In-situ Measurements of the Effect of Au Doping in Ni Catalysts on the Yield of Tubular Carbon Nanostructures

During the last five years, in-situ observations using environmental transmission microscopy (ETEM) has played a crucial role in revealing atomic level structural transformations occurring during their nucleation and growth. We have combined in-situ and ex-situ measurements to demonstrate the importance of temperature and precursor pressure in controlling the structure and morphology of the CNTs. We have also used the column of the ETEM for site-specific deposition of Fe catalyst particles and revealed the atomic-level structural transformations occurring during nucleation and growth of CNTs. Here we show that the yield of tubular carbon structures can be increased by doping a Ni catalyst with Au, and that their morphology and structure can be controlled by varying the reaction temperature. However, the exact role of Au for catalysis is not well understood. Au/Ni thin films (1-2 nm) with varying composition were co-deposited on perforated Si thin films supported on Mo grids and introduced into the column of a Tecnai F-20 environmental scanning/transmission electron microscope (ESTEM). The sample area of this microscope was used as a cold-wall CVD reactor. After preliminary characterization of as-deposited films, the samples were heated in vacuum up to reaction temperatures (500-650 °C). Acetylene was introduced in the sample region and pressures were kept constant during each experiment but varied between 1-3 mTorr for each individual experiment. Digital videos (15 frames/s) at low and high resolution were recorded at reaction temperatures and under flowing acetylene to measure growth rates and to follow the growth at atomic resolution. We observed increased activity in tubular structure formation for samples with low amounts of Au (~20% nominal composition). Moreover, tubes with herring-bone, stacked rings and multiwalled structure were observed to form depending upon the reaction temperature and pressure. Atomic resolution videos reveal that the graphene layers (the essential topological form of carbon for tubular growth) form at facets of the crystalline catalyst particles. Formation of different structures depended upon the rate of growth, which was in turn controlled by pressure and temperature. Both in-situ and ex-situ analysis of the composition of the catalyst particles, effect of temperature and pressure on the morphology and structural mechanism for the formation of various morphologies observed will be presented.