Synthesis of high-quality carbon nanotubes by using monodisperse spherical mesoporous silica encapsulating iron oxide nanoparticles

Well-graphitized carbon nanotubes (CNTs) were grown by using monodisperse spherical mesoporous silica encapsulating single iron oxide (Fe₃O₄) nanoparticles (MSEINPs) as catalytic templates by chemical vapor deposition (CVD) and using acetylene as carbon source. The catalytic templates were synthesized by a sol-gel method. The MSEINPs exhibited better activity and selectivity in CNT synthesis than bare Fe₃O₄ catalysts. The synthesized multiwall carbon nanotubes (MWCNTs) were analyzed by powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and Raman spectroscopy. The carbon deposits are rich in MWCNTs, as confirmed by FESEM and TGA. The wall thickness of the MWCNTs is controlled primarily by the size of the spherical mesoporous silica layer encapsulating the Fe₃O₄ NPs, while the inner diameter of the CNTs is determined by the size of the Fe₃O₄ NPs at the center of the MSEINPs. The average diameter of the MWCNTs increased significantly with increases in the growth temperature and acetylene flow rate. The analytical results show that the CNTs prepared on MSEINPs are well graphitized with a narrow size distribution in thickness, and straight and longer tubes are obtained without major defects as compared to the CNTs grown on bare Fe₃O₄ NPs.