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- All Subjects: Nanoscience
- All Subjects: Lithium
- Creators: Crozier, Peter
- Status: Published
After atomic resolution images of Ru nanoparticles observed during CO oxidation were obtained, the shape and surface structures of these particles was investigated. A Wulff model structure for Ru particles was compared to experimental images both by manually rotating the model, and by automatically determining a matching orientation using cross-correlation of shape signatures. From this analysis, it was determined that most Ru particles are close to Wulff-shaped during CO oxidation. While thick oxide layers were not observed to form on Ru during CO oxidation, thin RuO2 layers on the surface of Ru nanoparticles were imaged with atomic resolution for the first time. The activity of these layers is discussed in the context of the literature on the subject, which has thus far been inconclusive. We conclude that disordered oxidized ruthenium, rather than crystalline RuO2 is the most active species.
Lithium ion batteries are quintessential components of modern life. They are used to power smart devices — phones, tablets, laptops, and are rapidly becoming major elements in the automotive industry. Demand projections for lithium are skyrocketing with production struggling to keep up pace. This drive is due mostly to the rapid adoption of electric vehicles; sales of electric vehicles in 2020 are more than double what they were only a year prior. With such staggering growth it is important to understand how lithium is sourced and what that means for the environment. Will production even be capable of meeting the demand as more industries make use of this valuable element? How will the environmental impact of lithium affect growth? This thesis attempts to answer these questions as the world looks to a decade of rapid growth for lithium ion batteries.
The major objective of this research program is to investigate the spatial resolution of the monochromated energy-loss signal in the transmission-beam mode and correlate it to the excitation mechanism of the associated vibrational mode. The spatial variation of dipole vibrational signals in SiO2 is investigated as the electron probe is scanned across an atomically abrupt SiO2/Si interface. The Si-O bond stretch signal has a spatial resolution of 2 – 20 nm, depending on whether the interface, bulk, or surface contribution is chosen. For typical TEM specimen thicknesses, coupled surface modes contribute strongly to the spectrum. These coupled surface modes are phonon polaritons, whose intensity and spectral positions are strongly specimen geometry dependent. In a SiO2 thin-film patterned with a 2x2 array, dielectric theory simulations predict the simultaneous excitation of parallel and uncoupled surface polaritons and a very weak excitation of the orthogonal polariton.
It is demonstrated that atomic resolution can be achieved with impact vibrational signals from optical and acoustic phonons in a covalently bonded material like Si. Sub-nanometer resolution mapping of the Si-O symmetric bond stretch impact signal can also be performed in an ionic material like SiO2. The visibility of impact energy-loss signals from excitation of Brillouin zone boundary vibrational modes in hexagonal BN is seen to be a strong function of probe convergence, but not as strong a function of spectrometer collection angles. Some preliminary measurements to detect adsorbates on catalyst nanoparticle surfaces with minimum radiation damage in the aloof-beam mode are also presented.
Electron energy-loss spectroscopy (EELS) is used to sense variations in the local concentration of amine moieties (defects believed to facilitate interfacial charge transfer) at the surface of a g-CNxHy flake. Using an aloof-beam configuration, spatial resolution is maximized while minimizing damage thus providing nanoscale vibrational fingerprints similar to infrared absorption spectra. Structural disorder in g-CNxHys is further studied using transmission electron microscopy at low electron fluence rates. In-plane structural fluctuations revealed variations in the local azimuthal orientation of the heptazine building blocks, allowing planar domain sizes to be related to the average polymer chain length. Furthermore, competing factors regulating photocatalytic performance in a series of Pt/g-CNxHys is elucidated. Increased polymer condensation in the g-CNxHy support enhances the rate of charge transfer to reactants owing to higher electronic mobility. However, active site densities are over 3x lower on the most condensed g-CNxHy which ultimately limits its H2 evolution rate (HER). Based on these findings, strategies to improve the cocatalyst configuration on intrinsically active supports are given.
In TiO2/CeO2-x photocatalysts, the effect of the support particle size on the bulk
anoscale properties and photocatalytic performance is investigated. Small anatase supports facilitate highly dispersed CeO2-x species, leading to increased visible light absorption and HERs resulting from a higher density of mixed metal oxide (MMO) interfaces with Ce3+ species. Using monochromated EELS, bandgap states associated with MMO interfaces are detected, revealing electronic transitions from 0.5 eV up to the bulk bandgap onset of anatase. Overall, the electron microscopy/spectroscopy techniques developed and applied herein sheds light onto the relevant defects and limiting processes operating within these photocatalyst systems thus suggesting rational design strategies.