1,720,981 research outputs found
Surface-mediated mechanisms for defect engineering in zinc oxide
Get PDFThe technological usefulness of a solid often depends upon the types and concentrations of the defects it contains. In semiconducting metal oxides like zinc oxide, the concentration and diffusion of oxygen point defects, like interstitials and vacancies, play a central role in various physical phenomena, such as gas sensing, bipolar switching, photoluminescence and photocatalysis. Defect engineering in metal oxides aims at manipulating material properties through controlling the defects’ type, concentration, charge, spatial distribution, and mobility.
A specific challenge that inhibits performance improvement in metal oxide devices for microelectronics, photonics, and photocatalysis usages is that bulk oxygen vacancies (VO) are typically numerous and serve as carrier recombination centers or electron current scatterers. One solution suggested by our laboratory is to thermally inject highly mobile charged oxygen interstitials (Oi) through metal oxide surfaces from the gas phase to annihilate VO in the underlying bulk. Developing novel mechanisms to control such diffusion process would be crucial in tailoring material defect chemistry for real life applications. The present work demostrates two special surface-based control mechanisms for this purpose in the case of zinc oxide: near-surface electrostatics and the chemical state of surface active sites.Submission published under a 24 month embargo labeled 'Closed Access', the embargo will last until 2018-12-01The student, Ming Li, accepted the attached license on 2016-11-22 at 20:12.The student, Ming Li, submitted this Dissertation for approval on 2016-11-22 at 20:27.This Dissertation was approved for publication on 2016-11-23 at 13:24.DSpace SAF Submission Ingestion Package generated from Vireo submission #10302 on 2017-02-28 at 14:42:01Made available in DSpace on 2017-03-01T17:01:33Z (GMT). No. of bitstreams: 2
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Previous issue date: 2016-11-23Embargo set by: Seth Robbins for item 98701
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Surface Coverage Analysis Reveals Potential-Determining Heterolytic Reactions during Thermocatalytic Aerobic Glucose Oxidation
No full textElectric fields that form spontaneously at catalytic solid-liquid interfaces reflect the kinetics of surface reactions, coverage of reactive intermediates, and the nature of the microenvironments that encompass active sites. The measurement of these fields via the electrode potential of the catalyst and their interpretation can reveal mechanistic features of reactions that are not accessible by other methods. Here, the aqueous phase aerobic oxidation of glucose over carbon-supported platinum nanoparticles provides a representative model to demonstrate these concepts. Coupled analysis of steady-state rates for glucose oxidation and in situ open-circuit potentiometry obtained across a wide range of reactant concentrations (0.05-1 M glucose, 22-2170 kPa O2, 353 K) reveal the fundamental connections between the electrode potential of the catalyst (E cat) and the kinetics of surface reactions. The interpretation of these phenomena through networks of Faradaic and non-Faradaic elementary steps shows that differences in scaling relationships among E cat, rates, and reactant concentrations signify transitions among kinetic regimes and dominant surface intermediates. Redox reactions with high degrees of potential control (i.e., those with the greatest contribution to determining E cat) largely involve prevalent surface intermediates implied by the analysis of product formation rates. However, deviations between experimental observations and predictions obtained from mixed-potential theory show contributions from homolytic reaction pathways that emerge in response to high barriers for comparable heterolytic processes. These analyses demonstrate that concurrent interpretations of E cat and steady-state kinetics yield a deeper understanding of chemical phenomena at charged solid-liquid interfaces and reveal mechanistic features not otherwise evident.
Probing the electrochemical dynamics of soluble redox active polymers
Get PDFIt is an exciting time for exploring the synergism between the chemical and dimensional properties of redox nanomaterials for addressing the manifold performance demands faced by energy storage technologies. Our groups are interested in the development and implementation of a new strategy for non-aqueous flow batteries (NRFBs) for grid energy storage. Our motivation is to solve major challenges in NRFBs, such as the lack of membranes that simultaneously allow fast ion transport while minimizing redox active species crossover between anolyte (negative electrolyte) and catholyte (positive electrolyte) compartments. This pervasive crossover leads to deleterious capacity fade and materials underutilization.
Redox active polymers (RAPs) are highlighted as soluble nanoscopic energy storing units that enable the simple but powerful size-exclusion concept for NRFBs. Crossover of the redox component is suppressed by matching high molecular weight RAPs with simple and inexpensive nanoporous commercial separators. In contrast to the vast literature on the redox chemistry of electrode-confined polymer films, studies on the electrochemistry of solubilized RAPs are incipient. Here, viologen-, ferrocene- and nitrostyrene-based polymers in various formats exhibit properties that make amenable their electrochemical exploration as solution-phase redox couples.
A main finding is that RAP solutions store energy efficiently and reversibly while offering chemical modularity and size versatility. Beyond the practicality toward their use in NRFBs, the fundamental electrochemistry exhibited by RAPs is fascinating, showing clear distinctions in behavior from that of small molecules. Whereas RAPs conveniently translate the redox properties of small molecules into a nanostructure, they give rise to charge transfer mechanisms and electrolyte interactions that elicit distinct electrochemical responses. To understand how the electrochemical characteristics of RAPs depend on molecular features, including redox moiety, macromolecular size, and backbone structure, a range of techniques has been employed by our groups, including voltammetry at macro- and microelectrodes, rotating disk electrode (RDE) voltammetry, bulk electrolysis, and scanning electrochemical microscopy (SECM).
Herein, we characterize the charge transfer mechanisms, identify the impact of backbone tether length and structure, demonstrate the role of the composition of the supporting electrolyte, and explore how electrostatic interactions and polyelectrolyte dynamics all affect the reactivity of soluble RAPs. Finally, new tools to study the energy storage capabilities of bulk RAP solutions and the possibility to interrogate single entities using SECM methods are highlighted as promising technologies to advance the electrochemical characterization of nanostructured macromolecular redox architectures.Submission published under a 24 month embargo labeled 'Closed Access', the embargo will last until 2019-08-01The student, Mark Burgess, accepted the attached license on 2017-06-30 at 07:29.The student, Mark Burgess, submitted this Dissertation for approval on 2017-06-30 at 07:42.This Dissertation was approved for publication on 2017-06-30 at 12:13.DSpace SAF Submission Ingestion Package generated from Vireo submission #11286 on 2017-09-29 at 11:14:24Made available in DSpace on 2017-09-29T16:39:17Z (GMT). No. of bitstreams: 3
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Previous issue date: 2017-06-30Embargo set by: Colleen Fallaw for item 103396
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Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Effects of Catalyst Hydrophobicity on Rates and Selectivities of Alkene Epoxidation
Get PDFEpoxides are essential for the production of various plastics, fragrances, and pharmaceuticals, but conventional oxidants used for the large-scale synthesis of epoxides produce toxic co-products or over-oxidize to form large amounts of carbon dioxide, a greenhouse gas. Highly disperse titanium metal catalysts activate hydrogen peroxide (a green oxidant) for alkene epoxidation, but there is currently not an understanding of how the hydrophilicity (i.e., the silanol density) of such materials affects how quickly and selectively they catalyze epoxidation. Here, a series of titanium zeolite catalysts with varying silanol density but constant pore size and metal content is synthesized via post-synthetic modification of aluminum zeolite samples. The hydrophilicity of these materials is quantitatively determined, and a combination of kinetic, thermodynamic, and spectroscopic measurements are then used to show that the reactivity of these catalysts depends on their hydrophilicity. In particular, activation enthalpies for 1-octene epoxidation decrease with the density of silanols but are accompanied by a concomitant increase in entropy losses. For liquid-phase 1-octene epoxidation in acetonitrile, reaction barriers decrease by 7.7 kJ mol-1 but entropic losses increase by 22.4 J mol-1 K-1 when the catalyst structure is changed from highly-defective to defect-free, which manifests in turnover rates increasing by a factor of 500. Thus, the competing enthalpic and entropic factors must be balanced by carefully selecting the optimal silanol density for a given set of process conditions (e.g., reaction temperature). Together, these findings provide rational design criteria for the design of catalysts to be used in environmentally-friendly epoxide synthesis.Open Restriction set for Item 106009 on 2018-05-22T21:51:00Z with date null by [email protected] by Billy Tringali ([email protected]) on 2018-05-22T21:57:14Z
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Previous issue date: 2018-04Army Research Office #W911NF-18-1-0100Ope
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Development of new pyrochlore-type electrocatalysts for oxygen evolution reaction in acid
Get PDFA major challenge for the commercialization of water electrolyzer using proton exchange membrane (PEM) is the development of acid-stable, active catalysts for the oxygen evolution reaction (OER). In addition to discovering new catalysts, a fundamental understanding of OER activity and stability origin is also critical.
This dissertation focuses on: (1) identifying active and acid-stable OER catalysts in comparison to the state-of-the-art RuO2 and IrO2 catalysts; (2) studying the intrinsic material properties that yield high OER activity and stability; and (3) developing synthetic techniques to enhance the OER activity of a catalyst. The first part of the thesis concentrates on new material synthesis and methods to analyze activity and stability. Next, both theoretical calculations and experimental analysis are used to study the material properties. The last part discusses approaches that lead to high surface area and enhanced catalytic performance.
Specifically, pyrochlore-type ruthenium (Ru) and iridium (Ir) based catalysts were synthesized and characterized for their OER performance. Introducing yttrium (Y) in these pyrochlores increased the overall activity and stability. To explain this behavior, X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations were used to study the electronic and structural properties of these materials. Additionally, a range of iridium-based pyrochlores with lanthanide (La) series metals were studied to identify the important effects of the A-site metal atoms on the OER catalytic properties.
To synthesize high surface area, porous yttrium ruthenate pyrochlores, two techniques were utilized: polymeric entrapment synthesis and acid porogen strategy. The polymeric entrapment reduced the synthesis temperature, whereas the addition of perchloric acid generated pores during our established sol-gel synthesis. These materials showed enhanced OER activity, attributed to higher surface area.
The results in this dissertation provide new insights towards the design and development of active and stable OER catalysts.Submission published under a 24 month embargo labeled 'Closed Access', the embargo will last until 2021-08-01The student, Pei-Chieh Shih, accepted the attached license on 2019-07-03 at 22:14.The student, Pei-Chieh Shih, submitted this Dissertation for approval on 2019-07-03 at 22:24.This Dissertation was approved for publication on 2019-07-08 at 08:26.DSpace SAF Submission Ingestion Package generated from Vireo submission #14169 on 2019-11-26 at 14:01:33Made available in DSpace on 2019-11-26T20:58:40Z (GMT). No. of bitstreams: 3
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Previous issue date: 2019-07-08Embargo set by: Seth Robbins for item 113051
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Computational electronic structure studies of novel condensed matter phases
Get PDFThis dissertation compiles the bulk of my work as a PhD student in the research group of Professor Prashant K. Jain at University of Illinois at Urbana-Champaign. My research was exclusively in the field of theoretical chemistry and materials science: I employed high-performance computing tools to perform electronic structure investigations of novel crystalline materials synthesized, some for the very first time, in the group. My placement in the experimentally-focused Jain group afforded multiple opportunities in which the discoveries of my fellow group members prompted me to conduct stand-alone or collaborative theoretical investigations of new nanomaterials. A summary of the experimental backdrop to my work is presented in Chapter 1, along with a description of the theoretical methods that were the mainstay of my PhD research. Chapter 2 presents work in which my density functional theory (DFT) calculations improved our understanding of the metastability of a previously unobserved vacancy ordering in a Cu2Se. Chapter 3 presents a different direction of investigations that we conducted on Cu2Se, this time into its superionic properties. The nucleation, kinetics, and correlation of lattice strain to the order-disorder superionic phase transition were explored through a combination of transmission electron microscopy and DFT. The correlation between lattice strain and superionicity is expanded upon in Chapter 4 where Prashant and I developed a theoretical basis on which to understand compressively strain-stabilized superionicity in Cu2Se and Li2Se. Chapter 5 shifts away from Cu2Se on to HgSe. Additionally, the focus changes from the structure and transport of cations to the structure and transport of electrons, specifically the electron-conducting surface states found in topological phases of matter. Bulk band-structure calculations and charge density character analysis that I carried out led us to hypothesize that a hexagonal phase of HgSe newly-synthesized bny the Jain group was a 3-D topological insulator. The unique topological surface states (TSS) of HgSe and their dependence on strain, crystallographic symmetry, and surface faceting are determined by DFT and presented in Chapter 6. Particularly, the effect of lattice strain on the dispersion and spin texture circles back to the central theme in the studies of super-ionic crystals: that small amounts of strain can significantly alter the charge transport properties of a material.Submission published under a 24 month embargo labeled 'Closed Access', the embargo will last until 2022-05-01The student, Daniel Dumett Torres, accepted the attached license on 2020-04-21 at 14:56.The student, Daniel Dumett Torres, submitted this Dissertation for approval on 2020-04-21 at 15:04.This Dissertation was approved for publication on 2020-04-24 at 13:06.DSpace SAF Submission Ingestion Package generated from Vireo submission #15022 on 2020-08-25 at 17:40:29Made available in DSpace on 2020-08-27T00:49:58Z (GMT). No. of bitstreams: 2
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