Judith C. Yang - US grants

9902863
Yang

The goal is to gain new understanding of the stages of oxidation where metal surface conditions and impurities are highly controlled. A coordinated series of in-situ TEM experiments on copper and aluminum address open issues in transient oxidation, including the kinetics of initial oxide formation. Sample preparation, initial ex-situ characterization and oxidation experiments, as well as data analysis and modeling, are conducted at University of Pittsburgh. The initial stages of oxidation are followed by in-situ ultrahigh-vacuum transmission electron microscopy performed at the Materials Research Laboratory, University of Illinois at Urbana-Champaign (UIUC). Close interaction with the Corrosion Science and Technology (CS&T) Group at Oak Ridge National Laboratory (ORNL) is integral to this program. At present the behavior of oxygen atoms on metal surfaces are observed by surface techniques such as low energy electron diffraction (LEED) and ultra-high vacuum-scanning tunneling microscopy (UHV-STM), where the surfaces are highly controlled, or by studies of macroscopic oxide growth occurring at high and low temperatures, where surface cleanliness at the atomic level is extremely challenging to achieve. The planned experiments provide essential information that bridges the gap between these two extremes.
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Surface oxidation processes play critical roles in environmental stability, high temperature corrosion, electrochemistry, certain catalytic reactions and thin film growth as well as extreme fuel reactions. Much is known about oxygen interaction with metal surfaces and about the macroscopic growth of thermodynamically stable oxides. At present, however, the transient stages of oxidation, from the nucleation of the metal oxide to the formation of the thermodynamically stable oxide, represent a scientifically challenging and technologically important area.
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This project aims in applying in-situ transmission electron microscopy under ultra high vacuum conditions to develop a fundamental understanding of the oxidation processes at the nanoscale. Major objectives of the proposed study is to develop better understanding of the nanoscale stages of oxidation from the nucleation of the metal oxide to the formation of thermodynamically stable oxide. Systematic experimental studies will be made on thin films of copper and aluminum. The project involves collaborations with the Materials Research Laboratory at the University of Illinois at Urbana-Champaign (UIUC), Lawrence Livermore National Laboratory (LLNL) and the Institute for Materials Research and Engineering (IMRE), Singapore where a specialized in situ chamber for Al thin film deposition inside the UHV-TEM exist. This research has technological relevance where surface oxidation processes play critical roles in environmental stability, high temperature corrosion, electrochemistry, catalytic reactions, gate oxides and thin film growth as well as fuel reactions. The educational component incorporates the research tools developed in the project in the development of new courses at both the undergraduate and graduate levels in materials science and engineering (MS&E) curriculum as well as a new web site accessible for public.

The research develops a new understanding of the fundamental processes involved in nanoscale oxidation on thin metals that would have direct relevance in technologies where corrosion is the limiting factor.

NER: Coordinated Multi-scale Simulations and In situ Electron Microscopy to Elucidate the Mechanisms of Nanowire Formations

PI: Judith Yang, University of Pittsburgh
Co-PI: Kyeongjae Cho, Stanford University


To gain mechanistic understanding of nanowire formation via the vapor-liquid-solid (VLS) model as catalyzed by metal nanoparticles, a coordinated experimental, utilizing in situ and ex situ electron microscopy (Yang) and theoretical approach (Cho) will be conducted. We will focus on one model system, such as Au-catalyzed Si nanowire growth. Yang's team will apply in situ and ex situ electron microscopy to characterize the growth of the nanowire, with special attention to the metal/nanowire interface. Cho's team will utilize his hierarchical multi-scale simulations with first-principles accuracy for a specific catalytic reaction. This program will contribute to graduate education in terms of a thin films and theory/simulations classes as well as to undergraduates, via undergraduate summer research and senior projects.

TECHNICAL: Surface oxidation processes play critical roles in environmental stability, high temperature corrosion, electrochemistry, catalytic reactions, gate oxides and thin film growth as well as fuel reactions. At present, however, the nanoscale stages of oxidation - from the nucleation of the metal oxide to the formation of the thermodynamically stable oxide - represent a scientifically challenging and technologically important terra incognito. The objective is to fundamentally understand nanoscale oxidation processes by coordinated experimental (in situ UHV-TEM) and theoretical efforts, where the impact is potentially a new paradigm for oxidation. To meet this objective, directly correlated experimental and theoretical investigations of the initial stages of Cu and Cu alloy (Cu-Au and Cu-Ni) oxidation will be performed. The in-situ experiments of the initial stages of Cu have been performed by PI in the past. The extension to Cu alloy oxidation will be conducted in an in situ UHV-TEM at various temperatures, pressures and oxidizing atmospheres in Yang's laboratory where a unique in--situ UHV TEM exists. To bridge the temporal gap between simulations and experiments, a newly developed dynamic TEM (DTEM) with nanoseconds time resolution, will be used as well at Lawrence Livermore National Lab (LLNL). Determination of the Cu/Cu2O interface structure formed by in-situ oxidation by cross-sectional TEM and scanning TEM (STEM) methods, including high-resolution electron microscopy (HREM), Z-contrast imaging, tomography and electron energy loss spectroscopy (EELS) and the comparison with theoretical simulations will provide critical insights into the oxidation transformation mechanisms. The sample preparation and TEM/STEM studies will be conducted in the new Peterson Institute of NanoScience and Engineering (PINSE) at Univ. of Pittsburgh (UPitt). The in-situ and ex-situ experimental TEM results will be directly correlated to theoretical models, where a first-principles kinetic Monte Carlo, called Thin Film Oxidation (TFOx) is being developed. TFOx is a C++ code that presently simulates 2D nucleation and growth, and will be developed to simulate 3D island formation where computer clusters at CMU and UF and the supercomputer facility at UPitt will be utilized for significantly enhanced simulation speed. The direct comparison between these simulations and in-situ experiments will lead to new knowledge. NON-TECHNICAL: The combined experimental with theoretical partnership between UPitt, LLNL,UF and CMU will significantly enrich and broaden the education of all of the graduate and undergraduate students involved in this program. Yang has a strong track record in advising women students. Dissemination of results will include a web-site: www.tfox.org. Yang will integrate her research program in thin films, gas surface reactions and electron microscopy into several of the Mechanical Engineering and Materials Science department (MEMS) undergraduate and graduate-level courses, such as undergraduate crystallography/diffraction laboratory and graduate nano-courses, such as thin films, nanocharacterization and electron microscopy as well as nanomaterials, at UPitt.

Technical: This project is to study growth mechanism of nanostructures that are grown using encapsulation and controlled release of metal catalyst. The controlled catalyst release allows the exposed size/area of the catalyst be tailored, which leads to growth of unusual nanostructures, such as nanocones made of silicon carbide. The goal of the research project is to gain new mechanistic knowledge of all stages of the metal-catalyzed nano- and hetero-structure formation including the migration of the metal nanoparticle from the carbon shell and agglomeration behavior, the formation of the SiC at the metal/nanocone interface, and the nucleation and initial growth of the heterostructures. The primary research tool is transmission electron microscopy (TEM) and both in-situ and ex-situ TEM experiments will be carried out. Electrical and property measurements will be used to establish the synthesis-structure-property relationships. The potential outcome of the project will be a predictive tool for the manufacturing of specific nanowires with a precise distribution and geometry, as well as branched structures, and how processing conditions, such as temperature, size, and type of catalyst material, will alter the nanowire morphology, structure, and properties.
Non-technical: The project addresses fundamental research issues in a topical area of importance to materials science. It aims for a quantitative and mechanistic understanding of the catalytic reaction governing one-dimensional nanostructure formations. Such knowledge is necessary for the design-based production of nanodevices with specific architectures and properties. The project constitutes an effective integration of research and education through training of postdoctoral, graduate, and undergraduate students in nanoscience and nanotechnology. The in-situ TEM nanocharacterization represents a unique opportunity for the students. The research program will also contribute to a new suite of graduate-level courses in nanotechnology, particularly nanofabrication and nanocharacterization. Undergraduate education will be enhanced via undergraduate summer research and senior projects.

The research objective of this grant is to gain a detailed fundamental understanding of the complex interplay between the thermodynamics and kinetics governing early-stage surface-oxidation processes in single- and multi-oxidant environments. The study will use advanced high-resolution techniques to allow in situ interrogation of reaction-product interfaces for elucidating structural, chemical, and defect developments. Specifically, Ni, Ni-Cr, and, later, Ni-Cr-Al alloys in 700-1100¢ªC environments containing O2, H2O, CO2, or some combination of these as primary oxidants will be systematically examined. While the central focus is to establish science-based approaches for developing and/or improving advanced high-temperature alloys and coatings used in the many harsh environments found in practice, the scientific understanding gained will also contribute to elucidating the collective and coupled behaviors of surface reactions in general.

If successful, this interdisciplinary collaborative effort will lead to new paradigms in the important field of gas-solid surface reactions, such as corrosion, catalysis, sensors, fuel cells, and synthesis for a variety of electronic, magnetic, medical, and optical devices. The study will contribute to national economic competitiveness (novel materials and technologies created), development of a competitive STEM workforce (training and mentorship of graduate students and post-docs, interdisciplinary courses), the participation of women and underrepresented minorities; high school community outreach (Pennsylvania Junior Academy of Science), and national security (sustainable energy, energy generation, enhanced aircraft design). Materials-related classes at all levels will be developed or modified in multiple departments driven by the need to introduce students to the latest developments in materials-centered research and technology. Examples of classes include Gas-Metal Reactions, Crystallography, Nanocharacterization, Transmission Electron Microscopy, and Introduction to Materials Science.

Catalysts are used in the manufacturing of more than 60% of all synthesized chemicals and more than 90% of chemical industries use catalytic materials world-wide, with an estimated combined impact on the global economy of over $10 trillion per year. Furthermore, catalysis is essential to chemistry where reactants are efficiently converted to products while minimizing the production of by-products that are environmentally harmful. Yet, technological advancements in catalysis have frequently depended more on chemical intuition than fundamentals. The recent emergence of ?nano-characterization tools? has fundamentally changed this and is allowing the discovery of fundamental principles of catalysis via detailed characterization of catalysts and its correlation with their chemical reactivity.

In a collaborative program between the University of Pittsburgh, SUNY Binghamton, and Brookhaven National Laboratory, PIs Judith Yang, Goetz Veser and Guangwen Zhou will use state-of-the-art characterization tools including environmental transmission electron microscopy, in situ scanning tunneling microscopy, and X-ray photoelectron spectroscopy, complemented with reactivity studies using a specially designed spatially resolved microreactor in order to gain essential insights into catalytic structure-reactivity relationships. The PIs will focus on copper-containing catalysts, a class of catalysts with importance for existing and emerging energy technologies, such as partial oxidation of methanol and the water-gas-shift reaction. Experiments will be performed on simple model systems including Cu single crystals and Cu oxides produced by controlled oxidation of Cu surfaces in-situ. Correlations between the phases and surface and interface structure of Cu-based catalysts and their catalytic activity will be identified. The results will be compared with commercially available Cu/ZnO catalysts to provide a commercial base-line for these fundamental studies.

An important global topic such as energy production requires not only advances in scientific research, but trained people to aid in the transfer of these advances into industrial practice. The partnership between two major universities and a national laboratory will enrich the education of the students involved in this program. Graduate students will be trained in materials physics/chemistry and catalysis science and will learn about new microscopy, spectroscopy, kinetics and modeling techniques as well as materials issues that are at the forefront of current energy research. The training of graduate students in the broader area of clean energy technology, as well a fundamental scientific discipline (e.g., catalytic kinetics, materials science, physics, etc.), will result in future leaders that are better equipped to solve the complex energy and environmental problems that face society. Results from this project will also be incorporated into new graduate-level courses and high school outreach programs at both Universities.

1337731
Yang
Technical Abstract
This Major Research Instrumentation Award supports the acquisition of an environmental transmission electron microscope (ETEM), a high-resolution electron microscope with a simplified differential pumping system that permits gas introduction directly into the column. As a conventional TEM the instrument will enhance the current electron microscopy capabilities in teaching, user training and research. The capability to visualize structural changes at the sub-nanoscale level under operational environmental conditions will bring essential insights to many materials issues, such as the effect of dopants and impurities on processing and corrosion, defect (dislocation, grain boundaries and interface) migrations, thermally induced phase transformations, heterogeneous catalyses, nano-scale functional materials, and the environmental stability of materials in general. The low accelerating voltage minimizes damage of oxide species and soft materials, thereby providing the ability to visualize the dynamics of polymers, tissue engineering, drug delivery, biomaterials and biomineralization. This will be the first ETEM in the greater Western Pennsylvania area. It will be housed at the University of Pittsburgh in the Swanson School of Engineering (SSoE) and will be a part of the Nanofabrication and Characterization Facility (NFCF) in the Peterson Institute of NanoScience and Engineering (PINSE). It will be linked to the electron microscopy facility in the Structural Biology department, School of Medicine. The NFCF is a user facility available to all national and international researchers. The broad visibility and access to this instrument will contribute to nano- and meso-scale science and technology, biology, energy and sustainability research and to the training of post-docs, graduate students and undergraduate students. This instrument will also be used in teaching courses in Electron Microscopy, Nano-characterization, Thin Films, Biomaterials, Nanomaterials and Nanotechnology. As a part of PINSE, it will contribute to the education of high school students and under-represented groups, especially African-American, as part of the NSF-Nanotechnology Undergraduate Education (NUE) outreach program and Pitt Engineering Career Access Program (PECAP) program, respectively.
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This award from the Major Instrumentation program will support the installation of the first Environmental Transmission Electron Microscope (ETEM) in the greater Western Pennsylvania area. The instrument will be housed at the University of Pittsburgh in the Swanson School of Engineering (SSoE) as part of the Nanofabrication and Characterization Facility (NFCF), a user facility available to all national and international researchers. TEM is a powerful tool to visualize materials at the nanoscale and below. ETEM enables unique insights on the dynamic processing/structure/property relationships of nanomaterials with impact in a broad spectrum of science and engineering areas, such as oxidation, corrosion, catalysis, nano-processing, bio-mineralization, tissue engineering and drug delivery. The broad visibility and access to this instrument will contribute to nano- and meso-scale science and technology, biology, energy and sustainability research and education of post-docs, graduate students and undergraduate students. This instrumentation will also be used in teaching Electron Microscopy, Nano-characterization, Thin Films, Biomaterials, Nanomaterials and Nanotechnology courses. It will contribute to the education of high school students and under-represented groups, especially African-American, as part of the NSF-Nanotechnology Undergraduate Education outreach program and Pitt Engineering Career Access Program, respectively.

The NSF Chemical Catalysis Program supports the efforts of Professor Judith C. Yang to correlate experimental studies with first principles theoretical simulations in developing a fundamental understanding of the structure/chemical property relationship of a prototypical system based on platinum/gamma-alumina (Pt/gamma-Al2O3). Pt/gamma-Al2O3 is an important heterogeneous catalysts system used in oil refining, catalytic converters, and fuel cells. While much is known about the system, a significant gap exists between experiment and theory because commercially-available gamma-alumina is polycrystalline with ill-defined morphologies, crystallography and impurities. To bridge the gap, Yang's research group optimizes the oxidation conditions of single crystal nickel aluminum to produce reasonably flat, defect-free, single crystal gamma-Al2O3 films that are tens of nanometers thick (bulk-like). The characterization of size, morphologies, facets, dispersion and interfaces as well as electronic and chemical states under gas environments is accomplished by using the cutting-edge facilities at the Center for Functional Nanomaterials at Brookhaven National Laboratory. Direct comparison of the experimental data with theoretical modeling is accomplished through collaboration with Drs. Linlin Wang and Duane Johnson (Ames Laboratory and Iowa State University). This research project exposes graduate students to cutting edge research environments through combined university and national laboratory learning opportunities and collaborations. Undergraduates are also mentored in a team environment. Finally, Professor Yang serves and engages the local community via collaborations with the Pennsylvania Junior Academy of Science and the Materials Research Society.

NON-TECHNICAL SUMMARY

There is a great deal of knowledge about the oxidation of metal surfaces on macroscopic scales, but very little is known about how the process starts at the smallest molecular scales. Understanding the oxidation phenomena that occur at such nanometer (one billionth the size of a meter) length scales is of both scientific and technological importance, since more and more materials are being engineered at nanometer scales for practical applications. The stability of such nanoscale materials cannot necessarily be inferred directly from the knowledge about their bulk counterparts, and new theories are needed to predict material properties at such small scales. This collaborative project brings together a theoretical chemist who will model the individual reaction mechanisms of the oxidation of a copper surface, a theoretical physicist who will combine these mechanisms in a statistical model of the oxide growth kinetics, and finally an experimental microscopist who can watch the growth of nanometer scale oxide islands with an electron microscope. This research team will work together to understand the initial oxidation of copper surface, from the atomic scale on and up. New simulation methodologies will be developed as part of this effort. The software that will be developed and used to model the oxidation will be released freely to other researchers and to the public. The techniques will also be incorporated into graduate-level courses and as part of high-school outreach programs both in Austin, TX and Pittsburgh, PA.

TECHNICAL SUMMARY

This award supports a collaborative research and education effort between the University of Pittsburgh and the University of Texas at Austin for developing materials computational tools that can be used to model nano-oxidation, and to correlate computational predictions with experimental observations. The PIs will integrate versatile codes for modeling dynamics at surfaces and address three key challenges in the proposed research, which are (i) to use accelerated dynamics methods and off-lattice adaptive kinetic Monte Carlo (KMC) with empirical potentials and density functional theory to extract the reaction mechanisms of surface oxidation, (ii) to continue the development of the Thin Film Oxidation KMC approach, particularly taking it from 2 to 3 dimensions, and (iii) to develop a method for coarse graining the representations of reaction mechanisms found with adaptive KMC to provide the event tables for the three-dimensional Thin Film Oxidation code. These studies will provide realistic input parameters for the Thin Film Oxidation simulations, allow for critical insights to be made into the nucleation behavior, morphological evolution of oxide islands during nano-oxidation and coalescence, and provide the surface and interface energies required to understand island stability. This collaboration builds on existing infrastructure, including the experimental electron microscopy effort in Pittsburgh and the theoretical and software efforts in Austin and Pittsburgh. This research team will work together to understand the initial oxidation of a copper surface, from the atomic scale on and up. The software that will be used to model the oxidation will be released freely to other researchers and to the public. The techniques will also be incorporated into graduate-level courses and as part of high-school outreach programs both in Austin and Pittsburgh.

In this project, funded by the Designing Materials to Revolutionize and Engineer our Future (DMREF) Program of the Chemistry Division, Professors Graeme Henkelman and Richard Crooks at the University of Texas, Professor Anatoly Frenkel at Yeshiva University, and Professor Judith Yang of the University of Pittsburgh are combining experimental and computational methods aimed at discovering optimal bi-functional catalyst formulations and structures for the electrochemical oxidation of the poisonous gas, carbon monoxide. A key element of the research is the use of extremely small catalyst particles (containing only several hundred atoms) that are partially coated with a protective layer of molecules called dendrimers that keep the metal particles from coalescing. A variety of catalysts are being made, and their structures and catalytic activities are being analyzed. Computational studies are being performed to predict new catalyst structures that are then prepared and analyzed; the results from the experimental studies are used to refine the computational methods. The combined approach represents a new toolkit for the discovery and development of new catalytic materials having improved performance.The project participants are involved in educational and outreach activities to engage undergraduate students directly in various aspects of the project including an opportunity for some of the University of Texas - Austin students to spend up to a week at Brookhaven National Laboratory and the University of Pittsburgh.

The research team is conducting density functional theory (DFT) calculations of the entire nanoparticle structure for various combinations of metals - one which adsorbs carbon monoxide and one which adsorbs oxygen dissociatively. The DFT calculations of available reaction mechanisms, together with kinetic modeling and correlations to expected variations in catalyst structures and compositions, identify mechanisms and reactivity descriptors that will be used in subsequent screening-synthesis-characterization-evaluation cycles. Characterizations are being conducted by both in situ and scanning transmission electron microscopy (TEM/STEM) and extended X-ray absorption fine structure (EXAFS). Methods are being developed for combining the DFT calculations with the TEM and EXAFS data to improve the determination of nanoparticle structures at the atomic scale. From a scientific/technical standpoint, the study is advancing the dendrimer-aided approach to catalyst synthesis, with a clearer understanding of the nature of the particles that are produced and their catalytic activity. The study is extending previous work on bi-metallic particles to metal-metal oxide systems, with corresponding extension of computational and characterization efforts. The synthesis, characterization, and modeling tools developed during the course of this study have broad applicability to a wide range of reactions and catalyst formulations, and the software tools will be freely distributed to the catalysis science community.

NON-TECHNICAL SUMMARY
One of the most important properties for materials exposed to air or water is their environmental stability. As the dimensions of materials systems approach the nanoscale, it is critical to understand on a fundamental level how they interact with their environment at these length scales. Surprisingly, the initial stages are the least well-understood regime of oxidation. Classic models of oxidation assume uniform film growth. This is because classic oxidation analysis relied mostly on thermogravimetric techniques, which measure the weight change of the material during oxidation, and, hence, do not provide information on the materials' structure. Yet, structural changes are well-known to occur during metal oxidation. The potential impact of the proposed research project is the development of a fundamental understanding of nanoscale oxidation processes. This research team expands the experimental understanding of nanoscale oxidation using in situ and ex situ environmental transmission electron microscope to directly compare with a current theoretical effort on the atomistic simulation of oxidation of copper. The results from the in situ and ex situ experiments accelerates the development of computational tools needed to enhance the emergent field of predictive materials design for a critical reaction, oxidation. Oxidation is of world-wide importance, not only for corrosion but also as a bottom up approach to nano-oxide processing. Furthermore, a critical aspect of this project is the education and training of students and post-doc. The combined partnership between complementary experimental and theoretical tools, especially in situ, enriches the education of all participants involved in this project and the development of future leaders who are better equipped to bring to success the emergent field of predictive science and engineering. Results from this research are also incorporated into graduate courses and high school community outreach projects, such as Pennsylvania Junior Academy of Science workshop that has been held annually in Pittsburgh since 2007.


TECHNICAL SUMMARY
Much is known about oxygen interaction with metal surfaces and about the macroscopic growth of thermodynamically stable oxides. At present, however, the nanoscale stages of oxidation - from nucleation of the metal oxide to formation of the thermodynamically stable oxide - represent a scientifically challenging and technologically important terra incognito. As engineered materials approach the nanometer regime, control of their environmental stability at this scale becomes crucial. As environmental stability is an essential property of most engineered materials, many oxidation theories exist to explain its mechanisms. However, most classical oxidation theories assume a uniform growing film, where structural changes are not considered due to the lack of traditional experimental procedure to visualize this non-uniform growth under conditions that allow highly controlled surfaces and impurities. Yet, recent studies by this research team reveal that the Cu oxide islands form during the early stages of Cu oxidation, and thereby challenge the common assumption of a uniform oxide formation. This research team correlates experimental results with theoretical predictions where the impact could be a paradigm shift in the fundamental understanding of oxidation where surfaces and defects control the early stages of oxidation. Specifically, this research team integrates experimental in situ and ex situ transmission electron microscopy and X-ray photoelectron spectroscopy with theoretical simulations in order to gain critical insights into the nucleation behavior, morphological evolution of oxide islands during oxidation and coalescence, and quantitative fundamental physical parameters such as diffusion barriers. Although the focus is on oxygen-metal reactions, the methodologies developed are applicable to any epitaxial system and gas-surface reaction. The understanding obtained from combining the unique experimental results and directly correlated theoretical models leads to smarter design paradigms for nano- and mesoscale materials, devices, and processes that utilize surface gas-metal reaction. This is essential to many technical areas, such as high temperature corrosion, electrochemistry, gate oxides and thin film formation, catalysis used for environmental protection, energy generation and storage, and fuel cell reactions.