Atomic-scale imaging of carbon nanofibre growth

Abstract

The synthesis of carbon nanotubes with predefined structure and functionality plays a central role in the field of nanotechnology, whereas the inhibition of carbon growth is needed to prevent a breakdown of industrial catalysts for hydrogen and synthesis gas production. The growth of carbon nanotubes and nanofibres has therefore been widely studied. Recent advances in in situ techniques now open up the possibility of studying gas-solid interactions at the atomic level. Here we present time-resolved, high-resolution in situ transmission electron microscope observations of the formation of carbon nanofibres from methane decomposition over supported nickel nanocrystals. Carbon nanofibres are observed to develop through a reaction-induced reshaping of the nickel nanocrystals. Specifically, the nucleation and growth of graphene layers are found to be assisted by a dynamic formation and restructuring of mono-atomic step edges at the nickel surface. Density-functional theory calculations indicate that the observations are consistent with a growth mechanism involving surface diffusion of carbon and nickel atoms. The finding that metallic step edges act as spatiotemporal dynamic growth sites may be important for understanding other types of catalytic reactions and nanomaterial syntheses.

Future Challenges in Heterogeneous Catalysis: Understanding Catalysts under Dynamic Reaction Conditions

“…Prominent examples in which TEM was used to gain insights into dynamic structure changes of catalysts are the restructurings of Cu surfaces of a Cu/ZnO catalyst in different gas atmospheres (see also Figures 5 and 6),46, 47 carbon nanofiber growth on a Ni catalyst during methane decomposition,14 and the refacetting of Pt nanocrystals during CO oxidation in a nanoreactor at 1 bar and at elevated temperatures 89…”

“…The new boundary conditions contrast most of the former and current chemical processes that are continuously run in a single, more or less optimal operation window. Notwithstanding, even under steady‐state conditions, dynamic changes are omnipresent in heterogeneous catalysis and electrocatalysis 11, 12, 13, 14, 15, 16…”

Controlled Growth-Reversal of Catalytic Carbon Nanotubes under Electron-Beam Irradiation

“…Significant advances in the understanding of the growth processes have generally been linked to their in-situ observation [4][5][6][7][8] , usually in specially-modified electron microscopes. The successful incorporation of carbon nanotubes into electronic devices, by exploiting their high-aspect ratios, their electronic structure and their conduction properties, requires control over the growth process and the resulting diameter, length and chirality of the nanotubes.…”

“…The bulk diffusion model was proposed by Baker 4 , and explains the growth of the nanostructures as being driven by temperature and carbon solute concentration gradients within the catalyst. More importantly, the observations of Baker are based on direct observation of the growth process in a Controlled-Atmosphere Electron Microscope (CAEM) 4,5 , a transmission electron microscope (TEM) where the growth reaction can be observed in real time [6][7][8] . At the core of this model is the observation that the activation energies, derived from the measured growth rates, relate directly to the bulk diffusion of carbon into the respective catalyst (Ni, Co, Fe).…”

“…Again, crucial evidence has been provided recently 6,7 as a result of the direct observation of the growth process, with significantly higher spatial resolution. These showed a significant reorganization of the catalyst's surface at the initial stages of growth and the fact that the catalyst remained crystalline throughout the growth process 6 . The improvement of the existing models is complicated by a number of factors, amongst which most important are the two modes of catalytic growth (tip-based, where the catalyst is transported away from the substrate, and base-growth, where the catalyst remains firmly attached to the substrate) and the conflicting reports, about the chemical state of the catalyst (i.e.…”