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Rotation during reaction Determining changes in heterogeneous catalysts under reaction conditions can provide insight into mechanisms. Under reaction conditions, not only can metal nanoparticles change shape but their interaction with the oxide support could also be affected. Yuan et al. used aberration-corrected environmental transmission electron microscopy to study gold nanoparticles on titanium surfaces at low electron beam doses. During carbon monoxide (CO) oxidation at total pressures of a few millibars and 500°C, they observed that gold nanoparticles rotated by about 10° but returned to their original position when CO was removed. Density function theory calculations indicated that rotation was induced by changes in the coverage of adsorbed molecular oxygen at the interface. Science , this issue p. 517

In the present work, the degradation of size-selected Pt nanoclusters is studied under electrochemical conditions. This model catalyst mimics carbon supported Pt nanoclusters and nanoparticles typically employed in proton exchange membrane fuel cells (PEMFCs). Insight into the early stage of degradation is given by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and confirmed by transmission electron microscopy (TEM). In contrast to common assumptions, it is demonstrated that even extremely small Pt clusters exhibit a remarkable stability under electrochemical conditions. Such nanoclusters are then relevant to develop electrodes for energy applications. Furthermore, applying mixed cluster samples of Pt22 and Pt68, no preferential dissolution of Pt22 by Ostwald ripening - usually held responsible to be the main mechanism for activity loss in Pt fuel cell catalysts - is observed. In light of the findings reported, developing highly-dispersed subnanometer Pt clusters as catalyst for PEMFCs is a realistic approach provided the operation conditions are suitably adjusted. Furthermore, mitigation strategies to improve the stability of few-atoms catalyst under electrochemical operation will benefit from addressing cluster-support interactions.

Structural evolution of iron nanoparticles involving the formation and growth of iron carbide nuclei in the iron nanoparticle was directly visualized at the atomic level, using environmental transmission electron microscopy (TEM) under reactive conditions mimicking Fischer-Tropsch synthesis. Formation of the iron carbide nuclei and surface reconstruction of the iron nanoparticle play an essential role in carburization of the iron nanoparticle and consequent formation of Fe5C2. Identification of carbide and oxide intermediates evidenced by high-resolution TEM images, electron diffraction patterns and electron energy-loss spectra provides a detailed picture from initial activation to final degradation of iron under synthesis gas. (Chemical Equation Presented).

International audience; In order to controllably grow single-wall carbon nanotubes (SWCNTs), a better understanding of the growth processes and how they are influenced by external parameters such as catalyst and gaseous environment is required. Here, we present direct evidence of growth termination of individual SWCNTs and successive growth of additional SWCNTs on Co catalyst particles supported on MgO by means of environmental transmission electron microscopy. Such in situ observations reveal the plethora of solid carbon formations at the local scale while it is happening and thereby elucidate the multitude of configurations resulting from identical external synthesis conditions, which should be considered in the quest for controlled SWCNT growth. Using CO and a mixture of CO and H2 as carbon sources, we show that the growth of SWCNTs terminates with a reduced tube−catalyst adhesion strength. Two main reasons for the cessation are proposed: insufficient active carbon species and a certain amount of stress exerted at the tube−catalyst interface. Interestingly, it was observed that catalyst particles stayed active in terms of nucleating additional solid carbon structures after growth termination of the first SWCNT. These observations elucidate the importance of an in-depth understanding of the role of catalysts and carbon sources in the continued growth of SWCNTs. Furthermore, it serves as a guide for further control of carbon nanostructure synthesis via catalyst engineering and synthesis optimization.

One of the biggest challenges for in situ heating transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) is the ability to measure the local temperature of the specimen accurately. Despite technological improvements in the construction of TEM/STEM heating holders, the problem of being able to measure the real sample temperature is still the subject of considerable discussion. In this study, we review the present literature on methodologies for temperature calibration. We analyze calibration methods that require the use of a thermometric material in addition to the specimen under study, as well as methods that can be performed directly on the specimen of interest without the need for a previous calibration. Finally, an overview of the most important characteristics of all the treated techniques, including temperature ranges and uncertainties, is provided in order to provide an accessory database to consult before an in situ TEM/STEM temperature calibration experiment.

Imaging reactive surface water Recent developments in transmission electron microscopy (TEM) have enabled imaging of single atoms, but adsorbed gas molecules have proven more challenging because of a lack of sufficient image contrast. Yuan et al. adsorbed water and carbon monoxide (CO) on a reconstructed nanocrystalline anatase titanium dioxide (TiO 2 ) surface that has protruding TiO 3 ridges every four unit cells, which provide regions of distinct contrast. Water adsorption on this surface during environmental TEM experiments led to the formation of twinned protrusions. These structures developed dynamic contrast as the water reacted with coexposed CO to form hydrogen and carbon dioxide. Science , this issue p. 428

The growth kinetics play key roles in determining the chirality distribution of the grown single-walled carbon nanotubes (SWCNTs). However, the lack of comprehensive understandings on the SWCNT’s growth mechanism at the atomic scale greatly hinders SWCNT chirality-selective synthesis. Here, we establish a general model, where the dislocation theory is a specific case, to describe the etching agent–dependent growth kinetics of SWCNTs on solid catalyst particles. In particular, the growth kinetics of SWCNTs in the absence of etching agent is validated by both in situ environmental transmission electron microscopy and ex situ chemical vapor deposition growth of SWCNTs. On the basis of the new theory of SWCNT’s growth kinetics, we successfully explained the selective growth of (2n, n) SWCNTs. This study provides another degree of freedom for SWCNT controlled synthesis and opens a new strategy to achieve chirality-selective synthesis of (2n, n) SWCNTs using solid catalysts.

Oxide supported metal nanoparticles play an important role in heterogeneous catalysis. However, understanding the metal/oxide interface and their evolution under reaction conditions remains challenging. Herein, we investigate the interface between Au nanoparticles and a CeO2 substrate by environmental transmission electron microscopy with atomic resolution. We find that the Au nanoparticles have two preferential epitaxial relationships with the substrate, i.e. Type I (111)[-110]CeO2//(111)[-110]Au and Type II (111)[-110]CeO2//(111)[1-10]Au orientation relationships, where Type I is preferred. In situ observations in the presence of O-2 show that the gas can stimulate the supported Au nanoparticles to transform between these two orientations even at room temperature. Moreover, when increasing the temperature to 973 K, the transformation of an Au nanoparticle between the two orientation states and a non-crystalline state in the presence of O-2 is also observed. DFT calculations of the binding between Au and CeO2 in the two relationships are strongly influenced by the presence of oxygen vacancies. For a given position of a vacancy, there is a significant energy difference between the energy of the two types. However, for some positions, Type I is preferred, and for others, Type II, but the most favourable position of the vacancy for the two types has a very similar energy. This is consistent with the observation of both types of adhesion.

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

Platinum-yttrium alloy nanoparticles show both a high activity and stability for the oxygen reduction reaction. The catalysts were prepared by magnetron sputter aggregation and mass filtration providing a model catalyst system with a narrow size distribution. The structure and stability of nanostructured Pt-Y alloy catalysts were studied using transmission electron microscopy techniques. Using elemental X-ray mapping and high-resolution electron microscopy, the specific compositional structure and distribution of the individual nanoparticles was unraveled and the stability assessed. Studying the catalyst after reaction and after aging tests shows the development of a core-shell type structure after being exposed to reaction conditions.

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...

Recording atomic-resolution transmission electron microscopy (TEM) images is becoming increasingly routine. A new bottleneck is then analyzing this information, which often involves time-consuming manual structural identification. We have developed a deep learning-based algorithm for recognition of the local structure in TEM images, which is stable to microscope parameters and noise. The neural network is trained entirely from simulation but is capable of making reliable predictions on experimental images. We apply the method to single sheets of defected graphene, and to metallic nanoparticles on an oxide support., Comment: v2: Typo in author list corrected

The reduction of a metal oxide is often a critical preparation step for activating catalytic behaviour. This study addresses the reduction process of NiO in pure form and in a composite of NiO/yttria-stabilized zirconia (YSZ) in hydrogen relevant for solid oxide electrochemical cells by comparing results from environmental transmission electron microscopy (ETEM) with thermogravimetric analysis (TGA). The temperature dependent reduction profiles obtained from TGA confirm an inhibitive effect from YSZ on the NiO reduction. The ETEM images show the growth of Ni in decaying NiO and reveal the nanoscale morphological changes such as pore formation in NiO above 280°C and densification and collapse of the pore structures above 400°C. The accelerated Ni front in NiO illustrates the auto catalysis of the reaction. A rapid temperature ramping from room temperature to 780°C in hydrogen in 1 second resulted in immediate morphological changes at the nanoscale from dense NiO to dense Ni. The analysis suggests that the inhibitive effect of YSZ on the NiO reduction reaction is not due to a direct local interaction between YSZ and NiO, but instead due to gas and/or mass transport limitations.© 2014 Elsevier B.V. All rights reserved.

Strain analysis from high-resolution transmission electron microscopy (HRTEM) images offers a convenient tool for measuring strain in materials at the atomic scale. In this paper we present a theoretical study of the precision and accuracy of surface strain measurements directly from aberration-corrected HRTEM images. We examine the influence of defocus, crystal tilt and noise, and find that absolute errors of at least 1–2% strain should be expected. The model structures include surface relaxations determined using molecular dynamics, and we show that this is important for correctly evaluating the errors introduced by image aberrations. Electronic supplementary material The online version of this article (doi:10.1186/s40679-017-0047-0) contains supplementary material, which is available to authorized users.

When using bifunctional core@shell catalysts, the stability of both the shell and core–shell interface is crucial for catalytic applications. In the present study, we elucidate the stability of a CuO/ZnO/Al2O3@ZSM-5 core@shell material, used for one-stage synthesis of dimethyl ether from synthesis gas. The catalyst stability was studied in a hierarchical manner by complementary environmental transmission electron microscopy (ETEM), scanning electron microscopy (SEM) andin situhard X-ray ptychography with a specially designedin situcell. Both reductive activation and reoxidation were applied. The core–shell interface was found to be stable during reducing and oxidizing treatment at 250°C as observed by ETEM andin situX-ray ptychography, although strong changes occurred in the core on a 10 nm scale due to the reduction of copper oxide to metallic copper particles. At 350°C,in situX-ray ptychography indicated the occurrence of structural changes also on theµm scale, i.e. the core material and parts of the shell undergo restructuring. Nevertheless, the crucial core–shell interface required for full bifunctionality appeared to remain stable. This study demonstrates the potential of these correlativein situmicroscopy techniques for hierarchically designed catalysts.

The majority of the nanowire synthesis methods utilize catalyst particles to guide the nanowire geometry. In contrast, catalyst-free methods are attractive for facile fabrication of pure nanowires without the need for catalyst preparation. Nonetheless, how nanowire growth is guided without a catalyst is still widely disputed and unclear. Here, we show that the nanowire growth during metal oxidation is limited by a nucleation of a new layer. On the basis of in situ transmission electron microscope investigations we found that the growth occurs layer by layer at the lowest specific surface energy planes. Atomic layers nucleate at the edge of twin boundary ridges and form a long-range ordering along the twin boundary. We anticipate our study to be a starting point to employ defects for nanowire growth control and consequently shaping the geometry of nanowires in a similar manner as in the catalyst-assisted growth method.

Low-temperature chemical vapor deposition (CVD) growth of single-walled carbon nanotubes (SWNTs) was achieved on two different types of CoxMg1−xO catalysts prepared by different techniques: atomic layer deposition (ALD) and impregnation. The chirality distribution of SWNTs grown on the ALD-prepared CoxMg1−xO catalyst is wider than that of SWNTs grown on the impregnation-prepared CoxMg1−xO catalyst. The different chirality distributions of SWNTs are related to their different growth modes. The ALD-prepared CoxMg1−xO catalyzes the growth of SWNTs by “tip growth” mode, as revealed by in situ environmental transmission electron microscopy studies. In contrast, SWNTs grow on the impregnation-prepared CoxMg1−xO by “base growth” mode. “Base growth” is attributed to strong metal–support interactions between the epitaxially formed Co nanoparticles and the underlying MgO support, accounting for the synthesis of SWNTs with high chiral-selectivity. In addition, impregnation-prepared CoxMg1−xO catalysts calcinated at different temperatures were systematically studied and their catalytic performances in synthesizing carbon nanotubes were elucidated. This work illustrates the influence of metal–support interactions and catalyst reducibility on the chirality-distribution of the synthesized SWNTs.

The oxidation of nickel particles was studied in situ in an environmental transmission electron microscope in 3.2 mbar of O-2 between ambient temperature and 600 degrees C. Several different transmission electron microscopy imaging techniques, electron diffraction and electron energy-loss spectroscopy were used to study the evolution of the microstructure and the local chemical composition of the particles during oxidation. Our results suggest that built-in field effects control the initial stages of oxidation, with randomly oriented NiO crystallites and internal voids then forming as a result of outward diffusion of Ni2+ along NiO grain boundaries, self-diffusion of Ni2+ ions and vacancies, growth of NiO grains and nucleation of voids at Ni/NiO interfaces. We also observed the formation of transverse cracks in a growing NiO film in situ in the electron microscope. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Silica-supported, bimetallic Cu–Ni nanomaterials were prepared with different ratios of Cu to Ni by incipient wetness impregnation without a specific calcination step before reduction. Different in situ characterization techniques, in particular transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray absorption spectroscopy (XAS), were applied to follow the reduction and alloying process of Cu–Ni nanoparticles on silica. In situ reduction of Cu–Ni samples with structural characterization by combined synchrotron XRD and XAS reveals a strong interaction between Cu and Ni species, which results in improved reducibility of the Ni species compared with monometallic Ni. At high Ni concentrations silica-supported Cu–Ni alloys form a homogeneous solid solution of Cu and Ni, whereas at lower Ni contents Cu and Ni are partly segregated and form metallic Cu and Cu–Ni alloy phases. Under the same reduction conditions, the particle sizes of reduced Cu–Ni alloys decrease with increasing Ni content. Estimates of the metal surface area from sulfur chemisorption and from the XRD particle size generally agree well on the trend across the composition range, but show some disparity in terms of the absolute magnitude of the metal area. This work provides practical synthesis guidelines towards preparation of Cu–Ni alloy nanomaterials with different Cu/Ni ratios, and insight into the application of different in situ techniques for characterization of the alloy formation.

The crystallographic and compositional structure of heterostructured semiconductor nanowires has been studied by means of transmission electron microscopy. The native geometry of the studied InP-GaAs nanowires (80–100 nm in diameter) is in general too thick for reliable high-resolution TEM imaging. Nano Beam Electron Diffraction (NBED) is shown to be a powerful technique to reveal strain near the interface of compositional change in heterostructured semiconductor nanowires. Furthermore, the relative orientation of the nanowires is studied by means of NBED revealing the nanowires to be very flexible. NBED proves to be a good alternative to convergent beam electron diffraction and electron backscatter diffraction in terms of crystal orientation determination and mapping.

Photocatalysts for solar fuel production are subject to intensive investigation as they constitute one viable route for solar energy harvesting. Cuprous oxide (Cu2O) is a working photocatalyst for hydrogen evolution but it photocorrodes upon light illumination in an aqueous environment. Environmental transmission electron microscopy (ETEM) makes it possible to obtain insight into the local structure, composition and reactivity of catalysts in their working environment, which is of fundamental interest for sustainable energy research and is essential for further material optimization. Herein, photoreduction of Cu2O is studied in situ using a dedicated TEM specimen holder for light illumination.

Fundamental concepts of the crystal formation suggest that the growth and decomposition are determined by simultaneous embedding and removal of the atoms. Apparently, by changing the crystal formation conditions one can switch the regimes from the growth to decomposition. To the best of our knowledge, so far this has been only postulated, but never observed at the atomic level. By means of in situ environmental transmission electron microscopy we monitored and examined the atomic layer transformation at the conditions of the crystal growth and its decomposition using CuO nanowires selected as a model object. The atomic layer growth/decomposition was studied by varying an O2 partial pressure. Three distinct regimes of the atomic layer evolution were experimentally observed: growth, transition and decomposition. The transition regime, at which atomic layer growth/decomposition switch takes place, is characterised by random nucleation of the atomic layers on the growing {111} surface. The decomposition starts on the side of the nanowire by removing the atomic layers without altering the overall crystal structure, which besides the fundamental importance offers new possibilities for the nanowire manipulation. Understanding of the crystal growth kinetics and nucleation at the atomic level is essential for the precise control of 1D crystal formation.

Chiralities of single-walled carbon nanotubes grown on an atomic layer deposition prepared bimetallic FeCu/MgO catalyst were evaluated quantitatively using nanobeam electron diffraction. The results reveal that the growth yields nearly 90% semiconducting tubes, 45% of which are of the (6,5) type. The growth mechanisms as well as the roles of different components in the catalyst were studied in situ using environmental transmission electron microscopy and infrared spectroscopy. On the basis of the understanding of carbon nanotube growth mechanisms, an MgO-supported FeCu catalyst was prepared by impregnation, showing similar catalytic performance as the atomic layer deposition-prepared catalyst, yielding single-walled carbon nanotubes with a similar narrow chirality distribution.

This work investigates the use of the bulk carbides Mo2C, WC, and NbC as catalysts for the conversion of syngas into higher alcohols. K2CO3/WC produces mainly CH3OH and CH4 with a low activity. NbC has a very low activity in CO hydrogenation. K2CO3/Mo2C produces mixed alcohols with a reasonable activity and selectivity. In a 94 h test the activity and the specific surface area of the K2CO3/Mo2C catalyst decreased significantly, but X-ray diffraction and transmission electron microscopy did not indicate a strong sintering of the carbide. A likely cause for the deactivation is the formation of carbonaceous deposits on the catalyst. At the same general activity level Li, K, and Cs provide similar promotional effects for Mo2C, although K at a loading level of alkali metal/Mo = 0.164 mol/mol provides the better behavior at equal conditions. The effect of further additives on the K2CO3/Mo2C system was evaluated, but only Cu yielded an improvement.

The latest generation of environmental transmission electron microscopes incorporates aberration correctors and monochromators, allowing studies of chemical reactions and growth processes with improved spatial resolution and spectral sensitivity. Here, we describe the performance of such an instrument using examples taken from studies of fuel cells and supported catalysts. We discuss the challenges involved in performing in situ gas reaction experiments quantitatively and reliably and we highlight the care required to understand the effect of the electron beam on dynamic experiments performed in the electron microscope.

Carbon particle structures containing only a few graphene layers have been collected from a Bunsen (propane) diffusion flame in the low particle concentration pale yellow luminous regions close to the soot inception. These particles were sampled directly on transmission electron microscopy grids for structural and elemental analysis. They were found to be several hundreds of nanometers in size. Such large structures are not easily explained from gas-phase kinetic models, yet the sheets occurred relatively frequent in the images. Some pictures also showed interesting polygonal few-layered graphitic structures perpendicular to the graphene planes.

The development of a ternary AlxIn1-xP shell grown around GaAs nanowires epitaxially grown in the [(1) over bar (1) over bar (1) over bar] direction has been studied. Morphology and composition of the shell have been studied using cross-sectional transmission electron microscopy (TEM). The side facets of the shell are found to develop dominant {1 (1) over bar 0} macro facets with small (approx. 5 nm) {1 1 (2) over bar} facets independent of the GaAs core side facets. Phase segregation is observed as AIP developing from the {1 1 (2) over bar} facets, while Al0.5In0.5P is found in the rest of the ternary shell. (C) 2010 Elsevier B.V. All rights reserved.

We report the growth and characterization of GaSb nanowires grown by MOVPE. The structural properties of the nanowires are investigated by the means of transmission electron microscopy, X-ray diffraction and single nanowire photoluminescence. The measurements confirm a high material quality in the GaSb nanowires. Also, a back-gated nanowire transistor structure is used to extract values for the polarity and resistivity of the GaSb. Finally, a simple kinetic model is presented to explain the non-linear time dependence of the GaSb nanowire growth. (C) 2008 Elsevier B.V. All rights reserved.

We report Au-assisted growth of GaAs/GaSb nanowire heterostructures on GaAs(1 1 1)B-substrates by metal-organic vapor phase epitaxy. The growth is studied at various precursor molar fractions and temperatures, in order to optimize the growth conditions for the GaSb nanowire segment. In contrast to most other III-V nanowire systems, the GaSb nanowire growth is Group V-limited under most conditions. We found that depending on the TMSb molar fraction, the seed particle is either supersaturated AuGa or AuGa2 during GaSb growth. The high Ga content in the particle gives a characteristic diameter increase between the GaAs and GaSb segment. From TEM and XEDS measurements we conclude that the GaSb nanowire growth occurs along either the AuGa-GaSb or AuGa2-GaSb pseudo-binaries of the Au-Ga-Sb ternary phase diagram. Finally, the GaSb nanowires exhibit untapered radial growth on the {1 (1) over bar 0} side facets. (C) 2008 Elsevier B.V. All rights reserved. (Less)

A novel approach, which is based on the analysis of sequences of images recorded using energy-filtered transmission electron microscopy and can be used to assess the reaction of a solid with a gas at elevated temperature, is illustrated for the reduction of a NiO/ceramic solid oxide fuel cell anode in 1.3mbar of H2. Three-window elemental maps and jump-ratio images of the O K edge and total inelastic mean free path images are recorded as a function of temperature and used to provide local and quantitative information about the reaction kinetics and the volume changes that result from the reaction. Under certain assumptions, the speed of progression of the reaction front in all three dimensions is obtained, thereby providing a three-dimensional understanding of the reaction.

Elucidating the evolution of bimetallic catalyst for nucleating carbon nanotube has been challenging. In this work, acorn-like Pt-Fe2O3 nanoparticles are developed for the growth of single-walled carbon nanotubes (SWCNTs) by chemical vapor deposition. Using in situ environmental transmission electron microscopy, restructuring of the acorn-like Pt-Fe2O3 nanoparticles at reaction conditions is investigated. Upon heating to reaction temperature, ε-Fe2O3 is converted to β-Fe2O3, which can be subsequently reduced to metallic Fe once introducing CO. As Pt promotes the carburization of Fe, part of the metallic Fe reacts with active carbon atoms to form Fe2.5C instead of Fe3C, catalyzing the nucleation of carbon nanotubes. Nanobeam electron diffraction characterizations on SWCNTs grown under ambient pressure at 800 °C demonstrate that their chiral angle and diameter distributions are similar to those of SWCNTs grown on monometallic Fe. The results further indicate that the active components in both the catalysts, determining the chirality distribution of SWCNTs, are similar. In addition, Pt facilitates the reduction of Fe2O3, rendering SWCNT growth at a relatively low temperature of 700 °C. This work provides a profound understanding of the structural reconstruction in bimetallic catalyst, shedding more light on designing novel catalysts for the growth of SWCNTs.

Imaging in a differential pumped environmental TEM (ETEM) results in general in a degradation of the image quality. Scattering of electrons by gas molecules in the pressurized volume between the pole pieces blurs the image and decreases the signal-to-noise ratio of the acquired images. The somewhat simple picture of a plane wave interacting with the sample of interest is no longer valid. Furthermore, the exit wave from the sample is altered by scattering events taking place after the sample in the direction of propagation. In this chapter, the effect of the increased gas pressure between the pole pieces in an aberration-corrected high-resolution transmission electron microscope is discussed in order to shine some light on the additional phenomena occurring in ETEM compared to conventional HRTEM. Both direct effects on the image quality and more indirect effects rising from gas ionization are discussed.

In the chemical industry, approximately 85–90 % of all chemicals are made via catalytic processes (Chorkendorff and Niemantsverdriet 2003). Catalysts are used in processes ranging from synthesis of fine chemicals over fuel refinement to abatement of pollution. Heterogeneous catalysts often comprise metal nanoparticles supported on a substrate providing a high dispersion and stabilization of the particles. The nanoparticle surfaces hold the active sites responsible for the conversion of reactants to products. The atomic arrangement at the specific surfaces has large influence on the reaction rate, making it of high importance to be able to study such on a local scale. Something, which has become a reality with the advent of aberration corrected transmission electron microscopy (TEM) (Hansen and Wagner 2012; Yoshida et al. 2012). Whereas this information could be obtained from more conventional high vacuum microscopy (Honkala et al. 2005; Carlsson et al. 2006; Janssens et al. 2006), the atomic arrangement, and distribution of specific surfaces are most likely different when the catalyst is in a reactive environment.

The catalytic activity and mobility of silver nanoparticles used as catalysts in temperature programmed oxidation of soot:silver (1:5 wt:wt) mixtures have been investigated by means of flow reactor experiments and in situ environmental transmission electron microscopy (ETEM). The carbon oxidation temperature was significantly lower compared to uncatalyzed soot oxidation with soot and silver loosely stirred together (loose contact) and lowered further with the two components crushed together (tight contact). The in situ TEM investigations revealed that the silver particles exhibited significant mobility during the soot oxidation, and this mobility, which increases the soot/catalyst contact, is expected to be an important factor for the lower oxidation temperature. In the intimate tight contact mixture the initial dispersion of the silver particles is greater,,and the onset of mobility occurs at a lower temperature which is consistent with the lower oxidation temperature of the tight contact mixture. (C) 2015 Elsevier B.V. All rights reserved.

High-resolution transmission electron microscopy characterization of molybdenum oxide catalyst precursors is carried out for a better understanding of the mechanisms leading to the active catalytic material. The precipitated material consists of both large-unit cell crystals and randomly orientated clusters on a 3-5 nm scale. The crystalline clusters are embedded in non-crystalline material, preventing the formation of large well-defined inactive orthorhombic MoO(3) crystals during activation of the catalyst material.

We present the growth of homogeneous InAs(1-x)P(x) nanowires as well as InAs(1-x)P(x) heterostructure segments in InAs nanowires with P concentrations varying from 22% to 100%. The incorporation of P has been studied as a function of TBP/TBAs ratio, temperature, and diameter of the wires. The crystal structure of the InAs as well as the InAs(1-x)P(x) segments were found to be wurtzite as determined from high-resolution transmission electron microscopy. Furthermore, temperature-dependent electrical transport measurements were performed on individual heterostructured wires to extract the conduction band offset of InAs(1-x)P(x) relative to InAs as a function of composition. From these measurements we extract a value of the linear coefficient of the conduction band versus x of 0.6 eV and a nonlinear coefficient, or bowing parameter, of 0.2 eV. Finally, homogeneous InAs(0.8)P(0.2) nanowires were shown to have a nondegenerate n-type doping and function as field-effect transistors at room temperature.

Binary molybdenum oxides are active catalysts for propene oxidation to acrolein if they are structurally different from orthorhombic MoO 3 (o-Mo). The periodical structure of the as-synthesized precatalyst must be different from o-Mo to create the activity. Additional features typical for efficient catalysts are a bulk nanostructuring and a complex surface termination that are detectable in TEM examinations only. After transition into defective o-Mo the samples exhibit much reduced activities. The implications of these findings on the functional analysis of complex selective oxidation catalysts are discussed.

A set of Mo, V, and W mixed -oxide samples with additives P and Cs showing high performance in select ive partial oxidation of propene is studied by means of elemental mapping and crystal structure determination using scanning and transmission electron microscopy, energy-dispersive X-ray spectroscopy (EDS), and electron energy loss spectroscopy (EELS) as supplement to XRD measurements which are insufficient for phase determination. The set is divided into two subsets of samples denoted A and B, where subset A samples show the highest performance in select ive partial oxidation of propene. These high-performance catalysts are all predominated by structures of the Mo5O14 type in contrast to a negligible amount of the orthorhombic MoO3, which is known to favor total oxidation.

Over time, there has been an increasing interest in observing catalysts in their operating environment at high spatial resolution and ultimately to determine the structure of a catalytically active surface. One tool with the potential to do exactly this in direct space is the transmission electron microscope, and since its invention by Ernst Ruska, the idea of imaging samples under gaseous atmospheres was envisioned. However, microscopes have traditionally been operated in high vacuum due to sensitive electron sources, sample contamination, and electron scattering off gas molecules resulting in loss of resolution. Using suitably clean gases, modified pumping schemes, and short pathways through dense gas regions, these issues are now circumvented. Here we provide an account of best practice using environmental transmission electron microscopy on catalytic systems illustrated using select examples from the literature showing how in situ electron microscopy can provide new insight into the state of catalysts...

Metallic alloy nanoparticles (NPs) are synthesized in situ in an environmental transmission electron microscope. Atomic level characterization of the formed alloy NPs is carried out at synthesis conditions by use of high-resolution transmission electron microscopy, electron diffraction and electron energy-loss spectroscopy.

We report a pattern recognition approach to detect the atomic structure in high-resolution transmission electron microscopy images of graphene. The approach provides quantitative information such as carbon-carbon bond lengths and bond length variations on a global and local scale alike. © 2014 Elsevier Ltd. All rights reserved.

The increasing use of environmental transmission electron microscopy (ETEM) in materials science provides exciting new possibilities for investigating chemical reactions and understanding both the interaction of fast electrons with gas molecules and the effect of the presence of gas on high-resolution imaging. A gaseous atmosphere in the pole-piece gap of the objective lens of the microscope alters both the incoming electron wave prior to interaction with the sample and the outgoing wave below the sample. Whereas conventional TEM samples are usually thin (below 100 nm), the gas in the environmental cell fills the entire gap between the pole pieces and is thus not spatially localized. By using an FEI Titan environmental transmission electron microscope equipped with a monochromator and an aberration corrector on the objective lens, we have investigated the effects on imaging and spectroscopy caused by the presence of the gas.

In situ transmission electron microscopy is used to obtain atom-resolved images of copper nanocrystals on different supports. These are catalysts for methanol synthesis and hydrocarbon conversion processes for fuel cells. The nanocrystals undergo dynamic reversible shape changes in response to changes in the gaseous environment. For zinc oxide–supported samples, the changes are caused both by adsorbate-induced changes in surface energies and by changes in the interfacial energy. For copper nanocrystals supported on silica, the support has negligible influence on the structure. Nanoparticle dynamics must be included in the description of catalytic and other properties of nanomaterials. In situ microscopy offers possibilities for obtaining the relevant atomic-scale insight.

We have observed a previously undescribed stepwise oxidation of mono- and few layer suspended graphene by silver nanoparticles in situ at subnanometer scale in an environmental transmission electron microscope. Over the range of 600-850 K, we observe crystallographically oriented channelling with rates in the range 0.01-1 nm/s and calculate an activation energy of 0.557 ± 0.016 eV. We present a discrete statistical model for this process and discuss the implications for accurate nanoscale patterning of nanoscale systems.

We investigate the Au-assisted growth of InAs nanowires on two different kinds of heterostructured substrates: GaAs/AlGaAs structures capped by a 50 nm thick InAs layer grown by molecular beam epitaxy and a 2 μm thick InAs buffer layer on Si(111) obtained by vapor phase epitaxy. Morphological and structural properties of substrates and nanowires are analyzed by atomic force and transmission electron microscopy. Our results indicate a promising direction for the integration of III-V nanostructures on Si-based electronics as well as for the development of novel micromechanical structures incorporating nanowires as their active elements.

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

The effects of lamellar twinning in epitaxial particle-assisted GaAs B nanowires are investigated in an extensive high resolution electron microscopy (HRTEM) study of the low index zones , , and . As these directions are non-parallel to the (-1-1-1) twin planes we find that the twin segments exhibit two different zone axes as a consequence of twinning. In the first three cases the alternative zones were found to be and . These findings are supported by a comparison of experimental HRTEM images and multi-slice simulations along with fast Fourier transform mapping. From the appearance of non-overlapping regions we conclude that the nanowires are bound by {111} facets only. The twin formation and the development of the stable side facets are discussed (Less)

The strain distribution in heterostructured wurtzite InAs/InP nanowires is measured by a peak finding technique using high resolution transmission electron microscopy images. We find that nanowires with a diameter of about 20 nm show a 10 nm strained area over the InAs/InP interface and the rest of the wire has a relaxed lattice structure. The lattice parameters and elastic properties for the wurtzite structure of InAs and InP are calculated and a nanowire interface is simulated using finite element calculations. Both the method and the experimental results are validated using a combination of finite element calculations and image simulations.