Formation of Carbon Nanofibers and Thin Films Catalyzed by Palladium in Ethylene-Hydrogen Mixtures Mark A. Atwater, † Jonathan Phillips, †,‡ and Zayd C. Leseman* ,† UniVersity of New Mexico, Albuquerque, New Mexico 87131, and Los Alamos National Laboratories, Los Alamos, New Mexico 87545 ReceiVed: NoVember 9, 2009; ReVised Manuscript ReceiVed: February 2, 2010 The nature of observed growth of solid carbon on palladium from ethylene-hydrogen mixtures is consistent with the supposition that the primary source of carbon for growth is homogeneously generated radicals. Evidence includes the lack of growth in the absence of a reacting mixture, sharp maxima as a function of temperature, and dramatic differences in temperature of growth as a function of mixture composition. The finding that the structure of the support strongly influenced the morphology of the solid carbon, and the temperature regime for deposition, is also consistent with this model. Carbon nanofibers were found to form on sputtered palladium films and palladium nanopowder (ca. 700 °C), whereas planar carbon structures deposited on palladium micrometer powder and foil (ca. 600 °C). A radical species growth mechanism is consistent not only with observations made herein but also with data presented in earlier studies. 1. Introduction The present study of solid carbon deposition on palladium in various forms from ethylene-hydrogen mixtures was un- dertaken to test competing hypotheses regarding the mechanism of carbon growth on metal templates. The primary finding of this work is that the prevailing model of molecular decomposi- tion (MD), where fibers are postulated to grow via the simple, thermal decomposition of adsorbed hydrocarbons on a metal catalyst, such as that reported for acetylene decomposition over nickel 1 and cobalt, 2 where hydrogen was found to be a growth promoter, or for ethylene decomposition over iron being promoted by carbon monoxide and hydrogen, 3 cannot explain observations such as a peak in growth rate with temperature. Moreover, it cannot explain solid growth on foils as all catalytic material is clearly encapsulated. It is argued that an alternative model, growth from radicals (GR), originally put forth to explain carbon growth from ethylene-oxygen mixtures over a limited temperature range on platinum catalysts, 4-6 is more consistent with observations on growth. That is, carbon-containing radicals are formed homogeneously and these radicals are the primary source of carbon for fiber growth. The radicals adsorb on the surface, either carbon or catalytic metal, leaving carbon behind and releasing hydrogen and hydrocarbon species to the gas phase. It should be emphasized that the model is considered more evolutionary than revolutionary. Indeed, although the source of carbon is different in the two models, the mechanisms of growth once carbon is available are largely the same in the two models. Still, the difference is significant as it suggests very different approaches to optimizing features, for example, to attain maximum growth rate at minimum temperature. The GR model suggests that a maximum rate may be discovered at a low temperature and will require a reacting mixture. The older model (MD) implies that growth rate will increase with temperature and that mixtures are not necessary. A second finding of major significance is that, in ethylene- hydrogen mixtures, just as in fuel rich ethylene-oxygen mixtures, the morphology of the metal had an enormous impact on the morphology of the deposited carbon. For example, under identical conditions, filaments grow from sputtered films, whereas planar carbon structures grow on polycrystalline foils. Similar results were found in growth from fuel rich combustion mixtures. 7-9 Finally, it is important to note that a review of earlier literature suggests no major inconsistencies with predictions of the GR model, and there are earlier examples of sharp growth rate maxima as a function of temperature. 10-13 Also, the temperature of maximum growth rate in ethylene-oxygen mixtures 7 (∼550 °C) and ethylene-hydrogen mixtures (∼700 °C) is very different. This result is clearly consistent with GR theory. The temperature of radical production, according to combustion theory and experiment, clearly depends on gas mixture composition. 14,15 Also, other phenomena, such as the variation of solid carbon morphology with template structure, can also be explained with the GR model. 2. Experimental Methods Four forms of Pd were used in this study: sputtered film, foil, submicrometer powder, and nanopowder. Sputtered films were deposited onto single crystal Si(100) with a 0.6 μm oxidation layer. After oxidation, a thin adhesion layer of Cr (ca. 100 Å) was deposited onto the SiO 2 . A thicker layer (ca. 500 Å) of Pd was then sputtered on to this Cr layer. Pd foil (99.9%, 0.025 mm thick) was purchased from Alfa Aesar and used without modification. Two sizes of Pd powder (<25 nm and <1 μm, 99.9%) were purchased from Sigma Aldrich and used without modification. Pd foil and sputtered film samples were cleaned with methanol purchased from Burdick Jackson (HPLC grade) before being placed in the furnace. Samples were placed on ceramic boats, also cleaned using methanol. The reactor system consists of a single zone, horizontal tube furnace in which a 50 mm diameter quartz tube resides. Gases * To whom correspondence should be addressed. Telephone: (505) 277- 4940. Fax: (505) 277-1517. E-mail: zleseman@unm.edu. † University of New Mexico. ‡ Los Alamos National Laboratories. J. Phys. Chem. C 2010, 114, 5804–5810 5804 10.1021/jp9106734  2010 American Chemical Society Published on Web 03/05/2010