Reconstruction of supported nanocatalysts often occurs in gas–solid reactions and significantly affects the catalytic performance, yet it is much less explored in liquid-phase environment. Herein, we find that highly-dispersed Ag nanocatalysts, i.e., AgO_x clusters, supported on alumina, silica, and titania, can aggregate into larger Ag or Ag₂O particles after immersing in liquid-phase media at room temperature. The spontaneous aggregation of AgO_x clusters in liquid water is attributed to liquid-phase Ostwald ripening through dissolution of AgO_x clusters into water and subsequent redeposition to form Ag₂O particles. The immersion into organic solvents such as ethanol leads to reduction of AgO_x clusters and further growth into Ag particles. This work reveals that liquid-phase reaction media can induce substantial structural evolution of supported nanostructured catalysts, which should be carefully considered in liquid–solid interface catalytic reactions such as electrocatalysis, environmental catalysis, and organic synthesis in liquid phase.

Water is often involved in many catalytic processes, which can strongly affect structural evolution of catalysts during pretreatments and catalytic reactions. In this work, we demonstrate a promotional effect of H₂O on both oxidative dispersion and spontaneous aggregation of Ag nanocatalysts supported on alumina. Ag nanoparticles supported on γ-Al₂O₃ and Ag nanowires on Al₂O₃(0001) can be dispersed into nanoclusters via annealing in O₂ above 300 °C, which is accelerated by introduction of H₂O into the oxidative atmosphere. Furthermore, the formed highly dispersed Ag nanoclusters are subject to spontaneous aggregation in humid atmosphere at room temperature. Ex situ and in situ characterizations in both powder and model catalysts suggest that formation of abundant surface hydroxyls and/or water adlayer on the Al₂O₃ surface in the H₂O-containing atmosphere facilitates the surface migration of Ag species, thus promoting both dispersion and aggregation processes. The aggregation of the supported Ag nanostructures induced by the humid oxidative atmosphere enhances CO oxidation but inhibits selective catalytic reduction of NO with C₃H₆. This work illustrates the critical role of H₂O in structure and catalytic performance of metal nanocatalysts, which can be widely present in heterogeneous catalysis.

Understanding the effect of H₂O adsorption on reactant activation is of great importance in heterogeneous catalysis, which remains a grand challenge particularly in oxide catalyst systems with structural complexity. Herein, the effect of D₂O adsorption on D₂ activation over MgO nanocatalysts at different temperatures has been investigated by transmission Fourier transform infrared (FT-IR) and temperature-programmed desorption (TPD). Two sets of hydride and hydroxyl species produced from D₂ dissociation at more active and less active Mg-O pairs can be observed by FT-IR, which all desorb via the product of D₂ as confirmed by TPD experiments. We find that the physically adsorbed D₂O overlayer does not affect the dissociation of D₂ since D₂ may pass through the molecular layer and access the surface-active sites. When D₂O is partially dissociated on the MgO surface, D₂ can only dissociate at the remaining active sites until that dissociated -OD_w groups from D₂O occupy all active sites. These findings provide a fundamental understanding of the effect of water adsorption on D₂ activation on oxide catalysts.

The coupling of model batteries and surface-sensitive techniques provides an indispensable platform for interrogating the vital surface/interface processes in battery systems. Here, we report a sandwich-format nanopore-array model battery using an ultrathin graphite electrode and an anodized aluminum oxide (AAO) film. The porous framework of AAO regulates the contact pattern of the electrolyte with the graphite electrode from the inner side, while minimizing contamination on the outer surface. This model battery facilitates repetitive charge–discharge processes, where the graphite electrode is reversibly intercalated and deintercalated, and also allows for the in-situ characterizations of ion intercalation in the graphite electrode. The ion distribution profiles indicate that the intercalating Li ions accumulate in both the inner and outer surface regions of graphite, generating a high capacity of ~ 455 mAh·g⁻¹ (theory: 372 mAh·g⁻¹). The surface enrichment presented herein provides new insights towards the mechanistic understanding of batteries and the rational design strategies.

The synthesis of high-quality ultrathin overlayers is critically dependent on the surface structure of substrates, especially involving the overlayer–substrate interaction. By using in situ surface measurements, we demonstrate that the overlayer–substrate interaction can be tuned by doping near-surface Ar nanobubbles. The interfacial coupling strength significantly decreases with near-surface Ar nanobubbles, accompanying by an “anisotropic to isotropic” growth transformation. On the substrate containing near-surface Ar, the growth front crosses entire surface atomic steps in both uphill and downhill directions with no difference, and thus, the morphology of the two-dimensional (2D) overlayer exhibits a round-shape. Especially, the round-shaped 2D overlayers coalesce seamlessly with a growth acceleration in the approaching direction, which is barely observed in the synthesis of 2D materials. This can be attributed to the immigration lifetime and diffusion rate of growth species, which depends on the overlayer–substrate interaction and the surface catalysis. Furthermore, the “round to hexagon” morphological transition is achieved by etching-regrowth, revealing the inherent growth kinetics under quasi-freestanding conditions. These findings provide a novel promising way to modulate the growth, coalescence, and etching dynamics of 2D materials on solid surfaces by adjusting the strength of overlayer–substrate interaction, which contributes to optimization of large-scale production of 2D material crystals.

Understanding of thin film growth mechanism is crucial for tailoring film growth behaviors, which in turn determine physicochemical properties of the resulting films. Here, vapor-growth of tungsten carbide overlayers on W(110) surface is investigated by real time low energy electron microscopy. The surface growth is strongly confined by surface steps, which is in contrast with overlayer growth crossing steps in a so-called carpet-like growth mode for example in graphene growth on metal surfaces. Density functional theory calculations indicate that the step-confined growth is caused by the strong interaction of the forming carbide overlayer with the substrate blocking cross-step growth of the film. Furthermore, the tungsten carbide growth within each terrace is facilitated by the supply of carbon atoms from near-surface regions at high temperatures. These findings suggest the critical role of near-surface atom diffusion and step confinement effects in the thin film growth, which may be active in many film growth systems.

Ultrathin ZnO nanostructures present interesting two-dimensional (2D) graphene-like structure in contrast to wurtzite structure in bulk ZnO. Growth on Au(111) has been regarded as a well-established route to the 2D ZnO layers while controlled growth of uniform ZnO nanostructures remains as a challenge. Here, reactive deposition of Zn in O₃ and NO₂ was employed, which is investigated by scanning tunneling microscopy and X-ray photoelectron spectroscopy (XPS). We demonstrate that uniform ZnO monolayer nanoislands and films can be obtained on Au(111) using O₃ and uniform ZnO bilayer nanoislands and films form on Au(111) using NO₂, respectively. Formation of atomic oxygen overlayers on Au(111) via exposure to O₃ is critical to the formation of uniform ZnO monolayer nanostructures atop. Near ambient pressure XPS studies revealed that nearly full hydroxylation occurs on monolayer ZnO structures upon exposure to near ambient pressure water vapor or atomic hydrogen species, while partial surface hydroxylation happens on bilayer ZnO under the same gaseous exposure conditions.

Fundamental understanding of chemistry confined to nanospace remains a challenge since molecules encapsulated in confined microenvironments are difficult to be characterized. Here, we show that CO adsorption on Pt(111) confined under monolayer hexagonal boron nitride (h-BN) can be dynamically imaged using near ambient pressure scanning tunneling microscope (NAP-STM) and thanks to tunneling transparency of the top h-BN layer. The observed CO superstructures on Pt(111) in different CO atmospheres allow to derive surface coverages of CO adlayers, which are higher in the confined nanospace between h-BN and Pt(111) than those on the open Pt surface under the same conditions. Dynamic NAP-STM imaging data together with theoretical calculations confirm confinement-induced molecule enrichment effect within the 2D nanospace, which reveals new chemistry aroused by the confined nanoreactor.

Two-dimensional (2D) materials such as graphene and hexagonal boron nitride (h-BN) can be used as robust and flexible encapsulation overlayers, which effectively protect metal cores but allow reactions to occur between inner cores and outer shells. Here, we demonstrate this concept by showing that Pt@h-BN core–shell nanocatalysts present enhanced performances in H₂/O₂ fuel cells. Electrochemical (EC) tests combined with operando EC-Raman characterizations were performed to monitor the reaction process and its intermediates, which confirm that Pt-catalyzed electrocatalytic processes happen under few-layer h-BN covers. The confinement effect of the h-BN shells prevents Pt nanoparticles from aggregating and helps to alleviate the CO poisoning problem. Accordingly, embedding nanocatalysts within ultrathin 2D material shells can be regarded as an effective route to design high-performance electrocatalysts.

Core–shell nanostructures consisting of active metal cores and protective shells often exhibit enhanced catalytic performance, in which reactants can access a small part of the core surfaces through the pores in the shells. In this study, we show that Pt nanoparticles (NPs) can be embedded into few-layer hexagonal boron nitride (h-BN) overlayers, forming Pt@h-BN core–shell nanocatalysts. The h-BN shells not only protect the Pt NPs under harsh conditions but also allow gaseous molecules such as CO and O₂ to access a large part of the Pt surfaces through a facile intercalation process. As a result, the Pt@h-BN nanostructures act as nanoreactors, and CO oxidation reactions with improved activity, selectivity, and stability occur at the core–shell interfaces. The confinement effect exerted by the h-BN shells promotes the Pt-catalyzed reactions. Our work suggests that two-dimensional shells can function as robust but flexible covers on nanocatalyst surfaces and tune the surface reactivity.