In Situ Environmental TEM in Imaging Gas and Liquid Phase Chemical Reactions for Materials Research

Gas and liquid phase chemical reactions cover a broad range of research areas in materials science and engineering, including the synthesis of nanomaterials and application of nanomaterials, for example, in the areas of sensing, energy storage and conversion, catalysis, and bio‐related applications. Environmental transmission electron microscopy (ETEM) provides a unique opportunity for monitoring gas and liquid phase reactions because it enables the observation of those reactions at the ultra‐high spatial resolution, which is not achievable through other techniques. Here, the fundamental science and technology developments of gas and liquid phase TEM that facilitate the mechanistic study of the gas and liquid phase chemical reactions are discussed. Combined with other characterization tools integrated in TEM, unprecedented material behaviors and reaction mechanisms are observed through the use of the in situ gas and liquid phase TEM. These observations and also the recent applications in this emerging area are described. The current challenges in the imaging process are also discussed, including the imaging speed, imaging resolution, and data management.     

Formation of nanotubes and hollow nanoparticles based on Kirkendall and diffusion processes: a review

The Kirkendall effect is a consequence of the different diffusivities of atoms in a diffusion couple causing a supersaturation of lattice vacancies. This supersaturation may lead to a condensation of extra vacancies in the form of so-called "Kirkendall voids" close to the interface. On the macroscopic and micrometer scale these Kirkendall voids are generally considered as a nuisance because they deteriorate the properties of the interface. In contrast, in the nanoworld the Kirkendall effect has been positively used as a new fabrication route to designed hollow nano-objects. In this Review we summarize and discuss the demonstrated examples of hollow nanoparticles and nanotubes induced by the Kirkendall effect. Merits of this route are compared with other general methods for nanotube fabrication. Theories of the kinetics and thermodynamics are also reviewed and evaluated in terms of their relevance to experiments. Moreover, nanotube fabrication by solid-state reactions and non-Kirkendall type diffusion processes are covered.     

The Piezotronic Effect of Zinc Oxide Nanowires Studied by In Situ TEM

Piezotronics is a new field integrating piezoelectric effect into nanoelectronics, which has attracted much attention for the fundamental research and potential applications. In this paper, the piezotronic effect of zinc oxide (ZnO) nanowires, including the response of the electrical transport and photoconducting behaviors on the nanowire bending, has been investigated by in situ transmission electron microscopy (TEM), where the crystal structure of ZnO nanowires were simultaneously imaged. Serials of consecutively recorded current‐voltage (I–V) curves along with an increase of nanowire bending show the striking effect of bending on their electrical behavior. With increasing the nanowire bending, the photocurrent of ZnO nanowire under ultraviolet illumination (UV) drops dramatically and the photo response time becomes much shorter. In addition, the dynamic nanomechanics of ZnO nanowires were studied inside TEM. These phenomena could be attributed to the piezoelectric effect of ZnO nanowires, and they suggest the potential applications of ZnO nanowires on piezotronic devices.     

Approaching Elaborate Control of the Nano-Products of Carbothermal Reduction Reaction Through In Situ Identification

Atomic understanding of a chemical reaction can realize the programmable design and synthesis of desired products with specific compositions and structures. Through directly monitoring the phase transition and tracking the dynamic evolution of atoms in a chemical reaction, in situ transmission electron microscopy (TEM) techniques offer the feasibility of revealing the reaction kinetics at the atomic level. Nevertheless, such investigation is quite challenging, especially for reactions involving multi-phase and complex interfaces, such as the widely adopted carbothermal reduction (CTR) reactions. Herein, in-situ TEM is applied to monitor the CTR of Co₃O₄ nanocubes on reduced graphene oxide nanosheets. Together with the first-principle calculation, the migration route of Co atoms during the phase transition of the CTR reaction is revealed. Meanwhile, the interfacial edge-dislocations/stress-gradient is identified as a result of the atomistic diffusion, which in turn can affect the morphology variation of the reactants. Accordingly, controllable synthesis of Co-based nanostructure with a desirable phase and structure has been achieved. This work not only provides atomic kinetic insight into CTR reactions but also offers a novel strategy for the design and synthesis of functional nanostructures for emerging energy technologies.     

In Situ TEM Electromechanical Testing of Nanowires and Nanotubes

The emergence of one‐dimensional nanostructures as fundamental constituents of advanced materials and next‐generation electronic and electromechanical devices has increased the need for their atomic‐scale characterization. Given its spatial and temporal resolution, coupled with analytical capabilities, transmission electron microscopy (TEM) has been the technique of choice in performing atomic structure and defect characterization. A number of approaches have been recently developed to combine these capabilities with in‐situ mechanical deformation and electrical characterization in the emerging field of in‐situ TEM electromechanical testing. This has enabled researchers to establish unambiguous synthesis‐structure‐property relations for one‐dimensional nanostructures. In this article, the development and latest advances of several in‐situ TEM techniques to carry out mechanical and electromechanical testing of nanowires and nanotubes are reviewed. Through discussion of specific examples, it is shown how the merging of several microsystems and TEM has led to significant insights into the behavior of nanowires and nanotubes, underscoring the significant role in‐situ techniques play in the development of novel nanoscale systems and materials.     

CO Gas Induced Phase Separation in PtPb@Pt Catalyst and Formation of Ultrathin Pb Nanosheets Probed by In Situ Transmission Electron Microscopy

PtPb@Pt catalysts are very useful and widely applied in various industrial reactions. Here, the phase stability of such catalysts is compared in both CO gas and vacuum conditions at elevated temperatures using aberration‐corrected in situ transmission electron microscopy (TEM). A Pt aggregation process takes place affected by CO gas, which results in direct exposure of the PtPb core to CO. A phase separation process, in which Pb atoms are stripped off the original PtPb@Pt nanoparticles, is unambiguously identified in CO gas. At initial stages, the as nucleated Pb islands are amorphous. Once the ultrathin Pb islands reach ≈3.5 nm or higher, they suddenly became crystalline. The interaction between Pb and CO gas stabilizes the ultrathin Pb nanosheets, resulting in the formation of a large quantity of Pb nanosheets and Pb‐depleted PtPb_0.08 nanoparticles. In sharp contrast, when heated up in a vacuum, the PtPb@Pt catalyst remains intact. The results of this study shine light on the “toxic” effect of CO that results in failures of many Pt‐based catalysts and discloses formation mechanism of ultrathin Pb nanosheets.     

大型氧化风机房通风降噪工艺实践

氧化风机房是发电厂脱硫工艺中鼓风设备的厂房,厂房内噪声值达110 dB(A),严重影响工作人员的身心健康。分析某发电厂脱硫氧化风机房噪声及通风现状,探索通风降噪工艺,并应用于工程实践。结果表明：对于噪声较大、且需良好的空气流动性的氧化风机房,通过在机房内及氧化风机采取吸、隔声等噪声控制技术综合治理措施,优化通风系统,取得良好的通风降噪效果,极大的改善职工的工作环境,提高职业健康水平。     

Unveiling the Dynamical Assembly of Magnetic Nanocrystal Zig‐Zag Chains via In Situ TEM Imaging in Liquid

The controlled assembly of colloidal magnetic nanocrystals is key to many applications such as nanoelectronics, storage memory devices, and nanomedicine. Here, the motion and ordering of ferrimagnetic nanocubes in water via liquid‐cell transmission electron microscopy is directly imaged in situ. Through the experimental analysis, combined with molecular dynamics simulations and theoretical considerations, it is shown that the presence of highly competitive interactions leads to the formation of stable monomers and dimers, acting as nuclei, followed by a dynamic growth of zig‐zag chain‐like assemblies. It is demonstrated that such arrays can be explained by first, a maximization of short‐range electrostatic interactions, which at a later stage become surpassed by magnetic forces acting through the easy magnetic axes of the nanocubes, causing their tilted orientation within the arrays. Moreover, in the confined volume of liquid in the experiments, interactions of the nanocube surfaces with the cell membranes, when irradiated at relatively low electron dose, slow down the kinetics of their self‐assembly, facilitating the identification of different stages in the process. The study provides crucial insights for the formation of unconventional linear arrays made of ferrimagnetic nanocubes that are essential for their further exploitation in, for example, magnetic hyperthermia, magneto‐transport devices, and nanotheranostic tools.     

Platinum‐Based Nanowires as Active Catalysts toward Oxygen Reduction Reaction: In Situ Observation of Surface‐Diffusion‐Assisted,Solid‐State Oriented Attachment

Facile fabrication of advanced catalysts toward oxygen reduction reaction with improving activity and stability is significant for proton‐exchange membrane fuel cells. Based on a generic solid‐state reaction, this study reports a modified hydrogen‐assisted, gas‐phase synthesis for facile, scalable production of surfactant‐free, thin, platinum‐based nanowire‐network electrocatalysts. The free‐standing platinum and platinum–nickel alloy nanowires show improvements of up to 5.1 times and 10.9 times for mass activity with a minimum 2.6% loss after an accelerated durability test for 10k cycles; 8.5 times and 13.8 times for specific activity, respectively, compared to commercial Pt/C catalyst. In addition, combined with a wet impregnation method, different substrate‐materials‐supported platinum‐based nanowires are obtained, which paves the way to practical application as a next‐generation supported catalyst to replace Pt/C. The growth stages and formation mechanism are investigated by an in situ transmission electron microscopy study. It reveals that the free‐standing platinum nanowires form in the solid state via metal‐surface‐diffusion‐assisted oriented attachment of individual nanoparticles, and the interaction with gas molecules plays a critical role, which may represent a gas‐molecular‐adsorbate‐modified growth in catalyst preparation.     

In Situ Synthesis of CuO and Cu Nanostructures with Promising Electrochemical and Wettability Properties

A strategy is presented for the in situ synthesis of single crystalline CuO nanorods and 3D CuO nanostructures, ultra‐long Cu nanowires and Cu nanoparticles at relatively low temperature onto various substrates (Si, SiO₂, ITO, FTO, porous nickel, carbon cotton, etc.) by one‐step thermal heating of copper foam in static air and inert gas, respectively. The density, particle sizes and morphologies of the synthesized nanostructures can be effectively controlled by simply tailoring the experimental parameters. A compressive stress based and subsequent structural rearrangements mechanism is proposed to explain the formation of the nanostructures. The as‐prepared CuO nanostructures demonstrate promising electrochemical properties as the anode materials in lithium‐ion batteries and also reversible wettability. Moreover, this strategy can be used to conveniently integrate these nanostructures with other nanostructures (ZnO nanorods, Co₃O₄ nanowires and nanowalls, TiO₂ nanotubes, and Si nanowires) to achieve various hybrid hierarchical (CuO‐ZnO, CuO‐Co₃O₄, CuO‐TiO₂, CuO‐Si) nanocomposites with promising properties. This strategy has the potential to provide the nano society with a general way to achieve a variety of nanostructures.     

Intrinsic Effect of Nanoparticles on the Mechanical Rupture of Doubled‐Shell Colloidal Capsule via In Situ TEM Mechanical Testing and STEM Interfacial Analysis

The discovery of Pickering emulsion templated assembly enables the design of a hybrid colloidal capsule with engineered properties. However, the underlying mechanisms by which nanoparticles affect the mechanical properties of the shell are poorly understood. Herein, in situ mechanical compression on the transmission electron microscope and aberration‐corrected scanning transmission microscope are unprecedentedly implemented to study the intrinsic effect of nanoparticles on the mechanical properties of the calcium carbonate (CaCO₃)‐decorated silica (SiO₂) colloidal capsule. The stiff and brittle nature of the colloidal capsule is due to the interfacial chemical bonding between the CaCO₃ nanoparticles and SiO₂ inner shell. Such bonding strengthens the mechanical strength of the SiO₂ shell (166 ± 14 nm) from the colloidal capsule compared to the thicker single SiO₂ shell (310 ± 70 nm) from the silica hollow sphere. At elevated temperature, this interfacial bonding accelerates the formation of the single calcium silicate shell, causing shell morphology transformation and yielding significantly enhanced mechanical strength by 30.9% and ductility by 94.7%. The superior thermal durability of the heat‐treated colloidal capsule holds great potential for the fabrication of the functional additives that can be applied in the wide range of applications at elevated temperatures.     

On the Synthesis of In Situ Ni-SiO2 Nanocomposites by Isothermal and Non Isothermal Reduction Technique

In situ Ni-SiO₂ nanocomposites were prepared from silica gels containing NiCl₂ and dextrose by isothermal and non isothermal reduction treatment. Analysis of the isothermal kinetic data indicates that contracting geometry and nucleation and growth type of mechanism remain operative in the 600-750°C and 800-950°C temperature range respectively. However, analysis of non isothermal kinetic data reveals that contracting geometry type of mechanism solely remains operative over the entire temperature range of 600-950°C. Transmission electron microscopic (TEM) studies reveals that the microstructural features of the nanocomposites are sensitive to reduction conditions.     

Surface Oxidation of AuNi Heterodimers to Achieve High Activities toward Hydrogen/Oxygen Evolution and Oxygen Reduction Reactions

Although much attention has been paid to the exploration of highly active electrocatalysts, especially catalysts for hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), the development of multifunctional catalysts remains a challenge. Here, we utilize AuNi heterodimers as the starting materials to achieve high activities toward HER, OER and ORR. The HER and ORR activities in an alkali environment are similar to those of Pt catalysts, and the OER activity is very high and better than that of commercial IrO₂. Both the experimental and calculated results suggest that the surface oxidation under oxidative conditions is the main reason for the different activities. The NiO/Ni interface which exists in the as‐synthesized heterodimers contributes to high HER activity, the Ni(OH)₂‐Ni‐Au interface and the surface Ni(OH)₂ obtained in electrochemical conditons gives rise to promising ORR and OER activities, respectively. As a comparison, a Au@Ni core‐shell structure is also synthesized and examined. The core‐shell structure shows lower activities for HER and OER than the heterodimers, and reduces O₂ selectively to H₂O₂. The work here allows for the development of a method to design multifunctional catalysts via the partial oxidation of a metal surface to create different active centers.