Clusters of transition metals, W, Re, and Os, upon encapsulation within a single-walled carbon nanotube (SWNT) exhibit marked differences in their affinity and reactivity with the SWNT, as revealed by low-voltage aberration-corrected high-resolution transmission electron microscopy (AC-HRTEM). Activated by an 80 keV electron beam, W reacts only weakly with the SWNT, Re creates localized defects on the sidewall, and Os reacts readily causing extensive defect formation and constriction of the SWNT sidewall followed by total rupture of the tubular structure. AC-HRTEM imaging at the atomic level of structural transformations caused by metal−carbon bonding of π-and σ-character demonstrates what a crucial role these types of bonds have in governing the interactions between the transition metal clusters and the SWNT. The observed order of reactivity W < Re < Os is independent of the metal cluster size, shape, or orientation, and is related to the metal to nanotube bonding energy and the amount of electronic density transferred between metal and SWNT, both of which increase along the triad W, Re, Os, as predicted by firstprinciples density functional theory calculations. By selecting the appropriate energy of the electron beam, the metal−nanotube interactions can be controlled (activated or precluded). At an electron energy as low as 20 keV, no visible transformations in the nanotube in the vicinity of Os-clusters are observed. ■ INTRODUCTIONTransition metals (d-elements) form the largest block of the Periodic Table and offer the widest variety of magnetic, optical, catalytic, and other functional properties. The rich chemistry of transition metals when combined with the mechanical, electric, thermal, and chemical properties of carbon nanostructures, such as single-walled carbon nanotubes (SWNTs), may lead to the generation of new families of functional materials which harness the synergy of the resultant metal−nanocarbon interactions. Recent investigations of metal−SWNT heterostructures have opened new highly promising avenues for applications in catalysis, 1 hydrogen storage, 2 and electronic devices. 3 Therefore, the quest for complete understanding of the nature of bonding between carbon nanotubes and transition metals is becoming increasingly important as illustrated by a recent flurry of theoretical studies on interactions between transition metals and SWNTs. 4 However, experimental measurements are significantly impeded because of the typical polydispersity of nanotubes (i.e., SWNTs of different lengths, diameters and helicities are present in the same sample), the lack of their intrinsic solubility, and by ubiquitous impurities in SWNT samples (e.g., amorphous carbon, graphitic particles, residual metal catalyst). While conventional spectroscopic methods that integrate over larger volumes (e.g., XPS, Raman, etc.) can be applied for characterizing the bulk physicochemical properties, high-resolution transmission electron microscopy (HRTEM) is now rapidly becoming an excellent local-probe tool for study...

The cutting of single-walled carbon nanotubes by an 80 keV electron beam catalyzed by nickel clusters is imaged in situ using aberration-corrected high-resolution transmission electron microscopy. Extensive molecular dynamics simulations within the CompuTEM approach provide insight into the mechanism of this process and demonstrate that the combination of irradiation and nickel catalyst is crucial for the cutting process to take place.The atomistic mechanism of cutting is revealed by detailed analysis of irradiation-induced reactions of bonds reorganization and atom ejection in the vicinity of the nickel cluster, showing a highly complex interplay of different chemical transformations catalysed by the metal cluster. One of the most prevalent pathways includes three consecutive stages: formation of polyyne carbon chains from carbon nanotube, dissociation of the carbon chains into single and pairs of adatoms adsorbed on the nickel cluster, and ejection of these adatoms leading to the cutting of nanotube. Significant variations in the atom ejection rate are discovered depending on the process stage and nanotube diameter. The revealed mechanism and kinetic characteristics of cutting process provide fundamental knowledge for the development of new methodologies for control and manipulation of carbon structures at the nanoscale.

Catalysis of chemical reactions by nanosized clusters of transition metals holds the key to the provision of sustainable energy and materials. However, the atomistic behaviour of nanocatalysts still remains largely unknown due to uncertainties associated with the highly labile metal nanoclusters changing their structure during the reaction. In this study, we reveal and explore reactions of nm-sized clusters of 14 technologically important metals in carbon nano test tubes using time-series imaging by atomically-resolved transmission electron microscopy (TEM), employing the electron beam simultaneously as an imaging tool and stimulus of the reactions. Defect formation in nanotubes and growth of new structures promoted by metal nanoclusters enable the ranking of the different metals both in order of their bonding with carbon and their catalytic activity, showing significant variation across the Periodic Table of Elements. Metal nanoclusters exhibit complex dynamics shedding light on atomistic workings of nanocatalysts, with key features mirroring heterogeneous catalysis.

Carbonyl complexes of transition metals (M x (CO) y , where x ¼ 1, 2, or 3 and y ¼ 6, 10, or 12 for M ¼ W, Re, or Os, respectively) inserted into single walled carbon nanotubes (SWNT, diameter 1.5 nm) transform into metallic nanoparticles (MNPs) under heat treatment or electron beam irradiation. The host-nanotube acts as an efficient template, controlling the growth of MNPs to $1 nm in diameter. The only co-product of nanoparticle formation, carbon monoxide (CO) gas, creates pockets of high pressure between nanoparticles, thus preventing their collision and coalescence into larger structures. As a result, the MNPs stay largely spheroidal in shape and are uniformly distributed throughout the entire length of the SWNT. Despite their extremely small size (on average each MNP contains 30-90 atoms) and no protection of their surface by a capping layer of molecules, the metallic nanoparticles encapsulated in nanotubes are very stable under ambient conditions and even at elevated temperatures. Aberration-corrected high-resolution transmission electron microscopy reveals the crystalline nature of the MNPs, probes their interactions with the nanotube interior and illustrates the complex dynamics of confined MNPs in real-time and direct-space.