In situ transmission electron microscopy study of the growth of Ni nanoparticles on amorphous carbon and of the graphiti
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In a specially equipped transmission electron microscope (TEM), Ni particles were vapor deposited onto thin films of amorphous carbon (a-C), and subsequent reactions with the carbon support were observed at elevated temperatures. Particles deposited at temperatures around 370 °C developed a graphite shell at above 600 °C and subsequently spread and graphitized the substrate. This activity was enhanced by hydrogen. The speed of graphitization significantly increased during spreading of the metal, which is attributed to the concomitant increase of the length of the reaction front, as well as to a purifying effect of hydrogen. It is concluded that the driving force for spreading of the Ni is the interdiffusion and catalytic conversion of carbon to graphite at the reaction front. Particles deposited at 500 °C remained inactive at 670 °C. This is probably due to the formation of a rather stable carbidic or graphitic interlayer during deposition.
I. INTRODUCTION
It has been known for more than a hundred years that molten Ni can dissolve substantial amounts of carbon, which precipitates as graphite on cooling. More recently, graphitization of carbon supports by Ni particles has been observed by several authors to already start at 700 K.1–5 Also, graphite and/or carbidic layers can be formed on bulk Ni by vapor deposition of carbon at high temperatures, or during catalytic reactions, like the methanation from CO and H2.6–8 High-energy electron bombardment of Ni particles in carbon soot has also been found to yield graphitic shells.9 Actual interest in this subject has increased as Ni was found to catalyze the formation of carbon nanotubes and nanofibres.10,11 Much work has been done with the aim to clarify the reaction mechanism, up to the atomic scale. Different pathways have been discussed in the literature, including dissolution of carbon in the metal, bulk diffusion, and segregation as graphite layers at the particle surface, as well as diffusion on the Ni surface and nucleation at step edges. However, the detailed interplay of these paths is still not quite clear and may depend on the experimental conditions. In particular, the competing role of carbide formation seems to be ambiguous. In earlier work, we
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0229 J. Mater. Res., Vol. 20, No. 7, Jul 2005
investigated the graphitization of a-C films by particles of pure Ni (and Ni–Pd alloys).12 We observed that the graphene layers forming around the particle grow inwards, while the Ni core is driven out and starts to spread on the substrate, thereby converting it into graphite. The aim of our present work is to investigate the influence of active gases on the reaction by in situ analyzing the local kinetics. In particular, hydrogen could react with carbon at the Ni surface as well as at the metal–substrate interface to form methane gas. Such a reaction has already been observed on crystalline graphite at temperatures above 1050 K by other authors.13 First results of
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