Paper NS-ThM7
Growth Promotion and Etching of Carbon Nanotubes by CO₂ in Chemical Vapor Deposition using CH₄ Gas

Thursday, October 18, 2007, 10:00 am, Room 616

Oxidant has been used for growth promotion of carbon nanotubes (CNTs) in CVD since the work by Hata, et al.¹ We report the effect of CO₂ addition to CH₄ gas on CNT growth. The CVD chamber is evacuated down to 10^-6 Torr by a turbo-molecular pump followed by a rotary pump. We used Mo_(0.025)/Fe_(0.05)/MgO_(0.925) or Mo_(0.025)/Ni_(0.05)/MgO_(0.925) as a catalyst, H₂ gas as a reductant and CH₄ gas as a feedstock.² CO₂ gas was introduced during the CNT growth at a temperature of 800°C and its concentration in CH₄ was varied from 0.0037% (37 ppm) to 50%. The other experimental parameters are as follows: H₂ gas pressure = 100 Torr and flow rate = 100 sccm for reduction; CO₂/CH₄ gas pressure = 100 Torr and flow rate = 76 sccm for growth. The CNT yield and the G/D ratio in the Raman spectra of CNTs grown in 10%-CO₂/CH₄ were slightly higher (~86%) than that grown in CH₄ only (~71%). However, CNTs were hardly grown when the CO₂ concentration was more than 20%. Decreasing the CO₂ concentration down to 37 ppm, it was found that single-walled CNTs (SWCNTs) with a yield of ~10% were grown and that the yield for 60 min was 1.5 times higher than that for 10 min. This suggests that the SWCNT growth in a CO₂/CH₄ gas mixture continued for 60 min. The G/D ratio obtained from 37 ppm-CO₂/CH₄ was almost the same as that grown from CH₄ only. The radial breathing mode (RBM) obtained by a 632.8 nm excitation shows that the SWCNT diameter ranges from 0.9 to 1.3 nm and that a few metallic SWCNTs were eliminated but most of semiconducting SWCNTs remained.

¹K. Hata, et al, Science, 306 (2004) 1362-1364
²L.-P. Zhou, et al, J. Phys. Chem. B, 109 (2005) 4439-4447.