AC-HRTEM as a new tool to study chemical reactions of carbon and metals

February 05, 2016 - To unlock the full potential of carbon nanostructures for catalysis and electronic devices, the understanding of their bonds with metals is of great importance. AC-HRTEM is a tool to influence and study the formation of chemical bonds. In a new study conducted in the frame of the SALVE project, scientists have investigated the bonds between carbon and Group VIII metals on an atomic scale. With this they continued their study of the interaction of metals and carbon allotropes [1]. The study also provides new insights into radiation damage processes occurring in the presence of carbon and metal compounds under electron beam irradiation.

Recently, a number of significant advances have been achieved in terms of understanding the complexity of interactions between metals and carbon nanostructures. [2] A number of detailed systematic theoretical studies focused on 3d metals and examined trends within this period of the periodic table. It is expected that bonds between the π-electronic system of nanotubes or graphene and metal atoms consist of covalent and ionic components with a binding energy that depends on the type of metal and the curvature of the carbon nano-structure (for example, cylindrical nanotube vs. planar graphene). A DFT simulation of the binding energies of different transition metals with carbon revealed a non-uniform dependence of the π-bond energy on the position of the element within the 3d period of the periodic table that can be referred to as dual-maxima distribution. [3] Interestingly, graphene and nanotube surfaces turned out to have qualitatively similar trends of π bond with 3d metal atoms [3, 4]. 4d and 5d metals or trends within these groups of the PSE, however, remained largely unexplored.

Carbon nanostructures have different types of structural defects that significantly affect their properties. Even the simplest defects - single vacancies (SV, when a carbon atom has been removed from the graphene or CNT grid) - change the binding of metals with CNT or graphene dramatically [5 - 7]. For metal atoms bound to σ-bonds the covalent bond component of the energy is increased. The energy of such σ-bonds for 3d metals was examined in detail and it was shown that they show a tendency to form two maxima along the period [7] similar to the dependence, which was observed for π-bonds. No systematic analysis has been conducted for 4d and 5d metals, which are equally important for the synthesis and applications of CNT and graphene as 3d metals, until now. Moreover, theoretical studies of trends within the groups of the periodic table are so far limited to group XI metals (coinage metals), which thus leaves much room for further theoretical studies. [5]

In contrast to the theoretical work significantly fewer experimental studies exist. They are significantly less systematic and report usually about sporadic examples of metals from different places in the periodic table. Nevertheless, they provide some valuable information and can be effectively correlated with the theoretical predictions. [3 - 9] Systematic experimental measurements for different metals are hampered by technical difficulties in the deposition of transition metals on graphene or CNT surfaces.

In this new study, researchers from England and Germany headed by Andrey Khlobystov (University of Nottingham) and Ute Kaiser (University of Ulm), respectively, conducted for the first time systematic experiments to unravel the interaction of carbon structures with various metals (Fe, Ru, and Os), which have the same number of valence electrons, but belong to three different periods in the periodic table, where they represent 3d, 4d, and 5d metals. As in a previous study [1], they use single-walled carbon nanotubes (SWNTs) as a container for metals. Confinement of the metals in the tiny channels of SWNTs overcomes most of the challenges that has been highlighted in previous studies (for example oxidation of the metal, aggregation of the metal in large clusters, rapid unrestricted diffusion of metal atoms and any perturbing influence due to surface contamination). Previously the metal-CNT binding of adjacent metals from the same period in the PSE had been investigated. [1] Now they report the first comparative study of metals belonging to the same group of the periodic table. The scientists unravel a wealth of chemical transformations promoted by d-metals from different periods.

The electron microscopic studies were performed with an image-side C_S-corrected FEI Titan 80-300 transmission electron microscope operated at 80 kV with a modified filament extraction voltage for enhanced information limit and the SALVE II microscope, an image-side C_S-corrected Zeiss Libra 200MC equipped with a 0.15 eV monochromator. [12] The latter system has been specially modified for the low-voltage operation and was operated at 40 and 20 kV. [13]. AC-HRTEM images at different electron energies are shown in Fig. 1a-c and the supporting Information of the publication.

The direct removal of carbon atoms from the nanotube due to kinetic energy transfer from incident electrons is excluded by reducing the energy of the e-beam to below 80 keV in all experiments. If one works with electron energies < 80 keV the probability of knock-on damage is negligible (see supporting Information). Consequently almost all of the observed structural changes are caused by interactions between metal and CNT, which are due ultimately to the chemical properties of the encapsulated elements. [11, 14] However, CNT having structural defects, such as mono-vacancies, are more sensitive to e-beam damage (see supporting Information). To reduce this effect, the scientists used special SWNTs synthesized by electric arc discharge, which have fewer defects than the nanotubes produced by other methods. In addition, areas which were largely error-free at the beginning of the experiments were selected for the experiment. Since the speed of the chemical transformations depends strongly on energy and the electron dose, these parameters were carefully controlled during the experiments. The development of metal clusters and their interaction with carbon was performed with a continuous flow of electrons between 10⁶ - 10⁷ e^-nm^-2s^-1 and a cumulative dose ≈ 10¹⁰ e^-nm^-2 for each image sequence. Therefore, it is possible by comparing the time series of TEM images of various metals under the same conditions, to draw conclusions about the nature and the energy of the metal-CNT interaction for each element in the group VIII of the periodic table.

Within the group VIII triplet in the period table, Fe-Ru-Os, osmium has the highest cohesive energy (Table 1). As a result, Os cluster remain spheroidal and compact during TEM observations at 80 kV, with closed, partially faceted surfaces (Figure 2a). A slow rotation allows the clear visualization of the hexagonal lattice of Os (in accordance with the space group P6₃/mmc of the metal-solid), which confirms the metallic nature of the Os cluster. Osmium atoms are involved in the strongest interactions with the SWNT sidewalls. They facilitate the rapid formation of vacancy-type defects in SWNT sidewalls via the metal-assisted electron beam induced ejection (EBIE) mechanism, resulting in covalent σ bond of Os with the edges of the defects (Figure 2b). To compensate for the continuous loss of carbon atoms induced by the Os cluster, the nanotube undergoes extensive restructuring, resulting in the narrowing of the nanotube diameter (Figure 2c) and finally in the complete rupture of the SWNT at an electron dose of ≈ 10⁹ e^-nm^-2. The observed behavior clearly shows the high affinity of carbon for Os. Density functional theory (DFT) was used to confirm this experimental result. In agreement with the experiment the calculations showed that Os forms the strongest σ-bonds with carbon within the group VIII elements (Table 1), which is responsible for the formation of extensive sidewall defects in the host SWNT.

In contrast the researchers found that a carbon nanotube is able to connect electrically to Ru via π bonds, while for Fe it is most likely that a layer of iron carbide forms. Considering the chemical similarities between SWNT and graphene the same trend could apply for a graphene-metal interface.

The spatial resolution of TEM is sufficient for the direct visualization of isolated metal atoms on graphene or CNT, whereupon external parameters such as electron beam energy and temperature can be varied, so that their impact on the metal-carbon bond can be studied in detail at the atomic level. In this new study electron energies of 20, 40 and 80 keV were used to explore the two main effects responsible for metal-induced radiation damage, i.e. e-beam-induced ejection (EBIE, see Fig. 3b, a carbon atom becomes dislocated by the e-beam and leaves the SWNT in the presence of a metal) and e-beam induced restructuring (EBIR - see Fig. 3a, the dislocated or otherwise activated carbon atom can remain within the nanotube and can be engaged in bonding with the metal particle forming new carbon structures). EBIE processes are occurring frequently for Os clusters at 80 kV, EBIR processes rarely. In fact, the in the frame of the SALVE project previously conducted comparative study of the transition metals of period 6 (W-Re-Os) had also shown that no significant EBIR processes could be observed for each of the elements of this triad. [1]

"We modified the acceleration voltage of electrons to gain further insight in radiation damage processes", says SALVE scientist Ute Kaiser. "Although atomic resolution can currently not be achieved at 20 or 40 kV, the new study clearly shows that beam damage processes can be dramatically varied by the energy of the e-beam." By reducing the electron beam energy from 80 to 40 keV, the beam damage mechanism changes from EBIE to EBIR, i.e. the extent and the rate of transformation due to the metal in SWNT as a function of the kinetic energy of the e-beam is decreasing (Table 2). This means that the reactions in nanotubes in the presence of Os are primarily caused by the kinematic collisions of electrons of the e-beam with atoms (so-called knock-off effect) and less by ionization or phonon excitation, where the impact on radiation damage should increase with decreasing energy of the e-beam. One of the reasons, that the knock-on effect dominates the radiation damage in CNT and that ionization and phonon excitation are less significant is that SWNTs are excellent thermal and electrical conductors. On further reduction of beam energy to 20 keV, the energy of the electron is no longer sufficient to cause any significant metal-assisted transformation in the nanotube of neither EBIE- nor EBIR-type (Table 2).

"This study improves our understanding of the interaction and the links between SWNT and metals, which have major implications for nanotube-based electronic components and catalysis." said Khlobystov. "The recent developments in transmission electron microscopy (TEM) have led to a great importance of this method for the direct study of the interactions between transition metals and carbon nanostructures at the atomic level. Our study shows that radiation damage greatly decreases by reducing the electron energy in the system CNT / metal. This confirms that the further development of our SALVE TEM that is aiming to provide atomic resolution also at voltages < 80 kV is of major interest for materials science as well as to other research areas where transmission electron microscopy is applied."

Resource: Zoberbier, T., Chamberlain, T. W., Biskupek, J., Suyetin, M., Majouga, A. G., Besley, E., Kaiser, U. A., & Khlobystov, A. N. (2016). Investigation of the Interactions and Bonding between Carbon and Group VIII Metals at the Atomic Scale. small, 12: 1649-1657, doi: 10.1002/smll.201502210, [PDF], see also the supporting information.