We use an environmental transmission electron microscope to record atomic-scale movies showing how carbon atoms assemble together on a catalyst nanoparticle to form a graphene sheet that progressively lifts-off to convert into a nanotube. Time-resolved observations combined with theoretical calculations confirm that some nanoparticle facets act like a vice-grip for graphene, offering anchoring sites, while other facets allow the graphene to lift-off, which is the essential step to convert into a nanotube.

Rational catalyst design requires an atomic scale mechanistic understanding of the chemical pathways involved in the catalytic process. A heterogeneous catalyst typically works by adsorbing reactants onto its surface, where the energies for specific bonds to dissociate and/or combine with other species (to form desired intermediate or final products) are lower. Here, using the catalytic growth of single-walled carbon nanotubes (SWCNTs) as a prototype reaction, we show that the chemical pathway may in-fact involve the entire catalyst particle, and can proceed via the fluctuations in the formation and decomposition of metastable phases in the particle interior. We record in situ and at atomic resolution, the dynamic phase transformations occurring in a Cobalt catalyst nanoparticle during SWCNT growth, using a state-of-the-art environmental transmission electron microscope (ETEM). The fluctuations in catalyst carbon content are quantified by the automated, atomic-scale structural analysis of the time-resolved ETEM images and correlated with the SWCNT growth rate. We find the fluctuations in the carbon concentration in the catalyst nanoparticle and the fluctuations in nanotube growth rates to be of complementary character. These findings are successfully explained by reactive molecular dynamics (RMD) simulations that track the spatial and temporal evolution of the distribution of carbon atoms within and on the surface of the catalyst particle. We anticipate that our approach combining real-time, atomic-resolution image analysis and molecular dynamics simulations will facilitate catalyst design, improving reaction efficiencies and selectivity towards the growth of desired structure.

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The self-diffusion coefficients of carbon dioxide, nitrogen, and water in metal organic frameworks (IRMOF-1, Cu-BTC, and MIL-47) are calculated using molecular dynamics simulations at various temperatures for pure gases and in mixtures. The structure of the adsorbates with respect to the metal active sites in the metal organic frameworks and their intermolecular structural features are investigated through radial distribution functions and related to the self-diffusivity behavior. It is found that while in IRMOF-1 the three species maintain their mobility when they are adsorbed as pure components or in mixtures, the diffusivities of CO 2 and water in Cu-BTC are slower in the ternary mixture than when adsorbed individually, whereas the opposite behavior is observed in MIL-47 where the species diffuse faster in the mixture than as pure components. The behavior can be explained in terms of the strong interactions of water with the framework which slows down diffusion in Cu-BTC; however in MIL-47, the competition between CO 2 and water for the active vanadium sites increases the mobilities of both adsorbates.

Surface modification of Si anodes in Li-ion batteries by deposition of a thin alucone coating has demonstrated an effective way to help maintain a stable anode/electrolyte interface and good battery performance. In this work, we investigate the interactions and reactivity of the film with electrolyte components using ab initio molecular dynamics simulations. Adsorption of solvent molecules (ethylene carbonate, EC) and salt (LiPF) and reduction by two mechanisms depending on the Li content of the film (yielding open EC adsorbed on the film or CH + CO) take place near the film/electrolyte and film/anode interfaces. Reaction products incorporate into the structure of the film and create a new kind of solid-electrolyte interphase layer.