Modifying the Molecular Structure of Carbon Nanotubes through Gas-Phase Reactants

Abstract

Current approaches to carbon nanotube (CNT) synthesis are limited in their ability to control the placement of atoms on the surface of nanotubes. Some of this limitation stems from a lack of understanding of the chemical bond-building mechanisms at play in CNT growth. Here, we provide experimental evidence that supports an alkyne polymerization pathway in which short-chained alkynes directly incorporate into the CNT lattice during growth, partially retaining their side groups and influencing CNT morphology. Using acetylene, methyl acetylene, and vinyl acetylene as feedstock gases, unique morphological differences were observed. Interwall spacing, a highly conserved value in natural graphitic materials, varied to accommodate side groups, increasing systematically from acetylene to methyl acetylene to vinyl acetylene. Furthermore, attenuated total reflectance Fourier-transfer infrared spectroscopy (ATR-FTIR) illustrated the existence of intact methyl groups in the multiwalled CNTs derived from methyl acetylene. Finally, the nanoscale alignment of the CNTs grown in vertically aligned forests differed systematically. Methyl acetylene induced the most tortuous growth while CNTs from acetylene and vinyl-acetylene were more aligned, presumably due to the presence of polymerizable unsaturated bonds in the structure. These results demonstrate that feedstock hydrocarbons can alter the atomic-scale structure of CNTs, which in turn can affect properties on larger scales. This information could be leveraged to create more chemically and structurally complex CNT structures, enable more sustainable chemical pathways by avoiding the need for solvents and postreaction modifications, and potentially unlock experimental routes to a host of higher-order carbonaceous nanomaterials.

Acid Treatment in Alkyl-Functionalized Carbon Nanotubes: Soft Functionalization Techniques with Lower Environmental Footprint

“…It is important to note that vinylacetylene has a higher cost and would not likely be used for bulk CNT synthesis but instead to tailor a structure when the specificity is needed (e.g., to promote larger intertube spacing or a particular tortuosity, tube angle, or growth direction). 49 The anticipated limited use would eliminate the large environmental impact shown here (which assumes equimolar utilization). As a result, selecting a liquid HCl treatment of vinylacetylene for oxygen addition over a more standard liquid HNO 3 treatment of acetylene offered a mere 2% reduction in GWP, and an increase in ecotoxicity of 5%, suggesting that this new process scheme does not yield a favorable environmental footprint reduction.…”

“…The environmental savings offered by using HCl over HNO 3 for CNT functionalization are eliminated through the use of vinylacetylene due to a 2.5× greater GWP than acetylene on a molar basis (likely attributed to having twice the molar mass of acetylene) and a staggering 150× greater ecotoxicity potential (primarily due to the CuCl catalyst, which is produced industrially from solid Cu and Cl 2 gas). It is important to note that vinylacetylene has a higher cost and would not likely be used for bulk CNT synthesis but instead to tailor a structure when the specificity is needed (e.g., to promote larger intertube spacing or a particular tortuosity, tube angle, or growth direction) . The anticipated limited use would eliminate the large environmental impact shown here (which assumes equimolar utilization).…”

“…Liquid-based HCl treatment afforded CNTs an average of 0.05% chlorine (0.05 ± 0.03, 0.05 ± 0.02, and 0.05 ± 0.02% for Ac-, MAc-, and VAc-derived forests, respectively) compared to 0−0.01% in untreated blanks; this addition was observed with significance for vinylacetylene (p = 0.013; Figure 4c) and near significance for methylacetylene (p = 0.053; Figure 4b), suggesting that defect sites and/or addition to persistent vinyl groups may be important functionalization pathways. Vapor treatment with HCl was a more effective pathway, producing CNTs with 0.05−0.1% chlorine on average (1.0 ± 0.04, 0.06 ± 0.05, and 0.05 ± 0.03% for Ac-, MAc-and VAc-derived forests, respectively), with the greatest significance for acetylene-derived CNTs (p = 0.065; Figure 4d), which tend to be highly defective structures 49 and supporting the hypothesis that defect sites (or irregularities in the hexagonal C arrangement of the CNT lattice) serve as points of addition in functionalization schemes. 25 The Cl (2p) binding energies further suggest that MAc-and VAc-derived CNTs are prone to covalent Cl addition, supporting the persistence of vinyl groups in VAcderived CNTs (Figure S4).…”

“…The use of HCl would facilitate chlorine addition, which could be useful for further functionalization via nucleophilic substitution reactions. 48 Furthermore, variable CNT precursors that impart alkyl functionality to the CNT surface may yield CNT structures that are more amenable to HCl-mediated functionalization: for example, in vinylacetylene-derived CNTs, persistent alkene bonds 49 would favor halogenation. 50 This could provide a "handle" for subsequent derivatization of the CNT lattice, perhaps with less randomness than current state-of-the-art techniques.…”

“…■ METHODS CNT Synthesis. Vertically aligned CNT forests were grown in a cold-walled reactor with a resistively heated substrate to support the catalyst 51 following Johnson et al 38 and Giannetto et al 49 First, the reactor was flushed with 500 sccm helium (ultra high purity (UHP), 99.999%, Airgas) for 10 min before switching to the 600 sccm annealing mixture of 25% hydrogen (UHP, 99.99%, Airgas) and balance helium. The annealing mixture flowed for 2 min, and then, the reactor was heated to 700 °C and held for 2 min.…”