Date

2018

Document Type

Dissertation

Degree

Doctor of Philosophy

Department

Materials Science and Engineering

First Adviser

Kiely, Christopher J.

Abstract

Materials with nanoscale dimensions often display unique optical, electronic and chemical properties that depend on their size, structure and composition. For instance, semiconductor nanocrystals, or quantum dots (QDs), exhibit quantum confinement effects which can lead to tunable electronic and optical properties. Similarly, nanoscopic metal structures ranging from nanoparticles to sub-nm clusters and even individual atoms can display excellent catalytic properties when dispersed on a suitable support, due to their high surface area-to-volume ratio and modified electronic properties. In order to develop meaningful structure-property correlations and optimize the performance of such nanomaterials, their structural and compositional features need to be carefully characterized by state-of-the-art aberration corrected analytical electron microscopy (AC-AEM).The first half of this dissertation concerns the study of semiconductor QDs prepared via a novel aqueous-phase biomineralization route that offers lower environmental impact and decreased economic costs compared to more traditional chemical synthesis routes. In particular, we show how the optical properties of these biosynthesized QDs have been understood and improved by a process of ‘microscopy informed nanomaterial design’. Firstly, cadmium sulfide QDs biosynthesized using an engineered bacterial strain of Stenotrophomonas maltophilia have been structurally and chemically analyzed by aberration corrected STEM. This has allowed us to establish a direct correlation between mean nanocrystal size and the particle growth time, which in turn affords us good control and tunability over the band gap energy and absorption/fluorescence behavior of the resultant QDs. Next, a single enzyme produced by the bacterium, namely cystathionine ?-lyase (smCSE), is shown to be responsible for both inducing CdS mineralization and templating nanocrystal growth. The production of size- and structure-controlled CdSe and CdSe-CdS core-shell QDs is also shown to be possible using this same enzyme. The palette of QD materials available from this biomineralization approach is then further expanded to cover other metal sulfide nanocrystals, such as PbS and PbS-CdS core-shell QDs, and CuInS2 (ternary) and (CuInZn)S2 (quaternary) alloy QDs. Detailed HR-TEM, STEM and XEDS measurements on these PbS- and CuInS2-based QD systems are described and then correlated to their functional properties. Finally, it is shown that this aqueous, room temperature biomineralization strategy can also be adapted to produce cerium-based oxide materials by employing a modified form of silicatein as the biosynthesis agent. In particular, AC-AEM analysis is used to validate the successful biomineralization of sub-3 nm fluorite-type CeO2 and CeO2-ZrO2 nanocrystals, which are amongst the smallest reported to date. Their catalytic properties and thermal stability are also explored in relation to the CO oxidation reaction.The second half of this dissertation presents three case studies which serve to demonstrate how the ability to visualize the structure and dispersion of metallic nano-catalysts by AC-AEM, when coupled with spectroscopic information obtained from other complementary techniques, can be used to advance our mechanistic understanding of how these catalysts operate. In the first example, HAADF-STEM analysis is coupled with in-situ X-ray absorption fine structure (XAFS) analysis to study a gold-on-carbon catalyst which is commercially used for the hydrochlorination of acetylene to produce vinyl chloride monomer. We unequivocally demonstrate that a mixture of atomically dispersed Au+ and Au3+ cations constitute the active centers in this Au/C catalyst system. In a second example, physically separate cobalt and platinum nanoparticles supported on either ?-Al2O3 or BaZrO3 are examined as CO2 methanation catalysts. Detailed HAADF-STEM analysis is employed to show how the Pt re-distributes during catalyst activation and ends up atomically decorating the more strongly anchored Co particles. Finally, structure sensitivity is demonstrated for nickel nanoparticles supported on SiO2 which were designed as catalysts for CO2 hydrogenation. A systematic series of Ni/SiO2 catalysts with different Ni loadings has been carefully characterized by HAADF-STEM imaging to obtain reliable Ni particle size distribution data which take into account corrections for metal oxidation effects. The same set of Ni/SiO2 materials were then also characterized by operando FT-IR and quick X-ray absorption spectroscopy. By correlating the complementary data obtained from these three techniques two distinct, particle size dependent pathways are identified for this CO2 hydrogenation reaction.

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