Synthesis of Carbon nanotubes by CVD: Effect of acetylene pressure on nanotubes characteristics

M. ESCOBAR; M.S. MORENO; R.J. CANDAL; M.C. MARCHI; A. CASO; P. POLOSECKI; G.H. RUBIOLO; S. GOYANES

San Carlos de Bariloche, Argentina

Congreso; 13th International Conference on Solid Films and Surfaces; 2006

Chemical Vapor Deposition (CVD) is a widely used technique for the synthesis of carbon nanotubes (CNT). The quality of the product depends on several synthesis parameters: catalysts nanoparticles composition and size, carbon source, pressure, gas flow and temperature during synthesis between others.The value and the potential applications of CNT are function of their morphological characteristics, such as number and crystalline degree of the walls, internal channel diameter, amorphous carbon and impurities content.The aim of the present work is to determine the influence of acetylene (C2H2) pressure on the crystalline and morphological characteristics of MWCNT.The MWCNT were grown on iron nanoparticles included into a silica matrix prepared by sol-gel process. The nanoparticles displayed an average diameter of 6 nm, as determined by TEM. XRD analysis showed the presence of a-Fe and Fe2O3 (maghemite) in the SiO2 matrix.The general growing conditions were: 110 cm3/min flow rate, 690 °C synthesis temperature, 180 torr over pressure. Two gas compositions: 2.5% and 10% C2H2/N2 were studied The products were characterized by HR-TEM, SEM, IR, UV-Vis TGA-DTA and DRXIn all the cases the nanotubes were of the multiple wall type (MWCNT). The morphological and crystalline characteristics were notoriously dependent on the flow composition:i)      10% C2H2/N2: Most of the nanotubes displayed a regular shape, with an external diameter of 25-32 nm and an inner channel of 1.5-3 nm. The nanotubes walls are formed by a 5 nm thick crystalline layer, covered by a 5-10 nm thick amorphous layer. Few nanotubes displayed irregular shape. The inner channels narrowed in some points, until it is completely closed. In its wider point the internal diameter is around 9 nm.ii)     2.5% C2H2/N2: The nanotubes shown an external diameter of 14-22 nm, with a very regular inner channel of 4-7 nm. The walls were mostly crystalline with very low content of amorphous carbon adhere on the external surface. The results obtained by XRD and UV-visible analysis showed the typical pattern/spectrum associated with MWCNT. TGA and DTA analysis results can be explained in terms of the amorphous carbon content and crystalline degree. FTIR analysis showed only features corresponding to the backbone nanotubes.