Tetsuya Uchiyama

Atomic-Scale In-situ Observation of Carbon Nanotube Growth from Solid State Iron Carbide Nanoparticles

We have first observed the nucleation and growth process of carbon nanotubes (CNTs) from iron carbide (Fe 3C) nanoparticles in chemical vapor deposition with C 2H 2 by in situ environmental transmission electron microscopy. Graphitic networks are formed on the fluctuating iron carbide nanoparticles, and subsequently CNTs are expelled from them. Our atomic scale observations suggest that carbon atoms diffuse through the bulk of iron carbide nanoparticles during the growth of CNTs.

Systematic Morphology Changes of Gold Nanoparticles Supported on CeO2 during CO Oxidation

Gold, the most stable metallic element, shows remarkable catalytic activity for CO oxidation even at room temperature. Unlike platinum and palladium, gold must be supported in the form of nanoparticles on crystalline metal oxides such as TiO2 [1] and CeO2. [3] Despite extensive studies, the mechanism of catalysis by gold nanoparticles (GNPs) is still unclear, in particular in relation to CO oxidation at room temperature. In the present study we observed a real Au/CeO2 catalyst in CO/air mixtures by means of in situ environmental transmission electron microscopy (ETEM). The catalyst was also characterized by catalytic chemical analyses. In real GNP catalysts, the structures of the GNPs are not identical at the atomic scale. Hence, we examined a large number of GNPs in the Au/CeO2 catalyst using ETEM, and found that the majority of the GNPs behaved systematically, depending on the partial pressures of CO and O2 at room temperature. GNPs remained faceted during CO oxidation in CO/air and became rounded, or fluctuating multifaceted with decrease of the partial pressure of CO relative to air. We also examined GNPs supported on a non-oxide crystal (TiC) with ETEM. In contrast to GNPs supported on CeO2, switching the gases did not induce any morphology change of GNPs supported on TiC. These experimental results have provided a clue toward elucidation of the peculiar catalytic mechanism of supported GNPs. The interface between GNPs and CeO2 support most likely plays an important role in the catalytic activity, especially the dissociation of O2 molecules at room temperature. This work thus contributes to improving and developing real catalysts. The Au/CeO2 catalyst was prepared by the deposition precipitation method. The conversion of CO to CO2 reached 100 % at room temperature, and the turnover frequency (TOF) of the catalyst was measured as 0.24 molCO (molAusur) 1 s 1 at 303 K. The catalyst sample was examined in vacuum by conventional transmission electron microscopy before and after the oxidation of CO at atmospheric pressure and at 303 K for 5 h. As shown in Figure S1, it was confirmed that the average size and morphology of the GNPs remained unchanged after the oxidation of CO at atmospheric pressure. A detailed description of the catalyst is given in the Supporting Information. First, we summarize the typical morphology of a GNP supported on CeO2 in various environments at room temperature. During CO oxidation in 1 vol% CO/air gas mixture (1 vol% CO, 21 vol% O2, 78 vol% N2) at 1 mbar pressure, the GNP appeared to be faceted in the form of a stable polyhedron enclosed by the major {111} and {100} facets, as shown by Figure 1a. Unexpectedly, the GNP behaved differently, and became rounded in pure O2 gas. The GNP exhibited major facets in both inactive N2 gas at 1 mbar and in vacuum (Figure 1a). In N2 gas, N2 molecules collided with the surface of the GNP at a rate of 3 10 s 1 nm . By comparison of the GNP in N2 gas and in vacuum (Figure 1a), we consider that the impacts of inactive N2 molecules caused no significant

Atomic-scale analysis on the role of molybdenum in iron-catalyzed carbon nanotube growth.

We have elucidated the synergetic role played by molybdenum in iron-catalyzed chemical vapor deposition growth of carbon nanotubes (CNTs) by in situ environmental transmission electron microscopy. Molybdenum can be well accommodated by Fe-based carbide nanoparticle catalysts of M(23)C(6)-type structure (M = Fe and Mo). We have also shown that molybdenum suppresses the nucleation of iron compounds that are known to exhibit no catalytic activity for the growth of CNTs.

Environmental Transmission Electron Microscopy Observations of Swinging and Rotational Growth of Carbon Nanotubes

The swing and rotation of growing carbon nanotubes (CNTs) have been observed in situ for the first time by environmental transmission electron microscopy. We have also observed that a CNT grows discontinuously when the CNT comes in contact with a substrate. This means that the van der Waals force between the CNT and the substrate prevents the growth of CNTs. These in situ observations enable us to understand the suspended growth mechanism of CNTs.

Direct Observation of Carbon Nanotube Growth by Environmental Transmission Electron Microscopy

We have succeeded in direct observations of the growth of a multi-walled carbon nanotube (MWNT) in an ethanol gas by environmental transmission electron microscopy. A short MWNT has been grown by chemical vapor deposition from ethanol using Co as a catalyst. The catalyst nanoparticle has lifted off the substrate and, at the same time, a MWNT has been grown. After the interruption of the growth, the MWNT has been gradually transformed to a carbon onion. We consider that the transformation has been induced by knock-on displacement and Stone-Wales rearrangement.

In Situ Observation of Nucleation and Growth of Carbon Nanotubes from Iron Carbide Nanoparticles

Nucleation and growth processes of carbon nanotubes (CNTs) in iron catalyzed chemical vapor deposition (CVD) have been observed by means of in-situ environmental transmission electron microscopy. Our atomic scale observations demonstrate that solid state iron carbide (Fe 3 C) nanoparticles act as catalyst for the CVD growth of CNTs. Iron carbide nanoparticles are structurally fluctuated in CVD condition. Growth of CNTs can be simply explained by bulk diffusion of carbon atoms since nanoparticles are carbide.

Recent Advancement of Environmental TEM for Material Process Characterization

Recent technological advancements in TEM such as aberration correction of lenses and fast detection cameras have drastically boosted the capabilities of in-situ environmental transmission electron microscopy (E-TEM) for various types of materials characterization. Two major capabilities are: the characterization of material synthesis processes, and the characterization of functional materials and devices at operating conditions. On the former, a miniaturized reaction chamber reproduces the material synthesis conditions inside an E-TEM apparatus and therefore allows us to obtain atomic scale information throughout the processing time. On the latter, we can characterize the atomic scale of catalysts, batteries and electronic devices, in real environments, by combining the E-TEM apparatus with simultaneous optical, electronic, and/or thermal add-on equipment. Therefore, in-situ E-TEM has greatly extended the fields applicable by TEM. The purpose of this presentation is to show our recent contribution to E-TEM on the aforementioned aspects.

Environmental Transmission Electron Microscopy Study on the Role of Impurities in Carbon Nanotube Growth

Environmental transmission electron microscopy (ETEM) is one of the most promising experimental techniques to study solid-gas reaction at atomic scale. Along with the ETEM image simulation [1, 2], we have developed an ETEM based on a conventional 200 kV FEG TEM (FEI Tecnai F20). Using the ETEM, we have investigated the Fe-catalyzed growth of carbon nanotubes (CNTs) by chemical vapor deposition (CVD). CNTs have extraordinary properties that are structure sensitive. The key to the structure-controlled growth is deep understanding of their growth mechanism. Our previous ETEM observations have shown that nanoparticle catalysts are solid state iron carbide (Fe3C) in the CVD condition [3]. In this work, we have elucidated the role of Mo in Fe-catalyzed CVD growth of CNTs by ETEM [4].

Synthesis of single-walled carbon nanotube networks on silicon nanowires

Single-walled carbon nanotubes (SWNTs) have been synthesized on silicon nanowires (SiNWs) by ethanol chemical vapor deposition (CVD) using Co catalysts nanoparticles. The surface SiOx layers assist the formation of catalyst nanoparticles on SiNWs by inhibiting the diffusion of Co to Si. Co-Si compounds have formed in SiNWs readily when the surface SiOx layers are very thin. Therefore, the yield of SWNTs is strongly influenced by the thickness of the surface SiOx layers of SiNWs.

Junctions of Carbon Nanotubes and Silicon Nanowires Synthesized by ethanol-Co Chemical Vapor Deposition

Carbon nanotubes (CNTs) have been grown on silicon nanowires (SiNWs) by chemical vapor deposition using Co catalyst nanoparticles. Single-walled CNTs have been grown mainly when a thin Co film (0.1 nm thick) was deposited on SiNWs, while both SWNTs and MWNTs have been grown on SiNWs on which Co 0.5 nm thick was deposited. The correlation between the diameter of catalyst nanoparticles and that of CNTs has been investigated by transmission electron microscopy. The average diameter of CNTs is smaller than that of catalyst nanoparticles.