Research ArticleMATERIALS SCIENCE

High-precision solid catalysts for investigation of carbon nanotube synthesis and structure

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Science Advances  30 Sep 2020:
Vol. 6, no. 40, eabb6010
DOI: 10.1126/sciadv.abb6010
  • Fig. 1 Experimental setup for continuous catalyst NP generation, size selection, and collection.

    Location I represents atomized solution droplets as they enter the dryer, location II shows salt NPs (sNPs) after drying, location III shows metal oxide gas after calcination and vaporization in furnace, location IV shows oxide NPs (oNPs) after nucleation, and location V shows oNPs depositing onto substrate after size selection using a differential mobility analyzer (DMA). Example evolution of geometric mean particle diameter (dmean) and concentration (Ntot) of rhenium (Re) particles is shown in the middle for NPs (sNPs, oNPs, and size-selected oNPs) at locations II, IV, and V. Measurements from a scanning mobility particle sizer spectrometer are shown at the bottom with Ntot, dmean, and geometric SDs (σg).

  • Fig. 2 NPs after size selection and their size evolution during CNT growth.

    (A to C) AFM images of W oNPs homogenously deposited on SiO2/Si substrates. Particle populations having narrow size distributions with various mean diameters are shown including (A) ~1.2 nm, (B) ~1.9 nm, and (C) ~3.1 nm. Also shown are high-resolution transmission electron microscope (HRTEM) images of (D) as-produced polycrystalline Mo oNPs with a mobility-equivalent diameter of 10 nm corresponding to ~7 nm diameter measured by TEM and AFM, and (E) corresponding single-crystal Mo mNPs after reduction and reconstruction. mNP diameter is ~60% of the former oNP diameter. The plot in (F) presents the evolution in observed Mo NP diameter through various particle production steps and CNT growth for several oNP mobility-equivalent diameters, as prescribed using the DMA. The black arrows demonstrate the ~60% diameter ratio between (D) and (E). During the growth process, the resultant catalysts (here, Mo2C) retained the size of mNPs and strongly correlate to their nascent CNTs’ diameters.

  • Fig. 3 The constraint effect of catalysts on the areal density and diameter of CNTs.

    (A) XRD profile of real W, Mo, and Re catalysts during the growth process; all NPs are supported on alumina filters; SEM images of SWCNTs grown from low (B) and high (C) areal density of NPs on marked SiO2/Si substrate; with the small size of catalysts, i.e., (D) 1.4-nm Mo catalysts and (E) 2.6-nm W catalyst, high-quality SWCNTs are produced. Larger oNPs, i.e., (F) 4.5-nm Mo catalysts, produced few-walled CNTs with more defects. Tangential growth is the dominant mechanism. Catalysts are all observed to be wrapped by a carbon layer, forming a “pea pod” morphology, indicated by arrows in (E) and (F). With a size of less than 2 nm, catalysts are too small to be distinguished from the grid background, as happened in (D). a.u., arbitrary units.

  • Fig. 4 FCCVD growth of CNT using solid catalysts.

    (A) Schematic diagram of FCCVD setup and (B to D) the corresponding preliminary results of CNTs grown from FCCVD without size selection.

  • Fig. 5 Chirality distribution of SWCNTs grown from solid catalysts.

    Raman mapping results of SWCNTs grown from WC catalyst (diameter, ~1.5 nm). Raman spectra in the RBM region detected by (A) 532 nm, (B) 638 nm, and (C) 785 nm with all baselines subtracted. (D to F) The peak position abundance statistics results from three lasers are summarized with normalized scale. After converting peak position abundance to chirality abundance based on the Kataura plot (fig. S1), the results are displayed on a map (G). To guarantee unambiguous identification, the painting is done only for SWCNTs with 0.81 nm < diameter < 1.53 nm [160 cm−1 < RBM peak <295 cm−1, marked by the blue dash-dotted line in (A) to (G)]. Chirality cells that are not resonant with three lasers are left empty in (G). Chiralities near (2n,n) are most enriched (corresponding range of 19.1° ± 5° is marked by a black dashed line), and few can be found near the zigzag region. (H) The chiral angle statistics measured with nanobeam electron diffraction (ED) with the most dominant being in the same 19.1° ± 5° range; (inset) the example ED pattern of the (12,6) tube is shown with its simulation pattern (left, experiment; right, simulation). (I) The qualitative calculation of abundance based on interface thermodynamics, kinetic growth theory, and the extended chirality-dependent growth time determined from this work. All factors, particularly the chirality-dependent growth time theory (detailed in section S4), lead to further concentrated abundance around (2n,n).

Supplementary Materials

  • Supplementary Materials

    High-precision solid catalysts for investigation of carbon nanotube synthesis and structure

    Xiao Zhang, Brian Graves, Michael De Volder, Wenming Yang, Tyler Johnson, Bo Wen, Wei Su, Robert Nishida, Sishen Xie, Adam Boies

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    • Sections S1 to S4
    • Table S1
    • Figs. S1 to S9
    • References

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