In situ environmental transmission electron microscopy to determine transformation pathways in supported Ni nanoparticles

Selective atomic sieving across metal/oxide interface for super-oxidation resistance

Surface passivation, a desirable natural consequence during initial oxidation of alloys, is the foundation for functioning of corrosion and oxidation resistant alloys ranging from industrial stainless steel to kitchen utensils. This initial oxidation has been long perceived to vary with crystal facet, however, the underlying mechanism remains elusive. Here, using in situ environmental transmission electron microscopy, we gain atomic details on crystal facet dependent initial oxidation behavior in a model Ni-5Cr alloy. We find the (001) surface shows higher initial oxidation resistance as compared to the (111) surface. We reveal the crystal facet dependent oxidation is related to an interfacial atomic sieving effect, wherein the oxide/metal interface selectively promotes diffusion of certain atomic species. Density functional theory calculations rationalize the oxygen diffusion across Ni(111)/NiO(111) interface, as contrasted with Ni(001)/NiO(111), is enhanced. We unveil that crystal facet with initial fast oxidation rate could conversely switch to a slow steady state oxidation.

Visualization of the structural transformation of NiO/YSZ/BZY nanocomposite particles using in situ gas environmental transmission electron microscopy

This study focused on investigating the dynamic structural transformations of spherical NiO/YSZ/BZY triple-phase nanocomposite particles, commonly employed for cermet anodes, during the hydrogen reduction reaction. We utilized both spherical aberration (Cs) corrected transmission electron microscopy (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) observation modes under a controlled gaseous environment. The environmental gas pressure was set to 1 atm (760 Torr), mirroring real-world conditions. To elucidate pre- and post-hydrogen reduction compositional alterations, we conducted elemental mapping using energy-dispersive X-ray spectroscopy (EDS). Our findings indicated that NiO nanoparticles underwent reduction to Ni particles upon heat treatments in an environment containing H2 gas. Significantly, this reduction of NiO led to the migration of Ni along the external surface of each composite particle, ultimately resulting in the agglomeration at the interparticle spaces among the three NiO/YSZ/BZY nanocomposite particles.

Revealing Temperature-Dependent Oxidation Dynamics of Ni Nanoparticles via Ambient Pressure Transmission Electron Microscopy

Understanding the oxidation mechanism of metal nanoparticles under ambient pressure is extremely important to make the best use of them in a variety of applications. Through ambient pressure transmission electron microscopy, we in situ investigated the dynamic oxidation processes of Ni nanoparticles at different temperatures under atmospheric pressure, and a temperature-dependent oxidation behavior was revealed. At a relatively low temperature (e.g., 600 °C), the oxidation of Ni nanoparticles underwent a classic Kirkendall process, accompanied by the formation of oxide shells. In contrast, at a higher temperature (e.g., 800 °C), the oxidation began with a single crystal nucleus at the metal surface and then proceeded along the metal/oxide interface without voids formed during the whole process. Through our experiments and density functional theory calculations, a temperature-dependent oxidation mechanism based on Ni nanoparticles was proposed, which was derived from the discrepancy of gas adsorption and diffusion rates under different temperatures.

Environmental STEM Study of the Oxidation Mechanism for Iron and Iron Carbide Nanoparticles

The oxidation of solution-synthesized iron (Fe) and iron carbide (Fe₂C) nanoparticles was studied in an environmental scanning transmission electron microscope (ESTEM) at elevated temperatures under oxygen gas. The nanoparticles studied had a native oxide shell present, that formed after synthesis, an ~3 nm iron oxide (Fe_xO_y) shell for the Fe nanoparticles and ~2 nm for the Fe₂C nanoparticles, with small void areas seen in several places between the core and shell for the Fe and an ~0.8 nm space between the core and shell for the Fe₂C. The iron nanoparticles oxidized asymmetrically, with voids on the borders between the Fe core and Fe_xO_y shell increasing in size until the void coalesced, and finally the Fe core disappeared. In comparison, the oxidation of the Fe₂C progressed symmetrically, with the core shrinking in the center and the outer oxide shell growing until the iron carbide had fully disappeared. Small bridges of iron oxide formed during oxidation, indicating that the Fe transitioned to the oxide shell surface across the channels, while leaving the carbon behind in the hollow core. The carbon in the carbide is hypothesized to suppress the formation of larger crystallites of iron oxide during oxidation, and alter the diffusion rates of the Fe and O during the reaction, which explains the lower sensitivity to oxidation of the Fe₂C nanoparticles.

Reactivity and structural evolution of urchin-like Co nanostructures under controlled environments

In situ transmission electron microscopy (TEM) of samples in a controlled gas environment allows for the real time study of the dynamical changes in nanomaterials at high temperatures and pressures up to the ambient pressure (10⁵ Pa) with a spatial resolution close to the atomic scale. In the field of catalysis, the implementation and quantitative use of in situ procedures are fundamental for a better understanding of the behaviour of catalysts in their environments and operating conditions. By using a microelectromechanical systems (MEMS)-based atmospheric gas cell, we have studied the thermal stability and the reactivity of crystalline cobalt nanostructures with initial 'urchin-like' morphologies sustained by native surface ligands that result from their synthesis reaction. We have evidenced various behaviors of the Co nanostructures that depend on the environment used during the observations. At high temperature under vacuum or in an inert atmosphere, the migration of Co atoms towards the core of the particles is activated and leads to the formation of carbon nanostructures using as a template the initial multipods morphology. In the case of reactive environments, for example, pure oxygen, our investigation allowed to directly monitor the voids formation through the Kirkendall effect. Once the nanostructures were oxidised, it was possible to reduce them back to the metallic phase using a dihydrogen flux. Under a pure hydrogen atmosphere, the sintering of the whole structure occurred, which illustrates the high reactivity of such structures as well as the fundamental role of the present ligands as morphology stabilisers. The last type of environmental study under pure CO and syngas (i.e. a mixture of H₂ :CO = 2:1) revealed the metal particles carburisation at high temperature.

Direct in Situ TEM Observation of Modification of Oxidation by the Injected Vacancies for Ni-4Al Alloy Using a Microfabricated Nanopost

Vacancy injection and selective oxidation of one species in bimetallic alloy at high temperature is a well-known phenomenon. However, detailed understanding of the behavior of the injected vacancies and consequently their effect on oxidation remains elusive. The current research examines the oxidation of high-purity Ni doped with 4.1 at. % Al using in situ transmission electron microscopy (TEM). Experiments are performed on nanoposts fabricated from solution-annealed bulk material that are essentially single crystal samples. Initial oxidation is observed to occur by multisite oxide nucleation, formation of an oxide shell followed by cavity nucleation and growth at the metal/oxide interface. One of the most interesting in situ TEM observations is the formation of a cavity that leads to the faceting of the metal and subsequent oxidation occurring by an atomic ledge migration mechanism on the faceted metal surface. Further, it is directly observed that metal atoms diffuse through the oxide layer to combine with oxygen at the outer surface of the oxide. The present work indicates that injection of vacancies and formation of cavity will lead to a situation where the oxidation rate is essentially controlled by the low surface energy plane of the metal, rather than by the initial terminating plane at the metal surface exposed to the oxidizing environment.

Nanoparticle conversion chemistry: Kirkendall effect, galvanic exchange, and anion exchange

Conversion chemistry is a rapidly maturing field, where chemical conversion of template nanoparticles (NPs) into new compositions is often accompanied by morphological changes, such as void formation. The principles and examples of three major classes of conversion chemical reactions are reviewed: the Kirkendall effect for metal NPs, galvanic exchange, and anion exchange, each of which can result in void formation in NPs. These reactions can be used to obtain complex structures that may not be attainable by other methods. During each kind of conversion chemical reaction, NPs undergo distinct chemical and morphological changes, and insights into the mechanisms of these reactions will allow for improved fine control and prediction of the structures of intermediates and products. Conversion of metal NPs into oxides, phosphides, sulphides, and selenides often occurs through the Kirkendall effect, where outward diffusion of metal atoms from the core is faster than inward diffusion of reactive species, resulting in void formation. In galvanic exchange reactions, metal NPs react with noble metal salts, where a redox reaction favours reduction and deposition of the noble metal (alloying) and oxidation and dissolution of the template metal (dealloying). In anion exchange reactions, addition of certain kinds of anions to solutions containing metal compound NPs drives anion exchange, which often results in significant morphological changes due to the large size of anions compared to cations. Conversion chemistry thus allows for the formation of NPs with complex compositions and structures, for which numerous applications are anticipated arising from their novel catalytic, electronic, optical, magnetic, and electrochemical properties.

Analysis of catalytic gas products using electron energy-loss spectroscopy and residual gas analysis for operando transmission electron microscopy

Operando transmission electron microscopy (TEM) of catalytic reactions requires that the gas composition inside the TEM be known during the in situ reaction. Two techniques for measuring gas composition inside the environmental TEM are described and compared here. First, electron energy-loss spectroscopy, both in the low-loss and core-loss regions of the spectrum was utilized. The data were quantified using a linear combination of reference spectra from individual gasses to fit a mixture spectrum. Mass spectrometry using a residual gas analyzer was also used to quantify the gas inside the environmental cell. Both electron energy-loss spectroscopy and residual gas analysis were applied simultaneously to a known 50/50 mixture of CO and CO2, so the results from the two techniques could be compared and evaluated. An operando TEM experiment was performed using a Ru catalyst supported on silica spheres and loaded into the TEM on a specially developed porous pellet TEM sample. Both techniques were used to monitor the conversion of CO to CO2 over the catalyst, while simultaneous atomic resolution imaging of the catalyst was performed.

In-situ TEM visualization of vacancy injection and chemical partition during oxidation of Ni-Cr nanoparticles

Oxidation of alloy often involves chemical partition and injection of vacancies. Chemical partition is the consequence of selective oxidation, while injection of vacancies is associated with the differences of diffusivity of cations and anions. It is far from clear as how the injected vacancies behave during oxidation of metal. Using in-situ transmission electron microscopy, we captured unprecedented details on the collective behavior of injected vacancies during oxidation of metal, featuring an initial multi-site oxide nucleation, vacancy supersaturation, nucleation of a single cavity, sinking of vacancies into the cavity and accelerated oxidation of the particle. High sensitive energy dispersive x-ray spectroscopy mapping reveals that Cr is preferentially oxidized even at the initial oxidation, leading to a structure that Cr oxide is sandwiched near the inner wall of the hollow particle. The work provides a general guidance on tailoring of nanostructured materials involving multi-ion exchange such as core-shell structured composite nanoparticles.

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.

System for in situ UV-visible illumination of environmental transmission electron microscopy samples

A system for illuminating a sample in situ with visible and ultraviolet light inside a transmission electron microscope was devised to study photocatalysts. There are many mechanical and optical factors that must be considered when designing and building such a system. Some of the restrictions posed by the electron microscope column are significant, and care must be taken not to degrade the microscope's electron-optical performance or to unduly restrict the other capabilities of the microscope. We discuss the nature of the design considerations, as well as the practical implementation and characterization of a solution. The system that has been added to an environmental transmission electron microscope includes a high brightness broadband light source with optical filters, a fiber to guide the light to the sample, and a mechanism for precisely aligning the fiber tip.

Nanostructural transformations during the reduction of hollow and porous nickel oxide nanoparticles

Size-dependent nanostructural transformations occurring during the H(2)-mediated reduction of hollow and porous NiO nanoparticles were investigated for controlled nanoparticle sizes of ~10 to 100 nm. Transmission electron microscopy reveals that the location and number of reduction sites strongly depend on the nanoparticle size and structure.