Oxygen-functionalized alkyne precursors in carbon nanotube growth

Abstract

Functionalization of carbon nanotubes (CNTs) with heteroatoms enables covalent attachment, opening up a world of potential material structures. However, common functionalization techniques are hazardous and lack precision. Here, we evaluate an in situ functionalization technique using oxygen-containing alkyne precursors. CNTs were successfully derived from propargyl alcohol and propiolic acid, growing at a rate of 67 ± 7 and 19 ± 3 µm min–1, respectively. While there was no substantial increase in the oxygen content of resultant CNT structures (all less than 1% O), Fourier transform infrared spectroscopy revealed subtle incorporations of carboxyl and hydroxyl functionality. An analysis of reactor effluent showed that both oxygen-containing species shed oxygen groups, where propargyl alcohol yielded a reactive atmosphere high in methylacetylene, and propiolic acid thermally degraded to acetylene and CO2, potentially explaining the enhanced catalyst lifetimes (approximately 75 min). These results support the universality of alkyne-promoting chemistries and delineate the limits of stable, oxygen-bearing alkynes to support point-directed functionalization schemes. Evidence that carbon nanotubes (CNTs) can form from intact C2-C4 subunits, primarily as alkynes, in a polymerization-like growth mechanism opened the doors for functionalized alkynes to direct the placement of heteroatoms in CNT lattice structures. If possible, this would alleviate the need for ex situ functionalization, the costs and environmental burdens associated with high concentration acid processing, and eliminate the random nature currently limiting CNT functionalization. Here, we demonstrate that oxygen-functionalized alkynes dominantly shed their oxygen-containing groups prior to or simultaneously with incorporation in the CNT, resulting in limited incorporation of oxygen groups into the CNT structure. As a whole, the work sheds light on fundamental reaction processes that give rise to carbonaceous nanostructures through polymerization and defines the limits of current in situ functionalization strategies.