Charge redistribution at heterogeneous interfaces is a fundamental aspect of surface chemistry. Manipulating the amount of charges and the magnitude of dipole moments at the interface in a controlled way has attracted tremendous attention for its potential to modify the activity of heterogeneous catalysts in catalyst design. Two-dimensional ultrathin silica films with well-defined atomic structures have been recently synthesized and proposed as model systems for heterogeneous catalysts studies. R. Wlodarczyk et al. (Phys. Rev. B, 85, 085403 (2012)) have demonstrated that the electronic structure of silica/ Ru(0001) can be reversibly tuned by changing the amount of interfacial chemisorbed oxygen. Here we carried out systematic investigations to understand the underlying mechanism through which the electronic structure at the silica/Ru(0001) interface can be tuned. As corroborated by both in situ X-ray photoelectron spectroscopy and density functional theory calculations, the observed interface energy level alignments strongly depend on the surface and interfacial charge transfer induced dipoles at the silica/ Ru(0001) heterojunction. These observations may help to understand variations in catalytic performance of the model system from the viewpoint of the electronic properties at the confined space between the silica bilayer and the Ru(0001) surface. The same behavior is observed for the aluminosilicate bilayer, which has been previously proposed as a model system for zeolites.Jian-Qiang Zhong and Mengen Wang contributed equally to this work.

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Crack formation limits the growth of (AlxGa1−x)2O3 epitaxial films on Ga2O3 substrates. We employ first-principles calculations to determine the brittle fracture toughness of such films for three growth orientations of the monoclinic structure: [100], [010], and [001]. Surface energies and elastic constants are computed for the end compounds—monoclinic Ga2O3 and Al2O3—and used to interpolate to (AlxGa1−x)2O3 alloys. The appropriate crack plane for each orientation is determined, and the corresponding critical thicknesses are calculated based on Griffith’s theory, which relies on the balance between elastic energy and surface energy. We obtain lower bounds for the critical thickness, which compare well with available experiments. We also perform an in-depth analysis of surface energies for both relaxed and unrelaxed surfaces, providing important insights into the factors that determine the relative stability of different surfaces. Our study provides physical insights into surface stability, crack planes, and the different degrees of crack formation in (AlxGa1−x)2O3 films for different growth orientations.

The confinement of noble gases on nanostructured surfaces, in contrast to bulk materials, at non-cryogenic temperatures represents a formidable challenge. In this work, individual Ar atoms are trapped at 300 K in nano-cages consisting of (alumino)silicate hexagonal prisms forming a two-dimensional array on a planar surface. The trapping of Ar atoms is detected in situ using synchrotron-based ambient pressure X-ray photoelectron spectroscopy. The atoms remain in the cages upon heating to 400 K. The trapping and release of Ar is studied combining surface science methods and density functional theory calculations. While the frameworks stay intact with the inclusion of Ar atoms, the permeability of gasses (for example, CO) through them is significantly affected, making these structures also interesting candidates for tunable atomic and molecular sieves. These findings enable the study of individually confined noble gas atoms using surface science methods, opening up new opportunities for fundamental research.

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Subnanoscale spaces at the interface between weakly coupled thin films and their metal supports offer exciting opportunities for studying chemical reactions under confinement. Here, we investigated the kinetics of water formation (from hydrogen and chemisorbed oxygen) in the confined space at the interface between bilayer (BL) silica and a Ru(0001) support, compared to the reaction on the bare Ru(0001) surface. Ambient pressure X-ray photoelectron spectroscopy (AP-XPS) experiments were carried out at different temperatures at elevated pressures of H 2 to follow the reaction kinetics. The apparent activation energy at the BL-silica/Ru(0001) interface was found to be 0.38 eV lower than that on bare Ru( 0001), consistent with a recent report by Prieto et al. (Angew. Chem., Int. Ed. 2018, 57(28), 8749−8753) carried out at lower H 2 pressures using low-energy electron microscopy. Density functional theory calculations revealed that the ratelimiting step in the direct hydrogenation pathway on the Ru(0001) surface is the first hydrogen addition step (*H + *O ↔ *OH). The confinement at the BL-silica/Ru(0001) interface only marginally affects the energy barrier of the first hydrogen addition. Instead, it activates an alternative disproportionation reaction pathway (*H 2 O + *O ↔ 2*OH). On the bare Ru( 0001) surface, the disproportionation pathway can only occur at cryogenic temperatures or under high water vapor pressures. However, the presence of the BL-silica increases the desorption barrier for water molecules at the interface. The increased residence time allows trapped water molecules to react with chemisorbed oxygen to produce two *OH per H 2 O with an activation energy 0.25 eV lower than that of the first hydrogen addition step. This work reveals the origin of the observed accelerated water formation reaction at the BL-silica/ Ru(0001) interface in the low-temperature regime (T < 350 K) and points to a route to engineer chemical reaction pathways by leveraging subnanoscale confined spaces at metal−oxide interfaces.

The nanoscale confinement of noble gases at noncryogenic temperatures is crucial for many applications including noble gas separations, nuclear waste remediation, and the removal of radon. However, this process is extremely difficult primarily due to the weak trapping forces of the host matrices upon noble gas physisorption. Herein, the formation of 2D clathrate compounds, which result from trapping noble gas atoms (Ar, Kr, and Xe) inside nanocages of ultrathin silica and aluminosilicate crystalline nanoporous frameworks at 300 K, is reported. The formation of the 2D clathrate compounds is attributed to a novel activated physisorption mechanism, facilitated by ionization of noble gas atoms. Combined X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) studies provide evidence of an initial ionization process that significantly reduces the apparent trapping barrier. Noble gas ions become neutralized upon entering the cages, and their desorption requires unprecedentedly high temperatures, even in ultrahigh vacuum conditions. From 2D aluminosilicate films these temperatures are 348 K (Ar), 498 K (Kr), and 673 K (Xe). DFT calculations also predict that Rn can be trapped in 2D aluminosilicates with an even higher desorption temperature of 775 K. This work highlights a new ionization-facilitated trapping mechanism resulting in the thinnest family of clathrates ever reported.