Nano Focus: Iron catalyst reservoir doubles CNT growth on alumina
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Nano Focus Iron catalyst reservoir doubles CNT growth on alumina
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he most common way to synthesize tall carpets of vertically aligned carbon nanotubes (CNTs) is to heat samples with thin films of iron (catalyst) on top of alumina (underlayer) in the presence of hydrocarbon gases at temperatures between 700°C and 800°C. Now researchers at Bar Ilan University (BIU) in Israel have shown how a thin film of iron positioned below the alumina underlayer can substantially affect the growth of CNTs. The results may also help solve the longstanding puzzle: what makes CNTs grow? “The idea came after attending a presentation at a Materials Research Society Fall Meeting 2011, when Dr. Benji Maruyama explained how iron catalyst diffusion into the alumina underlayer was a factor in CNT growth termination. I thought that if the disappearance of iron on the top surface through porosity in the alumina is the cause for CNT termination, then we can replenish its supply from the bottom by using a reservoir of iron. We indeed doubled the growth but the underlying mechanism was very different from what we expected,” said Gilbert Daniel Nessim, principal investigator of this research, which was published in the February 7 issue of Nanoscale (DOI: 10.1039/ C3NR05240K; p. 1545). The researchers on Nessim’s team used the standard iron/alumina system as a benchmark. From the very first experiment, the new iron/alumina/iron reservoir system led to taller CNTs, with
“That our framework is held together by reversible, noncovalent supramolecular interactions ensures good solubility in water,” Li said. “The precise dimensional control of our solution-based processing facilitates the structural and chemical customization of our frameworks.” The tripods of these SOFs were made from aromatic bipyridine molecules whose trio of struts or arms was interlocked with the struts of their neighboring molecules through the macrocycles,
heights up to two times that of the iron/ alumina system. “From the start, the good results showed that we were onto something, but we started questioning the mechanism at play when we noticed that even very short growth experiments—30 seconds—led to taller CNTs on the sample with the iron reservoir,” said Efrat Shawat, a PhD candidate in the Nessim laboratory and lead author of their article. “It is only after extensive annealing experiments to observe the catalyst surface dewetting that we understood that the iron reservoir affected the morphology of the top surface,” she said. Annealing experiments performed using samples with increasing thicknesses of the alumina layer gave the research team the clue to understanding this mechanism. As thicker alumina layers did not show the height enhancement factor, it appeared that pinholes in the alumina underlayer were key to understanding the mechanism at play. “Only a few years back, observation of morphological size evolution of catalyst nanoparticles during CNT growth provided a significant hurdle to the realization of growing infinitely long, continuous CNTs,” said Cary Pint,
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