Unravelling the growth mechanism of confined carbyne by C-13 isotope engineering


A recent study in the group Electronic Properties of Materials led by Prof. Thomas Pichler and published in the Advanced Functional Materials allows for the first time to unravel the growth precursor and disentangle the growth processes of confined carbyne at high temperature.

Carbyne as a truly one-dimensional (1D) infinitely long polyynic nanocarbon has exceptional predicted properties, like being the world's strongest material. "Infinitely long" refers to a chain length long enough that the physical and chemical properties don't change anymore. However, carbyne is very unstable, and even its existence as the last missing carbon allotrope was doubted for more than a century since it was first mentioned by Adolf von Baeyer in 1885. Within the last 6 years, our group has achieved a series of breakthroughs regarding the synthesis and stability of ultra-long carbon chains confined inside double-walled carbon CNTs (DWCNTs). This includes the synthesis of the longest micrometer long carbon chains, i.e., proving the existence of carbyne (Nature Materials, 15, 634, 2016), carbyne with tailored properties (Nano Letters, 21, 1096, 2021), and isotope labeled carbyne (Angewandte Chemie-International Edition, 60, 9897, 2021). However, in all these studies, the precursor and growth mechanism of confined carbyne remained elusive. To solve this problem, we prepared and utilized ultra-clean 13C labeled DWCNTs as tailored hosts to trace the growth process of confined carbyne. As shown in the scheme from the publication on the right, we applied a series of experiments including oxidation and high temperature annealing in high vacuum to trace the relative distribution of 13C (orange atoms) as compared to 12C (blue atoms) using their different vibrational Raman responses.

As illustrated, we first filled the natural arc discharge single wall CNTs (SWCNTs) with isotopic C60 (81% 13C), which leads to the formation of so-called peapods. Then these peapods were transformed into DWCNTs by high temperature annealing in high vacuum, in which the C60 fullerenes were converted to inner tubes. Then the obtained DWCNTs (marked as IsoDWCNTs), whose outer walls with natural 13C abundant while the inner walls with 80% 13C content, were further treated to grow confined carbyne. The direct high temperature annealing of the IsoDWCNTs at the growth temperature of 1500 °C in high vacuum did not produce carbyne, which proves that such starting IsoDWCNTs were ultra-clean and no additional precursor carbon atoms were available to grow confined carbyne. Interestingly, an additional exchange of carbon atoms between the walls of the DWCNTs took place during the annealing. In order to grow carbyne, we oxidized the IsoDWCNTs in air before annealing. This not only introduced defects but also additional isotope engineered carbonaceous precursors from the IsoDWCNTs themselves. The two-step annealing yielded, concomitant to the atom exchange, the growth of isotope labelled confined carbyne with the up to now highest 13C concentration of 28.8%.

This work for the first time allowed us to unambiguously identify the growth precursor of confined carbyne and disentangle the different processes happening at high temperature annealing in high vacuum conditions. "This study not only provides new perspectives on the bulk scale synthesis of confined carbyne with tailored properties by selecting different fillers and different nanotube hosts, but also opens up new applications of isotope engineering as a novel tool to study the growth mechanism and optimize the synthesis of low dimensional nanostructures in general", states Dr. Weili Cui, the main author of this work.

The work in Vienna was supported by the Austrian Science Fund (FWF, P27769-N20) and received funding from the European Research Council (ERC) under the EU Horizon 2020 research and innovation program grant agreement No 951215 (MORE-TEM). 

Publication in "Advanced Functional Materials":
"Ultra-clean isotope engineered double-walled carbon nanotubes as tailored hosts to trace the growth of carbyne"
Weili Cui, Ferenc Simon, Yifan Zhang, Lei Shi, Paola Ayala, Thomas Pichler. Advanced Functional Materials, 2206491 (2022).