CoSO4/SiO2 catalyst for selective synthesis of (9, 8) single-walled carbon nanotubes: Effect of catalyst calcination

A sulfate-promoted CoSO4/SiO2 catalyst is developed for chirally selective synthesis of large-diameter single-walled carbon nanotubes needed for electronic applications. The catalyst shows tunable nanotube chiral selectivity by changing catalyst calcination temperature. After calcination in air at 400 °C, it has excellent single chiral selectivity toward large-diameter (9, 8) nanotubes at 1.17 nm. The selectivity shifts to small-diameter nanotubes, when it is calcined at 800 °C. Catalyst characterizations by hydrogen temperature-programed reduction, X-ray absorption spectroscopy at the cobalt K-edge and sulfur K-edge suggest that the presence of chelating bidentate SO42- bonded with cobalt on the silica surface may limit the aggregation of cobalt and/or form cobalt–sulfur compounds during nanotube growth, leading to the selectivity toward the (9, 8) nanotubes. Furthermore, the removal of sulfur at higher calcination temperatures may form surface cobalt oxides and silicates that shift the chiral selectivity toward small-diameter nanotubes.

Graphical abstractCalcination at different temperatures coverts CoSO4 supported on SiO2 into various Co species, which lead to tunable chiral selectivity in single walled carbon nanotube synthesis. After calcination at 400 °C it has excellent single chiral selectivity toward large-diameter (9, 8) nanotubes at 1.17 nm.Figure optionsDownload full-size imageDownload high-quality image (81 K)Download as PowerPoint slideHighlights► The CoSO4/SiO2 catalyst was prepared by impregnating Co (II) sulfate heptahydrate on fumed silica powders. ► Large-diameter single-chirality (9, 8) nanotubes were synthesized on the CoSO4/SiO2 catalyst calcined in air at 400 °C. ► The chiral selectivity of the catalyst can be tuned to small-diameter nanotubes when the catalyst is calcined above 700 °C. ► Calcination produces different Co species on catalysts, including chelating bidentate SO42- bonded Co species, Co oxides and Co silicates. ► Different Co species result in the chirality shift of SWCNTs produced.