Optimisation of gas mixture composition for the preparation of high quality MWCNT by catalytically assisted CVD

The achievement of high yields seems to be one of the most pressing demands for large-scale production of carbon nanotubes (CNT). In this work, a demonstration is given in which in the synthesis of CNT by chemical vapour deposition (CVD) higher yields are attained at the expense of crystalline quality and purity of CNT, thus suggesting that an accurate balance of competing factors is necessary for the optimisation of the preparation process. This is accomplished by comparatively discussing the results of morphologic and structural analysis carried out on multi-walled C nanotubes (MWCNT), prepared at 750 °C by CVD in C2H6-H2 mixture over 20% Fe/SiO2 catalyst, in order to clarify the role of the gas mixture composition on the issue of CNT synthesis. The results of Raman analysis, scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) observations are correlated with yield variations and understood in the light of current assessments on CNT growth. Higher ethane/hydrogen flow ratios result in enhanced carbon deposit with amorphous-phase inclusions. Among the values investigated, a dilution of ethane in hydrogen at 50% may represent the best compromise between yield and quality of reaction products.