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The dynamical structure of a catalyst determines the availability of active sites on its surface. However, how nanoparticle (NP) catalysts re-structure under reaction conditions and how these changes associate with catalytic activity remains poorly understood. Using operando transmission electron microscopy, we show that Pd NPs exhibit reversible structural and activity changes during heating and cooling in mixed gas environments containing O 2 and CO. Below 400 °C, the NPs form flat low index facets and are inactive towards CO oxidation. Above 400 °C, the NPs become rounder, and conversion of CO to CO 2 increases significantly. This behavior reverses when the temperature is later reduced. Pt and Rh NPs under similar conditions do not exhibit such reversible transformations. We propose that adsorbed CO molecules suppress the activity of Pd NPs at lower temperatures by stabilizing low index facets and reducing the number of active sites. This hypothesis is supported by thermodynamic calculations. How nanoparticle (NP) catalysts re-structure under reaction conditions and how these changes associate with catalytic activity remains poorly understood. Here, the authors present operando TEM studies of Pd NPs during CO oxidation, which show reversible changes in both structure and activity with temperature.

Metal–organic frameworks (MOFs) are crystalline nanoporous materials with great potential for a wide range of industrial applications. Understanding the nucleation and early growth stages of these materials from a solution is critical for their design and synthesis. Despite their importance, the pathways through which MOFs nucleate are largely unknown. Using a combination of in situ liquid-phase and cryogenic transmission electron microscopy, we show that zeolitic imidazolate framework-8 MOF nanocrystals nucleate from precursor solution via three distinct steps: 1) liquid–liquid phase separation into solute-rich and solute-poor regions, followed by 2) direct condensation of the solute-rich region into an amorphous aggregate and 3) crystallization of the aggregate into a MOF. The three-step pathway for MOF nucleation shown here cannot be accounted for by conventional nucleation models and provides direct evidence for the nonclassical nucleation pathways in open-framework materials, suggesting that a solute-rich phase is a common precursor for crystallization from a solution.

Galvanic replacement (GR) is a simple and widely used approach to synthesize hollow nanostructures for applications in catalysis, plasmonics, and biomedical research. The reaction is driven by the difference in electrochemical potential between two metals in a solution. However, transient stages of this reaction are not fully understood. Here, we show using liquid cell transmission electron microscopy that silver (Ag) nanocubes become hollow via the nucleation, growth, and coalescence of voids inside the nanocubes, as they undergo GR with gold (Au) ions at different temperatures. These direct in situ observations indicate that void formation due to the nanoscale Kirkendall effect occurs in conjunction with GR. Although this mechanism has been suggested before, it has not been verified experimentally until now. These experiments can inform future strategies for deriving such nanostructures by providing insights into the structural transformations as a function of Au ion concentration, oxidation state of Au, and temperature. Hollow nanoparticles can be synthesized by galvanic replacement or the Kirkendall effect, which are generally regarded as two separate processes. Here, the authors use liquid TEM to follow the entire galvanic replacement of Ag nanocubes, finding experimental evidence that the Kirkendall effect is a key intermediate stage during hollowing.

Two most common crystal structures in metals and metal alloys are body-centered cubic (bcc) and face-centered cubic (fcc) structures. The phase transitions between these structures play an important role in the production of durable and functional metal alloys. Despite their technological significance, the details of such phase transitions are largely unknown because of the challenges associated with probing these processes. Here, we describe the nanoscopic details of an fcc-to-bcc phase transition in PdCu alloy nanoparticles (NPs) using in situ heating transmission electron microscopy. Our observations reveal that the bcc phase always nucleates from the edge of the fcc NP, and then propagates across the NP by forming a distinct few-atoms-wide coherent bcc–fcc interface. Notably, this interface acts as an intermediate precursor phase for the nucleation of a bcc phase. These insights into the fcc-to-bcc phase transition are important for understanding solid − solid phase transitions in general and can help to tailor the functional properties of metals and their alloys. Phase transitions in crystals are challenging to study with atomic resolution. Here, the authors reveal that the transition from fcc to bcc occurs across a few-atoms-wide coherent interface, which serves as a precursor phase for the nucleation of the bcc phase.

Liquid phase (also called “liquid cell”) transmission electron microscopy (TEM) is a powerful platform for nanoscale imaging and characterization of physical and chemical processes of materials in liquids. It is a direct approach to address critical scientific questions on how materials form or transform in response to external stimuli, such as changes in chemical potential, applied electric bias, and interactions with other materials or their environment. Answers to these questions are essential for understanding and controlling nanoscale materials properties and advancing their applications. With the recent technical advances in TEM, such as the development of sample stages, detectors, and image processing toolkits, liquid phase TEM is transforming our ability to characterize materials and revolutionizing our understanding of many fundamental processes in materials science and other fields. In this article, we briefly review the current status, challenges, and opportunities in liquid phase TEM. More details of the development and applications of liquid cell TEM are discussed in the articles in this issue of MRS Bulletin.

Mono- or few-layer sheets of covalent organic frameworks (COFs) represent an attractive platform of two-dimensional materials that hold promise for tailor-made functionality and pores, through judicious design of the COF building blocks. But although a wide variety of layered COFs have been synthesized, cleaving their interlayer stacking to obtain COF sheets of uniform thickness has remained challenging. Here, we have partitioned the interlayer space in COFs by incorporating pseudorotaxane units into their backbones. Macrocyclic hosts based on crown ethers were embedded into either a ditopic or a tetratopic acylhydrazide building block. Reaction with a tritopic aldehyde linker led to the formation of acylhydrazone-based layered COFs in which one basal plane is composed of either one layer, in the case of the ditopic macrocyclic component, or two adjacent layers covalently held together by its tetratopic counterpart. When a viologen threading unit is introduced, the formation of a host–guest complex facilitates the self-exfoliation of the COFs into crystalline monolayers or bilayers, respectively.Layered COFs are attractive precursors for two-dimensional materials but they are difficult to cleave into mono- or few-layer sheets. Pseudorotaxane moieties have now been embedded into layered COFs to facilitate their cleavage into sheets of uniform thickness. Crown-ether macrocycles within the COF backbone bind to ionic viologen guests, leading to electrostatic repulsion between layers.

Nanoparticle (NP) catalysts are ubiquitous in energy systems, chemical production, and reducing the environmental impact of many industrial processes. Under reactive environments, the availability of catalytically active sites on the NP surface is determined by its dynamic structure. However, atomic-scale insights into how a NP surface reconstructs under reaction conditions and the impact of the reconstruction on catalytic activity are still lacking. Using operando transmission electron microscopy, we show that Pd NPs exhibit periodic round–to–flat transitions altering their facets during CO oxidation reaction at atmospheric pressure and elevated temperatures. This restructuring causes spontaneous oscillations in the conversion of CO to CO 2 under constant reaction conditions. Our study reveals that the oscillatory behavior stems from the CO-adsorption-mediated periodic restructuring of the nanocatalysts between high-index-faceted round and low-index-faceted flat shapes. These atomic-scale insights into the dynamic surface properties of NPs under reactive conditions play an important role in the design of high-performance catalysts. Atomic-scale insights into how a nanoparticle surface reconstructs under reaction conditions and the impact of the reconstruction on catalytic activity are still lacking. Here the authors reveal that Pd nanocatalysts display oscillatory changes in both their structure and activity during CO oxidation using operando TEM.

Membranes are ubiquitous in nature with primary functions that include adaptive filtering and selective transport of chemical/molecular species. Being critical to cellular functions, they are also fundamental in many areas of science and technology. Of particular importance are the adaptive and programmable membranes that can change their permeability or selectivity depending on the environment. Here, we explore implementation of such biological functions in artificial membranes and demonstrate two-dimensional self-assembled heterostructures of graphene oxide and polyamine macromolecules, forming a network of ionic channels that exhibit regulated permeability of water and monovalent ions. This permeability can be tuned by a change of pH or the presence of certain ions. Unlike traditional membranes, the regulation mechanism reported here relies on specific interactions between the membranes’ internal components and ions. This allows fabrication of membranes with programmable, predetermined permeability and selectivity, governed by the choice of components, their conformation and their charging state. Two-dimensional self-assembled heterostructures of graphene oxide and polyamine macromolecules are used to create membranes with tuneable permeability for water and ions.

Metal nanoparticle@metal−organic framework (NP@MOF) composites hold promise for potential applications in gas storage, catalysis, sensing, environmental monitoring, and biomedicine. Despite their importance, details of how MOFs encapsulate the NPs to form NP@MOF hybrid nanostructures are largely unexplored. Here, using ultra‐low electron‐flux in situ liquid phase transmission electron microscopy (LP‐TEM), the encapsulation of Au NPs with zeolitic imidazolate framework‐8 (ZIF‐8) is visualized. These observations reveal that the speeds at which MOFs nucleate on the NP's surface impact the shell's shape. At low concentrations of MOF precursor, NPs are encapsulated with well‐defined single‐crystalline MOF shells, while at high concentrations, MOFs tend to nucleate and grow from multiple sites on the NP surface, resulting in irregularly shaped polycrystalline MOF shells. This approach, which uses a very low electron flux to image the synthesis of Au@ZIF‐8 nanostructures, can be extended to imaging crucial processes in many other beam‐sensitive materials and help design hybrid systems for a broad range of applications.

At elevated temperatures, bimetallic nanomaterials change their morphologies because of the interdiffusion of atomic species, which also alters their properties. The Kirkendall effect (KE) is a well-known phenomenon associated with such interdiffusion. Here, we show how KE can manifest in bimetallic nanoparticles (NPs) by following core–shell NPs of Au and Pd during heat treatment with in situ transmission electron microscopy. Unlike monometallic NPs, these core–shell NPs did not evolve into hollow core NPs. Instead, nanoscale voids formed at the bimetallic interface and then, migrated to the NP surface. Our results show that: (1) the direction of vacancy flow during interdiffusion reverses due to the higher vacancy formation energy of Pd compared to Au, and (2) nanoscale voids migrate during heating, contrary to conventional assumptions of immobile voids and void shrinkage through vacancy emission. Our results illustrate how void behavior in bimetallic NPs can differ from an idealized picture based on atomic fluxes and have important implications for the design of these materials for high-temperature applications. At elevated temperatures, bimetallic nanomaterials often change their composition and morphology due to the interdiffusion of atomic species, which can affect their properties. Here, the authors use in situ transmission electron microscopy to provide new insights into the Kirkendall effect and void motion in core-shell nanoparticles of Au and Pd.