Abstract
Ir and Co catalysts are effective to obtain vertically aligned single-walled carbon nanotubes (VA-SWCNTs), but the growth condition of grown SWCNTs are much different; In alcohol catalytic chemical vapor deposition (ACCVD), VA-SWCNTs were grown from Co catalysts on Al2O3 support layers in the wide temperature range from 700 to 850°C [1], while VA-SWCNTs were obtained above 800°C with Ir catalysts on SiO2 support layers [2]. In addition, SWCNTs grown from Ir catalysts have diameters less than ~1 nm, which were much smaller than those from Co catalysts. This suggests that the reaction process between ethanol and catalysts is different between the two catalysts. To clarify it, we investigated growth process of CNTs by ACCVD from Ir catalysts on SiO2 and Co catalysts on Al2O3 by using in situ diffuse reflectance infrared Fourier Transform (DRIFT) measurements and in situ X-ray absorption fine structure (XAFS) spectroscopy. In situ DRIFT investigation showed that intermediate species formed on the catalysts during CNT growth were different between the two catalysts; formation of ethoxy was seen on Ir catalysts, while acetates were observed on Co catalysts. In addition, the amount of species adsorbed on Co catalysts were much larger than those on Ir catalysts. In situ XAFS result showed that Ir catalysts were kept metallic during SWCNT growth, while Co catalysts were partially carbonized (Fig. 1). From these results, we proposed a growth process model of SWCNTs from Ir and Co catalysts. We also performed ultra-violet photoelectron spectroscopy (UPS) to investigate the d band center of Ir 5d and Co 3d levels of catalyst particles. Our results showed that the d band center of Co 3d level is 0.8 eV lower than that of Ir 5d level. The lower binding energy of the d band center of Co catalysts would lead to high efficiency in feedstock dissociation, resulting in the wide temperature range in SWCNT growth.
[1] K. Cui et al. Nanoscale 8 (2016) 1608-1617.
[2] T. Maruyama et al. Appl. Surf. Sci. 509 (2020) 145340.
