Structures of iron and cobalt bimetallic clusters for optimized chemical vapor deposition growth of single-walled carbon nanotubes

Bimetallic catalysts are crucial for chirality-selective and/or efficient growth of single-walled carbon nanotubes (SWCNTs), yet the coupled effects of alloy composition, temperature, and catalyst structure remain unclear. Here, iron–cobalt (Fe–Co) alloys are systematically investigated by combining chemical vapor deposition (CVD) experiments, chirality-resolved spectroscopy, and molecular dynamics simulations based on machine learning force fields. Using zeolite-supported SWCNTs grown by alcohol CVD, absorption and photoluminescence spectroscopy with two-dimensional excitation–emission fitting were employed to quantify chirality-specific growth efficiency. Two distinct growth regimes were identified. At 600 °C, pure Co exhibits the highest activity, favoring (6,5) and (7,5) SWCNTs. At 850 °C, the Fe 0.75 Co 0.25 alloy shows superior growth efficiency compared with other compositions, producing larger-diameter tubes (0.9–1.1 nm). Consistent behavior on SiO 2 substrates enables detailed transmission electron microscopy (TEM) analysis, revealing that high yields correlate with small, uniform nanoparticles with Co-dominated shells, in agreement with simulations. Furthermore, a composition–diameter phase diagram further rationalizes the observed trends. This phase diagram for binary clusters sheds light on the role of metal catalysts in SWCNT growth. • TEM/EDS and MD reveal small, uniform nanoparticles with Co-rich surfaces. • MLFF-driven MD phase diagram explains composition-dependent growth trends. • Fe–Co composition and temperature jointly govern SWCNT yield and diameter. • Pure Co is most active at 600°C, favoring (6,5) and (7,5) SWCNT growth. • Fe0.75Co0.25 gives the highest yield at 850°C and larger SWCNTs.