Structural, Electronic and Catalytic Properties of Graphene-supported Platinum Nanoclusters

Carbon materials are predominantly used as catalytic supports due to their high surface area, excellent electrical conductivity, resistance to corrosion and structural stability. Graphene, a 2D monolayer of graphite, with its excellent thermal, electronic and mechanical features, has been considered a promising support material for next generation metal-graphene nanocatalysts. The main focus of this dissertation is to investigate the properties of such metal-graphene nanocomposites using computational methods, and to develop a comprehensive understanding of the experimentally observed enhanced catalytic activity of graphene-supported Platinum (Pt) clusters. In particular, we seek to understand the role of graphene supports on the ground-state morphology and the electronic structure of graphene-supported Pt nanoparticles, which correlate strongly with their catalytic activity. First, through a series of empirical potential and density functional theory (DFT) calculations, we determine low-energy isomers of Pt nanoclusters on pristine and defective graphene. Our results indicate that point defects in the graphene support enhance the cluster-support interaction, increasing their stability and significantly alter their electronic properties. Next, we investigate the support effects on CO and O adsorption on graphene-supported Pt13 nanoclusters. Defective-graphene-supported Pt13 nanoclusters bind CO and O more weakly than clusters on pristine graphene or unsupported clusters. Additional ab initio MD calculations on CO-saturated Pt13 nanoclusters show that support defects are crucial in stabilizing Pt13 clusters at high CO-coverages; in contrast, Pt13 clusters supported on pristine graphene desorb upon CO saturation, leading to potential catalyst loss. Finally, we examine the support effects on the CO oxidation reaction on graphene-supported Pt13 nanoclusters. A detailed study of the CO oxidation kinetics is undertaken in the high CO coverage regime, locating transition states and minimum energy pathways. The relevant kinetic mechanism is sampled at various surface sites on clusters bound at support defects and on unsupported clusters. The results of this study show that strong cluster-support interactions can substantially reduce the reaction barrier for CO oxidation on graphene-supported clusters compared to unsupported ones. Our studies suggest that defect engineering of graphene could serve to enhance the catalytic activity of ultra-small Pt clusters, opening up another dimension for rational design of catalytic materials

Density-Functional Tight-Binding Simulations of Curvature-Controlled Layer Decoupling and Band-Gap Tuning in Bilayer MoS2

Monolayer transition-metal dichalcogenides (TMDCs) display valley-selective circular dichroism due to the presence of time-reversal symmetry and the absence of inversion symmetry, making them promising candidates for valleytronics. In contrast, in bilayer TMDCs both symmetries are present and these desirable valley-selective properties are lost. Here, by using density-functional tight-binding electronic structure simulations and revised periodic boundary conditions, we show that bending of bilayer MoS2 sheets breaks band degeneracies and localizes states on separate layers due to bendinginduced strain gradients across the sheets. We propose a strategy for employing bending deformations in bilayer TMDCs as a simple yet effective means of dynamically and reversibly tuning their band gaps while simultaneously tuning valley-selective physics.peerReviewe