Catalyst Composition, Morphology and Reaction Pathway in the Growth of “Super‐Long” Carbon Nanotubes

The discovery by Hata et al. of “super-long” carbon nanotubes (CNTs) grown by catalyst-driven, water-assisted chemical vapor deposition represented a major synthetic breakthrough for the integration of CNTs into future device architectures, such as chemical and physical sensors, heterogeneous catalyst arrays, or 3D microand nanoelectromechanical systems (MEMS and NEMS). Their work was preceded by the development of the HiPCO process by Smalley et al. , which allowed, for the first time, a massive formation of unordered singlewalled CNTs (SWCNTs). This discovery was followed shortly thereafter by the first growth process of long (up to 4 mm) vertically aligned SWCNTs by Muruyama et al. Recently, Hata et al. found that CO2, ethers, ketones, and alcohols are, in the same way as water, advantageous for the growth of arranged “super-long” CNTs. All growth enhancers contain oxygen atoms, and act as weak oxidants, which are able to etch away a growing carbon layer, which deactivates the active catalyst, and thus being detrimental to the growth of “superlong” CNTs. 6] The composition of the active catalyst in the “super-long” growth of CNTs needs further investigation, despite experimental efforts in the field of in situ characterization of CNT growth. Herein we report our findings regarding the nature, role, and mechanism of the catalyst composition in the growth of “super-long” CNTs. Our studies allow an understanding of the morphology and structure of the catalyst nanoparticles by high-resolution scanning transmission electron microscopy (STEM) tomograms and of their chemical composition by a combination of spectroscopic and diffraction techniques. Besides catalyst dispersion, the properties of the substrate surface (roughness, active area, and electron-transfer ability to the catalyst surface) are critical for a high-yielding efficient CNT growth. Aluminum is known to work as an efficient buffer layer between catalyst and substrate under “super-long” CNT growth conditions. CNT growth starts with deposition of iron metal particles on a thin aluminum metal film deposited onto a Si wafer having a thin native SiO2 layer. [1] We have chosen two different procedures for catalyst sample preparation, which allow direct observation of the catalyst’s structure and morphology and characterization of its chemical composition. The first set of experiments was carried out on transmission electron microscopy (TEM) grids and the second on a Si wafer substrate. In both cases we deposited a 10 nm-thick aluminum film, followed by a 1 nm-thick layer of iron metal. These deposited films were then heated directly to 750 8C. The resulting catalysts were then tested and found to be active in the water assisted chemical vapor deposition (CVD) growth of CNTs in an independent set of experiments (for details and characterization of the CNTs, see the Supporting Information). The chemical composition of the catalyst system was further studied to clarify whether (a) FexAly intermetallic compound formation or (b) Fe–Al particle formation by cluster intermixing of aluminum and iron elements occurs under growth conditions. The latter are typically nonequilibrium structures, and thermal treatment and surrounding conditions may strongly influence their catalytic behavior. To confirm the shape and particle morphology of the Fe–Al nanocatalyst, firstly high-angle annular dark-field STEM images were recorded (Figure 1 A). The particles exhibit a core–shell contrast with facets. Their size ranges from a few nanometers up to 50 nm and the particles show sharp corners and edges with flat surface structures. Lattice fringes are observed, offering proof of the crystalline nature of the catalyst particles. High-resolution bright-field tomography of these particles was undertaken to clarify their three-dimensional shape. The tomogram data of the catalyst particles reveal a dome-shaped morphology with a void within the core, hence the donut-like contrast in the projection images. A tomogram slice showing the central void and well-defined facets is shown in Figure 1 B. Spatially resolved electron energy-loss spectroscopy (EELS) was performed in a high-resolution STEM to study the element distribution within the catalyst particles (Figure 1 C). Line profiles of 20 EEL spectra were taken across a number of catalyst particles. The evaluation of core-loss signals in the spectra allows plotting of the intensity of Fe and Al concentrations across the particle’s diameter. As can be seen from Figure 1 C, [a] R. Joshi, Dr. J. Engstler, Prof. Dr. J. J. Schneider Technische Universit t Darmstadt, Fachbereich Chemie Eduard-Zintl-Institut f r Anorganische und Physikalische Chemie Petersenstrasse 18, 64287 Darmstadt (Germany) Fax: (+ 49) 6151-163470 E-mail : joerg.schneider@ac.chemie.tu-darmstadt.de [b] Dr. L. Houben, Dr. M. Bar Sadan Ernst-Ruska-Zentrum f r Elektronenmikroskopie (ER-C) 52425 J lich (Germany) [c] Prof. Dr. A. Weidenkaff, Dr. P. Mandaliev Eidgençssische Material-Pr fungsanstalt, EMPA 8600 D bendorf (Switzerland) [d] Dr. A. Issanin Technische Universit t Darmstadt Fachbereich Materialund Geowissenschaften Fachgebiet Oberfl chenforschung Petersenstrasse 23, 64287 Darmstadt (Germany) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cctc.201000037.