Order Transitions in Liquid Crystals Coupled to Heterogeneous Catalytic Processes
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Amplification of catalytic processes into macroscopic scale can provide insights for designing programmable stimuli-responsive materials and enable rational designs of catalysts. Liquid crystals (LCs) are structured fluid phases that can amplify molecular and nanoscopic interactions, such as electrical double-layer interactions, binding of vesicles to proteins, and metal salt-ligand coordination interactions into optical outputs. Prior studies have shown that molecular transformations occurring at LC-catalytic interfaces can drive surface-ordering transitions in LCs. In this dissertation, we describe new designs of LC-catalytic interfaces that can report targeted catalytic transformations through ordering transitions in LC phases. The original research presented in this dissertation is organized in four chapters (Chapter 3-6). In Chapter 3, we introduce a new class of LC-solid interfaces, namely LC-photocatalytic interfaces, and explore if LC ordering can be coupled to surface photocatalytic processes. We demonstrate that surface-ordering transitions in LCs can report interfacial photocatalytic processes, such as the photo-oxidation of the LC to a new product, the co-adsorption of a reactant molecule (water) on the surface, and the non-monotonous effects of co-adsorption of water and LC on the reaction rate. Additionally, we report that the LC film introduces interfacial interactions on the photocatalytic surface, altering the local concentration and structure of water adsorbed at the LC-solid interface. These results establish that LCs offer a novel readout of photocatalytic transformations and provide approaches to modulate the photocatalytic activity. In Chapter 4 and 5, we demonstrate that tailored designs of chemically functionalized LCs can be used to study specific catalytic transformations. In Chapter 4, we report that alkyne-terminated LCs undergo sequential hydrogenation on palladium (Pd) surfaces and report their semi-hydrogenation to alkenes and total hydrogenation to alkanes through a surface-ordering transition and a bulk ordering transition in the LC, respectively. In Chapter 5, we study the hydrogenation of nitro-terminated LCs on Pd surfaces and demonstrate that the dynamic optical response of LCs can distinguish between the reactivity of metallic Pd and palladium hydride (PdHx) phases. In Chapter 6, we examine the effect of adsorbates- such as oxygen, hydrogen and PhCN (a surrogate molecule for a cyanobiphenyl LC)- on the surface structure of dilute Pd in gold (Au) alloys through a combination of electronic structure calculations and experimental measurements. Specifically, for PhCN, calculations predicted that in presence of 4/16 monolayers (ML) PhCN, Pd atoms preferred to be in an isolated state. Experimental measurements were performed to assess the surface structure of 0.1 ML Pd on Au surfaces in presence of adsorbates. Overall, these findings inform future studies exploring the effect of LC mesogens as adsorbates to induce aggregation/segregation in catalytic surfaces for novel designs of chemoresponsive LC sensors.