Nontronites as catalyst for synthesis of carbon nanotubes by catalytic chemical vapor deposition.

Abstract Multiwall carbon nanotubes (MWCNT) were synthesized by catalytic chemical vapor deposition (CCVD) in a horizontal tube reactor using acetylene as the carbon source in a gas mixture with argon at 700 °C and 30 minute reaction time. As a catalyst was used the sodium forms of natural platy nontronite particles Sampor or Washington and their iron modified forms. Additionally, hydrothermally synthesized hematite α-Fe 2 O 3 and/or its heterocoagulates with nontronite particles were used to test the catalytic activity. Catalyst nanoparticles were used to modify the conditioned Si wafer surface used as the catalyst support. Before CNT growth the catalyst nanoparticles were activated by applying a hydrogen stream in the tube reactor at 700 °C. The effects of catalyst type and the reaction conditions on MWCNT growth such as C 2 H 2 /Ar ratio, time and reaction temperature were investigated. The growth of MWCNT was affected by the density of catalyst particles covering the surface e.g. for hematite, the amount deposited on a silicon surface. Depending on the type of catalyst located on the Si substrate, the bamboo-like, curly shaped and straight individual MWCNT were formed. The quality of the synthesized MWCNT was investigated using Raman spectroscopy. According to the cation exchange to Fe-forms, the iron content in nontronites was increased by about 14.5 wt.%. However, by the addition of hematite particles, the iron content was increased by about 13.0 wt.% of the total iron present. Raman spectroscopy has proved that good quality ordered graphite structure of the carbon sheets in CNT was also achieved by using pure Na-forms of natural nontronites applicable as the low-cost catalyst nanoparticles. Therefore, no ion exchange modification using iron salt is necessary for this type of material.