Understanding the Structure-Function Relationship in Peptide-Enabled High Entropy Alloy Nanocatalysts

The structural complexity in high entropy alloy nanocatalysts (HEAs), afforded by the homogeneous mixing of five or more elements, has resulted in a limited understanding about the origin of their promising electrocatalytic properties. This thesis investigates the structure-function relationship in HEAs using advanced material characterization techniques. At first, a methodology for resolving the atomic-scale structure of peptide-enabled HEAs was developed using high-energy X-ray diffraction (HE-XRD) coupled with atomic pair distribution function (PDF) and reverse Monte Carlo (RMC) simulations, yielding structure models over the length scale of HEAs. Coordination analysis of the structure models revealed a multifunctional interplay of geometric and electronic attributes of surface atoms in HEAs that was responsible for the catalytic activity enhancement during the methanol electrooxidation reaction. Using the methodology for resolving the atomic scale structure of HEAs and peptide sequence engineering, the structure-function relationship of model PtPdAuCoSn HEAs during ethanol electrooxidation reaction (EOR) was studied. Compositional analysis of the PtPdAuCoSn HEA structure models revealed distinct miscibility characteristics that were attributed to the unique biotic-abiotic interactions. Analysis of the structure models identified the rapid dehydrogenation of CH3CHO intermediate into CH3COads in an optimized adsorption configuration as the contributing factor for the high selectivity towards CH3COO- in PtPdAuCoSn HEAs. Armed with these insights, a study was designed for understanding the effect of changing the concentration of Pt in the structure-function relationship of PtPdAuCoSn HEAs using spatiotemporal structural insights from in-situ PDF. The structure models demonstrated a degree of metastability as a function of their corresponding configurational entropy. Analysis of the structure models revealed that high selectivity towards CH3COO- in PtPdAuCoSn HEAs during EOR originates from the enhanced distribution of Pd and Co surface atoms. In summary, this thesis uses atomic PDF and RMC simulations to draw structure-function correlations in HEAs, presenting a path forward for developing strategies for the rational design of HEAs. Through collaborative efforts from theoreticians and experimentalists, the methodology presented here can form the basis for accelerating the discovery of promising HEA configurations for emerging electrocatalytic applications

Molybdenum Sulfide Prepared by Atomic Layer Deposition: Synthesis and Characterization

Molybdenum disulfide (MoS2) is the prototypical two-dimensional (2D) semiconductor. Like graphite, it has a layered structure containing weak van der Waals bonding between layers, while exhibiting strong covalent bonding within layers. The weak secondary bonding allows for isolation of these 2D materials to single layers, like graphene. While bulk MoS2 is an indirect band gap semiconductor with a band gap of ~1.3 eV, monolayer MoS2 exhibits a direct band gap of ~1.8 eV, which is an attractive property for many opto-electronic applications. Atomic layer deposition (ALD) has been used to grow amorphous films of MoS2 using molybdenum chlorides and carbonates, however many of these molybdenum chemistries require high temperature vapor transport as they are solids at room temperature. We demonstrate the first ALD of MoS2 at 200 ℃ using molybdenum hexafluoride (MoF6), a liquid at room temperature, and hydrogen sulfide (H2S). in situ quartz crystal microbalance measurements were used to demonstrate self-limiting chemistry for both precursors, which is the hallmark of ALD. The deposited films were amorphous, and after annealing in hydrogen, crystalline MoS2 was discernable. The nucleation and early stages of MoS2 ALD on metal oxide surfaces were investigated using in situ Fourier transform infrared (FTIR) spectroscopy. The formation of Al-F and MoOF4 seem to initially form, but after H2S is introduced sulfate species begin to appear. This competition for oxygen seems to inhibit growth initially, until the oxygen at the surface is consumed and steady state growth occurs. To understand the structure of the amorphous films, X-ray absorption spectroscopy (XAS) and high-energy X-ray diffraction (HE-XRD) experiments were performed at the Advanced Photon Source (APS) at Argonne National Laboratory (ANL). Contrary to previous findings, the MoS2 structure was found to be sulfur rich; however, the atomic coordinations of Mo and S atoms bond distances matched standards. Interestingly, the Mo-Mo coordinations were much lower than reference structures, which could explain the lack of or very weak Raman vibrational modes seen in many as-deposited ALD MoS2 films. Experimental data were consistent with films containing clusters of a sulfur rich [Mo3S(S6)2]2- phase, but after annealing in H2 and H2S, these clusters decompose forming a layered MoS2 structure. Understanding these complex surface interactions of nucleation, growth, and phase transformations is necessary to enable synthesis of high quality MoS2 for use in future microelectronics