Research ArticleChemistry

Atomic-scale structural identification and evolution of Co-W-C ternary SWCNT catalytic nanoparticles: High-resolution STEM imaging on SiO2

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Science Advances  24 May 2019:
Vol. 5, no. 5, eaat9459
DOI: 10.1126/sciadv.aat9459
  • Fig. 1 Overview of the proposed TEM method, produced SWCNTs, and the structure of the catalyst.

    (A) Schematic of the 3-mm-diameter Si/SiO2 TEM grid. (B) The enlarged schematic showing a 20-nm-thin suspended SiO2 serves as the TEM observation window, and the catalyst can be prepared directly on this film and dispersed uniformly. (C) Estimated distribution of charge density over the suspended SiO2 film, suggesting that electron accumulation at edges and corners of the window is less than 10% of the value in the center. (D) Chirality distribution of SWCNTs synthesized from the Co-W-C catalyst. (E) Typical SAED pattern of the reduced Co-W-C catalyst. (F) Intensity profile of SAED pattern in (E). (G and H) Standard powder diffraction file of metallic W (no. 04-0806) and Co6W6C (no. 23-0939).

  • Fig. 2 Atomic structure identification of Co-W-C.

    (A to D) Quantitative EDS mapping of the reduced Co-W-C catalyst on the suspended SiO2 film, suggesting that two types of particles are existing: (i) pure W and (ii) junctions of pure W and Co-W mixtures. (E) HAADF image of a junction particle viewed from the [111] direction of the bottom part, suggesting that it is a metallic bcc W phase. (F) HAADF- and (G) ABF-STEM image of the same particle tilted to the [310] axis of the upper half of the particle, suggesting a Co6W6C phase. (H) Structure model and comparison of experimental and simulated HAADF- and ABF-STEM images of Co6W6C from the direction of [310].

  • Fig. 3 Elemental evolution of Co-W-C after different CVD time.

    (A) Quantitative EDS mappings of reduced and reacted Co-W-C catalyst particles on Si/SiO2 grids. (B) Typical EDS spectra of different samples, showing a clear decrease of atomic ratio of W/Co along with CVD. a.u., arbitrary units.

  • Fig. 4 Structure evolution of Co-W-C during CVD.

    (A) HAADF- and (B) ABF-STEM images and (C) EDS mapping of the reacted Co-W-C catalyst after a 1-min CVD, showing that the particle is a junction of the W-enriched phase and Co6W6C, but the W-enriched phase is much smaller than the pure W phase in the reduced catalyst. (D) HAADF- and (E) ABF-STEM images and (F) structure model and comparison of simulated and experimental STEM images of the reacted Co-W-C catalyst after a 3-min CVD, showing that this particle contains only the Co6W6C phase and W-enriched phases disappeared after a 3-min CVD.

  • Fig. 5 Precipitation of Co and Co-enriched phase.

    (A) Atomic-resolution HAADF-STEM image of a 3-min reacted Co-W-C catalyst particle, showing that the particle is a junction of the Co6W6C and pure Co phase. (B to E) EDS mapping of the same particle (pink represents Co K and yellow represents W L) and HAADF-STEM image taken simultaneously during EDS mapping. (F) Comparison between simulated and experimental STEM image of Co6W6C. These results suggest an oversaturation and precipitation of Co or Co-enriched phase from Co6W6C when the CVD time is extended. (G) Bulk phase diagram of Co-W-C ternary system remade from (30). (H) Schematic showing the overall process revealed in this work and a possible nucleation mechanism of SWCNTs (pink and yellow).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/5/eaat9459/DC1

    Supplementary Materials and Methods

    Fig. S1. Comparison of SWCNTs grown on Si/SiO2 substrate and Si/SiO2 TEM grid.

    Fig. S2. Details of the relative surface charge estimation.

    Fig. S3. SEM images and Raman spectra of the SWCNTs.

    Fig. S4. Low-magnification TEM and SAED patterns of the Co-W catalyst.

    Fig. S5. STEM images of another reacted Co-W-C particle with a less W concentration.

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Materials and Methods
    • Fig. S1. Comparison of SWCNTs grown on Si/SiO2 substrate and Si/SiO2 TEM grid.
    • Fig. S2. Details of the relative surface charge estimation.
    • Fig. S3. SEM images and Raman spectra of the SWCNTs.
    • Fig. S4. Low-magnification TEM and SAED patterns of the Co-W catalyst.
    • Fig. S5. STEM images of another reacted Co-W-C particle with a less W concentration.

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