Calcium silicate as a metal-free catalyst for carbon nanotube synthesis
(Nanowerk Spotlight) Of all the methods that have been developed to produce carbon nanotubes (CNTs), including arc discharge, laser ablation, and chemical vapor deposition (CVD), CVD is the most technically important – since it can be achieved at low temperature and is upscalable – and the most widely used in industry (see Table 2 in our previous Nanowerk Spotlight"Global carbon nanotubes market – industry beckons"). Using the CVD process, manufacturers can combine a metal catalyst such as iron or nickel with reaction gases such as hydrocarbon to form carbon nanotubes inside a high-temperature furnace.
Semiconductor nanoparticles such as silicon carbide (SiC), germanium and silicon have also been used for single CNT catalysis. However, these catalyst materials are usually expensive and need to be of high purity in order to be useful for the growth of carbon nanotubes.
Researchers have now found that cheap and plentiful calcium silicate can absorb carbon species and grow multi-walled carbon nanotubes. Impurities inside the silicate have little effect on its catalytic properties but the temperature which controls the melting of silicate is very critical for the growth of CNTs. These finding will contribute to the still growing knowledge base about the growth mechanism of carbon nanotubes.
"We arrived at this novel, metal-free catalysts by accident" Jiangtao Zhu, now a Postdoctoral research associate in Arizona State University, tells Nanowerk. "Our initial goal was to try to get biomorphic SiC ceramic from bamboo charcoal by tetraethyl orthosilicate (TEOS) vapor. However, we got some CNTs, hollow or filled with silicate (see "Immobilization of CNT on Bamboo Charcoal by TEOS Vapor"). Then we used ethanol vapor instead of TEOS vapor but we still got hollow or filled CNTs (see "Synthesis of multiwalled carbon nanotubes from bamboo charcoal and the roles of minerals on their growth")."
Scanning electron microscopy image shows the high density of CNTs with a calcium silicate droplet tip grown on a calcium silicate film. (Image: Jiangtao Zhu, The Chinese University of Hong Kong).
As there are metals inside the bamboo charcoal, the researchers set out to verify the role of pure calcium silicate during CNT synthesis. In a recent paper in Carbon ("Metal-free synthesis of carbon nanotubes filled with calcium silicate"), first-authored by Zhu, led by Dickon H.L. Ng from The Chinese University of Hong Kong, showed that the pure calcium silicate did indeed act as a catalyst for the growth of CNTs. Dr. Juncai Jia, Dr. Fung Luen Kwong and Dr. Peter A. Crozier also made contributions to the paper as co-authors.
"Transmission electron microscopy (TEM) and energy dispersive X-ray spectrometry (EDS) examinations revealed that the tips of nanotubes synthesized at 1200-1400°C consist mainly of calcium silicate, thereby acting as effective catalysts for the nucleation of nanotubes," explains Zhu. "We found that the CNT formation follows the well-known vapor-liquid-solid (VLS) mechanism including an initial decomposition of ethanol vapor into carbon, dissolution of carbon inside molten silicate and final nucleation of carbon nanotubes."
The researchers explain that there are at least two possible applications for these findings. Firstly, the CNTs filled with silicate could provide a platform for the study of mineralogical formations in geophysics and geodynamics studies because CNTs will show to be high-pressure (higher than 40 gigapascals) cylinders under electron radiation.
Transmission Electron microscopy image shows a multi-walled CNT is filled with calcium silicate. (Image: Jiangtao Zhu, The Chinese University of Hong Kong).
Secondly, this work could provide applications in the cement industry to enhance the mechanical properties of cement. A large portion of cement is calcium silicate. The addition of CNTs to cement can greatly enhance its flexural and compressive strengths, as well as failure strain. Normally it is necessary to treat the surface of the CNTs to get a homogeneous dispersion inside the cement matrix because CNTs tend to agglomerate – which would worsen the mechanical properties of the matrix. In situ growth of CNTs in the silicate matrix – like in this work – would avoid this issue.