Studies On Bulk Synthesis Of Single-walled Carbon Nanotubes On Catalysts Of Osmium,Neodymium,Iridium,and Molybdenum

Owing to the unique structures and exceptional optical,electrical,mechanical,and field emission properties,single-walled carbon nanotubes have shown great potential in field-effect transistors,carbon-based chips,and quantum wires.However,the chiral structural diversity of single-walled carbon nanotubes has hindered their cutting-edge industrial applications.Nowadays,great efforts have been made in the chiral sorting of single-walled carbon nanotubes.To improve the separation efficiency and the yield of specific chirality single-walled carbon nanotubes,the chirality distribution of initial single-walled carbon nanotube powders should be as narrow as possible.Thus,the selectivity growth of single-walled carbon nanotubes with narrow chirality distribution by controlling the reaction conditions has shown great research value.Chemical vapor deposition offers unique advantages in the controlled synthesis of single-walled carbon nanotubes.Catalysts are the most important factor in the selectivity growth of single-walled carbon nanotubes during the chemical vapor deposition process.The iron group metals are the most commonly used catalysts.The iron group metals have a suitable binding energy to carbon which is responsible for their high catalytic activity.In addition to the extensive studies of iron group metals catalysts of single-walled carbon nanotube growth,investigations on chirality distribution of single-walled carbon nanotubes grown from other platinum group metals,early transition metals,and lanthanides metals are remained scarce.Although the catalyst containing different metal elements have been verified to successfully grow single-walled carbon nanotubes on flat substrate surfaces,the small surface area of the flat surfaces and the low yield of single-walled carbon nanotubes have made it difficult to establish the growth mechanism of single-walled carbon nanotubes.To fill this research gap,supported catalysts containing osmium,neodymium,iridium,and molybdenum were prepared by the impregnation method.Using the chemical vapor deposition technique,single-walled carbon nanotube powders are synthesized.The chiral distribution of single-walled carbon nanotubes are systemically characterized by optical characterization methods.(1)A magnesium oxide-supported osmium catalyst was prepared.After being calcinated at 1000 ~oC for 4 hours,a large number of osmium dioxide nanoparticles with epitaxial relationship and size less than 5 nm were formed on the surface of the magnesia carrier.In the chemical vapor deposition reaction using carbon monoxide as a carbon source,osmium dioxide was first reduced to form a high melting point metal osmium catalyst.Then,single-walled carbon nanotubes were grown at 1100 ~oC in vapor-solid-solid growth mode.The spectral analysis results show that the single-walled carbon nanotubes synthesized with osmium at high temperatures have a narrow chirality distribution.It is proved that the reduction of epitaxial osmium dioxide can facilitate the formation of small-size osmium nanoparticles,which provides a precondition for the growth of single-walled carbon nanotubes with a narrow chirality distribution at high temperatures.This work not only extends the development of unconventional catalysts but also opens up a new way for the high-temperature growth of single-walled carbon nanotubes with a narrow chirality distribution.(2)A magnesium oxide-supported neodymium catalyst was designed for growing single-walled carbon nanotubes by chemical vapor deposition.The porous magnesium oxide embedded with small-size neodymium trioxide nanoparticles provides a prerequisite for reducing neodymium metal and preparing high-quality single-walled carbon nanotubes.The characterization of hydrogen-temperature-programmed reduction showed that neodymium trioxide was reduced to neodymium metal at the reaction temperature.Subsequently,the carbon source adsorbed,cleaved,diffused,nucleated,and grew into single-walled carbon nanotubes on the neodymium surface.Absorption and fluorescence spectra show that the selectivity of(6,5)single-walled carbon nanotubes is up to 51.68%.Due to the low melting point of reduced neodymium nanoparticles,the nucleation growth of single-walled carbon nanotubes on neodymium catalyst follows the gas-liquid-solid mechanism.This work demonstrates the potential of chiral selective synthesis of single-walled carbon nanotubes by activated rare earth metals and provides a reference for designing rare earth catalysts with higher melting points.(3)A magnesium oxide supported by bimetallic iridium-molybdenum catalyst was developed for the growth of single-walled carbon nanotubes using carbon monoxide as a carbon source.Compared with single-metal molybdenum or iridium catalyst,bimetallic iridium molybdenum catalyst has higher catalytic activity.Thus,the yield of single-walled carbon nanotubes is higher and high-quality single-walled carbon nanotubes can be grown at 700-1000 ~oC.This work demonstrates the potential of bimetallic catalyst synergism in the growth of single-walled carbon nanotubes,paving the way for the design of high catalytic activity bimetallic catalyst synthesis of single-walled carbon nanotubes.