronic state as a result of a subtle rearrangement in the site occupancies and atom positions of copper ions results in the delocalization of the electrons in the charge ordered network, ultimately manifesting in metallic behavior. The switching behavior observed in individual nanowires of β'-CuvOv5xVv2Ov5 has parallels to the spiking behavior of neurons, and given the highly energy efficient electronic phase transition underpinned by preservation of the structural framework, is a potential vector for neuromorphic computing. Water splitting, the sum of water oxidation and hydrogen evolution half-reactions, remains a formidable challenge since it requires the concerted transfer of four electrons and four protons. The 2H polymorph of MoSvv2, a semiconducting transition metal dichalcogenide, has gained prominence as a catalyst for the hydrogen evolution reaction (HER) but requires a much greater overpotential as compared to Pt. Specifically, the edges of MoSv2 nanostructures are known to be the active sites for hydrogen evolution reaction. However, modification of edge sites to reduce the enthalpy of hydrogen adsorption has been stymied by the absence of a precise understanding of the atomistic and electronic structure of active sites. In the third section of this dissertation, first-principles DFT calculations along with element-specific X-ray absorption spectroscopy and imaging have been used to probe edge electronic structure, which has further been correlated to catalytic performance. Modeling of excited state spectra allows for assignment of X-ray absorption features to specific structural and bonding motifs. In addition, the final section demonstrates that the incorporation of Se-dopants provides a powerful means to modulate edge electronic structure and reactivity. The role of Se atoms is to increase the valance band maximum (VBM) for facile O2 evolution and decrease the conduction band minimum (CBM) for facile Hv2 evolution.
2D transition-metal chalcogenides
Parija, Abhishek (2019). Design of Low-Dimensional Materials for Energy Storage, Computing, and Catalysis Based on Electronic Structure Considerations. Doctoral dissertation, Texas A&M University. Available electronically from