Liquid Cell Transmission Electron Microscopy Research Articles

Investigating Charge-Induced Transformations of Metal Nanoparticles in a Radically-Inert Liquid: A Liquid-Cell TEM Study

We present a novel in situ liquid-cell transmission electron microscopy (TEM) approach to study the behavior of metal nanoparticles under high-energy electron irradiation. By utilizing a radically-inert liquid environment, we aim to minimize radiolysis effects and explore the influence of charge-induced transformations. We observed complex dynamics in nanoparticle behavior, including morphological changes and transitions between amorphous and crystalline states. These transformations are attributed to the delicate interplay between charge accumulation on the nanoparticles and enhanced radiolysis, suggesting a significant role for charge-assisted processes in nanoparticle evolution. Our findings provide valuable insights into the fundamental mechanisms driving nanoparticle behavior at the nanoscale and demonstrate the potential of liquid-cell TEM for studying complex physicochemical processes in controlled environments.

Micrometer-Scale Graphene-Based Liquid Cells of Highly Concentrated Salt Solutions for In Situ Liquid-Cell Transmission Electron Microscopy.

In situ liquid-cell transmission microscopy has attracted much attention as a method for the direct observations of the dynamics of soft matter. A graphene liquid cell (GLC) has previously been investigated as an alternative to a conventional SiN x liquid cell. Although GLCs are capable of scavenging radicals and providing high spatial resolutions, their production is fundamentally stochastic, and a significant compositional change in liquids encapsulated in GLCs has recently been pointed out. We found that graphene-based liquid cells were formed in nano- to micrometer sizes with high reproducibility when the concentration of the encapsulated aqueous salt solution was high. In contrast, when we revisited conventional fabrication methods, water-encapsulated GLC was formed with low yield, and any electron diffraction spots from ice were not confirmed by a cooling experiment. The reason for this was the presence of intrinsic defects in the graphene, the presence of which we confirmed by the etch-pit method. The shrinkage of a water-encapsulated cell and a decrease in the bubble area in an aqueous (NH4)2SO4 solution cell suggested that volatile water molecules and gas molecules can leak from the cells during the fabrication and observation processes. Further revision of the conditions for the formation of liquid cells and a reduction in the number of intrinsic graphene defects are expected to lead to the provision of graphene-based liquid cells capable of encapsulating dilute aqueous solutions or pure water.

Insight into the Internal Structure of High-Performance Multicore Magnetic Nanoparticles Used in Cancer Thermotherapy.

Multicore magnetic nanoparticles (MNPs), comprising iron oxide cores embedded in a sugar or starch matrix, are a class of nanomaterials with promising magnetic heating properties. Their internal structure, and particularly the strength of the internal core-core magnetic interactions, are believed to determine the functional properties, but there have been few detailed studies on this to date. We report here on an interlaboratory and multimodality transmission electron microscopy (TEM) and magnetic study of a high-performance MNP material (supplied by Resonant Circuits Limited, RCL) that is currently being used in a clinical study for the treatment of pancreatic cancer. TEM data were collected under a variety of conditions: conventional; high-resolution; scanning; cryogenic; and, for the first time, liquid phase. All the imaging modes showed mostly irregular dextran lamellae of lateral dimensions 30-90 nm, plus ca. 15% n/n of what appeared to be 30-60 nm long "nanorods", and a multitude of well-dispersed ca. 3.7 nm diameter iron oxide cores. Cryogenic electron tomography indicated that the nanorods were edge-on lamellae, but in dried samples, tomography showed rod- or lath-shaped forms, possibly resulting from the collapse of lamellae during drying. High-resolution TEM (HRTEM) showed the dextran to be crystallized in the low-temperature hydrated dextran polymorph. Magnetic remanence Henkel-plot analysis indicated a weak core-core interaction field of ca. 4.8 kA/m. Theoretical estimates using a point-dipole model associated this field with a core-to-core separation distance of ca. 5 nm, which tallies well with the ca. 4-6 nm range of separation distances observed in liquid-cell TEM data. On this basis, we identify the structure-function link in the RCL nanoparticles to be the unusually well-dispersed multicore structure that leads to their strong heating capability. This insight provides an important design characteristic for the future development of bespoke nanomaterials for this significant clinical application.

The rise and fall of adenine clusters in the gas phase: a glimpse into crystal growth and nucleation.

The emergence of a crystal nucleus from disordered states is a critical and challenging aspect of the crystallization process, primarily due to the extremely short length and timescales involved. Methods such as liquid-cell or low-dose focal-series transmission electron microscopy (TEM) are often employed to probe these events. In this study, we demonstrate that ion mobility spectrometry-mass spectrometry (IMS-MS) offers a complementary and insightful perspective on the nucleation process by examining the sizes and shapes of small clusters, specifically those ranging from n = 2 to 40. Our findings reveal the significant role of sulfate ions in the growth of adeninediium sulfate clusters, which are the precursors to the formation of single crystals. Specifically, sulfate ions stabilize adenine clusters at the 1:1 ratio. In contrast, guanine sulfate forms smaller clusters with varied ratios, which become stable as they approach the 1:2 ratio. The nucleation size is predicted to be between n = 8 and 14, correlating well with the unit cell dimensions of adenine crystals. This correlation suggests that IMS-MS can identify critical nucleation sizes and provide valuable structural information consistent with established crystallographic data. We also discuss the strengths and limitations of IMS-MS in this context. IMS-MS offers rapid and robust experimental protocols, making it a valuable tool for studying the effects of various additives on the assembly of small molecules. Additionally, it aids in elucidating nucleation processes and the growth of different crystal polymorphs.

Dynamic Evolution of Copper Nanowires during CO2 Reduction Probed by Operando Electrochemical 4D-STEM and X-ray Spectroscopy.

Nanowires have emerged as an important family of one-dimensional (1D) nanomaterials owing to their exceptional optical, electrical, and chemical properties. In particular, Cu nanowires (NWs) show promising applications in catalyzing the challenging electrochemical CO2 reduction reaction (CO2RR) to valuable chemical fuels. Despite early reports showing morphological changes of Cu NWs after CO2RR processes, their structural evolution and the resulting exact nature of active Cu sites remain largely elusive, which calls for the development of multimodal operando time-resolved nm-scale methods. Here, we report that well-defined 1D copper nanowires, with a diameter of around 30 nm, have a metallic 5-fold twinned Cu core and around 4 nm Cu2O shell. Operando electrochemical liquid-cell scanning transmission electron microscopy (EC-STEM) showed that as-synthesized Cu@Cu2O NWs experienced electroreduction of surface Cu2O to disordered (spongy) metallic Cu shell (Cu@CuS NWs) under CO2RR relevant conditions. Cu@CuS NWs further underwent a CO-driven Cu migration leading to a complete evolution to polycrystalline metallic Cu nanograins. Operando electrochemical four-dimensional (4D) STEM in liquid, assisted by machine learning, interrogates the complex structures of Cu nanograin boundaries. Correlative operando synchrotron-based high-energy-resolution X-ray absorption spectroscopy unambiguously probes the electroreduction of Cu@Cu2O to fully metallic Cu nanograins followed by partial reoxidation of surface Cu during postelectrolysis air exposure. This study shows that Cu nanowires evolve into completely different metallic Cu nanograin structures supporting the operando (operating) active sites for the CO2RR.

Understanding the Growth of Electrodeposited PtNi Nanoparticle Films Using Correlated In Situ Liquid Cell Transmission Electron Microscopy and Synchrotron Radiation.

Electrodeposition is a versatile method for synthesizing nanostructured films, but controlling the morphology of films containing two or more elements requires a detailed understanding of the deposition process. We used liquid cell transmission electron microscopy to follow the electrodeposition of PtNi nanoparticle films on a carbon electrode during cyclic voltammetry. These in situ observations show that the film thickness increases with each cycle, and by the fourth cycle, branched and porous structures could be deposited. Synchrotron studies using in situ transmission X-ray microscopy further revealed that Ni was deposited in the oxide phase. Ex situ studies of bulk electrodeposited PtNi nanoparticle films indicated the number of cycles and the scanning rate were the most influential parameters, resulting in a different thickness, a different homogeneity, a different nanoparticle size, and a different surface structure, while the precursor concentration did not have a significant influence. By varying the potential range, we were able to obtain films with different elemental compositions.

(Invited) In Situ Study of Catalyst Structural Evolution during CO2 Electroreduction by Development of Electrochemical Liquid Cell TEM

The electrocatalytic CO2 reduction reaction (CO2RR) has attracted significant interest of study in the exploration of sustainable energy and environment. Cu-based materials have demonstrated the capability of converting CO2 to multi-carbon products with high catalytic activity and low cost. Since the performance of heterogenous catalysts is highly susceptible to the catalyst structural changes, unveiling the structure and catalytic active sites during reactions is crucial to the understanding of catalytic mechanisms. Although lots of efforts have been made, it remains a challenge to directly image the atomic structure of a working catalyst in liquid electrolytes due to technical difficulties. Here, I will show our development of in-situ electrochemical liquid cell transmission electron microscopy (TEM) for the study of Cu catalysts during electrocatalytic CO2RR. The novel electrochemical liquid cell allows unprecedented atomic imaging of the catalyst evolution during electrochemical reactions. Our observations reveal that the Cu catalyst-electrolyte interaction under an applied potential (-1.1 v) leads to structural changes of Cu catalyst surfaces. Intermediate structure dynamics have been captured. The propagations and transformations of intermediates result in atomically rough Cu crystal surfaces. The intermediates are further found mediating the mass loss of Cu. Control experiments demonstrate the intermediates can be utilized for enhanced carbon-carbon coupling and the formation of two carbon products. Insights gained from this study are not only valuable to designing novel catalysts, but also useful for understanding of electrified solid-liquid interfaces in general.

(Invited) Multimodal Operando Studies of Dynamic Cu Nanocatalysts for CO2 Electroreduction

In an era of shifting the energy paradigm from fossil fuels to renewable energy, CO2 reduction reaction (CO2RR) emerges as a promising approach to covert greenhouse gas into valuable chemical fuels and close the carbon cycle for a sustainable energy supply. Since Cu remains the sole element for CO2RR to multicarbon products (C2+), significant efforts have been devoted to developing Cu electrocatalysts with higher selectivity and activity. However, the complex nature of active sites and the intrinsic structures under reaction conditions have remained largely elusive due to the lack of operando/in situ methods.1-3 In our previous studies, we reported that small Cu nanoparticles (sub-10 nm NPs) showed superior C2+ superior C2+ selectivity, relative to the larger sized Cu NPs, especially at low overpotentials.4,5 In this work, we present a comprehensive operando correlative study of dynamic evolution during the life cycle of a family of monodisperse Cu NP ensemble electrocatalysts under CO2RR.1 Operando electrochemical liquid-cell scanning transmission electron microscopy (EC-STEM) and 4D-STEM resolves microscopic dynamic morphological and structural evolution at the nm scale. Correlated operando high-energy-resolution fluorescence-detector (HERFD) X-ray absorption spectroscopy (XAS)6 reveals dynamic macroscopic changes in valence states and coordination environment. Statistical analysis of interparticle dynamics was probed by operando resonant soft X-ray-based small-angle X-ray scattering (SAXS).2 The operando correlative strategies, described herein, elucidates the longstanding enigmatic nature of Cu active sites as metallic Cu nanograins for selective CO2 electroreduction (Fig. 1). The strategy described herein can serve as a general platform to resolve the electrocatalytic interface of nanoparticle catalysts under real-time operating conditions across multiple time and length scales, thus serving the fundamental understanding necessary to development of many other electrochemical reactions for renewable energy technologies.

Temperature Dependent Growth Kinetics of Pd Nanocrystals: Insights from Liquid Cell Transmission Electron Microscopy.

Quantifying the role of experimental parameters on the growth of metal nanocrystals is crucial when designing synthesis protocols that yield specific structures. Here, the effect of temperature on the growth kinetics of radiolytically-formed branched palladium (Pd) nanocrystals is investigated by tracking their evolution using liquid cell transmission electron microscopy (TEM) and applying a temperature-dependent radiolysis model. At early times, kinetics consistent with growth limited is measured by the surface reaction rate, and it is found that the growth rate increases with temperature. After a transition time, kinetics consistent with growth limited by Pd atom supply is measured, which depends on the diffusion rate of Pd ions and atoms and the formation rate of Pd atoms by reduction of Pd ions by hydrated electrons. Growth in this regime is not strongly temperature-dependent, which is attributed to a balance between changes in the reducing agent concentration and the Pd ion diffusion rate. The observations suggest that branched rough surfaces, generally attributed to diffusion-limited growth, can form under surface reaction-limited kinetics. It is further shown that the combination of liquid cell TEM and radiolysis calculations can help identify the processes that determine crystal growth, with prospects for strategies for control during the synthesis of complex nanocrystals.

Extent of Radiolytic Damage from Liquid Cell TEM Experiments on Metal-Organic Frameworks via Post-Mortem 4D-STEM.

We report a systematic analysis of electron beam damage of the zeolitic imidazolate framework (ZIF-8) during liquid cell transmission electron microscopy (LCTEM). Our analysis reveals ZIF-8 morphology is strongly affected by solvent used (water vs dimethylformamide), electron flux applied, and imaging mode (i.e., TEM vs STEM), while ZIF-8 crystallinity is primarily affected by accumulated electron fluence. Our observations indicate that the stability of ZIF-8 morphology is higher in dimethylformamide (DMF) than in water. However, in situ electron diffraction indicates that ZIF-8 nanocrystals lose crystallinity at critical fluence of ∼80 e-Å-2 independent of the presence of solvent. Furthermore, 4D-STEM analysis as a post-mortem method reveals the extent of electron beam damage beyond the imaging area and indicates that radiolytic reactions are more pronounced in TEM mode than in STEM mode. These results illustrate the significance of radiolysis occurring while imaging ZIF-8 and present a workflow for assessing damage in LCTEM experiments.

In Situ Liquid Cell Transmission Electron Microscopy Observations of Growth and Anticorrosion Behaviors of AuCl3 Shell on Au Nanobipyramids

Comprehensive SummaryDesigning new materials and architectures to maintain activity and stability requires a better understanding on the anticorrosion dynamics of nanoparticles. Under‐coordinated atoms on the surface of nanoparticles can be protected by deposited shells. Real‐time observation on how protective shells grow and play a role is challenging but worthwhile. Here, protective effects of AuCl3 shells on Au nanobipyramids (NBPs) are studied in HAuCl4 aqueous solutions by in‐situ liquid cell transmission electron microscopy (LCTEM). This study is the first to observe the formation of Au‐AuCl3 core‐shell nanostructure and the corresponding anticorrosion behaviors of AuCl3 deposited shell. The presence of CTAB can substantially influence the growth mode and structure of AuCl3 shell, by a direct or indirect way, intervene the dissolution of Au NBP. These growth or dissolution kinetics here revealed at the nanoscale provide insights towards engineering of the surface anticorrosion to pursue Au nanoparticles with improved stability in acidic environment.

Operando Electrodeposition of Nonprecious Metal Copper Nanocatalysts on Low-Dimensional Support Materials for Nitrate Reduction Reactions.

Supported nonprecious metal catalysts such as copper (Cu) are promising replacements for Pt-based catalysts for a wide range of energy-related electrochemical reactions. Direct electrochemical deposition is one of the most straightforward and versatile methods to synthesize supported nonprecious metal catalysts. However, further advancement in the design of supported nonprecious metal catalysts requires a detailed mechanistic understanding of the interplay between kinetics and thermodynamics of the deposition phenomena under realistic reaction conditions. Here, we study the electrodeposition of Cu on carbon nanotubes and graphene derivatives under electrochemical conditions using in situ liquid cell transmission electron microscopy (TEM). By combining real-time imaging, electrochemical measurements, X-ray photoelectron spectroscopy (XPS), and finite-element analysis (FEA), we show that low-dimensional support materials, especially carbon nanotubes, are excellent for generating uniform and finely dispersed platinum group metal-(PGM)-free catalysts under mild electrochemical conditions. The electrodeposited Cu on graphene and carbon nanotubes is also observed to show good electrochemical activity toward nitrate reduction reactions (NO3RRs), further supported by density functional theory (DFT) calculations. Nitrogen doping plays an important role in guiding nonprecious metal deposition, but its low electrical conductivity may give rise to lower NO3RR activity compared to its nondoped analogue. The development of supported nonprecious metals through interfacial and surface engineering for the design of supported catalysts will substantially reduce the demand for precious metals and generate robust catalysts with better durability, thereby presenting opportunities for solving the critical problems in energy storage and electrocatalysis.

Reversible phase transformations between Pb nanocrystals and a viscous liquid-like phase.

Phase transformations have been a prominent topic of study for both fundamental and applied science. Solid-liquid reaction-induced phase transformations can be hard to characterize, and the transformation mechanisms are often not fully understood. Here, we report reversible phase transformations between a metal (Pb) nanocrystal and a viscous liquid-like phase unveiled by in situ liquid cell transmission electron microscopy. The reversible phase transformations are obtained by modulating the electron current density (between 1000 and 3000 electrons Å-2 s-1). The metal-organic viscous liquid-like phase exhibits short-range ordering with a preferred Pb-Pb distance of 0.5 nm. Assisted by density functional theory and molecular dynamics calculations, we show that the viscous liquid-like phase results from the reactions of Pb with the CH3O fragments from the triethylene glycol solution under electron beam irradiation. Such reversible phase transformations may find broad implementations.

Atomic dynamics of electrified solid-liquid interfaces in liquid-cell TEM.

Electrified solid-liquid interfaces (ESLIs) play a key role in various electrochemical processes relevant to energy1-5, biology6 and geochemistry7. The electron and mass transport at the electrified interfaces may result in structural modifications that markedly influence the reaction pathways. For example, electrocatalyst surface restructuring during reactions can substantially affect the catalysis mechanisms and reaction products1-3. Despite its importance, direct probing the atomic dynamics of solid-liquid interfaces under electric biasing is challenging owing to the nature of being buried in liquid electrolytes and the limited spatial resolution of current techniques for in situ imaging through liquids. Here, with our development of advanced polymer electrochemical liquid cells for transmission electron microscopy (TEM), we are able to directly monitor the atomic dynamics of ESLIs during copper (Cu)-catalysed CO2 electroreduction reactions (CO2ERs). Our observation reveals a fluctuating liquid-like amorphous interphase. It undergoes reversible crystalline-amorphous structural transformations and flows along the electrified Cu surface, thus mediating the crystalline Cu surface restructuring and mass loss through the interphase layer. The combination of real-time observation and theoretical calculations unveils an amorphization-mediated restructuring mechanism resulting from charge-activated surface reactions with the electrolyte. Our results open many opportunities to explore the atomic dynamics and its impact in broad systems involving ESLIs by taking advantage of the in situ imaging capability.

Real-time imaging reveal anisotropic dissolution behaviors of silver nanorods

The morphology and size control of anisotropic nanocrystals are critical for tuning shape-dependent physicochemical properties. Although the anisotropic dissolution process is considered to be an effective means to precisely control the size and morphology of nanocrystals, the anisotropic dissolution mechanism remains poorly understood. Here, using in situ liquid cell transmission electron microscopy, we investigate the anisotropic etching dissolution behaviors of polyvinylpyrrolidone (PVP)-stabilized Ag nanorods in NaCl solution. Results show that etching dissolution occurs only in the longitudinal direction of the nanorod at low chloride concentration (0.2 mM), whereas at high chloride concentration (1 M), the lateral and longitudinal directions of the nanorods are dissolved. First-principles calculations demonstrate that PVP is selectively adsorbed on the {100} crystal plane of silver nanorods, making the tips of nanorods the only reaction sites in the anisotropic etching process. When the chemical potential difference of the Cl− concentration is higher than the diffusion barrier (0.196 eV) of Cl− in the PVP molecule, Cl− penetrates the PVP molecular layer of {100} facets on the side of the Ag nanorods. These findings provide an in-depth insight into the anisotropic etching mechanisms and lay foundations for the controlled preparation and rational design of nanostructures.

4D-STEM Mapping of Nanocrystal Reaction Dynamics and Heterogeneity in a Graphene Liquid Cell.

Chemical reaction kinetics at the nanoscale are intertwined with heterogeneity in structure and composition. However, mapping such heterogeneity in a liquid environment is extremely challenging. Here we integrate graphene liquid cell (GLC) transmission electron microscopy and four-dimensional scanning transmission electron microscopy to image the etching dynamics of gold nanorods in the reaction media. Critical to our experiment is the small liquid thickness in a GLC that allows the collection of high-quality electron diffraction patterns at low dose conditions. Machine learning-based data-mining of the diffraction patterns maps the three-dimensional nanocrystal orientation, groups spatial domains of various species in the GLC, and identifies newly generated nanocrystallites during reaction, offering a comprehensive understanding on the reaction mechanism inside a nanoenvironment. This work opens opportunities in probing the interplay of structural properties such as phase and strain with solution-phase reaction dynamics, which is important for applications in catalysis, energy storage, and self-assembly.

Oriented Assembly of Lead Halide Perovskite Nanocrystals.

Cesium lead halide nanostructures have highly tunable optical and optoelectronic properties. Establishing precise control in forming perovskite single-crystal nanostructures is key to unlocking the full potential of these materials. However, studying the growth kinetics of colloidal cesium lead halides is challenging due to their sensitivity to light, electron beam, and environmental factors like humidity. In this study, in situ observations of CsPbBr3-particle dynamics were made possible through extremely low dose liquid cell transmission electron microscopy, showing that oriented attachment is the dominant pathway for the growth of single-crystal CsPbBr3 architectures from primary nanocubes. In addition, oriented assembly and fusion of ligand-stabilized cubic CsPbBr3 nanocrystals are promoted by electron beam irradiation or introduction of polar additives that both induce partial desorption of the original ligands and polarize the nanocube surfaces. These findings advance our understanding of cesium lead halide growth mechanisms, aiding the controlled synthesis of other perovskite nanostructures.

Revealing the Intrinsic Restructuring of Bi2O3 Nanoparticles into Bi Nanosheets during Electrochemical CO2 Reduction.

Bismuth is a catalyst material that selectively produces formate during the electrochemical reduction of CO2. While different synthesis strategies have been employed to create electrocatalysts with better performance, the restructuring of bismuth precatalysts during the reaction has also been previously reported. The mechanism behind the change has, however, remained unclear. Here, we show that Bi2O3 nanoparticles supported on Vulcan carbon intrinsically transform into stellated nanosheet aggregates upon exposure to an electrolyte. Liquid cell transmission electron microscopy observations first revealed the gradual restructuring of the nanoparticles into nanosheets in the presence of 0.1 M KHCO3 without an applied potential. Our experiments also associated the restructuring with solubility of bismuth in the electrolyte. While the consequent agglomerates were stable under moderate negative potentials (-0.3 VRHE), they dissolved over time at larger negative potentials (-0.4 and -0.5 VRHE). Operando Raman spectra collected during the reaction showed that under an applied potential, the oxide particles reduced to metallic bismuth, thereby confirming the metal as the working phase for producing formate. These results inform us about the working morphology of these electrocatalysts and their formation and degradation mechanisms.

Strategies to overcome electron-beam issues in liquid phase TEM: Study of chemical processes

AbstractLiquid phase (or liquid cell) transmission electron microscopy (TEM) has become a powerful platform for in situ investigation of various chemical processes at the nanometer or atomic level. The electron beam for imaging can also induce perturbation to the chemical processes. Thus, it has been a concern that the observed phenomena in a liquid cell could deviate from the real-world processes. Strategies have been developed to overcome the electron-beam-induced issues. This article provides an overview of the electron-beam effects, and discusses various strategies in liquid cell TEM study of nucleation, growth, and self-assembly of nanoscale materials, where an electron beam is often used to initiate the reactions, and highly electron-beam-sensitive electrochemical reactions. Graphical abstract

Corrosion Dynamics of Carbon-Supported Platinum Electrocatalysts with Metal-Carbon Interactions Revealed by In Situ Liquid Transmission Electron Microscopy.

Carbon support is essential for electrocatalysis, but limitations remain, as carbon corrosion can lead to electrocatalyst degradation and affect the long-term durability of electrocatalysts. Here, we studied the corrosion dynamics of carbon nanotubes (CNTs) and Vulcan carbon (VC) together with platinum (Pt) nanoparticles in real time by liquid cell (LC) transmission electron microscopy (TEM). The results showed that CNTs with a high degree of graphitization exhibited higher corrosion resistance compared to VC. Furthermore, we observed that the main degradation path of Pt nanoparticles in Pt/CNTs was ripening, while in Pt/VC, it was aggregation and coalescence, which was dominated by the interactions between Pt nanoparticles and different hybridization of carbon supports. Finally, we performed an ex situ CV stability test to confirm the conclusions obtained from in situ experiments. This work provides deep insights into the corrosion mechanism of carbon-supported electrocatalysts to optimize the design of electrocatalysts with a higher durability.