Probe Nanoscale Oxidation Mechanism of Metals Under Applied Stress
Although extensive interest in metal oxidation has existed for decades, very few work deals simultaneously with the oxidation mechanism and the environmental effects which are no longer described by the atmosphere only, but by all kinds of constraints (e.g. mechanical and thermal) an engineering component may undergo. Those constraints are not independent and the effects of the coupling of these different constraints cannot be deduced from classical oxidation studies in static conditions. We are investigating the stress dependence of nanoscale oxidation properties of metals by coupling mechanical loading and gas-surface reactions within specialized in situ environmental TEM and variable temperature ultrahigh vacuum STM chambers while simultaneously monitoring oxide nucleation and growth during the oxidation. We are particularly examining 1) the effect of applied stress on nucleation and growth of oxide islands; 2) correlations between applied stress and nanoscale morphologies of oxide islands; and 3) effect of applied stress on the coalescence behavior of oxide islands. The study is expected to provide the foundation for constructing atomistic oxidation models by taking into account environment effects, which are no longer described by the atmosphere only, but by all kinds of constraints (e.g. mechanical and thermal) an engineering component may undergo.

Correlation Between Oxidation Mechanism and Nanoscale Atomic Structure in Amorphous Oxide Film
The formation of thin amorphous oxide films by low-temperature oxidation of metals is of significant importance for many technological applications including heterogeneous catalysis, electronics, corrosion protection, and surface coatings. However, the field of low-temperature oxidation and the fundamental understanding of the mechanism of amorphous oxide formation have been experimentally handicapped by a lack of extensive data on simple systems. Several reasons contribute to this paucity of data: the difficulty of measurements due to slow oxidation kinetics at low temperatures, little control of impurities, incomplete characterization of the original surface, and the longstanding challenge in structural characterization of amorphous oxides. The limited understanding of low temperature oxidation has also been hindered by the inability of traditional techniques to perform in situ measurements of the structure and reaction kinetics at the nanoscale and below as the oxidation progresses. The main thrust of this project is to investigate the microscopic processes of amorphous oxide formation by utilizing the strong oxidation power of ozone (O3) to enhance the rate of oxide formation on metal surfaces and in situ ultrahigh vacuum (UHV) scanning probe microscopy to monitor the reaction sequence from oxygen surface chemisorption to oxide nucleation and growth. The in situ visualization experiment is complemented by fluctuation electron microscopy for establishing a correlation between oxidation mechanism and nanoscale atomic structure of the amorphous oxide film.

In Situ Visualization and Theoretical Modeling of the Early-stage Oxidation of Metals and Alloys
Oxidation of metal and alloy surfaces is a chemical reaction with significant technological impact. However, quantitative description of the oxide growth during early stages of oxidation of metals and alloys has received little attention. This is largely due to the inability of traditional experimental techniques to measure the structure, chemistry and kinetics at the nanoscale as the reaction progresses as well as a lack of universal theoretical approaches to address this issue due to the complexity associated with initial stages of oxide growth. The objective of this research is to utilize unique in situ microscopy techniques to visualize oxide formation on a nanometer scale and below. The in situ experiments are directly correlated with the theoretical approaches of percolation and dynamic scaling for a quantitative understanding of the complex oxide growth dynamics during early stages of oxidation of metals and alloys

Methanol Oxidation Catalyzed by Copper-based Materials Investigated by In Situ UHV-TEM
Catalytic methanol oxidation is an important reaction for hydrogen production and direct methanol fuel cells (DMFC) and is a promising sustainable energy source for low-temperature fuel cell applications. Copper-containing catalysts have successfully been utilized for partial oxidation of methanol (POM) that has high reaction rates and selectivity for H2 and CO2 production instead of CO. Many issues and controversies still exist regarding the reaction mechanism, such as the oxidation state of the catalytically active copper. We are utilizing a specialized in situ ultra-high vacuum (UHV)-transmission electron microscope (TEM) as a nano-laboratory, to produce various Cu-based nanostructures and carry out the catalytic reactions while simultaneously monitoring structural changes and gas species. This systematic investigation of the role of copper oxidation states, defect structures, surface orientation and roughness, as well as the particle size in Cu-catalyzed POM, will identify the important structural factors that control the catalytic activity.

Advanced Cathode Materials for Lithium-ion Batteries
Lithium ion batteries for future plug-in hybrid electric vehicles require high working voltages and long lifetimes. Existing cathode materials such as LiMn2O4 offer high working voltages but suffer from poor electrochemical stability such that the Mn3+ ions dissolve into the solution. To overcome this problem, we are collaborating with Argonne’s researchers to develop new interface materials which can prevent dissolution while maintaining electro/ion transport. We are developing new SPM and TEM techniques for in situ studies of cathode materials.