Chiral-selective CoSo₄/SiO₂ catalyst for (9,8) single-walled carbon nanotube growth

Electronic and optical properties of single-walled carbon nanotubes (SWCNTs) correlate with their chiral structures. Many applications need chirally pure SWCNTs that current synthesis methods cannot produce. Here, we show a sulfate-promoted CoSO₄/SiO₂ catalyst, which selectively grows large-diameter (9,8) nanotubes at 1.17 nm with 51.7% abundance among semiconducting tubes and 33.5% over all tube species. After reduction in H ₂ at 540 °C, the catalyst containing 1 wt % Co has a carbon yield of 3.8 wt %, in which more than 90% is SWCNT. As compared to other Co catalysts used for SWCNT growth, the CoSO₄/SiO₂ catalyst is unique with a narrow Co reduction window under H₂ centered at 470 °C, which can be attributed to the reduction of highly dispersed CoSO₄. X-ray absorption spectroscopy (XAS) results suggested the formation of Co particles with an average size of 1.23 nm, which matches the diameter of (9,8) tubes. Density functional theory study indicated that the diameter of structurally stable pure Co particles is scattered, matching the most abundant chiral tubes, such as (6,5) and (9,8). Moreover, the formation of such large Co particles on the CoSO₄/SiO₂ catalyst depends on sulfur in the catalyst. XAS results showed that sulfur content in the catalyst changes after catalyst reduction at different conditions, which correlates with the change in (n,m) selectivity observed. We proposed that the potential roles of sulfur could be limiting the aggregation of Co atoms and/or forming Co-S compounds, which enables the chiral selectivity toward (9,8) tubes. This work demonstrates that catalysts promoted with sulfur compounds have potentials to be further developed for chiral-selective growth of SWCNTs.