Aberration-corrected Environmental Transmission Electron Microscopy Research Articles

Catalytic mechanisms of nickel nanoparticles for the improved dehydriding kinetics of magnesium hydride

MgH2, albeit with slow desorption kinetics, has been extensively studied as one of the most ideal solid hydrogen storage materials. Adding such catalyst as Ni can improve the desorption kinetics of MgH2, whereas the catalytic role has been attributed to different substances such as Ni, Mg2Ni, Mg2NiH0.3, and Mg2NiH4. In the present study, Ni nanoparticles (Ni-NPs) supported on mesoporous carbon (Ni@C) have been synthesized to improve the hydrogen desorption kinetics of MgH2. The utilization of Ni@C largely decreases the dehydrogenation activation energy from 176.9 to 79.3 kJ mol−1 and the peak temperature of dehydrogenation from 375.5 to 235 °C. The mechanism of Ni catalyst is well examined by advanced aberration-corrected environmental transmission electron microscopy and/or x-ray diffraction. During the first dehydrogenation, detailed microstructural studies reveal that the decomposition of MgH2 is initially triggered by the Ni-NPs, which is the rate-limiting step. Subsequently, the generated Mg reacts rapidly with Ni-NPs to form Mg2Ni, which further promotes the dehydrogenation of residual MgH2. In the following dehydrogenation cycle, Mg2NiH4 can rapidly decompose into Mg2Ni, which continuously promotes the decomposition of MgH2. Our study not only elucidates the mechanism of Ni catalyst but also helps design and assemble catalysts with improved dehydriding kinetics of MgH2.

Direct observation of oxygen vacancy formation and migration over ceria surface by in situ environmental transmission electron microscopy

The extremely high structural tolerance of ceria to oxygen vacancies (Ov) has made it a desirable catalytic material for the hydrocarbon oxidation to chemicals and pharmaceuticals and the reduction of gaseous pollutants. It is proposed that the formation and diffusion of Ov originate from its outstanding reduction property. However, the formation and diffusion process of Ov over the surface of ceria at the atomic level is still unknown. Herein, the structural and valence evolution of CeO2 (111) surfaces in reductive, oxidative and vacuum environments from room temperature up to 700 °C was studied with in situ aberration-corrected environmental transmission electron microscopy (ETEM) experiments. Ov is found to form under a high vacuum at elevated temperatures; however, the surface can recover to the initial state through the adsorption of oxygen atoms in an oxygen-contained environment. Furthermore, in hydrogen environment, the step-CeO2 (111) surface is not stable at elevated temperatures; thus, the steps tend to be eliminated with increasing temperature. Combined with first-principles density function calculations (DFT), it is proposed that O-terminated surfaces would develop in a hypoxic environment due to the dynamic diffusion of Ov from the outer surface to the subsurface. Furthermore, in a reductive environment, H2 facilitates the formation and diffusion of Ov while Ce-terminated surfaces develope. These results reveal dynamic atomic-scale interplay between the nanoceria surface and gas, thereby providing fundamental insights into the Ov-dependent reaction of nano-CeO2 during catalytic processes.

MOF-Derived MnO/C Nanocomposites for High-Performance Supercapacitors.

As ordered porous materials, metal-organic frameworks (MOFs) have attracted tremendous attention in the field of energy conversion and storage due to their high specific surface area, permanent porosity, and tunable pore sizes. Here, MOF-derived MnO/C nanocomposites with regular octahedral shape were synthesized using a Mn-based analogue of the MIL-100 framework (Mn-MIL-100, MIL: Matérial Institut Lavoisier) as the precursor. Using aberration-corrected environmental transmission electron microscopy (ETEM), MnO nanocages with a diameter of approximately 20 nm were recognized in the MnO/C nanocomposites fabricated, dispersed in a microporous carbon matrix homogeneously. The nanocages are composed of MnO nanoparticles with a diameter of approximately 2 nm and with a single crystal structure. The specific surface area of the as-prepared MnO/C octahedra decreases to 256 m2 g-1 from 507 m2 g-1 of the Mn-MIL-100 precursor, whereas the total pore volume increases to 0.245 cm3 g-1, which is approximately 29% higher than that of the precursor (0.190 cm3 g-1). Additionally, when utilized as an electrode for supercapacitors, the MOF-derived MnO/C nanocomposite demonstrates a towering specific capacitance of 421 F g-1 at 0.5 A g-1 and good cycle stability (94%) after 5000 cycles. Our work reveals that the MnO nanoparticles in MOF-derived MnO/C nanocomposites exhibit nanocage structure characteristics, which might be inherited from the Mn-MIL-100 precursor with analogous supertetrahedron units.

Atomic-Scale Evidence of Catalyst Evolution for the Structure-Controlled Growth of Single-Walled Carbon Nanotubes.

ConspectusKnowing how nanomaterials nucleate and dynamically evolve at the nanoscale is crucial to understanding and in turn controlling the structure and properties of a wide variety of materials, among which single-walled carbon nanotubes (SWCNTs) with chirality-dependent properties is a typical example. Catalyst takes a central role in guiding the SWCNT growth. An in-depth understanding of the growth mechanism of SWCNTs requires knowledge of the catalyst dynamic behavior during the chemical vapor deposition process, where real-time atomic-scale observations are needed. The high spatial, temporal, and energy resolution makes the state-of-the-art aberration-corrected environmental transmission electron microscope (ETEM) a superior tool for tracking the catalyst evolution and the SWCNT growth.Several key factors and processes, including the catalyst stability, carbon diffusion pathway, nucleation site, and growth modes of nanotubes, greatly influence the structure of SWCNTs. This Account summarizes our recent progress in the ETEM investigation of the dynamic catalyst behavior and nucleation of SWCNTs. We first compare the different growth modes of SWCNTs on two types of catalyst-stable solid intermetallic Co7W6 and unstable monometallic catalysts. Then we address the origin of different growth modes and chirality selectivity by revealing the atomic-scale stability and evolution of catalysts under carbon feed conditions and the observation of the in situ growth of SWCNTs on catalysts. We also discuss the catalyst-support interaction and the possible influence on SWCNT growth. In the end, we summarize the present achievements and future challenges.We carefully compare the difference in the ordinary Co catalyst and Co7W6 catalyst which has shown great chirality selectivity in SWCNT growth. Direct imaging by ETEM demonstrated that solid catalysts initiated the growth of SWCNTs with diameters smaller (dNT) than those of the catalyst particles (dNP) (dNT < dNP), whereas molten catalyst nanoparticles produced SWCNTs with similar diameters (dNT ≈ dNP). ETEM combined with in situ synchrotron X-ray absorption spectroscopy demonstrated that the Co7W6 catalyst maintained a solid stable structure under carbon feed conditions at 700-1000 °C, demonstrating the feasibility in acting as a structure template to grow SWCNTs. By contrast, the state and composition of the Co catalyst were changing during SWCNT growth. The near-surface lattice spacings of Co7W6 remained unchanged under carbon feed condition with carbon diffusion on the surface, whereas the solid Co catalyst underwent dynamic expansion and contraction due to carbon penetration into and precipitation out of Co nanoparticles. These two different pathways of carbon diffusion on or in catalysts indicate the distinctly different growth mechanisms of SWCNTs: the epitaxial growth of SWCNTs with specified chirality on the facets of Co7W6 nanocrystals and the nonselective growth of SWCNTs by the Co catalyst with Co/CoC3 as the active species. Besides the SWCNT-catalyst interface, the catalyst-support interface is also of importance in SWCNT growth. The atomic-scale information on catalyst dynamics provides a deep mechanistic understanding of SWCNT growth and will boost the development of the structure-controlled synthesis of SWCNTs and other nanomaterials.

Growth modes of single-walled carbon nanotubes on catalysts.

Understanding the growth mechanism of single-walled carbon nanotubes (SWCNTs) and achieving selective growth requires insights into the catalyst structure-function relationship. Using an in situ aberration-corrected environmental transmission electron microscope, we reveal the effects of the state and structure of catalysts on the growth modes of SWCNTs. SWCNTs grown from molten catalysts via a vapor-liquid-solid process generally present similar diameters to those of the catalysts, indicating a size correlation between nanotubes and catalysts. However, SWCNTs grown from solid catalysts via a vapor-solid-solid process always have smaller diameters than the catalysts, namely, an independent relationship between their sizes. The diameter distribution of SWCNTs grown from crystalline Co7W6, which has a unique atomic arrangement, is discrete. In contrast, nanotubes obtained from crystalline Co are randomly dispersed. The different growth modes are linked to the distinct chiral selectivity of SWCNTs grown on intermetallic and monometallic catalysts. These findings will enable rational design of catalysts for chirality-controlled SWCNTs growth.

In Situ Structural Dynamics of Atomic Defects in Tungsten Oxide.

Atomic defects are critical to tuning the physical and chemical properties of functional materials such as catalysts, semiconductors, and 2D materials. However, direct structural characterization of atomic defects, especially their formation and annihilation under practical conditions, is challenging yet crucial to understanding the underlying mechanisms driving defect dynamics, which remain mostly elusive. Here, through in situ atomic imaging by an aberration-corrected environmental transmission electron microscope (AC-ETEM), we directly visualize the formation and annihilation mechanism of planar defects in monoclinic WO3 on the atomic scale in real time. We captured the atomistic process of the nucleation dynamics of the dislocation core in the [010] direction, followed by its propagation to form a planar defect. Corroborated by density functional theory-based calculations, we rationalize the formation of dislocation through O extraction from bridge sites followed by an atomic channeling process. These in situ observations shed light on the defect dynamics in oxides and provide atomic insights into forming and manipulating defects in functional materials.

Revealing synergetic structural activation of a CuAu surface during water–gas shift reaction

Bimetallic alloy catalysts show strong structural and compositional dependence on their activity, selectivity, and stability. Often referred to as the “synergetic effect” of two metal elements in the alloys, their detailed dynamic information, structurally and chemically, of catalyst surface under reaction conditions remains largely elusive. Here, using aberration-corrected environmental transmission electron microscopy, we visualize the atomic-scale synergetic surface activation of CuAu under a water–gas shift reaction condition. The unique “periodic” structural activation largely determines the dominating reaction pathway, which is related to a possible “carboxyl” reaction route corroborated by density functional theory–based calculation and ab initio molecular dynamics simulation. These results demonstrate how the alloy surface is activated and catalyzes the chemical reaction, which provides insights into catalyst design with atom precision.

Atom-Resolved Investigation on Dynamic Nucleation and Growth of Platinum Nanocrystals.

Understanding the mechanism of nucleation and growth of nanocrystals is crucial for designing and regulating the structure and properties of nanocrystals. However, the process from molecules to nanocrystals remains unclear because of the rapid and complicated dynamics of evolution under reaction conditions. Here, the complete evolution process of solid-phase chloroplatinic acid during the electron beam irradiation triggered reduction and nucleation of platinum nanocrystals is recorded. Aberration-corrected environmental transmission electron microscopy is used for direct visualization of the dynamic evolution from H2 PtCl6 to Pt nanocrystals at the atomic scale, including the formation and growth of amorphous clusters, crystallization, and growth of clusters, and the ripening of Pt nanocrystals. At the first two stages, there exists a critical size of ≈2.0nm, which represents the start of crystallization. Crystallization from the center and density fluctuation are observed in the second stage of the crystallization of a few clusters with a size obviously larger than the critical size. The work provides valuable information to understand the kinetics of the early stage of nanocrystal nucleation and crystallization at atomic scale.

Rotation during reaction

Catalysis 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][1] [1]: /lookup/doi/10.1126/science.abe3558

In situ manipulation of the active Au-TiO2 interface with atomic precision during CO oxidation.

The interface between metal catalyst and support plays a critical role in heterogeneous catalysis. An epitaxial interface is generally considered to be rigid, and tuning its intrinsic microstructure with atomic precision during catalytic reactions is challenging. Using aberration-corrected environmental transmission electron microscopy, we studied the interface between gold (Au) and a titanium dioxide (TiO2) support. Direct atomic-scale observations showed an unexpected dependence of the atomic structure of the Au-TiO2 interface with the epitaxial rotation of gold nanoparticles on a TiO2 surface during carbon monoxide (CO) oxidation. Taking advantage of the reversible and controllable rotation, we achieved in situ manipulation of the active Au-TiO2 interface by changing gas and temperature. This result suggests that real-time design of the catalytic interface in operating conditions may be possible.

Atomic origins of the strong metal-support interaction in silica supported catalysts.

Silica supported metal catalysts are most widely used in the modern chemical industry because of the high stability and tunable reactivity. The strong metal–support interaction (SMSI), which has been widely observed in metal oxide supported catalysts and significantly affects the catalytic behavior, has been speculated to rarely happen in silica supported catalysts since silica is hard to reduce. Here we revealed at the atomic scale the interfacial reaction induced SMSI in silica supported Co and Pt catalysts under reductive conditions at high temperature using aberration-corrected environmental transmission electron microscopy coupled with in situ electron energy loss spectroscopy. In a Co/SiO2 system, the amorphous SiO2 migrated onto the Co surface to form a crystallized quartz-SiO2 overlayer, and simultaneously an interlayer of Si was generated in-between. The metastable crystalline SiO2 overlayer subsequently underwent an order-to-disorder transition due to the continuous dissociation of SiO2 and the interfacial alloying of Si with the underlying Co. The SMSI in the Pt–SiO2 system was found to remarkably boost the catalytic hydrogenation. These findings demonstrate the universality of the SMSI in oxide supported catalysts, which is of general importance for designing catalysts and understanding catalytic mechanisms.

Gas adsorbate-induced Au atomic segregation and clustering from Cu(Au)

Surface compositional and phase segregation in an alloy can change its functionality, especially for applications where surface structure and chemistry play a vital role. For instance, the surface status of alloy catalysts significantly affects their catalytic performance for both heterogeneous and electrochemical processes. Surface segregation is believed to be driven by the difference in surface energy to reduce the total free energy of the alloy. However, the atomistic processes during the segregation process remain elusive, especially for gas molecule-induced segregation, where both structural and chemical reordering may occur. Herein, we achieved in-situ atomic-scale visualization of the surface segregation behaviors of a solid solution Cu(Au) alloy under the CO gas by an aberration-corrected environmental transmission electron microscope. CO-induced Cu(Au) surface ordering structures largely change the surface chemistry of the alloy. Further gas exposure at elevated temperature could facilitate Au atom diffusion through a specific “atomic channel” structure for dealloying and clustering on the surface. The segregated Au nanoparticles show rich phase and morphological dynamics interacting with the alloy surface, where the gas adsorption also plays an important role. These atomic insights provide direct evidence for the surface segregation and dealloying mechanisms of bimetallic alloys, and highlight the role of gas adsorbate in these surface processes.

Direct Visualization of Atomic-Scale Graphene Growth on Cu through Environmental Transmission Electron Microscopy.

The functionalities of two-dimensional (2D) materials are solely determined by their perfect single-layer lattice or precisely stacking of multiple lattice planes, which is predominately determined during their growth process. Although the growth of graphene has been successfully achieved on different substrates with a large area up to millimeters, direct visualization of atomic-scale graphene growth in real time still lacks, which is vital to decipher atomistic mechanisms of graphene growth. Here, we employ aberration-corrected environmental transmission electron microscopy (AC-ETEM) to visualize the nucleation and growth of graphene at the atomic scale in real time. We find a unique lateral epitaxial growth process of graphene on Cu edges under the CO2 atmosphere with a ledge-flow process. The nucleation of graphene nuclei from amorphous carbon atoms also has been found to proceed with a gradual ordering of in-plane carbon atoms. The coalescence of smaller graphene nanoislands to form large ones is thermodynamically favored, and the evolution of atomic structures at grain boundaries is also revealed in great details. These atomic insights obtained from real-time observations can provide direct evidence for the growth mechanisms of graphene, which can be extended to other 2D materials.

Atomic-Scale Dynamic Interaction of H_{2}O Molecules with Cu Surface.

Atomic-scale interaction of water vapor with metal surfaces beyond surface adsorption under technologically relevant conditions remains mostly unexplored. Using aberration-corrected environmental transmission electron microscopy, we reveal the dynamic surface activation of Cu by H_{2}O at elevated temperature and pressure. We find a structural transition from flat to corrugated surface for the Cu(011) under low water-vapor pressure. Increasing the water-vapor pressure leads to the surface reaction of Cu with dissociated H_{2}O, resulting in the formation of a metastable "bilayer" Cu─O─H phase. Corroborated by density functional theory and abinitio molecular dynamics calculations, the cooperative O and OH interaction with Cu is responsible for the formation and subsurface propagation of this phase.

Understanding the structural evolution of Au/WO2.7 compounds in hydrogen atmosphere by atomic scale in situ environmental TEM

Hydrogen energy is a resuscitated clean energy source and its sensitive detection in air is crucial due to its very low explosive limit. Metal oxide decorated with noble metal nanoparticles has been used for the enhancement of gas detection and exhibits superior sensitivity. Understanding the intrinsic mechanism of the detection and the enhancement mechanism is thus becoming a fundamental issue for the further development of novel metal/oxide compound gas-sensing materials. However, the correlation between the microstructural evolution, the charge transport and the complex sensing process has not yet been directly revealed and its atomic mechanism is still debatable. In this study, an Au/WO2.7 compound was synthesized and exhibited a strongly enhanced gas sensitivity to many reductive gases, especially H2. Aberration-corrected environmental transmission electron microscopy was used to investigate the atomic-scale microstructural evolution in situ during the reaction between H2 and Au/WO2.7 compound. Swing and sintering processes of the Au particles on the WO2.7 surface were observed under heating and gaseous environments, and no injection of hydrogen atoms was suggested. First principle calculations verified the swing and sintering processes, and they can be explained by the enhancement of H2 sensitivity.

Direct visualization of dynamic atomistic processes of Cu2O crystal growth through gas-solid reaction

Crystal growth involves “mysterious” long-range ordering of atoms or molecules through local-only interaction, which requires repeated addition of atoms or molecules to the growing surface from the liquid or vapour phase. Although the canonical theory has successfully explained crystal growth behaviors, detailed atomistic mechanisms, especially for the growth of crystals involving chemical reactions (covalent bond formation), remain mostly elusive. Herein, we reveal such atomistic mechanisms of Cu2O growth during the oxidation process by real-time atomic imaging in an aberration-corrected environmental transmission electron microscope (AC-ETEM). We directly visualize the atomistic process of sequential bonding between Cu and O atoms onto the growing oxide surface. Besides, we also reveal the unique role of surface defects during the crystal growth process and the effect of growing kinetics on the ending morphology of the oxide crystals. The obtained results provide the fundamental understanding of the crystal growth process and further enrich the metal oxidation theory.

Real-Time Atomic-Scale Visualization of Reversible Copper Surface Activation during the CO Oxidation Reaction.

By using in situ aberration-corrected environmental transmission electron microscopy, for the first time at atomic level, the dynamic evolution of the Cu surface is captured during CO oxidation. Under reaction conditions, the Cu surface is activated, typically involving 2-3 atomic layers with the formation of a reversible metastable phase that only exists during catalytic reactions. The distinctive role of CO and O2 in the surface activation is revealed, which features CO exposure to lead to surface roughening and consequently formation of low-coordinated Cu atoms, while O2 exposure induces a quasi-crystalline CuOx phase. Supported by DFT calculations, it is shown that crystalline CuOx reversibly transforms into the amorphous phase, acting as an active species to facilitate the interaction of gas reactants and catalyzing CO oxidation.

Real-Time Imaging of the Electrochemical Process in Na-O2 Nanobatteries Using Pt@CNT and Pt0.8Ir0.2@CNT Air Cathodes.

Compared to lithium-oxygen batteries, sodium-oxygen (Na-O2) batteries exhibit a number of advantages: extremely low cost, low charging overpotential, and stability under nitrogen. However, accumulation of insoluble discharge products and failure of catalysts often result in poor performance of Na-O2 batteries and limit their cycling life. In this work, electrochemical reactions of Na-O2 batteries were directly investigated in situ by assembling a solid-state Na-O2 nanobattery in an aberration-corrected environmental transmission electron microscope. During discharge, NaO2 hollow spheres formed and expanded continuously, accompanying their partial decomposition into Na2O2. These spheres shrank and collapsed into Na2O2 nanoparticles during the charging process. Carbon nanotubes doped with Pt and bimetallic Pt/Ir nanoscale catalyst can promote product formation and reversible evolution. In-depth investigation of the electrochemical reaction mechanism in Na-O2 cells helps to accelerate the development of metal-air devices.

(Invited) In Situ Study of Cobalt-Tungsten and Cobalt Nanocrystals Under Reactive Environment

Revealing the catalyst structure in the working environment is crucial for understanding the controlled growth mechanism of single-walled carbon nanotubes (SWNTs). We used the aberration-corrected environmental transmission electron microscope (ETEM) combined with electron energy loss spectroscopy to detect the Co7W6 catalyst structure and composition in the reaction environment. With the quantitative analysis of high-resolution image, we showed that the Co7W6 catalyst structure keep stable with carbon feeding at 700-1100 ○C. Carbon caps were clearly observed to nucleate from the Co7W6 nanocrystals from ETEM. We also established in situ synchrotron X-ray absorption spectroscopy operated in the atmospheric-pressure CVD and demonstrated the stable local coordination structure of Co7W6. No carbides or carbon diffusion induced structural change was observed from these in situ characterization. It reveals that Co7W6 catalysts act as the stable structure template for the chirality controlled growth of SWNTs. Cobalt catalyzed SWNT growth is generally believed to carry out via a vapor–liquid–solid mechanism. We investigated the synergetic role of Co3C in Co catalyzed SWNT growth by ETEM. The carbon atoms disperse from surface into bulk of Co nanoparticle. The metallic Co is effective for catalyzing the formation of a cap, while Co3C, depending on the carbon feeding rate, is essential for subsequently elongating of the carbon cap. The growth of SWNTs is realized only by the cooperation of Co with Co3C in proper carbon feeding condition. These findings are helpful for the rational design of catalysts with high activity for SWNT growth.

Real-time, in situ, atomic scale observation of soot oxidation

The oxidation of soot is a complex process due to the heterogeneous structure of the material. Several mechanisms have been hypothesized based on ex situ studies, but need confirmation from in situ observation; furthermore, deeper insight is needed to develop and validate structure-dependent reaction mechanisms. In this work, soot oxidation was for the first time observed at atomic scale in situ, in real-time, using a spherical aberration-corrected Environmental Transmission Electron Microscope. The transformation of individual soot particles was followed through from initiation to complete conversion. Observations clearly showed the existence of different burning modes and particle fragmentation previously hypothesized in the literature. Furthermore, transitioning between the modes—affected by temperature and O2 pressure—was unambiguously observed, explaining previous observations regarding structure-dependent and time-varying oxidation rates. A new mode of burning in which oxidation happens rapidly in the bulk phase with the disruption of long-range lamellar order was observed and is suspected to be dominant at practically relevant conditions. The ability to unambiguously relate different burning modes in terms of nanostructure will be of importance for optimizing both soot emission abatement and properties of nanoparticulate carbon products.