Atomic-scale electron microscopy at ambient pressure

Visualizing Dynamic Single Atom Catalysis

Many industrial chemical processes, including for producing fuels, foods, pharmaceuticals, chemicals and environmental controls, employ heterogeneous solid state catalysts at elevated temperatures in gas or liquid environments. Dynamic reactions at the atomic level play a critical role in catalyst stability and functionality. In situ visualization and analysis of atomic-scale processes in real time under controlled reaction environments can provide important insights into practical frameworks to improve catalytic processes and materials. This review focuses on innovative real time in situ electron microscopy (EM) methods, including recent progress in analytical in situ environmental (scanning) transmission EM (E(STEM), incorporating environmental scanning TEM (ESTEM) and environmental transmission EM (ETEM), with single atom resolution for visualizing and analysing dynamic single atom catalysis under controlled flowing gas reaction environments. ESTEM studies of single atom dynamics of reactions, and of sintering deactivation, contribute to a better-informed understanding of the yield and stability of catalyst operations. Advances in in situ technologies, including gas and liquid sample holders, nanotomography, and higher voltages, as well as challenges and opportunities in tracking reacting atoms, are highlighted. The findings show that the understanding and application of fundamental processes in catalysis can be improved, with valuable economic, environmental, and societal benefits.

In-situ TEM study from the perspective of holders

During the in situ transmission electron microscopy (TEM) observations, the diverse functionalities of different specimen holders play a crucial role. We hereby provide a comprehensive overview of the main types of holders, associated technologies and case studies pertaining to the widely employed heating and gas heating methods, from their initial developments to the latest advancement. In addition to the conventional approaches, we also discuss the emergence of holders that incorporate a micro-electro-mechanical system (MEMS) chip for in situ observations. The MEMS technology offers a multitude of functions within a single chip, thereby enhancing the capabilities and versatility of the holders. MEMS chips have been utilized in environmental-cell designs, enabling customized fabrication of diverse shapes. This innovation has facilitated their application in conducting in situ observations within gas and liquid environments, particularly in the investigation of catalytic and battery reactions. We summarize recent noteworthy studies conducted using in situ liquid TEM. These studies highlight significant advancements and provide valuable insights into the utilization of MEMS chips in environmental-cells, as well as the expanding capabilities of in situ liquid TEM in various research domains.

Operando Electron Microscopy of Catalysts: The Missing Cornerstone in Heterogeneous Catalysis Research?

Heterogeneous catalysis in thermal gas-phase and electrochemical liquid-phase chemical conversion plays an important role in our modern energy landscape. However, many of the structural features that drive efficient chemical energy conversion are still unknown. These features are, in general, highly distinct on the local scale and lack translational symmetry, and thus, they are difficult to capture without the required spatial and temporal resolution. Correlating these structures to their function will, conversely, allow us to disentangle irrelevant and relevant features, explore the entanglement of different local structures, and provide us with the necessary understanding to tailor novel catalyst systems with improved productivity. This critical review provides a summary of the still immature field of operando electron microscopy for thermal gas-phase and electrochemical liquid-phase reactions. It focuses on the complexity of investigating catalytic reactions and catalysts, progress in the field, and analysis. The forthcoming advances are discussed in view of correlative techniques, artificial intelligence in analysis, and novel reactor designs.

Label-Free Imaging of Catalytic H2O2 Decomposition on Single Colloidal Pt Nanoparticles Using Nanofluidic Scattering Microscopy

Single-particle catalysis aims at determining factors that dictate the nanoparticle activity and selectivity. Existing methods often use fluorescent model reactions at low reactant concentrations, operate at low pressures, or rely on plasmonic enhancement effects. Hence, methods to measure single-nanoparticle activity under technically relevant conditions and without fluorescence or other enhancement mechanisms are still lacking. Here, we introduce nanofluidic scattering microscopy of catalytic reactions on single colloidal nanoparticles trapped inside nanofluidic channels to fill this gap. By detecting minuscule refractive index changes in a liquid flushed trough a nanochannel, we demonstrate that local H2O2 concentration changes in water can be accurately measured. Applying this principle, we analyze the H2O2 concentration profiles adjacent to single colloidal Pt nanoparticles during catalytic H2O2 decomposition into O2 and H2O and derive the particles' individual turnover frequencies from the growth rate of the O2 gas bubbles formed in their respective nanochannel during reaction.

On-chip gas reaction nanolab for in situ TEM observation

The catalysis reaction mechanism at nano/atomic scale attracted intense attention in the past decades. However, most in situ characterization technologies can only reflect the average information of catalysts, which leads to the inability to characterize the dynamic changes of single nanostructures or active sites under operando conditions, and many micro-nanoscale reaction mechanisms are still unknown. The combination of in situ transmission electron microscopy (TEM) holder system with MEMS chips provides a solution for it, where the design and fabrication of MEMS chips are the key factors. Here, with the aid of finite element simulation, an ultra-stable heating chip was developed, which has an ultra-low thermal drift during temperature heating. Under ambient conditions within TEM, atomic resolution imaging was achieved during the heating process or at high temperature up to 1300 °C. Combined with the developed polymer membrane seal technique and nanofluidic control system, it can realize an adjustable pressure from 0.1 bar to 4 bar gas environment around the sample. By using the developed ultra-low drift gas reaction cells, the nanoparticle's structure evolution at atomic scale was identified during reaction.

Recent advances in in-situ transmission electron microscopy techniques for heterogeneous catalysis

The process of heterogeneous catalytic reaction under working conditions has long been considered a "black box", which is mainly because of the difficulties in directly characterizing the structural changes of catalysts at the atomic level during catalytic reactions. The development of in situ transmission electron microscopy (TEM) techniques offers opportunities for introducing a realistic chemical reaction environment in TEM, making it possible to uncover the mystery of catalytic reactions. In this article, we present a comprehensive overview of the application of in situ TEM techniques in heterogeneous catalysis, highlighting its utility for observing gas-solid and liquid-solid reactions during thermal catalysis, electrocatalysis, and photocatalysis. in situ TEM has a unique advantage in revealing the complex structural changes of catalysts during chemical reactions. Revealing the real-time dynamic structure during reaction processes is crucial for understanding the intricate relationship between catalyst structure and its catalytic performance. Finally, we present a perspective on the future challenges and opportunities of in situ TEM in heterogeneous catalysis.

Templated Synthesis of SiO2 Nanotubes for Lithium-Ion Battery Applications: An In Situ (Scanning) Transmission Electron Microscopy Study

One of the weaknesses of silicon-based batteries is the rapid deterioration of the charge-storage capacity with increasing cycle numbers. Pure silicon anodes tend to suffer from poor cycling ability due to the pulverization of the crystal structure after repeated charge and discharge cycles. In this work, we present the synthesis of a hollow nanostructured SiO₂ material for lithium-ion anode applications to counter this drawback. To improve the understanding of the synthesis route, the crucial synthesis step of removing the ZnO template core is shown using an in situ closed gas-cell sample holder for transmission electron microscopy. A direct visual observation of the removal of the ZnO template from the SiO₂ shell is yet to be reported in the literature and is a critical step in understanding the mechanism by which these hollow nanostructures form from their core-shell precursors for future electrode material design. Using this unique technique, observation of dynamic phenomena at the individual particle scale is possible with simultaneous heating in a reactive gas environment. The electrochemical benefits of the hollow morphology are demonstrated with exceptional cycling performance, with capacity increasing with subsequent charge-discharge cycles. This demonstrates the criticality of nanostructured battery materials for the development of next-generation Li⁺-ion batteries.

Chemical Electron Microscopy (CEM) for Heterogeneous Catalysis at Nano: Recent Progress and Challenges

Chemical electron microscopy (CEM), a toolbox that comprises imaging and spectroscopy techniques, provides dynamic morphological, structural, chemical, and electronic information about an object in chemical environment under conditions of observable performance. CEM has experienced a revolutionary improvement in the past years and is becoming an effective characterization method for revealing the mechanism of chemical reactions, such as catalysis. Here, we mainly address the concept of CEM for heterogeneous catalysis in the gas phase and what CEM could uniquely contribute to catalysis, and illustrate what we can know better with CEM and the challenges and future development of CEM.

In Situ Kinetic Observations on Crystal Nucleation and Growth

Nucleation and growth are critical steps in crystallization, which plays an important role in determining crystal structure, size, morphology, and purity. Therefore, understanding the mechanisms of nucleation and growth is crucial to realize the controllable fabrication of crystalline products with desired and reproducible properties. Based on classical models, the initial crystal nucleus is formed by the spontaneous aggregation of ions, atoms, or molecules, and crystal growth is dependent on the monomer's diffusion and the surface reaction. Recently, numerous in situ investigations on crystallization dynamics have uncovered the existence of nonclassical mechanisms. This review provides a summary and highlights the in situ studies of crystal nucleation and growth, with a particular emphasis on the state-of-the-art research progress since the year 2016, and includes technological advances, atomic-scale observations, substrate- and temperature-dependent nucleation and growth, and the progress achieved in the various materials: metals, alloys, metallic compounds, colloids, and proteins. Finally, the forthcoming opportunities and challenges in this fascinating field are discussed.

X-Ray absorption spectroscopy on airborne aerosols

Here we demonstrate a method for performing X-ray absorption spectroscopy (XAS) on airborne aerosols. XAS provides unique insight into elemental composition, chemical and phase state, local coordination and electronic structure of both crystalline and amorphous matter. The aerosol is generated from different salt solutions using a commercial atomizer and dried using a diffusion drier. Embedded in a carrier gas, the aerosol is guided into the experimental chamber for XAS analysis. Typical particle sizes range from some 10 to a few 100 nm. Inside the chamber the aerosol bearing gas is then confined into a region of about 1-2 cm³ in size, by a pure flow of helium, generating a stable free-flowing stream of aerosol. It is hit by a monochromatic X-ray beam, and the emitted fluorescent light is used for spectroscopic analysis. Using an aerosol generated from CaCl₂, KCl, and (NH₄)₂SO₄ salt solutions, we demonstrate the functionality of the system in studying environmentally relevant systems. In addition, we show that the detection limits are sufficient to also observe subtle spectroscopic signatures in XAS spectra with integration times of about 1-2 hours using a bright undulator beamline. This novel setup opens new research opportunities for studying the nucleation of new phases in multicomponent aerosol systems in situ, and for investigating (photo-) chemical reactions on airborne matter, as relevant to both atmospheric science and also for general chemical application.

In Situ TEM under Optical Excitation for Catalysis Research

In situ characterization of materials in their operational state is a highly active field of research. Investigating the structure and response of materials under stimuli that simulate real working environments for technological applications can provide new insight and unique input to the synthesis and design of novel materials. Over recent decades, experimental setups that allow different stimuli to be applied to a sample inside an electron microscope have been devised, built, and commercialized. In this review, we focus on the in situ investigation of optically active materials using transmission electron microscopy. We illustrate two different approaches for exposing samples to light inside the microscope column, explaining the importance of different aspects of their mechanical construction and choice of light source and materials. We focus on the technical challenges of the setups and provide details of the construction, providing the reader with input on deciding which setup will be more useful for a specific experiment. The use of these setups is illustrated using examples from the literature of relevance to photocatalysis and nanoparticle synthesis.

The Progress and Outlook of Metal Single-Atom-Site Catalysis

Single-atom-site catalysts (SASCs) featuring maximized atom utilization and isolated active sites have progressed tremendously in recent years as a highly prosperous branch of catalysis research. Varieties of SASCs have been developed that show excellent performance in many catalytic applications. The major goal of SASC research is to establish feasible synthetic strategies for the preparation of high-performance catalysts, to achieve an in-depth understanding of the active-site structures and catalytic mechanisms, and to develop practical catalysts with industrial value. This Perspective describes the up-to-date development of SASCs and related catalysts, such as dual-atom-site catalysts (DASCs) and nano-single-atom-site catalysts (NSASCs), analyzes the current challenges encountered by these catalysts for industrial applications, and proposes their possible future development path.

Effects of the Encapsulation Membrane in Operando Scanning Transmission Electron Microscopy

Nanoscale tailoring of catalytic materials and Li-battery alternatives has elevated the importance of in situ gas-phase electron microscopy. Such advanced techniques are often performed using an environmental cell inserted into a conventional S/TEM setup, as this method facilitates concurrent electrochemical and temperature stimulations in a convenient and cost-effective manner. However, these cells are made by encapsulating gas between two insulating membranes, which introduces additional electron scattering. We have evaluated strengths and limitations of the gas-phase E-cell S/TEM technique, both experimentally and through simulations, across a variety of practical parameters. We reveal the degradation of image quality in an E-cell setup from various components and explore opportunities to improve imaging quality through intelligent choice of experimental parameters. Our results underscore the benefits of using an E-cell STEM technique, due to its versatility and excellent ability to suppress the exotic contributions from the membrane device.

Precise Drift Tracking for In Situ Transmission Electron Microscopy via a Thon-Ring Based Sample Position Measurement

Visualizing how a catalyst behaves during chemical reactions using in situ transmission electron microscopy (TEM) is crucial for understanding the activity origin and guiding performance optimization. However, the sample drifts as temperature changes during in situ reaction, which weakens the resolution and stability of TEM imaging, blocks insights into the dynamic details of catalytic reaction. Herein, a Thon-ring based sample position measurement (TSPM) was developed to track the sample height variation during in situ TEM observation. Drifting characteristics for three commercially available nanochips were studied, showing large biases in aspects of shifting modes, expansion heights, as well as the thermal conduction hysteresis during rapid heating. Particularly, utilizing the TSPM method, for the first time, the gas layer thickness inside a gas-cell nanoreactor was precisely determined, which varies with reaction temperature and gas pressure in a linear manner with coefficients of ~8 nm/°C and ~50 nm/mbar, respectively. Following drift prediction of TSPM, fast oxidation kinetics of a Ni particle was tracked in real time for 12 s at 500°C. This TSPM method is expected to facilitate the functionality of automatic target tracing for in situ microscopy applications when feedback to hardware control of the microscope.

A cell design for correlative hard X-ray nanoprobe and electron microscopy studies of catalysts under in situ conditions

To improve the understanding of catalysts, and ultimately the ability to design better materials, it is crucial to study them during their catalytic active states. Using in situ or operando conditions allows insights into structure-property relationships, which might not be observable by ex situ characterization. Spatially resolved X-ray fluorescence, X-ray diffraction and X-ray absorption near-edge spectroscopy are powerful tools to determine structural and electronic properties, and the spatial resolutions now achievable at hard X-ray nanoprobe beamlines make them an ideal complement to high-resolution transmission electron microscopy studies in a multi-length-scale analysis approach. The development of a system to enable the use of a commercially available gas-cell chip assembly within an X-ray nanoprobe beamline is reported here. The novel in situ capability is demonstrated by an investigation of the redox behaviour of supported Pt nanoparticles on ceria under typical lean and rich diesel-exhaust conditions; however, the system has broader application to a wide range of solid-gas reactions. In addition the setup allows complimentary in situ transmission electron microscopy and X-ray nanoprobe studies under identical conditions, with the major advantage compared with other systems that the exact same cell can be used and easily transferred between instruments. This offers the exciting possibility of studying the same particles under identical conditions (gas flow, pressure, temperature) using multiple techniques.

Development of a gas environmental heating specimen holder system using differential pumping

We developed a gas environmental heating specimen holder system by applying differential pumping effect to a specimen holder for the insitu transmission electron microscopy observation and electron energy loss spectroscopy (EELS) analysis of catalytic materials. In the insitu experiments, using two small orifices and O-rings, the maximum formed gas pressure was ∼20 Pa. Also, using a heater membrane, the maximum obtained heating temperature was ∼1000°C. We could actually observe/analyze the Pt and Ni nanoparticles with an atomic scale using a double-aberration-corrected microscope and an EELS instrument in the reaction gases at high temperatures.

Facet-Dependent Oxidative Strong Metal-Support Interactions of Palladium-TiO2 Determined by In Situ Transmission Electron Microscopy

The strong metal-support interaction (SMSI) is widely used in supported metal catalysts and extensive studies have been performed to understand it. Although considerable progress has been achieved, the surface structure of the support, as an important influencing factor, is usually ignored. We report a facet-dependent SMSI of Pd-TiO₂ in oxygen by using in situ atmospheric pressure TEM. Pd NPs supported on TiO₂ (101) and (100) surfaces showed encapsulation. In contrast, no such cover layer was observed in Pd-TiO₂ (001) catalyst under the same conditions. This facet-dependent SMSI, which originates from the variable surface structure of the support, was demonstrated in a probe reaction of methane combustion catalyzed by Pd-TiO₂ . Our discovery of the oxidative facet-dependent SMSI gives direct evidence of the important role of the support surface structure in SMSI and provides a new way to tune the interaction between metal NPs and the support as well as catalytic activity.

Simultaneous ambient pressure x-ray photoelectron spectroscopy and grazing incidence x-ray scattering in gas environments

We have developed an experimental system to simultaneously measure surface structure, morphology, composition, chemical state, and chemical activity for samples in gas phase environments. This is accomplished by simultaneously measuring x-ray photoelectron spectroscopy (XPS) and grazing incidence x-ray scattering in gas pressures as high as the multi-Torr regime while also recording mass spectrometry. Scattering patterns reflect near-surface sample structures from the nano-scale to the meso-scale, and the grazing incidence geometry provides tunable depth sensitivity of structural measurements. Scattered x rays are detected across a broad range of angles using a newly designed pivoting-UHV-manipulator for detector positioning. At the same time, XPS and mass spectrometry can be measured, all from the same sample spot and under ambient conditions. To demonstrate the capabilities of this system, we measured the chemical state, composition, and structure of Ag-behenate on a Si(001) wafer in vacuum and in O₂ atmosphere at various temperatures. These simultaneous structural, chemical, and gas phase product probes enable detailed insights into the interplay between the structure and chemical state for samples in gas phase environments. The compact size of our pivoting-UHV-manipulator makes it possible to retrofit this technique into existing spectroscopic instruments installed at synchrotron beamlines. Because many synchrotron facilities are planning or undergoing upgrades to diffraction limited storage rings with transversely coherent beams, a newly emerging set of coherent x-ray scattering experiments can greatly benefit from the concepts we present here.

In situandoperandoelectron microscopy in heterogeneous catalysis-insights into multi-scale chemical dynamics

This review features state-of-the-artin situandoperandoelectron microscopy (EM) studies of heterogeneous catalysts in gas and liquid environments during reaction. Heterogeneous catalysts are important materials for the efficient production of chemicals/fuels on an industrial scale and for energy conversion applications. They also play a central role in various emerging technologies that are needed to ensure a sustainable future for our society. Currently, the rational design of catalysts has largely been hampered by our lack of insight into the working structures that exist during reaction and their associated properties. However, elucidating the working state of catalysts is not trivial, because catalysts are metastable functional materials that adapt dynamically to a specific reaction condition. The structural or morphological alterations induced by chemical reactions can also vary locally. A complete description of their morphologies requires that the microscopic studies undertaken span several length scales. EMs, especially transmission electron microscopes, are powerful tools for studying the structure of catalysts at the nanoscale because of their high spatial resolution, relatively high temporal resolution, and complementary capabilities for chemical analysis. Furthermore, recent advances have enabled the direct observation of catalysts under realistic environmental conditions using specialized reaction cells. Here, we will critically discuss the importance of spatially-resolvedoperandomeasurements and the available experimental setups that enable (1) correlated studies where EM observations are complemented by separate measurements of reaction kinetics or spectroscopic analysis of chemical species during reaction or (2) real-time studies where the dynamics of catalysts are followed with EM and the catalytic performance is extracted directly from the reaction cell that is within the EM column or chamber. Examples of current research in this field will be presented. Challenges in the experimental application of these techniques and our perspectives on the field's future directions will also be discussed.

A Novel Method to Maintain the Sample Position and Pressure in Differentially Pumped Systems Below the Resolution Limit of Optical Microscopy Techniques

We present a new method to maintain constant gas pressure over a sample during in situ measurements. The example shown here is a differentially pumped high-pressure X-ray photoelectron spectroscopy system, but this technique could be applied to many in situ instruments. By using the pressure of the differential stage as a feedback source to change the sample position, a new level of consistency has been achieved. Depending on the absolute value of the sample-to-aperture distance, this technique allows one to maintain the distance within several hundred nanometers, which is below the limit of typical optical microscopy systems. We show that this method is well suited to compensate for thermal drift. Thus, X-ray photoelectron spectroscopy data can be acquired continuously while the sample is heated and maintaining constant pressure over the sample. By implementing a precise manipulator feedback system, pressure variations of less than 5% were reached while the temperature was varied by 400 ℃. The system is also shown to be highly stable under significant changes in gas flow. After changing the flow by a factor of two, the pressure returned to the set value within 60 s.

Operando Surface Characterization on Catalytic and Energy Materials from Single Crystals to Nanoparticles

Modern surface science faces two major challenges, a materials gap and a pressure gap. While studies on single crystal surface in ultrahigh vacuum have uncovered the atomic and electronic structures of the surface, the materials and environmental conditions of commercial catalysis are much more complicated, both in the structure of the materials and in the accessible pressure range of analysis instruments. Model systems and operando surface techniques have been developed to bridge these gaps. In this Review, we highlight the current trends in the development of the surface characterization techniques and methodologies in more realistic environments, with emphasis on recent research efforts at the Korea Advanced Institute of Science and Technology. We show principles and applications of the microscopic and spectroscopic surface techniques at ambient pressure that were used for the characterization of atomic structure, electronic structure, charge transport, and the mechanical properties of catalytic and energy materials. Ambient pressure scanning tunneling microscopy and X-ray photoelectron spectroscopy allow us to observe the surface restructuring that occurs during oxidation, reduction, and catalytic processes. In addition, we introduce the ambient pressure atomic force microscopy that revealed the morphological, mechanical, and charge transport properties that occur during the catalytic and energy conversion processes. Hot electron detection enables the monitoring of catalytic reactions and electronic excitations on the surface. Overall, the information on the nature of catalytic reactions obtained with operando spectroscopic and microscopic techniques may bring breakthroughs in some of the global energy and environmental problems the world is facing.

Progress in environmental high-voltage transmission electron microscopy for nanomaterials

A new environmental high-voltage transmission electron microscope (E-HVEM) was developed by Nagoya University in collaboration with JEOL Ltd. An open-type environmental cell was employed to enable in-situ observations of chemical reactions on catalyst particles as well as mechanical deformation in gaseous conditions. One of the reasons for success was the application of high-voltage transmission electron microscopy to environmental (in-situ) observations in the gas atmosphere because of high transmission of electrons through gas layers and thick samples. Knock-on damages to samples by high-energy electrons were carefully considered. In this paper, we describe the detailed design of the E-HVEM, recent developments and various applications. This article is part of a discussion meeting issue 'Dynamic in situ microscopy relating structure and function'.

Quo Vadis Micro-Electro-Mechanical Systems for the Study of Heterogeneous Catalysts Inside the Electron Microscope?

During the last decade, modern micro-electro-mechanical systems (MEMS) technology has been used to create cells that can act as catalytic nanoreactors and fit into the sample holders of transmission electron microscopes. These nanoreactors can maintain atmospheric or higher pressures inside the cells as they seal gases or liquids from the vacuum of the TEM column and can reach temperatures exceeding 1000 °C. This has led to a paradigm shift in electron microscopy, which facilitates the local characterization of structural and morphological changes of solid catalysts under working conditions. In this review, we outline the development of state-of-the-art nanoreactor setups that are commercially available and are currently applied to study catalytic reactions in situ or operando in gaseous or liquid environments. We also discuss challenges that are associated with the use of environmental cells. In catalysis studies, one of the major challenge is the interpretation of the results while considering the discrepancies in kinetics between MEMS based gas cells and fixed bed reactors, the interactions of the electron beam with the sample, as well as support effects. Finally, we critically analyze the general role of MEMS based nanoreactors in electron microscopy and catalysis communities and present possible future directions.

Nanoscale Visualization of Phase Transition in Melting of Sn-Bi Particles by In situ Hard X-ray Ptychographic Coherent Diffraction Imaging

The phase transition in the melting of Sn–Bi eutectic solder alloy particles was observed by in situ hard X-ray ptychographic coherent diffraction imaging with a pin-point heating system. Ptychographic diffraction patterns of micrometer-sized Sn–Bi particles were collected at temperatures from room temperature to 540 K. The projection images of each particle were reconstructed at a spatial resolution of 25 nm, showing differences in the phase shifts due to two crystal phases in the Sn–Bi alloy system and the Sn/Bi oxides at the surface. By quantitatively evaluating the Bi content, it became clear that the nonuniformity of the composition of Sn and Bi at the single-particle level exists when the particles are synthesized by centrifugal atomization.

Structural changes in noble metal nanoparticles during CO oxidation and their impact on catalyst activity

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 O₂ 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 CO₂ 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.

Introducing and Controlling Water Vapor in Closed-Cell In Situ Electron Microscopy Gas Reactions

Protocols for conducting in situ transmission electron microscopy (TEM) reactions using an environmental TEM with dry gases have been well established. However, many important reactions that are relevant to catalysis or high-temperature oxidation occur at atmospheric pressure and are influenced by the presence of water vapor. These experiments necessitate using a closed-cell gas reaction TEM holder. We have developed protocols for introducing and controlling water vapor concentrations in experimental gases from 2% at a full atmosphere to 100% at ~17 Torr, while measuring the gas composition using a residual gas analyzer (RGA) on the return side of the in situ gas reactor holder. Initially, as a model system, cube-shaped MgO crystals were used to help develop the protocols for handling the water vapor injection process and confirming that we could successfully inject water vapor into the gas cell. The interaction of water vapor with MgO triggered surface morphological and chemical changes as a result of the formation of Mg(OH)2, later validated with mass spectra obtained with our RGA system with and without water vapor. Integrating an RGA with an in situ scanning/TEM closed-cell gas reaction system can thus provide critical measurements correlating gas composition with dynamic surface restructuring of materials during reactions.

Liquid cell transmission electron microscopy and its applications

Transmission electron microscopy (TEM) has long been an essential tool for understanding the structure of materials. Over the past couple of decades, this venerable technique has undergone a number of revolutions, such as the development of aberration correction for atomic level imaging, the realization of cryogenic TEM for imaging biological specimens, and new instrumentation permitting the observation of dynamic systems in situ. Research in the latter has rapidly accelerated in recent years, based on a silicon-chip architecture that permits a versatile array of experiments to be performed under the high vacuum of the TEM. Of particular interest is using these silicon chips to enclose fluids safely inside the TEM, allowing us to observe liquid dynamics at the nanoscale. In situ imaging of liquid phase reactions under TEM can greatly enhance our understanding of fundamental processes in fields from electrochemistry to cell biology. Here, we review how in situ TEM experiments of liquids can be performed, with a particular focus on microchip-encapsulated liquid cell TEM. We will cover the basics of the technique, and its strengths and weaknesses with respect to related in situ TEM methods for characterizing liquid systems. We will show how this technique has provided unique insights into nanomaterial synthesis and manipulation, battery science and biological cells. A discussion on the main challenges of the technique, and potential means to mitigate and overcome them, will also be presented.

Nanoscale temperature measurement during temperature controlled in situ TEM using Al plasmon nanothermometry

Over recent years, the advent of microelectromechanical system (MEMS)-type microheaters has pushed the limits of temperature controlled in situ transmission electron microscopy (TEM). In particular, by enabling the observation of the structure of materials in their application environments, temperature controlled TEM provides unprecedented insights into the link between the properties of materials and their structure in real-world problems, a clear knowledge of which is necessary for rational development of functional materials with new or improved properties. While temperature is the key parameter in such experiments, accessing the precise temperature of the sample at the nanoscale during observations still remains challenging. In the present work, we have applied aluminium plasmon nanothermometry technique that monitors the temperature dependence of the volume plasmon of Al nanospheres using electron energy loss spectroscopy for in situ local temperature determination over MEMS-type microheaters. With access to local temperatures between room temperature to 550 ^∘C, we have assessed the spatial and temporal stabilities of these microheaters when they operate at different setpoint temperatures both under vacuum and in the presence of a static H₂ gas environment. Temperature comparisons performed under the two environments show discrepancies between local and setpoint temperatures.

Observing the colloidal stability of iron oxide nanoparticles in situ

Colloidal processes such as nucleation, growth, ripening, and dissolution are fundamental to the synthesis and application of engineered nanoparticles, as well as numerous natural systems. In nanocolloids consisting of a dispersion of nanoparticles in solution, colloidal stability is influenced by factors including the particle surface facet and capping layer, and local temperature, chemistry, and acidity. In this paper, we investigate colloidal stability through the real-time manipulation of nanoparticles using in situ liquid cell Scanning Transmission Electron Microscopy (STEM). In a distribution of uniform iron oxide nanoparticles, we use the electron beam to precisely control the local chemistry of the solution and observe the critical role that surface chemistry plays in nanoparticle stability. By functionalizing the nanoparticle surfaces with charged amino acids and peptides, stability can be tuned to promote dissolution, growth, or agglomeration, either permanently or reversibly. STEM imaging is used to quantify kinetics of individual nanoparticles subject to local variations in chemistry. These measurements of dissolution and growth rates of iron oxide nanoparticles provide insights into nanoparticle stability relevant to synthesis and functionalization for biomedical applications.

Single Electron Precision in the Measurement of Charge Distributions on Electrically Biased Graphene Nanotips Using Electron Holography

We use off-axis electron holography to measure the electrostatic charge density distributions on graphene-based nanogap devices that have thicknesses of between 1 and 10 monolayers and separations of between 8 and 58 nm with a precision of better than a single unit charge. Our experimental measurements, which are compared with finite element simulations, show that wider graphene tips, which have thicknesses of a single monolayer at their ends, exhibit charge accumulation along their edges. The results are relevant for both fundamental research on graphene electrostatics and applications of graphene nanogaps to single nucleotide detection in DNA sequencing, single molecule electronics, plasmonic antennae, and cold field emission sources.

Atomic Mechanism in Layer-by-Layer Growth via Surface Reconstruction

Layer-by-layer growth played a critical role in the fine design of novel materials and devices. Although it has been widely studied during materials synthesis, the atomic mechanism of the growth remains unclear due to the lack of direct observation at the atomic scale. Here, we report a new mode in layer-by-layer growth via surface reconstruction on MoO₂ (011) by environmental transmission electron microscopy and density functional theory calculations. Our in situ environmental transmission electron microscopy results demonstrate that the layer-by-layer growth of MoO₂ experiences two steps that occur in an oscillatory manner: (1) the formation of an atomic ledge by transforming a section of the reconstructed layer to the intrinsic surface layer and then (2) the spontaneous reconstruction of the newly formed intrinsic surface section. Thus, the surface reconstruction can be considered as an intermediated phase during the layer-by-layer growth of MoO₂. A similar phenomenon was also observed in the MoO₂ dissolution procedure.

Nanofluidic and monolithic environmental cells for cryogenic microscopy

We present a device capable of combining nanofluidics and cryogenic transmission electron microscopy (cryo-TEM) to allow inspection of water-soluble samples under near-native conditions. The devices can be produced in a multitude of designs, but as a general rule, they consist of channels or chambers enclosed between two electron-transparent silicon nitride windows. With the appropriate design, those devices can allow screening of multiple samples in parallel and remove the interaction between the sample and the environment (no air-water interface). We demonstrate channel sizes from 80 to 500 nm in height and widths from 100 to 2000 μm. The presented fabrication flow allows producing hollow devices on a single wafer eliminating the need of aligning or bonding two half-cavities from separate wafers, which provides additional resistance to thermal stress. Taking advantage of a single-step through-membrane exposure with a 100 keV electron beam, we introduced arrays of thin (10-15 nm) electron-transparent silicon nitride membrane windows aligned between top and bottom (200-250 nm) carrier membranes. Importantly, the final devices are compatible with standard TEM holders. Furthermore, they are compatible with rapid freezing of samples, which is crucial for the formation of vitreous water, hence avoiding the formation of crystalline ice, that is detrimental for TEM imaging. To demonstrate the potential of this technology, we tested those devices in imaging experiments verifying their applicability for cryo-TEM applications and proved that vitreous water could be prepared through conventional plunge freezing of the chips.

Toward 3D imaging of corrosion at the nanoscale: Cross-sectional analysis of in-situ oxidized TEM samples

Understanding the effect of microstructural features on corrosion behavior will allow for significant improvements to alloy design for harsh environments. Recently, in-situ TEM has been recognized to offer significant data on corrosion behavior at the nanoscale, but in order for the best information to be acquired, a three dimensional view of the oxidation process is needed so that oxide structure and phase can be identified. Described herein is a new method of sample preparation for transmission electron microscopy (TEM) using a focused ion beam (FIB) to cross-section a previously FIB prepared sample. In-situ TEM was used to oxidize a sample using an environmental cell, and this in-situ sample is cross-sectioned to study oxide depth and oxide structure. This technique provides a new method to investigate in-situ TEM samples in 3D.

Three-Dimensional Quantification of the Facet Evolution of Pt Nanoparticles in a Variable Gaseous Environment

Pt nanoparticles play an essential role in a wide variety of catalytic reactions. The activity of the particles strongly depends on their three-dimensional (3D) structure and exposed facets, as well as on the reactive environment. High-resolution electron microscopy has often been used to characterize nanoparticle catalysts but unfortunately most observations so far have been either performed in vacuum and/or using conventional (2D) in situ microscopy. The latter however does not provide direct 3D morphological information. We have implemented a quantitative methodology to measure variations of the 3D atomic structure of nanoparticles under the flow of a selected gas. We were thereby able to quantify refaceting of Pt nanoparticles with atomic resolution during various oxidation-reduction cycles. In a H₂ environment, a more faceted surface morphology of the particles was observed with {100} and {111} planes being dominant. On the other hand, in O₂ the percentage of {100} and {111} facets decreased and a significant increase of higher order facets was found, resulting in a more rounded morphology. This methodology opens up new opportunities toward in situ characterization of catalytic nanoparticles because for the first time it enables one to directly measure 3D morphology variations at the atomic scale in a specific gaseous reaction environment.

Revealing Electron-Phonon Interactions and Lattice Dynamics in Nanocrystal Films by Combining in Situ Thermal Heating and Femtosecond Laser Excitations in 4D Transmission Electron Microscopy

We present a comparative investigation on static equilibrium and transient structural dynamics of nanocrystalline gold films on silicon nitride supports performed at various in situ temperatures and by ultrafast laser excitations in a four-dimensional ultrafast transmission electron microscope (4D-UTEM). The change of relative diffraction intensity and lattice spacing with rising temperatures was systematically measured for {220} Debye-Scherrer rings via the in situ heating technique, which leads to a precise determination of the actual Debye temperature and a finding of significant depression of lattice expansions in the films. The diffraction intensity/lattice spacing-temperature relationship calibrated by the static, thermally equilibrium observations was then employed for investigating ultrafast transient dynamics on the same specimen region. The electron-phonon coupling constant g was determined to be 7.2 × 10¹⁵ W/m³ K in combination with simple two-temperature model analysis. We found a marked variation of temperature rise maximum (at quasi-equilibrium states) in between the temporal evolutions of lattice spacing and diffraction intensity, a phenomenon which may only be explained by the effect of nonthermal equilibrium relaxation dynamics following femtosecond laser excitations. The method demonstrated here can thus be applied to quantitative evaluations of nonthermal equilibrium contributions during the electron-lattice thermalization.