Dynamic Atomic-scale Metal Oxidation to Correlate with Multi-scale Simulations

University Of Pittsburgh, Pittsburgh PA

Investigators

Abstract

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.