“Recent advances in in situ techniques now allow gas-solid interactions to be studied at the atomic-level in the course of a catalytic reaction or a nanomaterial synthesis,” Stig Helveg told nanotechweb.org. “The movies directly show elementary steps involved in the catalytic growth reaction.”
The videos revealed that the graphitic nanofibres grew from nickel nanoclusters about 5 to 20 nm in diameter. The nickel catalyst particles actually changed shape during the growth process, becoming periodically more and less elongated. The nanocrystals elongated until they reached a length:width ratio of up to about four, before contracting to a spherical shape within less than half a second. The elongation appeared to correspond to the formation of more graphene sheets at the graphene-nickel interface: the scientists say the reshaping of the nanocrystals assists the alignment of graphene layers into a tubular structure. If the graphene layers completely surrounded the nickel particle, growth of the nanofibre stopped.
The TEM images also showed the presence of mono-atomic steps at the nickel surface, with a graphene sheet terminating at each of the steps. These nickel step edges appeared to play a key role in the nucleation and growth of graphene sheets, with graphene layers growing between pairs of step edges as the steps moved towards the ends of the nickel cluster and vanished.
“By combining the atomic-scale observations with density functional theory (DFT) calculations, we derived a detailed and coherent growth mechanism describing carbon nanofibre formation in terms of atomic-scale surface transport and restructuring of the nickel nanocrystals,” said Helveg. “What’s more, the direct observations and DFT calculations show that the active site/growth centre is associated with step edges at the nickel surface, mainly because carbon binds more strongly to such sites than to sites at close-packed facets.”
The scientists, who reported their work in Nature, believe that their discovery that the metallic step sites exhibit spatiotemporal dynamic behaviour “may be important for understanding catalytic reactions and nanomaterial syntheses, which usually assume a fixed number of stationary active sites.”