Scientists to explore metal−nanotube interactions in the TEM

Defects in TEM
Figure 1. Elementary defects.
Defects in TEM
Figure 2. Transformations in SWNTs.
Defects in TEM
Figure 3. Os atoms in CNT.

January 17, 2012 - The full understanding of the interaction of transition metals and carbon nanotubes is of great importance for the development of new advanced materials, e.g. in electronic components and catalysts. A group of researchers from the Universities of Ulm (Germany) and Nottingham (UK) as well as the company Zeiss (Germany) has now for the first time studied the interaction of the transition metals W, Re and Os with the interior of CNTs by imaging at the single-atom level in direct space and real time using LV-AC-HRTEM combined with DFT simulations.

The full understanding of the nature of the bond between carbon nanotubes and transition metals is increasingly a focus of nano-materials science today.[1-5] This is due to the promising opportunities for the applications of the metal SWNT heterostructures in catalysis,[6-9] hydrogen storage,[10] and microelectronics.[11-15] High-resolution transmission electron microscopy (HRTEM) developed with the advent of systematic investigations at low voltages to a reference method for studies at atomic resolution. New research results can now show for the first time the complex nature of the nanotube-metal bond with atomic resolution in real time.[16-21]

From the atoms to the edge

The outer surface and the inner surface of carbon nanotubes have so far been investigated to different degrees.[22] The chemistry of the CNT's inside has remained largely unexplored. Now, a team of scientists has investigated the behavior of the three transition metals, W, Re and Os, after they had encapsulated them in the nanotubes.[16-21] The electron beam enabled a detailed study of the metal clusters and, at the same time, provided energy in the form of the kinetic energy of the electron beam for possible chemical transformations in the sample.[23,24]

At a standard energy of 200–300 kV, which is typically used in HRTEM experiments, damage to the nanotube structure occurs too quickly and it cannot be investigated at the atomic level.[25,26] By reducing the energy of the electron beam to 80 kV or less, the maximal transferable energy from an incident electron to a carbon atom is below the minimum energy required for a direct removal of a carbon atom from a nanotube making a detailed atomic-scale investigation possible.

The metal atoms coordinated to the inner surface of the nanotube were found to increase the reactivity of the CC bonds near the clusters and lead to the removal of the carbon atoms (Figure 1). The coupling of the outermost metal atoms with the SWNT (Figure 2) and the redistribution of electrons between nanotubes and metal can stretch and thus weaken the CC bonds of the ligand molecules.[27] Out of the three elements, Os activates aromatic molecules to a greater degree than W or Re.[28]

The scientists then also analyzed the complete cycle of chemical reactions until the metal atoms are released from the MWNT. A number of previous reports has already shown a significantly stronger bond of metal atoms to vacancy defects in graphene compared to an untouched graphene structure.[29-33] Transition metals appeared to bond more strongly with dangling carbon bonds than with the π-electron system of an intact MWNT.[34] (Figure 3) As they later discovered, the model could also explain the observed formation of a cap in the process. Their experimental AC-HRTEM observations showed that from one point on, the nanotube defect largely reconstructs until a fully sealed cap with no dangling bonds and six pentagons (required for a closed cap topology) is formed (Figure 1g). Once the metal cluster has separated from the nanotube, no further transformations could take place in the SWNT structure.

At 80 kV, the research relies on observations made with the CS-corrected FEI Titan 80-300 TEM and the Zeiss Libra 200MC TEM. For 20 kV AC HRTEM experiments, special modifications of the corrector and base structure of the Zeiss Libra 200MC TEM were made as part of the SALVE project.[35] For instance, an improved monochromator (0.15 eV) was made to shift the information limit. Moreover, the Contrast improved through loss-free energy filtering (5 eV). The results were published on January 17 in the Journal of the American Chemical Society.

"High-resolution transmission electron microscopy (HRTEM) is rapidly developing into an excellent local probe tool for studying chemical reactions in nanotubes by mapping transformations in direct space and in real time down to the individual atom level," said SALVE Prof. Ute Kaiser, head of the SALVE project.

The recently demonstrated ability for HR-TEM to map the electron density distribution in carbon- and nitrogen-containing structures[36] is to be extended to organometallic systems in the future in order to further understand the complexity of the nanotube-metal bond at the subatomic level.

Resource: Zoberbier, T., Chamberlain, T. W., Biskupek, J., Kuganathan, N., Eyhusen, S., Bichoutskaia, E., Kaiser, U. & Khlobystov, A. N. (2012). Interactions and reactions of transition metal clusters with the interior of single-walled carbon nanotubes imaged at the atomic scale. Journal of the American Chemical Society, 134, 3073-3079, doi: 10.1021/ja208746z, [PDF], see also the [Supporting information].

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