Date of Award
Doctor of Philosophy (PhD)
The discovery of bronze as an alloy of copper and tin is arguably the earliest form of material design, dating back thousands of years. In contrast, two-dimensional materials are new to the 21st century. The research presented in this dissertation is at the intersection of alloying and two-dimensional materials. I specifically study a class of two-dimensional materials known as transition metal dichalcogenides (TMDCs). Because of the large number of transition metals, there are many combinations of TMDCs that can be alloyed, making experimental exploration of the phase space of possible alloys unwieldly. Instead, I have applied first-principles methods to study the stability and properties of TMDC alloys. The primary focus of my work is the design of TMDC alloys for electrocatalysis applications, but other properties are also explored. Cumulatively, this work advances both the area of electrocatalysis and the general possibilities associated with TMDC alloys.
In the first section, I discuss my research on isostructural quasibinary TMDC alloys. First-principles methods are combined with thermodynamic analysis to predict the stability of alloys using special quasirandom structures: these predictions include new, miscible TMDC alloys and guidelines for the synthesis of immiscible TMDC alloys. These predictions are confirmed with temperature-dependent synthesis attempts performed by experimental collaborators. Our collaborators discover that one of the alloys I predicted to be miscible, Nb0.5Ta0.5S2, has excellent electrocatalytic performance for CO2 reduction and in a Li-air battery. The alloy is also shown to have a high breakdown voltage and a high thermal stability. As a follow-up, similar techniques are used to predict the stability of heterostructural TMDC alloys in the second section. These alloys are combinations of two TMDCs with different ground-state structural phases. Where available, results are compared to previous works obtained using cluster expansion methods.
In the next section, the concept of high entropy alloying is applied explicitly to layered TMDCs. I develop a simple metric to evaluate the stability of the alloys based on the temperature at which the equimolar high entropy alloy (HEA) has a lower free energy than all other equimolar decompositions. I use this to determine the stability of a small number of 4- and 5-component TMDC HEAs. Our experimental collaborators successfully synthesized the first layered HEAs based on these predictions. It is found that the 5-component TMDC HEA, (MoWVNbTa)S2, has excellent CO2 reduction activity with a low overpotential. I use first-principles calculations to show that the improved catalytic performance is caused by ultra-fast activity at a small number of active sites with exceptionally low energetic barriers.
In the penultimate section, a comprehensive theory on mapping the stability of HEAs from first-principles calculations is presented. This method involves the interpolation of a high-dimension and high-order polynomial to the calculated enthalpy of various equimolar alloys. This fitted enthalpy is then used to get an analytical expression for the Gibbs free energy which can be analyzed to determine temperature and composition dependent phase transition boundaries.
In the last section, we make a departure from alloy stability and electrocatalytic performance. In this section, the phase-dependent band gap engineering of Mo0.5Nb0.5S2 is studied. This immiscible alloy is synthesized at high temperatures, forming flakes of two distinct phases: 1T and 2H. The two phases are observed to be semiconducting and metallic, respectively. I use first-principles calculations to show that the band gap opening in the 1T phase results from electron transfer from Nb to Mo, facilitated by degeneracy lifting caused by local distortions. Electron diffraction patterns and annular bright-field scanning transmission electron microscope images are used to determine the existence of an incommensurate strain pattern in the 1T phase. We attribute this to a nearly-commensurate charge density wave phase and show results from temperature-dependent resistivity measurements that hint at further transitions to commensurate charge density wave phases at low temperatures.
Chair and Committee
Li Yang, Erik Henriksen, Sheng Ran, Bryce Sadtler,
Cavin, John Douglas, "Computational Design of Two-Dimensional Transition Metal Dichalcogenide Alloys and Their Applications" (2021). Arts & Sciences Electronic Theses and Dissertations. 2401.