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.

Figure 1

Alignment and growth analyses of CNT forests. (A) Representative SEM images taken from the side of CNT forests grown with either acetylene (Ac), methyl acetylene (MAc), or vinyl acetylene (VAc). Insets are synchrotron-based wide-angle X-ray scattering (WAXS) patterns of entire cross sections of these CNT forests. (B) Average Hermans orientation factor (H) values calculated from WAXS patterns of experimental triplicates. Error bars represent ±1 standard deviation. The p-value on the left was calculated from a one-way ANOVA across values from all three alkynes, while the p-value on the right comparing methyl acetylene CNTs and vinyl acetylene CNTs was calculated from a pairwise, two-tailed t test with Bonferroni correction. The other pairwise p-values were all much greater than 0.1 (SI Table S1). (C) Growth rates, (D) terminal heights, and (E) catalyst lifetimes of the CNT forests. (C and D calculated from growth curves in SI Figure S1). There are at least 3 experimental replicates for all analyses.

Figure 2

Atomic- and nanoscale properties determined by synchrotron-based X-ray scattering. (A) Interwall spacing between CNT walls and (B) total CNT wall thickness (outer diameter – inner diameter) as determined by WAXS. (C) Number of CNT walls was calculated from the interwall spacing and stack height (i.e., calculated from A and B). (D) CNT diameter, (E) number density throughout an array (i.e., calculated from D and F), and (F) mass density throughout an array as determined by small-angle X-ray scattering (SAXS) and X-ray attenuation. Error bars represent ±1 standard deviation of experimental triplicates. p-values were calculated with a one-way ANOVA across all three alkynes. Ac represents acetylene, MAc represents methyl acetylene, and VAc represents vinyl acetylene. Previous work has demonstrated that the outer wall CNT diameter is consistent with that of the supporting particle (Shi et al.).

Figure 3

Raman spectroscopy reveals differences in defects among substituted alkyne-grown CNTs. (A) Raman spectra taken from the sidewalls of CNT forests. Black curves represent average spectral data of experimental triplicate forests, normalized to the height of the D band, and the thickness of the black curve represents ±1 standard deviation. The green, gray, and orange curves are the average Lorentzian fits to the D (1339 cm^–1), G (1578 cm^–1), and D′ (1612 cm^–1) bands, respectively. The vertical black lines indicate the average fitted peak location across all spectra. The fitted curves have been slightly separated, vertically, from the spectral data to aid with visualization. (B) D/G band and (C) D′/G band intensity ratios obtained from the Lorentzian fits (A). The p-values were calculated with a one-way ANOVA across experimental triplicates of all three alkynes. (D) Full-width half-max (fwhm; cm^–1) of the Lorentzian fits for the D, G, and D′ bands. All error bars in (B, C) represent ±1 standard deviation of experimental triplicates. Ac represents acetylene, MAc represents methyl acetylene, and VAc represents vinyl acetylene.

Figure 4

Methyl groups in methyl acetylene-grown CNTs detected in ATR-FTIR spectra of CNT forests. Dark, solid lines represent the average of experimental triplicates, while the shaded regions represent ±1 standard deviation. The peaks in the methyl acetylene-doped CNT spectra at 2915 and 2850 cm^–1 are due to the asymmetric CH₂ stretching and symmetric CH₂ stretching vibrations, respectively. A noteworthy feature at 2150 cm^–1 is associated with alkynyl groups persistent in the solid structures. Ac represents acetylene, MAc represents methyl acetylene, and VAc represents vinyl acetylene.

Figure 5

SEM image of the morphological evolution in forest sidewall at the junction where (A) acetylene was replaced by methyl acetylene and (B) methyl acetylene was replaced by acetylene.

Figure 6

Variable alkynes exhibit unique influence on final nanotube structures mixed-mode polymerization, where unsaturated substituents on small alkynes (less than C4) can incorporate into a CNT lattice (forming sp² bonds) and saturated substitutes protrude from the lattice, introducing defects (or sp³ bonding).