Direct Visualization of Chemical Transport in Solid-State Chemical Reactions by Time-of-Flight Secondary Ion Mass Spectrometry

Systematic control and design of solid-state chemical reactions are required for modifying materials properties and in novel synthesis. Understanding chemical dynamics at the nanoscale is therefore essential to revealing the key reactive pathways. Herein, we combine focused ion beam-scanning electron microscopy (FIB-SEM) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) to track the migration of sodium from a borate coating to the oxide scale during in situ hot corrosion testing. We map the changing distribution of chemical elements and compounds from 50 to 850 °C to reveal how sodium diffusion induces corrosion. The results are validated by in situ X-ray diffraction and post-mortem TOF-SIMS. We additionally retrieve the through-solid sodium diffusion rate by fitting measurements to a Fickian diffusion model. This study presents a step change in analyzing microscopic diffusion mechanics with high chemical sensitivity and selectivity, a widespread analytical challenge that underpins the defining rates and mechanisms of solid-state reactions.

Identification of a quasi-liquid phase at solid-liquid interface

An understanding of solid–liquid interfaces is of great importance for fundamental research as well as industrial applications. However, it has been very challenging to directly image solid–liquid interfaces with high resolution, thus their structure and properties are often unknown. Here, we report a quasi-liquid phase between metal (In, Sn) nanoparticle surfaces and an aqueous solution observed using liquid cell transmission electron microscopy. Our real-time high-resolution imaging reveals a thin layer of liquid-like materials at the interfaces with the frequent appearance of small In nanoclusters. Such a quasi-liquid phase serves as an intermediate for the mass transport from the metal nanoparticle to the liquid. Density functional theory-molecular dynamics simulations demonstrate that the positive charges of In ions greatly contribute to the stabilization of the quasi-liquid phase on the metal surface.

Solid–liquid interfaces are ubiquitous in natural and technological processes, but their imaging at the atomic scale has been challenging. The authors, using liquid-phase transmission electron microscopy, identify a quasi-liquid phase and the mass transport between the surface of In and Sn nanocrystals and an aqueous solution.

Graphene Liquid Cell Electron Microscopy: Progress, Applications, and Perspectives

Graphene liquid cell electron microscopy (GLC-EM), a cutting-edge liquid-phase EM technique, has become a powerful tool to directly visualize wet biological samples and the microstructural dynamics of nanomaterials in liquids. GLC uses graphene sheets with a one carbon atom thickness as a viewing window and a liquid container. As a result, GLC facilitates atomic-scale observation while sustaining intact liquids inside an ultra-high-vacuum transmission electron microscopy chamber. Using GLC-EM, diverse scientific results have been recently reported in the material, colloidal, environmental, and life science fields. Here, the developments of GLC fabrications, such as first-generation veil-type cells, second-generation well-type cells, and third-generation liquid-flowing cells, are summarized. Moreover, recent GLC-EM studies on colloidal nanoparticles, battery electrodes, mineralization, and wet biological samples are also highlighted. Finally, the considerations and future opportunities associated with GLC-EM are discussed to offer broad understanding and insight on atomic-resolution imaging in liquid-state dynamics.

Development of in situ optical spectroscopy with high temporal resolution in an aberration-corrected transmission electron microscope

Exploring the corresponding relation between structural and physical properties of materials at the atomic scale remains the fundamental problem in science. With the development of the aberration-corrected transmission electron microscopy (AC-TEM) and the ultrafast optical spectroscopy technique, sub-angstrom-scale spatial resolution and femtosecond-scale temporal resolution can be achieved, respectively. However, the attempt to combine both their advantages is still a great challenge. Here, we develop in situ optical spectroscopy with high temporal resolution in AC-TEM by utilizing a self-designed and manufactured TEM specimen holder, which has the capacity of sub-angstrom-scale spatial resolution and femtosecond-scale temporal resolution. The key and unique design of our apparatus is the use of the fiber bundle, which enables the delivery of focused pulse beams into TEM and collection of optical response simultaneously. The generated focused spot has a size less than 2 µm and can be scanned in plane with an area larger than 75 × 75 µm². Most importantly, the positive group-velocity dispersion caused by glass fiber is compensated by a pair of diffraction gratings, thus resulting in the generation of pulse beams with a pulse width of about 300 fs (@ 3 mW) in TEM. The in situ experiment, observing the atomic structure of CdSe/ZnS quantum dots in AC-TEM and obtaining the photoluminescence lifetime (∼4.3 ns) in the meantime, has been realized. Further ultrafast optical spectroscopy with femtosecond-scale temporal resolution could be performed in TEM by utilizing this apparatus.

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.

Real-time visualization of diffusion-controlled nanowire growth in solution

This Letter shows that copper nanowires grow through the diffusion-controlled reduction of dihydroxycopper(I), Cu(OH)2(-). A combination of potentiostatic coulometry, UV-visible spectroscopy, and thermodynamic calculations was used to determine the species adding to growing Cu nanowires is Cu(OH)2(-). Cyclic voltammetry was then used to measure the diffusion coefficient of Cu(OH)2(-) in the reaction solution. Given the diameter of a Cu nanowire and the diffusion coefficient of Cu(OH)2(-), we calculated the dependence of the diffusion-limited growth rate on the concentration of copper ions to be 26 nm s(-1) mM(-1). Independent measurements of the nanowire growth rate with dark-field optical microscopy yielded 24 nm s(-1) mM(-1) for the growth rate dependence on the concentration of copper. Dependence of the nanowire growth rate on temperature yielded a low activation energy of 11.5 kJ mol(-1), consistent with diffusion-limited growth.

Visualizing the electrochemical reaction of ZnO nanoparticles with lithium by in situ TEM: two reaction modes are revealed

The lithiation reaction of ZnO as an anode in a lithium-ion battery (LIB) is unclear. The electrochemical behavior of ZnO was investigated inside a transmission electron microscope (TEM) by constructing a nano-LIB using an individual ZnO/graphene sheet as the electrode. The lithiation reaction of ZnO/graphene was monitored by simultaneous determination of the structure with high-resolution TEM, electron diffraction pattern and electron energy-loss spectroscopy. Two kinds of reaction modes were revealed in terms of different reaction rates. One was the violent reaction mode, in which one particle can evolve into an aggregate of many nanoparticles within the Li2O matrix in 1-2 min. The other was the peaceful evolution mode, in which each ZnO nanoparticle evolves into a core-shell particle with multi-domains constituted of Zn and LiZn nanograins. Abnormally large Zn nanocrystals grow quickly in the violent reaction mode, which can suppress the formation of LiZn and impair the reversible capacity. Our observations give direct evidence and important insights for the lithiation mechanism of metal oxide anodes in LIBs.

Recent developments and applications of electron microscopy to heterogeneous catalysis

Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) are popular and powerful techniques used to characterize heterogeneous catalysts. Rapid developments in electron microscopy--especially aberration correctors and in situ methods--permit remarkable capabilities for visualizing both morphologies and atomic and electronic structures. The purpose of this review is to summarize the significant developments and achievements in this field with particular emphasis on the characterization of catalysts. We also highlight the potential and limitations of the various methods, describe the need for synergistic and complementary tools when characterizing heterogeneous catalysts, and conclude with an outlook that also envisions future needs in the field.

Wurtzite CuInS₂ and CuInxGa₁-xS₂ nanoribbons: synthesis, optical and photoelectrical properties

Single crystalline wurtzite ternary and quaternary semiconductor nanoribbons (CuInS(2), CuIn(x)Ga(1-x)S(2)) were synthesized through a solution-based method. The structure and composition of the nanoribbons were characterized by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), the corresponding fast Fourier transform (FFT) and nanoscale-resolved elemental mapping. Detailed investigation of the growth mechanism by monitoring the structures and morphologies of the nanoribbons during the growth indicates that Cu(1.75)S nanocrystals are formed first and act as a catalyst for the further growth of the nanoribbons. The high mobility of Cu(+) promotes the generation of Cu(+) vacancies in Cu(1.75)S, which will facilitate the diffusion of Cu, In or Ga species from solution into Cu(1.75)S to reach supersaturated states. The supersaturated species in the Cu(1.75)S catalyst, Cu-In-S and Cu-In-Ga-S species, start to condense and crystallize to form wurtzite CuInS(2) or CuIn(x)Ga(1-x)S(2) phases, firstly resulting in two-sided nanoparticles. Successive crystallizations gradually impel the Cu(1.75)S catalyst head forward and prolong the length of the CuInS(2) or CuIn(x)Ga(1-x)S(2) body, forming heterostructured nanorods and thus nanoribbons. The optical band gaps of CuIn(x)Ga(1-x)S(2) nanoribbons can be continuously adjusted between 1.44 eV and 1.91 eV, depending on the Ga concentration in nanoribbons. The successful preparation of those ternary and quaternary semiconductor nanoribbons provide us an opportunity to study their photovoltaic properties. The primary photoresponsive current measurements demonstrate that wurtzite CuIn(x)Ga(1-x)S(2) nanoribbons are excellent photoactive materials. Furthermore, this facile method could open a new way to synthesize other various nano-structured ternary and quaternary semiconductors, such as CuInSe(2) and CuIn(x)Ga(1-x)Se(2), for applications in solar cells and other fields.

Electron microscopy of specimens in liquid

Imaging samples in liquids with electron microscopy can provide unique insights into biological systems, such as cells containing labelled proteins, and into processes of importance in materials science, such as nanoparticle synthesis and electrochemical deposition. Here we review recent progress in the use of electron microscopy in liquids and its applications. We examine the experimental challenges involved and the resolution that can be achieved with different forms of the technique. We conclude by assessing the potential role that electron microscopy of liquid samples can play in areas such as energy storage and bioimaging.

In situ transmission electron microscopy observation of microstructure and phase evolution in a SnO₂ nanowire during lithium intercalation

Recently we have reported structural transformation features of SnO(2) upon initial charging using a configuration that leads to the sequential lithiation of SnO(2) nanowire from one end to the other (Huang et al. Science2010, 330, 1515). A key question to be addressed is the lithiation behavior of the nanowire when it is fully soaked into the electrolyte (Chiang Science2010, 330, 1485). This Letter documents the structural characteristics of SnO(2) upon initial charging based on a battery assembled with a single nanowire anode, which is fully soaked (immersed) into an ionic liquid based electrolyte using in situ transmission electron microscopy. It has been observed that following the initial charging the nanowire retained a wire shape, although highly distorted. The originally straight wire is characterized by a zigzag structure following the phase transformation, indicating that during the phase transformation of SnO(2) + Li ↔ Li(x)Sn + Li(y)O, the nanowire was subjected to severe deformation, as similarly observed for the case when the SnO(2) was charged sequentially from one end to the other. Transmission electron microscopy imaging revealed that the Li(x)Sn phase possesses a spherical morphology and is embedded into the amorphous Li(y)O matrix, indicating a simultaneous partitioning and coarsening of Li(x)Sn through Sn and Li diffusion in the amorphous matrix accompanied the phase transformation. The presently observed composite configuration gives detailed information on the structural change and how this change takes place on nanometer scale.

Surface energy driven agglomeration and growth of single crystal metal wires

We introduce a novel wire growth technique that involves simply heating a multilayer film specifically designed to take advantage of the different surface energies of the substrate and film components. In all cases the high surface energy component is extruded as a single crystal nanowire. Moreover we demonstrate that patterning the bilayer film generates localized surface agglomeration waves during the anneal that can be exploited to position the grown wires. Examples of Au and Cu nanowire growth are presented, and the generalization of this method to other systems is discussed.