Understanding the mechanisms of anisotropic dissolution in metal oxides by applying radiolysis simulations to liquid-phase TEM.

Iron-based redox-active minerals are ubiquitous in soils, sediments, and aquatic systems. Their dissolution is of great importance for microbial impacts on carbon cycling and the biogeochemistry of the lithosphere and hydrosphere. Despite its widespread significance and extensive prior study, the atomic-to-nanoscale mechanisms of dissolution remain poorly understood, particularly the interplay between acidic and reductive processes. Here, we use in situ liquid-phase-transmission electron microscopy (LP-TEM) and simulations of radiolysis to probe and control acidic versus reductive dissolution of akaganeite (ß-FeOOH) nanorods. Informed by crystal structure and surface chemistry, the balance between acidic dissolution at rod tips and reductive dissolution at rod sides was systematically varied using pH buffers, background chloride anions, and electron beam dose. We find that buffers, such as bis-tris, effectively inhibited dissolution by consuming radiolytic acidic and reducing species such as superoxides and aqueous electrons. In contrast, chloride anions simultaneously suppressed dissolution at rod tips by stabilizing structural elements while promoting dissolution at rod sides through surface complexation. Dissolution behaviors were systematically varied by shifting the balance between acidic and reductive attacks. The findings show LP-TEM combined with simulations of radiolysis effects can provide a unique and versatile platform for quantitatively investigating dissolution mechanisms, with implications for understanding metal cycling in natural environments and the development of tailored nanomaterials.

Unveiling Corrosion Pathways of Sn Nanocrystals through High-Resolution Liquid Cell Electron Microscopy.

Unveiling materials' corrosion pathways is significant for understanding the corrosion mechanisms and designing corrosion-resistant materials. Here, we investigate the corrosion behavior of Sn@Ni3Sn4 and Sn nanocrystals in an aqueous solution in real time by using high-resolution liquid cell transmission electron microscopy. Our direct observation reveals an unprecedented level of detail on the corrosion of Sn metal with/without a coating of Ni3Sn4 at the nanometric and atomic levels. The Sn@Ni3Sn4 nanocrystals exhibit "pitting corrosion", which is initiated at the defect sites in the Ni3Sn4 protective layer. The early stage isotropic etching transforms into facet-dependent etching, resulting in a cavity terminated with low-index facets. The Sn nanocrystals under fast etching kinetics show uniform corrosion, and smooth surfaces are obtained. Sn nanocrystals show "creeping-like" etching behavior and rough surfaces. This study provides critical insights into the impacts of coating, defects, and ion diffusion on corrosion kinetics and the resulting morphologies.

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, usingin situliquid 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.

Revealing Dominant Oxidative Species in Reactive Oxygen Species-Driven Rapid Chemical Etching.

Reactive oxygen species (ROS) widely participate in a variety of chemical reactions in biological and chemical applications. However, due to the extremely short lifetime of most ROS, conventional ROS-detecting techniques cannot show real-time dynamic changes of ROS-driven chemical reactions and identify the actual role of individual reactive species in these reactions. Herein, using in situ liquid cell TEM complemented by ex situ experiments, we directly visualize ROS-driven rapid etching of Prussian bule (PB) in real time and identify the dominant reactive species in etching processes. The results reveal that highly oxidative •OH is the dominant reactive radical in ROS-driven rapid chemical etching and hollow mesoporous PB nanoparticles can be synthesized on a minute-level time scale via •OH-dominated rapid etching. This work provides insight into ROS-related oxidation, which can continuously improve our understanding of ROS chemistry and make ROS more widely applicable in advanced chemical etching.

Intermediate states of molecular self-assembly from liquid-cell electron microscopy.

Traditional single-molecule methods do not report whole-molecule kinetic conformations, and their adaptive shape changes during the process of self-assembly. Here, using graphene liquid-cell electron microscopy with electrons of low energy at low dose, we show that this approach resolves the time dependence of conformational adaptations of macromolecules for times up to minutes, the resolution determined by motion blurring, with DNA as the test case. Single-stranded DNA molecules are observed in real time as they hybridize near the solid surface to form double-stranded helices; we contrast molecules the same length but differing in base-pair microstructure (random, blocky, and palindromic hairpin) whose key difference is that random sequences possess only one stable final state, but the others offer metastable intermediate structures. Hybridization is observed to couple with enhanced translational mobility and torsion-induced rotation of the molecule. Prevalent transient loops are observed in error-correction processes. Transient melting and other failed encounters are observed in the competitive binding of multiple single-stranded molecules. Among the intermediate states reported here, some were predicted but not observed previously, and the high incidence of looping and enhanced mobility come as surprises. The error-producing mechanisms, failed encounters, and transient intermediate states would not be easily resolved by traditional single-molecule methods. The methods generalize to visualize motions and interactions of other organic macromolecules.

In Situ TEM Observation of Stagnant Liquid Layer Activation in Nanochannel.

The kinetics of mass transfer in a stagnant fluid layer next to an interface govern numerous dynamic reactions in diffusional micro/nanopores, such as catalysis, fuel cells, and chemical separation. However, the effect of the interplay between stagnant liquid and flowing fluid on the micro/nanoscopic mass transfer dynamics remains poorly understood. Here, by using liquid cell transmission electron microscopy (TEM), we directly tracked microfluid unit migration at the nanoscale. By tracking the trajectories, an unexpected mass transfer phenomenon in which fluid units in the stagnant liquid layer migrated two orders faster during gas-liquid interface updating was identified. Molecular dynamics (MD) simulations indicated that the chemical potential difference between nanoscale liquid layers led to convective flow, which greatly enhanced mass transfer on the surface. Our study opens up a pathway toward research on mass transfer in the surface liquid layers at high spatial and temporal resolutions.

Using Single-Crystal Graphene to Form Arrays of Nanocapsules Enabling the Observation of Light Elements in Liquid Cell Transmission Electron Microscopy.

We have designed and fabricated a TEM (transmission electron microscopy) liquid cell with hundreds of graphene nanocapsules arranged in a stack of two Si3N4-x membranes. These graphene nanocapsules are formed on arrays of nanoholes patterned on the Si3N4-x membrane by focused ion beam milling, allowing for better resolution than for the conventional graphene liquid cells, which enables the observation of light elements, such as atomic structures of silicon. We suggest that multiple nanocapsules provide opportunities for consecutive imaging under the same conditions in a single liquid cell. The use of single-crystal graphene windows offers an excellent signal-to-noise ratio and high spatial resolution. The motion of silicon nanoparticles (a low atomic number (Z) material) interacting with nanobubbles was observed, and analyzed, in detail. Our approach will help advance liquid-phase TEM observations by providing a straightforward method to encapsulate liquid between monolayers of various 2-dimensional materials.

Supramolecular systems chemistry through advanced analytical techniques.

Supramolecular chemistry is the quintessential backbone of all biological processes. It encompasses a wide range from the metabolic network to the self-assembled cytoskeletal network. Combining the chemical diversity with the plethora of functional depth that biological systems possess is a daunting task for synthetic chemists to emulate. The only route for approaching such a challenge lies in understanding the complex and dynamic systems through advanced analytical techniques. The supramolecular complexity that can be successfully generated and analyzed is directly dependent on the analytical treatment of the system parameters. In this review, we illustrate advanced analytical techniques that have been used to investigate various supramolecular systems including complex mixtures, dynamic self-assembly, and functional nanomaterials. The underlying theme of such an overview is not only the exceeding detail with which traditional experiments can be probed but also the fact that complex experiments can now be attempted owing to the analytical techniques that can resolve an ensemble in astounding detail. Furthermore, the review critically analyzes the current state of the art analytical techniques and suggests the direction of future development. Finally, we envision that integrating multiple analytical methods into a common platform will open completely new possibilities for developing functional chemical systems.

Controlling the Facet of ZnO during Wet Chemical Etching Its (000 1 ¯ ) O-Terminated Surface.

Special surface plays a crucial role in nature as well as in industry. Here, the surface morphology evolution of ZnO during wet etching is studied by in situ liquid cell transmission electron microscopy and ex situ wet chemical etching. Many hillocks are observed on the (000 1 ¯ ) O-terminated surface of ZnO nano/micro belts during in situ etching. Nanoparticles on the apex of the hillocks are observed to be essential for the formation of the hillocks, providing direct experimental evidence of the micromasking mechanism. The surfaces of the hillocks are identified to be {01 1 ¯ 3 ¯ } crystal facets, which is different from the known fact that {01 1 ¯ 1 ¯ } crystal facets appear on the (000 1 ¯ ) O-terminated surface of ZnO after wet chemical etching. O2 plasma treatment is found to be the key factor for the appearance of {01 1 ¯ 3 ¯ } instead of {01 1 ¯ 1 ¯ } crystal facets after etching for both ZnO nano/micro belts and bulk materials. The synergistic effect of acidic etching and O-rich surface caused by O2 plasma treatment is proposed to be the cause of the appearance of {01 1 ¯ 3 ¯ } crystal facets. This method can be extended to control the surface morphology of other materials during wet chemical etching.

A Universal Nano-capillary Based Method of Catalyst Immobilization for Liquid-Cell Transmission Electron Microscopy.

A universal nano-capillary based method for sample deposition on the silicon nitride membrane of liquid-cell transmission electron microscopy (LCTEM) chips is demonstrated. It is applicable to all substances which can be dispersed in a solvent and are suitable for drop casting, including catalysts, biological samples, and polymers. Most importantly, this method overcomes limitations concerning sample immobilization due to the fragility of the ultra-thin silicon nitride membrane required for electron transmission. Thus, a straightforward way is presented to widen the research area of LCTEM to encompass any sample which can be externally deposited beforehand. Using this method, Nix B nanoparticles are deposited on the µm-scale working electrode of the LCTEM chip and in situ observation of single catalyst particles during ethanol oxidation is for the first time successfully monitored by means of TEM movies.

In Situ Liquid Cell TEM Reveals Bridge-Induced Contact and Fusion of Au Nanocrystals in Aqueous Solution.

During nanoparticle coalescence in aqueous solution, dehydration and initial contact of particles are critically important but poorly understood processes. In this work, we used in situ liquid-cell transmission electron microscopy to directly visualize the coalescence process of Au nanocrystals. It is found that the Au atomic nanobridge forms between adjacent nanocrystals that are separated by a ∼0.5 nm hydration layer. The nanobridge structure first induces initial contact of Au nanocrystals over their hydration layers and then surface diffusion and grain boundary migration to rearrange into a single nanocrystal. Classical density functional theory calculations and ab initio molecular dynamics simulations suggest that the formation of the nanobridge can be attributed to the accumulation of auric ions and a higher local supersaturation in the gap, which can promote dehydration, contact, and fusion of Au nanocrystals. The discovery of this multistep process advances our understanding of the nanoparticle coalescence mechanism in aqueous solutions.

Unravelling the Mechanisms of Gold-Silver Core-Shell Nanostructure Formation by in Situ TEM Using an Advanced Liquid Cell Design.

The growth of silver shells on gold nanorods is investigated by in situ liquid cell transmission electron microscopy using an advanced liquid cell architecture. The design is based on microwells in which the liquid is confined between a thin Si3N4 membrane on one side and a few-layer graphene cap on the other side. A well-defined specimen thickness and an ultraflat cell top allow for the application of high-resolution TEM and the application of analytical TEM techniques on the same sample. The combination of high-resolution data with chemical information is validated by radically new insights into the growth of silver shells on cetrimonium bromide stabilized gold nanorods. It is shown that silver bromide particles already formed in the stock solution play an important role in the exchange of silver ions. The Ag shell growth can be directly correlated with the layer-by-layer dissolution of AgBr nanocrystals, which can be controlled by the electron flux density via distinctly generated chemical species in the solvent. The derived model framework is confirmed by in situ UV-vis absorption spectroscopy evaluating the blue shift in the longitudinal surface plasmon resonance of anisotropic NRs in a complementary batch experiment.

Dynamics of Nanoscale Dendrite Formation in Solution Growth Revealed Through in Situ Liquid Cell Electron Microscopy.

Formation mechanisms of dendrite structures have been extensively explored theoretically, and many theoretical predictions have been validated for micro- or macroscale dendrites. However, it is challenging to determine whether classical dendrite growth theories are applicable at the nanoscale due to the lack of detailed information on the nanodendrite growth dynamics. Here, we study iron oxide nanodendrite formation using liquid cell transmission electron microscopy (TEM). We observe "seaweed"-like iron oxide nanodendrites growing predominantly in two dimensions on the membrane of a liquid cell. By tracking the trajectories of their morphology development with high spatial and temporal resolution, it is possible to explore the relationship between the tip curvature and growth rate, tip splitting mechanisms, and the effects of precursor diffusion and depletion on the morphology evolution. We show that the growth of iron oxide nanodendrites is remarkably consistent with the existing theoretical predictions on dendritic morphology evolution during growth, despite occurring at the nanoscale.

In†Situ Observation of Dynamic Galvanic Replacement Reactions in Twinned Metallic Nanowires by Liquid Cell Transmission Electron Microscopy.

Galvanic replacement is a versatile approach to prepare hollow nanostructures with controllable morphology and elemental composition. The primary issue is to identify its fundamental mechanism. In this study, in situ liquid cell transmission electron microscopy was employed to monitor the dynamic reaction process and to explore the mechanism of galvanic replacement. The detailed reaction process was revealed based on in situ experiments in which small Au particles first appeared around Ag nanowires; they coalesced, grew, and adhered to Ag nanowires. After that, small pits grew from the edge of Ag nanowires to form tubular structures, and then extended along the Ag nanowires to obtain hollowed structures. All of our experimental observations from the viewpoint of electron microscopy, combined with DFT calculations, contribute towards an in-depth understanding of the galvanic replacement reaction process and the design of new materials with hollow structures.

Driving reversible redox reactions at solid-liquid interfaces with the electron beam of a transmission electron microscope.

Liquid-cell transmission electron microscopy (LCTEM) has opened up a new way to study chemical reactions at the interface between solids and liquids. However, understanding the effects of the electron beam in the liquid cell has been clearly identified as one of the most important challenges to assess correctly and quantitatively LCTEM data. Here we show that the electron beam can be used to drive reversible deposition/dissolution cycles of copper shells over gold nanoparticles in methanol. Besides revealing the influence of irreversible processes on the kinetic of growth/etching cycles, this study of nanostructure behaviour as a function of the dose rate highlights the possibility to switch the oxidising or reducing nature of liquid environment only with the electron beam. The chemical and electronic processes possibly involved in these tunable redox reactions are qualitatively discussed together with their possible impacts on electrochemical LCTEM experiments.

Exploring the Formation of Symmetric Gold Nanostars by Liquid-Cell Transmission Electron Microscopy.

The shape-dependent properties of gold nanostars (NSs) have motivated massive research efforts in the field of colloidal chemistry to gain a better control over the morphology of these promising nanostructures. Nevertheless, this challenge requires a better understanding of the atomic-scale processes leading to the formation of stellated nanoparticles. We hereby report an unprecedented in situ study focused on the seed-mediated synthesis of symmetric gold NSs performed by radiolysis in methanol. We take advantage of the spatial and temporal resolutions of liquid-cell transmission electron microscopy to unravel the key effects of the growth speed, seed-crystal morphology, and dimethylamine functionalization on the formation mechanisms, shape, and stability of NSs enclosed by high-index facets. Surprisingly, the stellation processes transforming icosahedral nanoparticles into NSs with 20 sharp arms entails a continuous restructuring of NS facets driven by surface diffusion, which provide a fresh look at faceting mechanisms.

Tracking Nanoparticle Diffusion and Interaction during Self-Assembly in a Liquid Cell.

Nanoparticle self-assembly has been well studied theoretically, but it remains challenging to directly observe and quantify individual nanoparticle interactions. With our custom image analysis method, we track the trajectories of nanoparticle movement with high precision from a stack of relatively noisy images obtained using liquid cell transmission electron microscopy. In a time frame of minutes, Pt-Fe nanoparticles self-assembled into a loosely packed hcp lattice. The energetics and stability of the dynamic assembly were studied quantitatively. From velocity and diffusion measurements, we experimentally determined the magnitude of forces between single particles and the related physical properties. The results illustrate that long-range anisotropic forces drive the formation of chains, which then clump and fold to maximize close range van der Waals interactions.

Liquid Cell Transmission Electron Microscopy.

Liquid cell transmission electron microscopy (TEM) has attracted significant interest in recent years. With nanofabricated liquid cells, it has been possible to image through liquids using TEM with subnanometer resolution, and many previously unseen materials dynamics have been revealed. Liquid cell TEM has been applied to many areas of research, ranging from chemistry to physics, materials science, and biology. So far, topics of study include nanoparticle growth and assembly, electrochemical deposition and lithiation for batteries, tracking and manipulation of nanoparticles, catalysis, and imaging of biological materials. In this article, we first review the development of liquid cell TEM and then highlight progress in various areas of research. In the study of nanoparticle growth, the electron beam can serve both as the illumination source for imaging and as the input energy for reactions. However, many other research topics require the control of electron beam effects to minimize electron beam damage. We discuss efforts to understand electron beam-liquid matter interactions. Finally, we provide a perspective on future challenges and opportunities in liquid cell TEM.

Giant Radiolytic Dissolution Rates of Aqueous Ceria Observed in Situ by Liquid-Cell TEM.

The dynamics of cerium oxide nanoparticle aqueous corrosion are revealed in situ. We use innovative liquid-cell transmission electron microscopy (TEM) combined with deliberate high-intensity electron-beam irradiation of nanoparticle suspensions. This enables life video-recording of materials reactions in liquid, with nm resolution. We introduce image quantification to measure detailed rates of dissolution as a function of time and particle size to be compared with literature data. Giant dissolution rates, exceeding any previous reports for chemical dissolution rates at room temperature by many orders of magnitude, are discovered. The reasons for this accelerated dissolution are outlined, including the importance of the radiolysis of water preceding the ceria attack. Electron-water interaction generates radicals, ions, and hydrated electrons, which assist in hydration and reductive dissolution of oxide minerals. The presented methodology has the potential to become a novel accelerated testing procedure to compare multiple nanoscale materials for relative aqueous durability. The ceria-water system is of crucial importance for the fields of catalysis, abrasive polishing, environmental remediation, and as simulant for actinide oxide behaviour in contact with liquid for nuclear engineering.

Exploring dynamic surface processes during silicate mineral (wollastonite) dissolution with liquid cell TEM.

Most liquid cell transmission electron microscopy (LC TEM) studies focus on nanoparticles or nanowires, in large part because the preparation and study of materials in this size range is straightforward. By contrast, this is not true for samples in the micrometre size range, in large part because of the difficulties associated with sample preparation starting from a 'bulk' material. There are also many advantages inherent to the study of micrometre-sized samples compared to their nanometre-sized counterparts. Here, we present a liquid cell transmission electron study that employed an innovative sample preparation technique using focused ion beam (FIB) milling to fabricate micrometre-sized electron transparent lamellae that were then welded to the liquid cell substrate. This technique, for which we have described in detail all of the fabrication steps, allows for samples having dimensions of several square micrometres to be observed by TEM in situ in a liquid. We applied this technique to test whether we could observe and measure in situ dissolution of a crystalline material called wollastonite, a calcium silicate mineral. More specifically, this study was used to observe and record surface dynamics associated with step and terrace edge movement, which are ultimately linked to the overall rate of dissolution. The wollastonite lamella underwent chemical reactions in pure deionized water at ambient temperature in a liquid cell with a 5-µm-spacer thickness. The movement of surface steps and terraces was measured periodically over a period of almost 5 h. Quite unexpectedly, the one-dimensional rates of retreat of these surface features were not constant, but changed over time. In addition, there were noticeable quantitative differences in retreat rates as a function crystallographic orientation, indicating that surface retreat is anisotropic. Several bulk rates of dissolution were also determined (1.6-4.2 • 10-7 mol m-2 s-1 ) using the rates of retreat of representative terraces and steps, and were found to be within one order of magnitude of dissolution rates in the literature based on aqueous chemistry data.