Ultrathin two-dimensional (2D) semiconducting layered materials offer a great potential to extend the Moore's Law (1). One key challenge for 2D semiconductors is to avoid the formation of charge scattering and trap sites from adjacent dielectrics. The insulating van der Waals layer, hexagonal boron nitride (hBN), is an excellent interface dielectric to 2D semiconductors, efficiently reducing charge scatterings (2, 3). Recent studies have shown the growth of single-crystal hBN films on molten Au surfaces (4) or bulk Cu foils (5). However, using molten Au is not favored in industry due to high cost, cross-contamination, and potential issues of process control and scalability. Cu foils may be suitable for roll-to-roll processes, but unlikely to be compatible with advanced microelectronic fabrication on Si wafers. Thus, only a reliable approach to grow single-crystal hBN on wafers can help realize the broad adoption of 2D layered materials in industry. Previous efforts on growing hBN triangular monolayers on Cu (111) metals have failed to achieve mono-orientation, resulting in unwanted grain boundaries when they merge as films (6,7). Growing singlecrystal hBN on such a high-symmetry surface planes (5,8) is commonly believed to be impossible even in theory. In stark contrast, we have successfully realized the epitaxial growth of single-crystal hBN monolayers on a Cu ( 111) thin film across a 2-inch c-plane sapphire wafer. This surprising result is corroborated by our first-principles calculations, suggesting that the epitaxy to the underlying Cu lattice is enhanced by the lateral docking to Cu (111) steps, to ensure the mono-orientation of hBN monolayers. The obtained singlecrystal hBN, incorporated as an interface layer between MoS2 and HfO2 in a bottom-gate configuration, has enhanced the electrical performance of transistors based on monolayer MoS2. This reliable approach of producing wafer-scale single-crystal hBN truly paves the way for developing futuristic 2D electronics.First, a single-crystal Cu (111) thin film on a wafer is needed. Single-crystal Cu in thick foils can be achieved through recrystallization induced by implanted seeds (5,9). However, for the formation of Cu (111) thin film on a wafer, the crystallinity strongly relies on the underlying substrate lattices. Here we used a c-plane sapphire as the substrate, on which a 500-nm-thick polycrystalline Cu film was sputtered followed by extensive thermal annealing to achieve singlecrystal Cu (111) films (10). One challenge is that Cu (111) tends to form twin grains separated by twin grain boundaries, through kinetic growth processes. Fig. 1a illustrates the atomic arrangements for the typical twinned Cu (111) structure. We find that the post-annealing at a high temperature (1,040 -1,070 °C) in the presence of hydrogen is the key to removing the twin grains, consistent with recent reports (10,11). Figures 1b and 1c show the optical micrographs (OMs) and electron backscatter diffraction (EBSD) patterns for the Cu (111) thin films after annealing at 1,000 °...

Encapsulation of metal nanoparticles by support-derived materials known as the classical strong metal–support interaction (SMSI) often happens upon thermal treatment of supported metal catalysts at high temperatures (≥500 °C) and consequently lowers the catalytic performance due to blockage of metal active sites. Here, we show that this SMSI state can be constructed in a Ru–MoO3 catalyst using CO2 hydrogenation reaction gas and at a low temperature of 250 °C, which favors the selective CO2 hydrogenation to CO. During the reaction, Ru nanoparticles facilitate reduction of MoO3 to generate active MoO3–x overlayers with oxygen vacancies, which migrate onto Ru nanoparticles’ surface and form the encapsulated structure, that is, Ru@MoO3–x . The formed SMSI state changes 100% CH4 selectivity on fresh Ru particle surfaces to above 99.0% CO selectivity with excellent activity and long-term catalytic stability. The encapsulating oxide layers can be removed via O2 treatment, switching back completely to the methanation. This work suggests that the encapsulation of metal nanocatalysts can be dynamically generated in real reactions, which helps to gain the target products with high activity.

Stacking order has strong influence on the coupling between the two layers of twisted bilayer graphene (BLG), which in turn determines its physical properties. Here, we report the investigation of the interlayer coupling of the epitaxially grown singlecrystal 30° twisted BLG on Cu(111) at the atomic scale. The stacking order and morphology of BLG is controlled by a rationally designed two-step growth process, that is, the thermodynamically controlled nucleation and kinetically controlled growth. The crystal structure of the 30°-twisted bilayer graphene (30°-tBLG) is determined to have the quasicrystal like symmetry. The electronic properties and interlayer coupling of the 30-tBLG is investigated using scanning tunneling microscopy (STM) and spectroscopy (STS). The energy-dependent local density of states (DOS) with in-situ electrostatic doping shows that the electronic states in two graphene layers are decoupled near the Dirac point. A linear dispersion originated from the constituent graphene monolayers is discovered with doubled degeneracy.This study contributes to controlled growth of twist-angle-defined BLG, and provides insights of the electronic properties and interlayer coupling in this intriguing system.

Identifying the dynamic structure of heterogeneous catalysts is crucial for the rational design of new ones. In this contribution, the structural evolution of Fe(0) catalysts during CO 2 hydrogenation to hydrocarbons has been investigated by using several (quasi) in situ techniques. Upon initial reduction, Fe species are carburized to Fe 3 C and then to Fe 5 C 2 . The by-product of CO 2 hydrogenation, H 2 O, oxidizes the iron carbide to Fe 3 O 4 . The formation of Fe 3 O 4 @(Fe 5 C 2 +Fe 3 O 4 ) core-shell structure was observed at steady state, and the surface composition depends on the balance of oxidation and carburization, where water plays a key role in the oxidation. The performance of CO 2 hydrogenation was also correlated with the dynamic surface structure. Theoretical calculations and controll experiments reveal the interdependence between the phase transition and reactive environment. We also suggest a practical way to tune the competitive reactions to maintain an Fe 5 C 2 -rich surface for a desired C 2+ productivity.

Metal nanoparticles anchored on perovskite through in situ exsolution under reducing atmosphere provide catalytically active metal/oxide interfaces for CO2 electrolysis in solid oxide electrolysis cell. However, there are critical challenges to obtain abundant metal/oxide interfaces due to the sluggish diffusion process of dopant cations inside the bulk perovskite. Herein, we propose a strategy to promote exsolution of RuFe alloy nanoparticles on Sr2Fe1.4Ru0.1Mo0.5O6−δ perovskite by enriching the active Ru underneath the perovskite surface via repeated redox manipulations. In situ scanning transmission electron microscopy demonstrates the dynamic structure evolution of Sr2Fe1.4Ru0.1Mo0.5O6−δ perovskite under reducing and oxidizing atmosphere, as well as the facilitated CO2 adsorption at RuFe@Sr2Fe1.4Ru0.1Mo0.5O6−δ interfaces. Solid oxide electrolysis cell with RuFe@Sr2Fe1.4Ru0.1Mo0.5O6−δ interfaces shows over 74.6% enhancement in current density of CO2 electrolysis compared to that with Sr2Fe1.4Ru0.1Mo0.5O6−δ counterpart as well as impressive stability for 1000 h at 1.2 V and 800 °C.

Reduction behaviors, oxygen vacancies and hydroxyl groups play decisive roles in the surface chemistry and catalysis of oxides. Employing isothermal H2 reduction we simultaneously reduced CeO2 nanocrystals with different morphologies, created oxygen vacancies and produced hydroxyl groups. The morphology of CeO2 nanocrystals was observed to strongly affect the reduction process and the resultant oxygen vacancy structure. The resultant oxygen vacancies are mainly located on the surfaces of CeO2 cubes and rods but in the subsurface/bulk of CeO2 octahedra. The reactivity of isolated bridging hydroxyl groups on CeO2 nanocrystals was found to depend on the local oxygen vacancy concentration, in which they reacted to produce water at low local oxygen vacancy concentrations but to produce both water and hydrogen with increasing local oxygen vacancy concentration. These results reveal a morphology-dependent interplay among the reduction behaviors, oxygen vacancies and hydroxyl reactivity of CeO2 nanocrystals, which deepens the fundamental understanding of the surface chemistry and catalysis of CeO2.

Hydroxyl (OH) groups are widely present on an oxide surface, which have been recognized as a key factor affecting surface properties of the oxide and interaction of the metal overlayer with the oxide. Here, γ-alumina (γ-Al2O3) supports with different OH contents are prepared by calcinating pseudo-boehmite (AlOOH) at different temperatures. The surface OH effect on oxidative redispersion of supported Ag nanoparticles including surface migration and anchoring of Ag species has been explored using in situ X-ray diffraction and UV–visible spectroscopy, as well as ex situ X-ray photoelectron spectroscopy and transmission electron microscopy. We reveal that the dispersion capacity, i.e., the amount of anchored Ag species associated with the steady dispersion state is thermodynamically determined by the surface OH contents, while the dispersion rate related to the surface migration process is kinetically limited by surface OH densities at low Ag loading. The higher dispersion ability is observed on the support with higher OH contents, and the quicker dispersion occurs on the support with higher OH densities. The results reveal that both OH contents and OH densities are critical in the redispersion of metal particles on oxide surfaces, which can be used to manipulate the Ag-catalyzed CO oxidation reaction.

Oxidative dispersion has been widely used in regeneration of sintered metal catalysts and fabrication of single atom catalysts, which is attributed to an oxidation-induced dispersion mechanism. However, the interplay of gas-metal-support interaction in the dispersion processes, especially the gas-metal interaction has not been well illustrated. Here, we show dynamic dispersion of silver nanostructures on silicon nitride surface under reducing/oxidizing conditions and during carbon monoxide oxidation reaction. Utilizing environmental scanning (transmission) electron microscopy and near-ambient pressure photoelectron spectroscopy/photoemission electron microscopy, we unravel a new adsorption-induced dispersion mechanism in such a typical oxidative dispersion process. The strong gas-metal interaction achieved by chemisorption of oxygen on nearly-metallic silver nanoclusters is the internal driving force for dispersion. In situ observations show that the dispersed nearly-metallic silver nanoclusters are oxidized upon cooling in oxygen atmosphere, which could mislead to the understanding of oxidation-induced dispersion. We further understand the oxidative dispersion mechanism from the view of dynamic equilibrium taking temperature and gas pressure into account, which should be applied to many other metals such as gold, copper, palladium, etc. and other reaction conditions.