Thermally Selective Formation of Subsurface Oxygen in Ag(111) and Consequent Surface Structure

New insights into the structure of the Ag(111)-p(4 × 4)-O phase: high-resolution STM and DFT study

The atomic structure of the Ag(111)-p(4 × 4)-O phase was reexamined with scanning tunneling microscopy (STM) and density functional theory. We discovered two different phases with the same (4 × 4) periodicity and demonstrated that the accepted Ag6 model is incompatible with high-resolution oxygen-sensitive STM images. Using bias dependencies of the STM images, we have shown that the p(4 × 4) phase is highly nonuniform, with local oxygen coverage varying from 1/8 ML up to 1/2 ML.

Automated search for optimal surface phases (ASOPs) in grand canonical ensemble powered by machine learning

The surface of a material often undergoes dramatic structure evolution under a chemical environment, which, in turn, helps determine the different properties of the material. Here, we develop a general-purpose method for the automated search of optimal surface phases (ASOPs) in the grand canonical ensemble, which is facilitated by the stochastic surface walking (SSW) global optimization based on global neural network (G-NN) potential. The ASOP simulation starts by enumerating a series of composition grids, then utilizes SSW-NN to explore the configuration and composition spaces of surface phases, and relies on the Monte Carlo scheme to focus on energetically favorable compositions. The method is applied to silver surface oxide formation under the catalytic ethene epoxidation conditions. The known phases of surface oxides on Ag(111) are reproduced, and new phases on Ag(100) are revealed, which exhibit novel structure features that could be critical for understanding ethene epoxidation. Our results demonstrate that the ASOP method provides an automated and efficient way for probing complex surface structures that are beneficial for designing new functional materials under working conditions.

Growth and auto-oxidation of Pd on single-layer AgOx/Ag(111)

Pd is auto-oxidized during deposition onto AgO_x and undergoes a transition from single to bi-layer growth at low Pd coverage.

Adsorption of molecular oxygen on the Ag(111) surface: A combined temperature-programmed desorption and scanning tunneling microscopy study

The adsorption of O₂ on Ag(111) between 300 and 500 K has been studied with temperature-programmed desorption (TPD) and scanning tunneling microscopy (STM). At the first stage of adsorption, the disordered local oxide phase (commonly looking in STM as an array of black spots) is formed on the surface irrespective of the substrate temperature. The maximum concentration of black spots was found to be ≈0.11 ML, which corresponds to an oxygen coverage of ≈0.66 ML. Taking into account that the nucleation of the Ag(111)-p(4 × 4)-O phase starts after the saturation of the disordered phase, one can conclude that its coverage is at least not less than 0.66 ML. The analysis of STM and TPD data shows that the thermodesorption peak (m/e = 32) at 570 K is related exclusively to the decomposition of the p(4 × 4) phase, while the local oxide phase does not contribute to desorption.

Exposure of Pt(5 5 3) and Rh(1 1 1) to atomic and molecular oxygen: do defects enhance subsurface oxygen formation?

Subsurface oxygen is known to form in transition metals, and is thought to be an important aspect of their ability to catalyze chemical reactions. The formation of subsurface oxygen is not, however, equivalent across all catalytically relevant metals. As a result, it is difficult to predict the stability and ease of the formation of subsurface oxygen in metals, as well as how the absorbed oxygen affects the chemical and physical properties of the metal. In comparing how a stepped platinum surface, Pt(5 5 3), responds to exposure to gas-phase oxygen atoms under ultra-high vacuum conditions to planar Rh(1 1 1), we are able to determine what role, if any, steps have on the capacity of a metal for subsurface oxygen formation. Despite the presence of regular defects, we found that only surface-bound oxygen formed on Pt(5 5 3). Alternatively, on the Rh(1 1 1) surface, oxygen readily absorbed into the selvedge of the metal. These results suggest that defects alone are insufficient for the formation of subsurface oxygen, and the ability of the metal to absorb oxygen is the primary factor in the formation and stabilization of subsurface oxygen.

Surface chemistry of group IB metals and related oxides

Catalytic surface chemistry of IB metals are reviewed with an attempt to bridge model catalysts and powder catalysts.

Orientation-free and differentially pumped addition of a low-flux reactive gas beam to a surface analysis system

We describe an example of a piecewise gas chamber that can be customized to incorporate a low flux of gas-phase radicals with an existing surface analysis chamber for in situ and stepwise gas-surface interaction experiments without any constraint in orientation. The piecewise nature of this gas chamber provides complete angular freedom and easy alignment and does not require any modification of the existing surface analysis chamber. In addition, the entire gas-surface system is readily differentially pumped with the surface chamber kept under ultra-high-vacuum during the gas-surface measurements. This new design also allows not only straightforward reconstruction to accommodate the orientation of different surface chambers but also for the addition of other desired features, such as an additional pump to the current configuration. Stepwise interaction between atomic oxygen and a highly ordered pyrolytic graphite surface was chosen to test the effectiveness of this design, and the site-dependent O-atom chemisorption and clustering on the graphite surface were resolved by a scanning tunneling microscope in the nm-scale. X-ray photoelectron spectroscopy was used to further confirm the identity of the chemisorbed species on the graphite surface as oxygen.