Oxidation of metals and metallic alloys plays a crucial role in many technologically important materials, processes, and devices including high-temperature resistant coatings, heterogeneous catalysis, and microelectronics. In particular, oxidation of copper surfaces is of fundamental and practical importance. Copper, CuO, and Cu2O are candidates to replace rare and expensive noble metal catalysts for applications including methane synthesis, catalytic conversion of nitrogen oxides, water-gas shift, and preventing CO poisoning in fuel cells.
In this research program we closely interact with two experimental groups lead by Professor Judith Yang at the Univeristy of Pittsburg and Professor Guangwen Zhou at Binghamton University.
Authors Qing Zhu, Wissam A. Saidi, and Judith C. Yang
Surface defects such as step edges play an important role in determining the surface properties, affecting immensely the growth mechanisms and morphologies of the nanostructures in epitaxial film growth processes. Here, we probe the dynamics of the oxidation on stepped Cu(100) using molecular dynamic simulations in conjunction with a reactive force field, and we elucidate the mechanisms and energy barriers affecting the oxidation process. Molecular dynamic simulations show that the adsorbed oxygen adatoms are unevenly distributed on the stepped surface, favoring the top terrace. We show that this behavior is due to Ehrlichâ€“Schwöbel (ES) barrier effect. However, differently from the reduced interlayer self-diffusion in descending a step as in a conventional ES barrier effect, we find instead that the ES barrier reduces the ascending diffusion barrier for oxygen, promoting its transport across the step edge and enhancing oxidation of the upper terrace. Additionally, we find that the ES barrier is step-height dependent, where higher step edges reduce more the oxygen-ascending diffusion barrier and favor more oxidation of the upper terraces of stepped surfaces.
Authors Liang Li, Na Cai, Wissam A Saidi, Guangwen Zhou
Surface steps are typically assumed as a source of adatoms for oxygen-chemisorption induced surface reconstruction, but few microscopic observations have been made in the vicinity of steps on reconstructing surfaces. Using in situ scanning tunneling microscopy, we provide direct evidence that surface steps are the source of Cu adatoms for the Cu(1 1 0)single bond(2 × 1)-O restructuring. Using density functional theory, we show that the role of oxygen is to stabilize Cu adatoms detached from step edges via the barrier-less formation of Cusingle bondO dimers on terraces. Incorporating this atomic process of capturing Cu adatoms into kinetic Monte Carlo simulations reproduces the experimentally observed (2 × 1)-O reconstruction.
Authors Qianqian Liua, Liang Lia, Na Caia, Wissam A. Saidib, Guangwen Zhou
From an interplay between variable temperature scanning tunneling microscopy and densityâ€“functional theory calculations, the evolution of oxygen chemisorption-induced surface reconstructions of the Cu(110) surface is determined. The surface reconstructions proceed via a sequential pathway with increasing oxygen surface coverage. The (2 × 1) reconstruction occurs first and then transits to the c(6 × 2) phase with a higher oxygen coverage through a mechanism that consumes the existing (2 × 1) phase with the supply of Cu adatoms from step edges and terraces. The temperature dependence of the (2 × 1) ? c(6 × 2) transition demonstrates that the surface phase transition is an activated process for breaking up added Cuâ€“Oâ€“Cu rows in the (2 × 1) structure. Comparison between the experimental observations and the theoretical surface phase diagram obtained from first-principles thermodynamic calculations reveals that the (2 × 1) ? c(6 × 2) transition takes place at the oxygen chemical potentials that are far above the chemical potential for Cu2O bulk oxide formation, reflecting the existence of kinetic limitations to the surface phase transition and the bulk oxide formation.
Authors Liang Li, Langli Luo, Jim Ciston, Wissam A. Saidi, Eric A. Stach, Judith C. Yang, and Guangwen Zhou
We report in situ atomic-resolution transmission electron microscopy observations of the oxidation of stepped Cu surfaces. We find that the presence of surface steps both inhibits oxide film growth and leads to the oxide decomposition, thereby resulting in oscillatory oxide film growth. Using atomistic simulations, we show that the oscillatory oxide film growth is induced by oxygen adsorption on the lower terrace along the step edge, which destabilizes the oxide film formed on the upper terrace.
Authors Liang Li, Qianqian Liu, Jonathan Li, Wissam A. Saidi, and Guangwen Zhou
Oxygen chemisorption induced surface reconstructions are widely observed, but the atomic processes leading to transitions among oxygen chemisorbed phases are largely unknown. Using ab initio molecular dynamics and density-functional theory, we study the kinetic process of the Cu(110)-(2 × 1) ? c(6 × 2) phase transition upon increasing oxygen surface coverage. We show that the phase transition involves initially Cuâ€“O dimer and Cuâ€“Oâ€“Cu trimer formation with a kinetic barrier of ?0.13 eV, followed by a barrierless process of forming a four Cuâ€“Oâ€“Cuâ€“O chains configuration that transitions to the c(6 × 2) reconstruction via concerted movement of three Cu atoms with an associated energy barrier of ?1.41 eV. The larger kinetic barrier is suggested as the origin of the kinetic hindrance that is inferred from the significant discrepancy between the experimentally observed temperature and pressure dependent (2 × 1) ? c(6 × 2) phase transition and the equilibrium thermodynamics prediction.
Authors Guangwen Zhou, Langli Luo, Liang Li, Jim Ciston, Eric A. Stach, Wissam A. Saidi and Judith C. Yang
Oxidation of Cu occurs via Cu2O islanding on an oxide wetting layer at a critical thickness of two atomic layers. The transition from 2D wetting-layer growth to 3D oxide islanding is driven energetically arising from the Cuâ€“Cu2O interfacial interaction.
Authors Wissam A. Saidi, Minyoung Lee, Liang Li, Guangwen Zhou, and Alan J. H. McGaughey
Using an ab initio atomistic thermodynamics framework, we identify the stable surface structures during the early stages of Cu(100) oxidation at finite temperature and pressure conditions. We predict the clean surface, the 0.25 monolayer oxygen-covered surface, and the missing-row reconstruction as thermodynamically stable structures in range of 100â€“1000 K and 10?15â€“105atm, consistent with previous experimental and theoretical results. We also investigate the thermodynamic stabilities of possible precursors to Cu2O formation including missing-row reconstruction structures that include extra on- or subsurface oxygen atoms as well as boundary phases formed from two missing-row nanodomains. While these structures are not predicted to be thermodynamically stable for oxygen chemical potentials below the nucleation limit of Cu2O, they are likely to exist due to kinetic hindrance.