Environmental Transmission Electron Microscopy Observations Research Articles

Atomic origin of CO2-promoted oxidation dynamics of chromia-forming alloys

The development of atomic imperfections within oxide films from high-temperature oxidation of heat-resistant alloys significantly limits the self-protectiveness of the surface oxide, contributing to the failure of energy generating system components such as turbines, engines, and heat exchanges. Directly probing the dynamics of such atomic defects is challenging because of the extreme thermochemical conditions of high-temperature oxidation. Using environmental transmission electron microscopy observations, here we directly capture atomic-scale dynamics of vacancies in growing Cr2O3 film during high-temperature oxidation of NiCr alloy in CO2. Coordinated with theory modeling, we delineate the atomistic mechanisms associated with the effect of interstitial carbon derived from CO2 on promoting the formation, migration and clustering of atomic vacancies to result in the enhanced alloy oxidation. The identified oxidation mechanism can find broader applicability in utilizing the atmosphere to tune the formation and evolution of atomic-scale defects, thereby affecting the mass transport properties of the growing oxide film.

Atomic Structure of the Fe3O4/Fe2O3 Interface During Phase Transition from Hematite to Magnetite.

Phase transition between iron oxides practically defines their functionalities in both physical and chemical applications. Direct observation of the atomic rearrangement and a quantitative description of the dynamic behavior of the phase transition, however, are rare. Here, we monitored the structure evolution from a rod-shaped hematite nanoparticle to magnetite during H2 reduction at elevated temperatures. Environmental transmission electron microscopy observations, along with selected area electron diffraction experiments, identified that the reduction preferentially commenced with Fe3O4 nucleation on the surface defective sites, followed by laterally growing into a Fe3O4 film until fully covering the particle surface. The Fe3O4 phase then propagated toward the bulk particle via a Fe3O4/α-Fe2O3 interface with the relationship α-Fe2O3(0001)//Fe3O4(111) in an aligned orientation of [112]Fe3O4||[112̅0]α-Fe2O3. Upon this Fe3O4/α-Fe2O3 interface, the Fe-O octahedra in Fe3O4(111) (as layer A) matches that of α-Fe2O3(0001) at a rotation angle of 30°, and the reduction proceeds in such a pattern that two-thirds of the FeOh in the adjacent layer (layer B) is transformed into FeTe.

Oxidation and hydrogenation of Pd: suppression of oxidation by prolonged H2 exposure.

We investigate the phase transition of a Pd surface in both oxidizing and reducing environments by environmental transmission electron microscopy (ETEM). ETEM allows us to study sequential exposure of Pd to O2 and H2 in the same TEM conditions. First, under ETEM observation, oxidation occurs at step edges but it can also occur at terraces. Second, as the most important result, we observed a novel process where previous exposure to H2 suppresses new oxidation of the Pd surface. Third, we show by electron energy loss spectroscopy (EELS) that this process, suppression of oxidation by previous exposure to H2, is not due to the formation of bulk β-phase Pd hydride. We also demonstrate that this process is not present in Pt. Finally, we discuss the hypothesis to explain this phenomenon: formation of surface–Pd–hydride suppresses the new oxidation. This observation, suppression of oxidation by H2 exposure, may eventually lead to new breakthroughs.

Reversible gas–solid reaction in an electronically-stimulated palladium nanogap

We investigated a nanogap between a pair of palladium electrode tips with gas (nitrogen, hydrogen, and oxygen) and a biasing voltage using in situ atomic resolution environmental transmission electron microscopy (ETEM). We found an unexpected gas-solid (nitrogen-palladium) reaction that occurs on the surface of the positive electrode tip. A palladium nitride compound was synthesized with gaseous nitrogen at low pressure at room temperature. The nitridation of palladium was previously reported and predicted to occur only under high pressure and at high temperature. The reaction in ETEM apparatus was reversible with the change in the magnitude of an electric field in the nanogap. Additionally, the asymmetrical surface dynamics on the pair of electrode tips in gas (nitrogen, hydrogen, and oxygen) were revealed by ETEM observation. It is likely that the electrons in the gap induce the reversible reaction. This study has opened a new route toward creating nanoscale materials because the creation, stabilization, and annihilation of the material in a nanogap can be controlled electrically and electronically on demand for various applications.

Visualizing Redox Dynamics of a Single Ag/AgCl Heterogeneous Nanocatalyst at Atomic Resolution

Operando characterization of gas-solid reactions at the atomic scale is of great importance for determining the mechanism of catalysis. This is especially true in the study of heterostructures because of structural correlation between the different parts. However, such experiments are challenging and have rarely been accomplished. In this work, atomic scale redox dynamics of Ag/AgCl heterostructures have been studied using in situ environmental transmission electron microscopy (ETEM) in combination with density function theory (DFT) calculations. The reduction of Ag/AgCl to Ag is likely a result of the formation of Cl vacancies while Ag(+) ions accept electrons. The oxidation process of Ag/AgCl has been observed: rather than direct replacement of Cl by O, the Ag/AgCl nanocatalyst was first reduced to Ag, and then Ag was oxidized to different phases of silver oxide under different O2 partial pressures. Ag2O formed at low O2 partial pressure, whereas AgO formed at atmospheric pressure. By combining in situ ETEM observation and DFT calculations, this structural evolution is characterized in a distinct nanoscale environment.

Vibrational and optical spectroscopies integrated with environmental transmission electron microscopy.

Here, we present a measurement platform for collecting multiple types of spectroscopy data during high-resolution environmental transmission electron microscopy observations of dynamic processes. Such coupled measurements are made possible by a broadband, high-efficiency, free-space optical system. The critical element of the system is a parabolic mirror, inserted using an independent hollow rod and placed below the sample holder which can focus a light on the sample and/or collect the optical response. We demonstrate the versatility of this optical setup by using it to combine in situ atomic-scale electron microscopy observations with Raman spectroscopy. The Raman data is also used to measure the local temperature of the observed sample area. Other applications include, but are not limited to: cathodo- and photoluminescence spectroscopy, and use of the laser as a local, high-rate heating source.

Environmental transmission electron microscopy for catalyst materials using a spherical aberration corrector

Atomic resolution has been obtained using environmental transmission electron microscopy (ETEM) by installing a spherical aberration corrector (Cs-corrector) on the objective lens. Simultaneously, the technology for controlling the environment around a specimen in ETEM has advanced significantly in the past decade. Quantification methodology has recently been established for deriving relevant experimental data in catalyst materials from substantial and systematic ETEM observation at the atomic scale. With this background, this paper summarizes aspects of the evolutional microscopy technique: necessary conditions for atomic resolution in ETEM; reduction of the scattering of electrons by the medium surrounding a specimen; and an environmental cell for structural imaging of a crystalline specimen. The high spatial resolution of a Cs-corrected ETEM is demonstrated for different observation conditions. After statistical analysis combined with numerical image analysis of ETEM data is briefly described, the recent applications of the Cs-corrected ETEM to catalyst materials are reviewed. For gold nanoparticulate catalysts, the structural information on the reaction sites and adsorption sites are deduced. For Pt nanoparticulate catalysts, ETEM studies elucidate the correlation between the catalytic activity and the morphology of the nanoparticles. These studies also reveal oxidation and reduction on the topmost Pt surface layer at the atomic scale. Finally, current issues and the future perspectives of Cs-corrected ETEM are summarized, including the reproducibility of ETEM observation data, the control of environments, the critical evaluation of electron irradiation effects, the full implementation of transmission electron microscopy technology in ETEM, and the safety issues for an ETEM laboratory.

Fabrication of tin-filled carbon nanofibres by microwave plasma vapour deposition and their in situ heating observation by environmental transmission electron microscopy

Sn-filled carbon nanofibres (CNFs) are fabricated by microwave plasma chemical deposition. Scanning electron microscopy observations revealed the existence of a Sn island under the CNFs. The structure of the CNFs is investigated, and the behaviour of Sn in the internal space of CNFs is revealed by performing in situ heating observations by environmental transmission electron microscopy (ETEM). ETEM observations reveal that they have low-crystallized carbon wall and Sn occupies not only the CNF’s internal space but also its carbon wall. The Sn inside the CNF is completely covered by the carbon wall. Further, the in situ heating observations reveal that Sn within the internal space and the carbon wall of the CNFs diffused to the outside during heating. Moreover, it is found that higher membered carbon rings and defects in the graphite layer act as diffusion routes between disordered carbon layers.

Dynamic environmental transmission electron microscopy observation of platinum electrode catalyst deactivation in a proton-exchange-membrane fuel cell

Spherical-aberration-corrected environmental transmission electron microscopy (AC-ETEM) was applied to study the catalytic activity of platinum/amorphous carbon electrode catalysts in proton-exchange-membrane fuel cells (PEMFCs). These electrode catalysts were characterized in different atmospheres, such as hydrogen and air, and a conventional high vacuum of 10−5 Pa. A high-speed charge coupled device camera was used to capture real-time movies to dynamically study the diffusion and reconstruction of nanoparticles with an information transfer down to 0.1 nm, a time resolution below 0.2 s and an acceleration voltage of 300 kV. With such high spatial and time resolution, AC-ETEM permits the visualization of surface-atom behaviour that dominates the coalescence and surface-reconstruction processes of the nanoparticles. To contribute to the development of robust PEMFC platinum/amorphous carbon electrode catalysts, the change in the specific surface area of platinum particles was evaluated in hydrogen and air atmospheres. The deactivation of such catalysts during cycle operation is a serious problem that must be resolved for the practical use of PEMFCs in real vehicles. In this paper, the mechanism for the deactivation of platinum/amorphous carbon electrode catalysts is discussed using the decay rate of the specific surface area of platinum particles, measured first in a vacuum and then in hydrogen and air atmospheres for comparison.

Atomic-resolution environmental TEM for quantitativein-situmicroscopy in materials science

We have compiled our recent in-situ quantitative environmental transmission electron microscopy (ETEM) studies on typical gold nanoparticulate catalysts for the low-temperature oxidation of CO to describe the issues surrounding the application of ETEM, with a special regard to catalyst chemistry. Thanks to the recent development of high-resolution environmental transmission electron microscopes that can work robustly to accumulate observation data in controlled environments, we can deal with the electron irradiation effects and heterogeneity of real catalysts. We established a structural evolution diagram that summarizes the structure of catalysts under electron irradiation as a function of the electron current density ϕ and the electron dose, D. By extrapolating to ϕ = 0, D = 0, we could deduce the intrinsic catalysis structure (without electron irradiation) in various environments, including reaction environments. By numerically and statistically analyzing a substantial number of ETEM images of gold nanoparticles, we established a morphology phase diagram that summarizes how the majority of gold nanoparticles change their morphology systematically as a function of the partial pressures of CO and O(2). Similar diagrams will be helpful in elucidating the phenomena that directly correlate with the catalytic activity determined from ETEM observations. Using these quantitative analyses, we could analyze Cs-corrected ETEM images of the catalysts. The surfaces of gold nanoparticles were structurally reconstructed under reaction conditions, via interactions with CO molecules. CO molecules were observed on the surfaces of catalysts under reaction conditions using high-resolution ETEM. Finally, we discuss the potential of environmental transmission electron microscopy for quantitative in-situ microscopy at the atomic scale.

Environmental transmission electron microscopy observations of the growth of carbon nanotubes under nanotube–nanotube and nanotube–substrate interactions

The growth process of carbon nanotubes (CNTs) under CNT–CNT and CNT–substrate interactions has been observed directly by environmental transmission electron microscopy. Even a free standing CNT occasionally swings during the growth until it touches the substrate. In addition, we show that the growth direction of CNTs changes due to the interaction between CNTs.

Interaction of Oxide Surfaces with Water: Environmental Transmission Electron Microscopy of MgO Hydroxylation

Environmental transmission electron microscopy (ETEM) is opening an important window for in situ studies of interaction of water with oxides. Studies of MgO smoke nanocrystals under partial pressures of water ranging from 10 mTorr to 10 Torr found their {100} neutral surfaces to be extremely resistant to dissociative adsorption of water and hydroxylation, in agreement with recent theoretical predictions. ETEM observations of electron irradiation driven MgO smoke nanocrystal hydroxylation displayed the anticipated volume expansion, but revealed complex shape changes with elongations toward oxide corners. The reaction rate was found to increase with electron flux at constant water pressure. In situ selected area diffraction studies of MgO single crystals showed that the hydroxide grows with its basal (0001) plane parallel to the polar MgO (111) planes. This is the same crystallographic relationship as in dehydroxylation experiments, but with four variants. Electron energy loss spectroscopy found oxygen K-edge changes consistent with bulk hydroxylation.

In situreal‐time environmental TEM of gas phase Ziegler–Natta catalytic polymerization of propylene

In situ gas phase polypropylene (PP) formation over heterogeneous ballmilled TiCl4‐MgCl2 Ziegler–Natta (Z–N) catalysts (containing no electron donors or selectivity control agents) has been observed for the first time in real time using environmental transmission electron microscopy (ETEM). The MgCl2 support and associated Z–N catalysts were characterized with electron diffraction and high‐resolution electron microscopy. The support was found to be composed of highly defective β‐MgCl2 nanocrystallites, 3 nm in size. Ball milling these supports in TiCl4 to create the pro‐catalyst increased the degree of disorder and resulted in a higher fraction of amorphous material. The surfaces of the catalysts consist of amorphous material, various terminations of β‐MgCl2, and other contaminant phases. The ETEM observations revealed initial nanoscale PP growth in the form of globules and continuous non‐uniform polymer layers. The growth rate was measured to be 0.2 nm s−1 at 1 Torr of C3H6 pressure. The continuous growth implies that the active Ti sites are located on both the crystalline and non‐crystalline regions of the MgCl2 surface. These active Ti sites appear to be located at the polymer/MgCl2 buried interface and there does not appear to be any significant diffusion limitation for polymer layers up to 150 nm in thickness. Our data and calculations show that only 1 in 105 collisions with the active site results in monomer insertion into the polymer chain.