Environmental Transmission Electron Microscopy Study Research Articles

Nanocarbon oxidation in the environmental transmission electron microscope - Disentangling the role of the electron beam

Environmental transmission electron microscopy (ETEM) can provide unique insights into nanocarbon oxidation processes through atomic resolution and real time imaging of materials at high temperatures in reactive atmospheres. However, the electron beam can also influence the reaction rates, and even alter the processes entirely, complicating the interpretation of the in situ observations. Many mechanisms have been proposed to account for the impact of the electron beam, predominantly involving ionization of the oxidative gases to form more reactive species. However, these mechanisms have not been critically evaluated and compared to predictions from theory. Here, we evaluate the impact of the electron beam both qualitatively (oxidation mode and spatial extent) and quantitatively (oxidation rates), using high resolution imaging and electron energy loss spectroscopy, at different electron energies and dose rates. We demonstrate that transient defects generated by elastic scattering, forming highly active sites for carbon abstraction by oxygen, is the main mechanism for the enhanced oxidation rates observed in situ. This is evident from an insensitivity to electron energy and saturation of the effects at high electron dose rates. To avoid undue influence of the electron beam in future ETEM studies, we therefore recommend conditions where the intrinsic oxidation dominates over the beam-enhanced oxidation (note that no conditions are completely “safe”) and extensive comparisons with other methods.

Contrasting Pr1–xCaxMnO3 OER Catalysts with Different Valences and Covalences

Perovskite oxides are promising materials for catalyzing the oxygen evolution reaction (OER). However, understanding the properties of active sites that enable high OER activity without corrosive electrode reduction is still elusive. In this work, we use combined electrochemical and environmental transmission electron microscopy (ETEM) studies to compare the OER stability of the (001) surfaces of Pr1–xCaxMnO3 perovskite films with doping of x = 0.1 and x = 0.33. Notably, electrochemical analysis in alkaline conditions, as well as ETEM studies in H2O vapor, shows parallel trends in the stability of both systems: Mn leaching for the Pr1–xCaxMnO3 (PCMO) x = 0.33 system due to oxygen vacancy formation at the surface and higher stability of PCMO x = 0.1, where the oxygen vacancy redox peak is absent in cyclovoltammetry. Electron energy loss spectroscopy reveals the preservation of the Mn valence state in H2O for the PCMO x = 0.1 system, whereas for x = 0.33, Mn is reduced. We interpret this enhanced stability for the low-doped system in terms of a modified Mn 3d–O 2p hybridization, i.e., covalence. For a system with charge localization due to Jahn–Teller polarons, the covalence determines to what degree redox processes of lattice oxygen can arise that can finally lead to corrosive oxygen vacancy formation.

Deep learning detection of nanoparticles and multiple object tracking of their dynamic evolution during in situ ETEM studies

In situ transmission electron microscopy (TEM) studies of dynamic events produce large quantities of data especially under the form of images. In the important case of heterogeneous catalysis, environmental TEM (ETEM) under gas and temperature allows to follow a large population of supported nanoparticles (NPs) evolving under reactive conditions. Interpreting properly large image sequences gives precious information on the catalytic properties of the active phase by identifying causes for its deactivation. To perform a quantitative, objective and robust treatment, we propose an automatic procedure to track nanoparticles observed in Scanning ETEM (STEM in ETEM). Our approach involves deep learning and computer vision developments in multiple object tracking. At first, a registration step corrects the image displacements and misalignment inherent to the in situ acquisition. Then, a deep learning approach detects the nanoparticles on all frames of video sequences. Finally, an iterative tracking algorithm reconstructs their trajectories. This treatment allows to deduce quantitative and statistical features about their evolution or motion, such as a Brownian behavior and merging or crossing events. We treat the case of in situ calcination of palladium (oxide) / delta-alumina, where the present approach allows a discussion of operating processes such as Ostwald ripening or NP aggregative coalescence.

An Environmental Transmission Electron Microscopy Study of the Stability of the TiO2 (1 × 4) Reconstructed (001) Surface

The anatase TiO2 nanocrystals with dominant (001) facets attracted tremendous attention in the past decade. However, the reported intrinsic property of the (001) surface is still a cause of controversy. A crucial reason is that the (001) surface usually undergoes a (1 × 4) reconstruction, which may result in a remarkable difference in property. Herein, we performed an in situ environmental transmission electron microscopy (ETEM) study regarding the formation and stability of the (1 × 4)-(001) surface of TiO2 nanocrystals. The systematic ETEM studies confirmed that the (1 × 4)-(001) surface could be generated at elevated temperature (above 300 °C) when the surface contaminants were removed and the formed reconstruction could survive under different conditions, which indicate the surface reconstruction should be taken into account in related property research. In addition, an oriented layer-by-layer beam damage process on the (001) surface is confirmed, and optimal imaging conditions were also investigated,...

2D & 3D in situ study of the calcination of Pd nanocatalysts supported on delta-Alumina in an Environmental Transmission Electron Microscope

The quality of metallic nanoparticles (NPs) used in heterogeneous catalysis relies through many aspects on their small size, on the homogeneity of their spatial distribution on their supports and on their ability to resist to sintering or coalescence. It is thus very important to quantify these parameters and understand the mechanisms controlling the growth of NPs during the genesis process of the catalyst. Whereas conventional Transmission Electron Microscopy (TEM) is currently used for these purposes, it most frequently remains a ‘static’ method where results are obtained in high vacuum and post mortem, i.e. after the typical drying, calcination and reduction steps without the possibility to follow directly the evolution of both NPs and supports during those treatments. Environmental TEM (ETEM) unlocks this blocking and allows elementary mechanisms, such as Ostwald Ripening and coalescence to be unravelled through direct in situ observations. We report here an ETEM study of the preparation of Pd-based narrow NPs, less than 5 nm in size, deposited on a δ-alumina support. We focused on 3 main objectives: (i) quantifying the sizes of NPs at each preparation step performed in situ under environmental (i.e. respectively oxygen or air and hydrogen atmospheres at working temperatures) and comparing them to post mortem measurements; (ii) identify the oxidation state of the NPs through an in situ High Resolution imaging study of their crystallographic structure; (iii) explore the possibilities of environmental tri-dimensional (3D) studies by tilt series based Electron Tomography. This last item represents a challenging breakthrough in the characterization of nanocatalysts; it will be demonstrated that the use of modern instruments (microscope and accessories) allows tomographic acquisitions to be performed very fast, within a few minutes and even seconds, which opens the way to the 3D tracking of microstructures almost in real time during their evolution under gas and at high temperature.

In Situ TEM Study of the Degradation of PbSe Nanocrystals in Air

PbSe nanocrystals have attracted widespread attention due to a variety of potential applications. However, the practical utility of these nanocrystals has been hindered by their poor air stability, which induces undesired changes in the optical and electronic properties. An understanding of the degradation of PbSe nanocrystals when they are exposed to air is critical for improving the stability and enhancing their applications. Here, we use in situ transmission electron microscopy (TEM) with an environmental cell connected to air to study PbSe nanocrystal degradation triggered by air exposure. We have also conducted a series of complementary studies, including in situ environmental TEM study of PbSe nanocrystals exposed to pure oxygen and PbSe nanocrystals in H2O using a liquid cell, and ex situ experiments, such as O2 plasma treatment and thermal heating of PbSe nanocrystals under different air exposure. Our in situ observations reveal that when PbSe nanocrystals are exposed to air (or oxygen) under elec...

Heating-Induced Transformations of Atmospheric Particles: Environmental Transmission Electron Microscopy Study.

Environmental transmission electron microscopy was employed to probe transformations in the size, morphology, and composition of individual atmospheric particles as a function of temperature. Two different heating devices were used and calibrated in this work: a furnace heater and a Micro Electro Mechanical System heater. The temperature calibration used sublimation temperatures of NaCl, glucose, and ammonium sulfate particles, and the melting temperature of tin. Volatilization of Suwanee River Fulvic Acid was further used to validate the calibration up to 800 °C. The calibrated furnace holder was used to examine both laboratory-generated secondary organic aerosol particles and field-collected atmospheric particles. Chemical analysis by scanning transmission X-ray microscopy and near-edge fine-structure spectroscopy of the organic particles at different heating steps showed that above 300 °C particle volatilization was accompanied by charring. These methods were then applied to ambient particles collected in the central Amazon region. Distinct categories of particles differed in their volatilization response to heating. Spherical, more-viscous particles lost less volume during heating than particles that spread on the imaging substrate during impaction, due to either being liquid upon impaction or lower viscosity. This methodology illustrates a new analytical approach to accurately measure the volume fraction remaining for individually tracked atmospheric particles at elevated temperatures.

Atomic scale environmental transmission electron microscopy study of the surface mobility of ceria nanocubes

Journal Article Atomic scale environmental transmission electron microscopy study of the surface mobility of ceria nanocubes Get access M Bugnet, M Bugnet University of Lyon, INSA-Lyon, UCBL Lyon 1, MATEIS, UMR 5510 CNRS, 69621 Villeurbanne Cedex, France Search for other works by this author on: Oxford Academic Google Scholar S H Overbury, S H Overbury Chemical Science Division, Center for Nanophase Materials Science, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA Search for other works by this author on: Oxford Academic Google Scholar Z Wu, Z Wu Chemical Science Division, Center for Nanophase Materials Science, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA Search for other works by this author on: Oxford Academic Google Scholar F J C S Aires, F J C S Aires University of Lyon, UCBL Lyon 1, IRCELYON, UMR 5256 CNRS, 69100 Villeurbanne, France Search for other works by this author on: Oxford Academic Google Scholar T Epicier T Epicier University of Lyon, INSA-Lyon, UCBL Lyon 1, MATEIS, UMR 5510 CNRS, 69621 Villeurbanne Cedex, France Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 23, Issue S1, 1 July 2017, Pages 898–899, https://doi.org/10.1017/S1431927617005153 Published: 04 August 2017

Electron microscopic studies of natural gas oxidation catalyst – Effects of thermally accelerated aging on catalyst microstructure

Structural changes of PtPd nanoparticles in a natural gas oxidation catalyst were studied at elevated temperatures in air and low-oxygen conditions and in situ using environmental transmission electron microscopy (ETEM). The fresh catalyst shows <5nm, PtPdOx particles on the γ-Al2O3 support. At 700°C, the noble metal oxide decomposes and Pt gets trapped by PdO particles followed by formation of metallic Pd and Pt containing particles. At 1000°C, the particles had a metallic Pd and Pt containing core surrounded by PdO particles. In addition, the presence of <10nm sized particles was always observed. The activity measurements indicate the decrease in activity at the elevated temperatures. ETEM studies showed significant mobility of the noble metal particles above 850°C. Above 1100°C, PtPd particles were mobile and smaller particles were trapped by larger ones by a particle coalescence mechanism.

In Situ TEM Observation on Redox Cycling of SOFC Anode

Solid Oxide Fuel Cell (SOFC) technology is a promising energy conversion option which generates electricity with high efficiency. SOFCs have additional advantages including fuel flexibility (e. g. CO, H2 and CH4), distributed power generation, and reduction of pollutants such as NOx and SOx. The standard SOFC design is based on the use of a porous cermet composite anode made of yttria (Y2O3)-stabilized zirconia (YSZ) and Ni. NiO particles are sintered with YSZ and then reduced to metallic Ni before normal operation of the cell. Ni-based anode-supported cells display sufficient performance and steady state durability, however, the anode exhibits structural instability in successive redox cycles: although Ni is kept in its reduced state under normal operating conditions, the Ni may reoxidize due to factors such as seal leakage, shutdown in the absence of protecting gas, or high fuel utilization. It has been suggested that coalescence and sintering of Ni during reduction and the formation of porosity on reoxidation may account for the instability of Ni on redox cycling. However, there have been only a few reports on the reduction and oxidation processes of Ni-zirconia cermets observed directly with nanometer spatial resolution in real time. Environmental transmission electron microscopy (ETEM) can be used to provide dynamic in situ observations of chemical reactions at the nano- to micrometric scale. It allows gases at pressures above 10 mbar to be introduced into the electron microscope column with the sample at elevated temperature and without compromising other aspects of the performance of the microscope. In this study, in situ reduction and reoxidation for Ni and Sc2O3-stabilized zirconia (ScSZ) cermet anode were performed in a differentially pumped FEI TitanTM ETEM with the specimen mounted in a NanoExTM heating holder. This configuration allows in situ studies in the microscope in gaseous atmospheres up to 20 mbar and at temperatures up to 1000℃. The SOFC anode samples for the ETEM examination were prepared by a focused ion beam (FIB) lamella lift-out method on MEMS Chips for the NanoEX holder using a FEI Versa 3D. Prior to heating the sample, H2 or O2was introduced into the column, corresponding to measured pressures of 3-5 mbar at the sample. After that, scanning TEM (STEM) high-angle annular dark-field and bright-field (BF) images of the SOFC lamella were continuously taken by the BB FlashBack software. The temperature was kept at 300 ℃, 400 ℃, 500 ℃, 600 ℃, and 700 ℃ for 5 minutes during heating up to 800 ℃. In the range between 400 ℃ and 800 ℃, the specimen was heated at a ramp of 1 ℃/sec, although it was possible to heat and to cool it immediately in the holder. In the case of heating as-sintered NiO-ScSZ cermet in a H2 atmosphere of 3 mbar, nanovoids were observed at around 400 ℃ at the grain boundaries of NiO/ScSZ and NiO/NiO. Nano-pores were formed in the NiO grains to accommodate the volume shrinkage that resulted from the reduction of NiO to Ni. When the temperature was over 500 ℃, the NiO-free surface was reduced directly, and porosity formed uniformly across the NiO grains. At higher temperatures, coalescence of the porosity was observed to result from Ni reorganization. This sample was reoxidized with heating again up to 800 ℃ under an O2atmosphere of 3 mbar in the ETEM. When the Ni-ScSZ cermet samples were heated in the O2 atmosphere of 3-5 mbar, both the grain boundary of Ni/ScSZ and the Ni free surface were reduced at around 500 ℃. Electron energy loss spectroscopy revealed that Ni particles were completely oxidized to NiO after heating up to 800 ℃. From these ETEM studies, it was found that the formation of porous NiO and the coarsening of Ni during the redox cycling result in the reduction of triple-phase boundaries in the cells. This work is partly supported by the Japan Science and Technology Agency (JST) through its Center of Innovation (COI) Program. International Institute for Carbon-Neutral Energy Research (I2CNER) is supported by World Premier International Research Center Initiative (WPI), MEXT, Japan. Figure 1

Fe[sbnd]Ti[sbnd]O based catalyst for large-chiral-angle single-walled carbon nanotube growth

Catalyst selection is very crucial for controlled growth of single-walled carbon nanotubes (SWNTs). Here we introduce a well-designed FeTiO solid solution for SWNT growth with a high preference to large chiral angles. The FeTiO catalyst was prepared by combining Ti layer deposition onto premade Fe nanoparticles with subsequent high-temperature air calcination, which favours the formation of a homogeneous FeTiO solid solution. Using CO as the carbon feedstock, chemical vapour deposition growth of SWNTs at 800 °C was demonstrated on the FeTiO catalyst. Nanobeam electron diffraction characterization on a number of individual SWNTs revealed that more than 94% of SWNTs have chiral angles larger than 15°. In situ environmental transmission electron microscopy study was carried out to reveal the catalyst dynamics upon reduction. Our results identify that the phase segregation through reducing FeTiO catalyst leads to the formation of TiOx-supported small Fe nanoparticles for SWNT growth. The strong metal-support interactions induced by partial reduction of TiOx support promote the wettability of Fe nanoparticle, accounting for the preferential growth of large-chiral-angle SWNTs. This work opens a new avenue for chiral angle selective growth of SWNTs.

Cover Picture: Environmental Transmission Electron Microscopy Study of Diesel Carbon Soot Combustion under Simulated Catalytic‐Reaction Conditions (ChemPhysChem 7/2015)

Cover Picture: Environmental Transmission Electron Microscopy Study of Diesel Carbon Soot Combustion under Simulated Catalytic‐Reaction Conditions (ChemPhysChem 7/2015)

Environmental Transmission Electron Microscopy Study of Diesel Carbon Soot Combustion under Simulated Catalytic‐Reaction Conditions

AbstractThe front cover artwork is provided by the groups of Prof. Kohsuke Mori and Prof. Hiromi Yamashita (Osaka University) in collaboration with Keitaro Watanabe and Dr. Takeshi Sato (Hitachi High‐Technologies). The cover picture shows the time‐lapsed TEM images acquired by environmental transmission electron microscopy (ETEM) observation during the exposure of the catalysts and soot to O2 at elevated temperatures. Read the full text of the article at 10.1002/cphc.201402818.

Environmental TEM Study of Electron Beam Induced Electrochemistry of Pr0.64Ca0.36MnO3 Catalysts for Oxygen Evolution

Environmental transmission electron microscopy (ETEM) studies offer great potential for gathering atomic scale information on the electronic state of electrodes in contact with reactants. It also poses big challenges due to the impact of the high energy electron beam. In this article, we present an ETEM study of a Pr0.64Ca0.36MnO3 (PCMO) thin film electrocatalyst for water splitting and oxygen evolution in contact with water vapor. We show by means of off-axis electron holography and electrostatic modeling that the electron beam gives rise to a positive electric sample potential due to secondary electron emission. The value of the electric potential depends on the primary electron flux, the sample’s electric conductivity and grounding, and gas properties. We present evidence that two observed electrochemical reactions are driven by a beam induced electrostatic potential of the order of a volt. The first reaction is an anodic oxidation of oxygen depleted amorphous PCMO which results in recrystallization of the oxide. The second reaction is oxygen evolution which can be detected by the oxidation of a silane additive and formation of SiO2–y at catalytically active surfaces. The quantification of beam induced potentials is an important step for future controlled electrochemical experiments in an ETEM.

Environmental Transmission Electron Microscopy Study of Diesel Carbon Soot Combustion under Simulated Catalytic-Reaction Conditions.

Environmental transmission electron microscopy (ETEM) is used to monitor the catalytic combustion of diesel carbon soot upon exposure to molecular oxygen at elevated temperatures by using a gas-injection specimen heating holder. The reaction conditions simulated in the ETEM experiments reconstruct real conditions effectively. This study demonstrated for the first time that soot combustion occurs at the soot-catalyst interface for both Ag/CeO2 and Cu/BaO/La2 O3 catalysts.

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.

Novel Hybrid Sample Preparation Method for In Situ Liquid Cell TEM Analysis

In situ environmental transmission electron microscopy (TEM) research has increased dramatically in recent years [1-3]. However, due to the difficulty in sample preparation, liquid environmental TEM studies have been limited to materials such as nanoparticles or nanowires [3-5]. To date, no work has been reported on conventional metals/alloys in liquid environment. This is due to the significant difficulty in preparation of suitable “bulk” metal specimens that are compatible with the requirements of the liquid cell in situ specimen holders. We demonstrate a new sample preparation method for the in situ liquid cell TEM study of cold-rolled Type 304 austenitic stainless steel. The proposed new procedure takes advantage of electropolishing and focused ion beam (FIB) sample preparation techniques to extract large electron-transparent site-specific areas with minimum contamination. The liquid cell TEM holder used in these experiments was a Protochips Poseidon 200 liquid cell system, but this novel method can be extended as a general procedure applicable to other types of in situ environmental (E)-TEM studies.

In situ environmental transmission electron microscopy study of oxidation of two-dimensional Ti3C2and formation of carbon-supported TiO2

Two-dimensional Ti3C2, also known as “MXene”, was oxidized in air under two different oxidizing regimes in order to produce carbon-supported TiO2. In situ TEM analysis coupled with Raman spectroscopy revealed the formation of either anatase nanoparticles or planar rutile nanocrystals, which were controlled by the time, temperature and heating rate.

Restructuring Transition Metal Oxide Nanorods for 100% Selectivity in Reduction of Nitric Oxide with Carbon Monoxide

Transition metal oxide is one of the main categories of heterogeneous catalysts. They exhibit multiple phases and oxidation states. Typically, they are prepared and/or synthesized in solution or by vapor deposition. Here we report that a controlled reaction, in a gaseous environment, after synthesis can restructure the as-synthesized transition metal oxide nanorods into a new catalytic phase. Co3O4 nanorods with a preferentially exposed (110) surface can be restructured into nonstoichiometric CoO1-x nanorods. Structure and surface chemistry during the process were tracked with ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and environmental transmission electron microscopy (E-TEM). The restructured nanorods are highly active in reducing NO with CO, with 100% selectivity for the formation of N2 in temperatures of 250-520 °C. AP-XPS and E-TEM studies revealed the nonstoichiometric CoO1-x nanorods with a rock-salt structure as the active phase responsible for the 100% selectivity. This study suggests a route to generate new oxide catalysts.

Environmental Transmission Electron Microscopy Study of the Origins of Anomalous Particle Size Distributions in Supported Metal Catalysts

In this Environmental Transmission Electron Microscopy (ETEM) study we examined the growth patterns' of uniform distributions of nanoparticles (NPs) using model catalysts. Pt/SiO2 was heated at 550 degrees C in 560 Pa of O-2 while Pd/carbon was heated in vacuum at 500 degrees C and in 300 Pa of 5%H-2 in Argon at temperatures up to 600 degrees C. Individual NPs of Pd were tracked to determine the operative sintering mechanisms. We found anomalous growth Of NPs occurred during the early stages of catalyst sintering wherein some particles started to grow significantly larger than the mean, resulting in a broadening of the particle size distribution (PSD). The abundance of the larger particles did not fit the log normal distribution. We can rule out sample nonuniformity as a cause for the growth of these large particles, since images were recorded prior to heat treatments. The anomalous growth of these particles may help explain PSDs in heterogeneous catalysts which often show particles that are significantly larger than the mean, resulting in a long tail to the right. It has been suggested previously that particle migration and coalescence could be the likely cause for such broad size distributions. We did not detect any random migration of the NPs leading to coalescence. A directed migration process was seen to occur at elevated temperatures for Pd/carbon under H-2. This study shows that anomalous growth of NPs can occur under conditions where Ostwald ripening is the primary sintering mechanism.