Comparison of atomic scale dynamics for the middle and late transition metal nanocatalysts

Unearthing Atomic Dynamics in Nanocatalysts

Being able to access the rich atomic-scale phenomenology, which occurs during the reactions pathways, is a pressing need toward the pursued knowledge-based design of more efficient nanocatalysts, precisely tailored atom by atom for each reaction. However, to reach this goal of achieving maximum optimization, it is mandatory, first, to address how exposure to the experimental conditions, which will be needed to activate the processes, affects the internal configuration of the nanoparticles at the atomic level. In particular, the most critical experimental parameter is probably the temperature, which among other unwanted effects can induce nanocatalyst aggregation. This work highlights the high potential of experimental techniques such as the scanning probe microscopies, which are able to investigate matter in real space with atomic resolution, to reach the key challenge in heterogeneous catalysis of achieving access to the atomic-scale processes taking place in the nanocatalysts. Specifically, the phenomenology occurring in a nanoparticle system during annealing is studied with atomic precision by scanning tunneling microscopy. As a result, the existence of an internal atomic restructuring, occurring already at relatively low temperatures, within Ir nanoparticles grown over h-BN/Ru(0001) surfaces is demonstrated. Such restructuration, which reduces the undercoordination of the outer Ir atoms, is expected to have a significant effect on the reactivity of the nanoparticles. Going a step further, an internal restructuring of the nanoparticles during their involvement as catalysts has also been also identified.

Directional growth and reconstruction of ultrafine uranium oxide nanorods within single-walled carbon nanotubes

Understanding the atomic structures and dynamic evolution of uranium oxides is crucial for the reliable operation of fission reactors. Among them, U4O9-as an important intermediate in the oxidation of UO2 to UO2+x -plays an important role in the nucleation and conversion of uranium oxides. Herein, we realize the confined assembly of uranyl within SWCNTs in liquid phase and reveal the directional growth and reconstruction of U4O9 nanorods in nanochannels, enabled by in situ scanning transmission electron microscopy (STEM) e-beam stimulation. The nucleation and crystallization of U4O9 nanorods in nanochannels obey the "non-classical nucleation" mechanism and exhibit remarkably higher growth rate compared to those grown outside. The rapid growth process is found to be accompanied by the formation and elimination of U atom vacancies and strain, aiming to achieve the minimum interfacial energy. Eventually, the segments of U4O9 nanorods in SWCNTs merge into single-crystal U4O9 nanorods via structural reconstruction at the interfaces, and 79% of them exhibit anisotropic growth along the specific 〈11̄0〉 direction. These findings pave the way for tailoring the atomic structures and interfaces of uranium oxides during the synthesis process to help improve the mechanical properties and stability of fission reactors.

Electron beam-induced demetallation of Fe, Co, Ni, Cu, Zn, Pd, and Pt metalloporphyrins: insights in e-beam chemistry and metal cluster formations

Electron beams are versatile tools for nanoscale fabrication processes, however, the underlying e-beam chemistry remains in its infancy. Through operando transmission electron microscopy investigations, we elucidate a redox-driven cargo release of individual metal atoms triggered by electron beams. The chosen organic delivery molecule, tetraphenylporphyrin (TPP), proves highly versatile, forming complexes with nearly all metals from the periodic table and being easily processed in solution. A comprehensive cinematographic analysis of the dynamics of single metal atoms confirms the nearly instantaneous ejection of complexed metal atoms under an 80 kV electron beam, underscoring the system's broad versatility. Providing mechanistic insights, we employ density functional theory to support the proposed reductive demetallation pathway facilitated by secondary electrons, contributing novel perspectives to electron beam-mediated chemical reaction mechanisms. Lastly, our findings demonstrate that all seven metals investigated form nanoclusters once ejected from TPP, highlighting the method's potential for studying and developing sustainable single-atom and nanocluster catalysts.

Sorting of Cluster-Confined Metallic Single-Walled Carbon Nanotubes for Fabricating Atomically Vacant Uranium Oxide

Single-walled carbon nanotube (SWCNT) heterostructures have shown great potential in catalysis, magnetism, and nanofluidics, in which host SWCNTs with certain conductivity (metallic or semiconducting) are highly required. Herein, inspired by the large molecular weight and redox properties of polyoxometalate (POM) clusters, we reported the selective separation of POM encapsulated metallic SWCNTs (POM@m-SWCNTs) with a uniform diameter through density gradient ultracentrifugation (DGU). The confined POMs increased the SWCNT density and amplified the nanotubes' density difference, thus greatly lowering the centrifugal force (70,000g) of DGU. With this strategy, a series of POM@m-SWCNTs of ∼1.2 nm with high purity were sorted. The mechanism supported by theoretical and experimental evidence showed that the separation of m-SWCNTs depended on not only nanotube/cluster size but also the conductivity of SWCNTs. The smallest 1.2 nm m-SWCNT that can exactly accommodate a 0.9 nm-{Mo6} cluster exhibited the maximum electron transfer to inner clusters; thus, intertube π-π stacking of such m-SWCNTs was greatly loosened, leading to the preferential dispersion into individual ones and partitioning in the uppermost layer after DGU. As a proof-of-concept application, this sorting strategy was extended to separate heavy-element 238U-encapsulated m-SWCNTs. The phase-pure, tiny (1-2.5 nm) U4O9 crystals with atomic vacancy clusters were fabricated on m-SWCNTs through growth kinetic control. This work may provide a general way to construct desired actinide materials on specific SWCNTs.

Time-resolved imaging and analysis of the electron beam-induced formation of an open-cage metallo-azafullerene

The visualization of single-molecule reactions provides crucial insights into chemical processes, and the ability to do so has grown with the advances in high-resolution transmission electron microscopy. There is currently a limited mechanistic understanding of chemical reactions under the electron beam. However, such reactions may enable synthetic methodologies that cannot be accessed by traditional organic chemistry methods. Here we demonstrate the synthetic use of the electron beam, by in-depth single-molecule, atomic-resolution, time-resolved transmission electron microscopy studies, in inducing the formation of a doubly holed fullerene-porphyrin cage structure from a well-defined benzoporphyrin precursor deposited on graphene. Through real-time imaging, we analyse the hybrid's ability to host up to two Pb atoms, and subsequently probe the dynamics of the Pb-Pb binding motif in this exotic metallo-organic cage structure. Through simulation, we conclude that the secondary electrons, which accumulate in the periphery of the irradiated area, can also initiate chemical reactions. Consequently, designing advanced carbon nanostructures by electron-beam lithography will depend on the understanding and limitations of molecular radiation chemistry.

Anisotropic Growth of One-Dimensional Carbides in Single-Walled Carbon Nanotubes with Strong Interaction for Catalysis

Tungsten and molybdenum carbides have shown great potential in catalysis and superconductivity. However, the synthesis of ultrathin W/Mo carbides with a controlled dimension and unique structure is still difficult. Here, inspired by the host-guest assembly strategy with single-walled carbon nanotubes (SWCNTs) as a transparent template, we reported the synthesis of ultrathin (0.8-2.0 nm) W₂C and Mo₂C nanowires confined in SWCNTs deriving from the encapsulated W/Mo polyoxometalate clusters. The atom-resolved electron microscope combined with spectroscopy and theoretical calculations revealed that the strong interaction between the highly carbophilic W/Mo and SWCNT resulted in the anisotropic growth of carbide nanowires along a specific crystal direction, accompanied by lattice strain and electron donation to the SWCNTs. The SWCNT template endowed carbides with resistance to H₂O corrosion. Different from normal modification on the outer surface of SWCNTs, such M₂C@SWCNTs (M = W, Mo) provided a delocalized and electron-enriched SWCNT surface to uniformly construct the negatively charged Pd catalyst, which was demonstrated to inhibit the formation of active PdH_x hydride and thus achieve highly selective semihydrogenation of a series of alkynes. This work could provide a nondestructive way to design the electron-delocalized SWCNT surface and expand the methodology in synthesizing unusual 1D ultrathin carbophilic-metal nanowires (e.g., TaC, NbC, β-W) with precise control of the anisotropy in SWCNT arrays.

In Situ TEM Observation of the Atomic Transport Process during the Coalescence of Au Nanoparticles

In practical applications, the coalescence of metal nanoparticles (NPs) is a major factor affecting their physical chemistry properties. Currently, due to a lack of understanding of the atomic-level mechanisms during the nucleation and growth stages of coalescence, the correlation between the different dynamic factors in the different stages of NP coalescence is unclear. In this study, we used advanced in situ characterization techniques to observe the formation of atomic material transport channels (Au chains) during the initiation of coalescence nucleation. We focused on the movement and migration states of Au atoms and discovered an atomic ordered arrangement growth mechanism that occurs after the completion of nucleation. Simultaneously, we used density functional theory to reveal the formation principle of Au chains. These findings improve our understanding of the atomic-scale coalescence process, which can provide a new perspective for further research on coalescence atomic dynamics.

Atomic-Scale Structure Dynamics of Nanocrystals Revealed By In Situ and Environmental Transmission Electron Microscopy

Nanocrystals are of great importance in material sciences and industry. Engineering nanocrystals with desired structures and properties is no doubt one of the most important challenges in the field, which requires deep insight into atomic-scale dynamics of nanocrystals during the process. The rapid developments of in situ transmission electron microscopy (TEM), especially environmental TEM, reveal insights into nanocrystals to digest. According to the considerable progress based on in situ electron microscopy, a comprehensive review on nanocrystal dynamics from three aspects: nucleation and growth, structure evolution, and dynamics in reaction conditions are given. In the nucleation and growth part, existing nucleation theories and growth pathways are organized based on liquid and gas-solid phases. In the structure evolution part, the focus is on in-depth mechanistic understanding of the evolution, including defects, phase, and disorder/order transitions. In the part of dynamics in reaction conditions, solid-solid and gas-solid interfaces of nanocrystals in atmosphere are discussed and the structure-property relationship is correlated. Even though impressive progress is made, additional efforts are required to develop the integrated and operando TEM methodologies for unveiling nanocrystal dynamics with high spatial, energy, and temporal resolutions.

Single-Walled Carbon Nanotubes Encapsulated within Metallacycles

Mechanically interlocked derivatives of carbon nanotubes (MINTs) are interesting nanotube products since they show high stability without altering the carbon nanotube structure. So far, MINTs have been synthesized using ring-closing metathesis, disulfide exchange reaction, H-bonding or direct threading with macrocycles. Here, we describe the encapsulation of single-walled carbon nanotubes within a palladium-based metallosquare. The formation of MINTs was confirmed by a variety of techniques, including high-resolution transmission electron microscopy. We find the making of these MINTs is remarkably sensitive to structural variations of the metallo-assemblies. When a metallosquare with a cavity of appropriate shape and size is used, the formation of the MINT proceeds successfully by both templated clipping and direct threading. Our studies also show indications on how supramolecular coordination complexes can help expand the potential applications of MINTs.

Improved Carrier Lifetime in BiVO4 by Spin Protection

Mechanistic understanding of the effect bulk defects have on carrier dynamics at the quantum level is crucial to suppress associated midgap mediated charge recombination in semiconductors yet many questions remain unexplored. Here, by employing ab initio quantum dynamics simulation and taking BiVO₄ with oxygen vacancies (Ov) as a model system we demonstrate a spin protection mechanism for suppressed charge recombination. The carrier lifetime is significantly improved in the high spin defect system. The lifetime can be optimized by tuning the Ov concentration to minimize the nonradiative relaxation. Our work addresses literature ambiguities and contradictions about the role of bulk Ov in charge recombination and provides a route for defect engineering of semiconductors with enhanced carrier dynamics.

Direct Visualization of the Evolution of a Single-Atomic Cobalt Catalyst from Melting Nanoparticles with Carbon Dissolution

Transition metal single-atom catalysts (SACs) are of immense interest, but how exactly they are evolved upon pyrolysis of the corresponding precursors remains unclear as transition metal ions in the complex precursor undergo a series of morphological changes accompanied with changes in oxidation state as a result of the interactions with the carbon support. Herein, the authors record the complete evolution process of Co SAC during the pyrolysis a Co/Zn-containing zeolitic imidazolate framework. Aberration-corrected environmental TEM coupled with in-situ EELS is used for direct visualization of the evolution process at 200-1000 °C. Dissolution of carbon into the nanoparticles of Co is found to be key to modulating the wetting behavior of nanoparticles on the carbon support; melting of Co nanoparticles and their motion within the zeolitic architecture leads to the etching of the framework structure, yielding porous C/N support onto which Co-single atoms reside. This uniquely structured Co SAC is found to be effective for the oxidation of a series of aromatic alkanes to produce selective ketones among other possible products. The carbon dissolution and melting/sublimation-driven structural dynamics of transition metal revealed here will expand the methodology in synthesizing SACs and other high-temperature processes.

Dynamic hetero-metallic bondings visualized by sequential atom imaging

Traditionally, chemistry has been developed to obtain thermodynamically stable and isolable compounds such as molecules and solids by chemical reactions. However, recent developments in computational chemistry have placed increased importance on studying the dynamic assembly and disassembly of atoms and molecules formed in situ. This study directly visualizes the formation and dissociation dynamics of labile dimers and trimers at atomic resolution with elemental identification. The video recordings of many homo- and hetero-metallic dimers are carried out by combining scanning transmission electron microscopy (STEM) with elemental identification based on the Z-contrast principle. Even short-lived molecules with low probability of existence such as AuAg, AgCu, and AuAgCu are directly visualized as a result of identifying moving atoms at low electron doses.

The dynamic assembly and disassembly of atoms and molecules is challenging to characterize in real time, with atomic resolution and elemental identification. Here, the authors report direct observation of more than twenty homo and hetero-metallic compounds, including labile Ag-Cu dimers and Au-Ag-Cu trimers.

Catalyzing Bond-Dissociation in Graphene via Alkali-Iodide Molecules

Atomic design of a 2D-material such as graphene can be substantially influenced by etching, deliberately induced in a transmission electron microscope. It is achieved primarily by overcoming the threshold energy for defect formation by controlling the kinetic energy and current density of the fast electrons. Recent studies have demonstrated that the presence of certain species of atoms can catalyze atomic bond dissociation processes under the electron beam by reducing their threshold energy. Most of the reported catalytic atom species are single atoms, which have strong interaction with single-layer graphene (SLG). Yet, no such behavior has been reported for molecular species. This work shows by experimentally comparing the interaction of alkali and halide species separately and conjointly with SLG, that in the presence of electron irradiation, etching of SLG is drastically enhanced by the simultaneous presence of alkali and iodine atoms. Density functional theory and first principles molecular dynamics calculations reveal that due to charge-transfer phenomena the CC bonds weaken close to the alkali-iodide species, which increases the carbon displacement cross-section. This study ascribes pronounced etching activity observed in SLG to the catalytic behavior of the alkali-iodide species in the presence of electron irradiation.

Engineering of Microcage Carbon Nanotube Architectures with Decoupled Multimodal Porosity and Amplified Catalytic Performance

New approaches for the engineering of the 3D microstructure, pore modality, and chemical functionality of hierarchically porous nanocarbon assemblies are key to develop the next generation of functional aerogel and membrane materials. Here, interfacially driven assembly of carbon nanotubes (CNT) is exploited to fabricate structurally directed aerogels with highly controlled internal architectures, composed of pseudo-monolayer, CNT microcages. CNT Pickering emulsions enable engineering at fundamentally different length scales, whereby the microporosity, mesoporosity, and macroporosity are decoupled and individually controlled through CNT type, CNT number density, and process energy, respectively. In addition, metal nanocatalysts (Cu, Pd, and Ru) are embedded within the architectures through an elegant sublimation and shock-decomposition approach; introducing the first approach that enables through-volume functionalization of intricate, pre-designed aerogels without microstructural degradation. Catalytic structure-function relationships are explored in a pharma-important amidation reaction; providing insights on how the engineered frameworks enhance catalyst activity. A sophisticated array of advanced tomographic, spectroscopic, and microscopic techniques reveal an intricate 3D assembly of CNT building-blocks and their influence on the functional properties of the enhanced nanocatalysts. These advances set a basis to modulate structure and chemistry of functional aerogel materials independently in a controlled fashion for a variety of applications, including energy conversion and storage, smart electronics, and (electro)catalysis.

Single-molecule imaging and kinetic analysis of intermolecular polyoxometalate reactions

We induce and study reactions of polyoxometalate (POM) molecules, [PW12O40]3- (Keggin) and [P2W18O62]6- (Wells-Dawson), at the single-molecule level. Several identical carbon nanotubes aligned side by side within a bundle provided a platform for spatiotemporally resolved imaging of ca. 100 molecules encapsulated within the nanotubes by transmission electron microscopy (TEM). Due to the entrapment of POM molecules their proximity to one another is effectively controlled, limiting molecular motion in two dimensions but leaving the third dimension available for intermolecular reactions between pairs of neighbouring molecules. By coupling the information gained from high resolution structural and kinetics experiments via the variation of key imaging parameters in the TEM, we shed light on the reaction mechanism. The dissociation of W-O bonds, a key initial step of POM reactions, is revealed to be reversible by the kinetic analysis, followed by an irreversible bonding of POM molecules to their nearest neighbours, leading to a continuous tungsten oxide nanowire, which subsequently transforms into amorphous tungsten-rich clusters due to progressive loss of oxygen atoms. The overall intermolecular reaction can therefore be described as a step-wise reductive polycondensation of POM molecules, via an intermediate state of an oxide nanowire. Kinetic analysis enabled by controlled variation of the electron flux in TEM revealed the reaction to be highly flux-dependent, which leads to reaction rates too fast to follow under the standard TEM imaging conditions. Although this presents a challenge for traditional structural characterisation of POM molecules, we harness this effect by controlling the conditions around the molecules and tuning the imaging parameters in TEM, which combined with theoretical modelling and image simulation, can shed light on the atomistic mechanisms of the reactions of POMs. This approach, based on the direct space and real time chemical reaction analysis by TEM, adds a new method to the arsenal of single-molecule kinetics techniques.

Direct Imaging of Atomic Permeation Through a Vacancy Defect in the Carbon Lattice

Porous graphene has shown promise as a new generation of selective membrane for sieving atoms, ions and molecules. However, the atomistic mechanisms of permeation through defects in the graphenic lattice are still unclear and remain unobserved in action, at the atomic level. Here, the direct observation of palladium atoms from a nanoparticle passing through a defect in a single‐walled carbon nanotube one‐by‐one has been achieved with atomic resolution in real time, revealing key stages of the atomic permeation. Bonding between the moving atom and dangling bonds around the orifice, immediately before and after passing through the subnano‐pore, plays an important role in the process. Curvature of the graphenic lattice crucially defines the direction of permeation from concave to convex side due to a difference in metal‐carbon bonding at the curved surfaces as confirmed by density functional theory calculations, demonstrating the potential of porous carbon nanotubes for atom sieving.

Noise2Atom: unsupervised denoising for scanning transmission electron microscopy images

We propose an effective deep learning model to denoise scanning transmission electron microscopy (STEM) image series, named Noise2Atom, to map images from a source domain [Formula: see text] to a target domain [Formula: see text], where [Formula: see text] is for our noisy experimental dataset, and [Formula: see text] is for the desired clear atomic images. Noise2Atom uses two external networks to apply additional constraints from the domain knowledge. This model requires no signal prior, no noise model estimation, and no paired training images. The only assumption is that the inputs are acquired with identical experimental configurations. To evaluate the restoration performance of our model, as it is impossible to obtain ground truth for our experimental dataset, we propose consecutive structural similarity (CSS) for image quality assessment, based on the fact that the structures remain much the same as the previous frame(s) within small scan intervals. We demonstrate the superiority of our model by providing evaluation in terms of CSS and visual quality on different experimental datasets.

High-precision solid catalysts for investigation of carbon nanotube synthesis and structure

The direct growth of single-walled carbon nanotubes (SWCNTs) with narrow chiral distribution remains elusive despite substantial benefits in properties and applications. Nanoparticle catalysts are vital for SWCNT and more generally nanomaterial synthesis, but understanding their effect is limited. Solid catalysts show promise in achieving chirality-controlled growth, but poor size control and synthesis efficiency hampers advancement. Here, we demonstrate the first synthesis of refractory metal nanoparticles (W, Mo, and Re) with near-monodisperse sizes. High concentrations (N = 10⁵ to 10⁷ cm^-3) of nanoparticles (diameter 1 to 5 nm) are produced and reduced in a single process, enabling SWCNT synthesis with controlled chiral angles of 19° ± 5°, demonstrating abundance >93%. These results confirm the interface thermodynamics and kinetic growth theory mechanism, which has been extended here to include temporal dependence of fast-growing chiralities. The solid catalysts are further shown effective via floating catalyst growth, offering efficient production possibilities.

Dynamic co-catalysis of Au single atoms and nanoporous Au for methane pyrolysis

Nanocatalysts and single-atom catalysts are both vital for heterogeneous catalysis. They are recognized as two different categories of catalysts. Nevertheless, recent theoretical works have indicated that Au nanoparticles/clusters release Au single atoms in CO oxidation, and they co-catalyze the oxidation. However, to date, neither experimental evidence for the co-catalysis nor direct observations on any heterogeneous catalysis process of single-atom catalysts are reported. Here, the dynamic process of nanoporous Au to catalyze methane pyrolysis is monitored by in situ transmission electron microscopy with high spatial–temporal resolutions. It demonstrates that nanoporous Au surfaces partially disintegrate, releasing Au single atoms. As demonstrated by DFT calculation, the single atoms could co-catalyze the reaction with nanoporous Au. Moreover, the single atoms dynamically aggregate into nanoparticles, which re-disintegrate back to single atoms. This work manifests that under certain conditions, the heterogeneous catalysis processes of nanocatalysts and single-atom catalysts are not independent, where their dynamic co-catalysis exists.

Nanocatalysts and single‐atom catalysts are generally considered as two categories with distinct performances. Here, in situ TEM study of catalytic methane pyrolysis over nanoporous Au reveals a highly dynamic process where co‐catalysis exists among various catalyst forms.

Imaging an unsupported metal-metal bond in dirhenium molecules at the atomic scale

Metallic bonds remain one of the most important and least understood of the chemical bonds. In this study, we generated Re₂ molecules in which the Re-Re core is unsupported by ligands. Real-time imaging of the atomic-scale dynamics of Re₂ adsorbed on a graphitic lattice allows direct measurement of Re-Re bond lengths for individual molecules that changes in discrete steps correlating with bond order from one to four. Direct imaging of the Re-Re bond breaking process reveals a new bonding state with the bond order less than one and a high-amplitude vibrational stretch, preceding the bond dissociation. The methodology, based on aberration-corrected transmission electron microscopy imaging, is shown to be a powerful analytical tool for the investigation of dynamics of metallic bonding at the atomic level.

Effect of Temperature on the Nucleation and Growth of Precious Metal Nanocrystals

Understanding the effect of physical parameters (e.g., temperature) on crystallisation dynamics is of paramount importance for the synthesis of nanocrystals of well-defined sizes and geometries. However, imaging nucleation and growth is an experimental challenge owing to the resolution required and the kinetics involved. Here, by using an aberration-corrected transmission electron microscope, we report the fabrication of precious metal nanocrystals from nuclei and the identification of the dynamics of their nucleation at three different temperatures (20, 50, and 100 °C). A fast, and apparently linear, acceleration of the growth rate is observed against increasing temperature (78.8, 117.7, and 176.5 pm min^-1 , respectively). This work appears to be the first direct observation of the effect of temperature on the nucleation and growth of metal nanocrystals.

Direct Correlation of Carbon Nanotube Nucleation and Growth with the Atomic Structure of Rhenium Nanocatalysts Stimulated and Imaged by the Electron Beam

Subnanometer Re clusters confined in a single-walled carbon nanotube are activated by the 80 keV electron beam to promote the catalytic growth of a new carbon nanotube. Transmission electron microscopy images the entire process step-by-step, with atomic resolution in real time, revealing details of the initial nucleation followed by a two-stage growth. The atomic dynamics of the Re cluster correlate strongly with the nanotube formation process, with the growth accelerating when the catalyst becomes more ordered. In addition to the nanotube growth catalyzed by Re nanoclusters, individual atoms of Re released from the nanocluster play a role in the nanotube formation.