Effect of synthesis conditions and sequential post-treatments on morphology, thermal behaviour, and gas adsorption properties of self-aligned multi-wall carbon nanotubes assemblies

D. ZILLI; P. R. BONELLI; A. L. CUKIERMAN

Nanotechnology Research: New Nanostructures, Nanotubes and Nanofibers

NOVA SCIENCE PUBLISHERS INC.

Lugar: NEW YORK, USA; Año: 2008; p. 195 - 219

The present chapter deals with the synthesis, purification, and characterization of self-aligned multi-wall carbon nanotubes (MWCNTs) assemblies obtained via in situ chemical vapour deposition of a cobalt-based organometallic compound. The effect of key process variables and of a sequence of post-treatments applied for purification, on morphology, thermal behaviour, and N2 adsorption capacity of the MWCNTs assemblies are examined. Increasing the reaction temperature in the range 850-1100 C leads to reduce density and degree of alignment of the assemblies structures, affecting dimensions of the constituent individual carbon nanotubes (CNTs). For otherwise constant conditions, higher temperatures promote growth of longer CNTs with larger inner and outer diameters. Increases in nanotubes average length appear to respond to a thermal activation process governing CNTs growth rate, with an estimated activation energy that points to bulk diffusion rate of C atoms in bulk Co as a main controlling factor. Possible enhancements in agglomeration of metallic nanoparticles, occasioned by their greater mobility, and in cleavage rates of C fragments from the precursor may explain the trend for the inner and outer diameters with temperature rise, respectively. In turn, lowering the total gas flow rate promotes CNTs of larger diameters, likely because of an excess of metallic and carbon species caused by the longer residence time, that could favour agglomeration of Co nanoparticles and non-graphitic carbon deposition. Thermal analysis performed in oxidation conditions, from room temperature up to 1000 C, for the pristine assemblies shows that growing reaction temperatures lead to enhance their thermal stability. The opposite is found for those obtained with a lower gas flow rate, the results also indicating a greater presence of Co nanoparticles. Besides, as determined from N2 (- 196 C) adsorption isotherms, increasing the synthesis temperature renders MWCNTs assemblies with lower BET surface Area and total pore volume because of the progressively larger interstitial spaces between neighbouring nanotubes, which contribute predominantly to N2 adsorption for assemblies built of relatively large inner diameter CNTs as the present ones. Sequential post-treatments of a batch of MWCNTs synthesized for conditions deliberately selected to emphasize impurities mainly include oxidation with O2 (10% v/v) at 375 C, acid treatment of the oxidized assemblies, followed by dispersion in ethanol and further ultrasonication. As judged from TEM images and thermal analysis, the sequence of post-treatments applied to the raw assemblies is effective for removal of cobalt nanoparticles and C based impurities, without extensively damaging the nanotubes and keeping their dimensions almost unchanged. It also leads to improve thermal stability and N2 adsorption capacity of the MWCNTs assemblies. The latter is mainly attributed to the additional contribution of CNTs endohedral spaces as a result of impurities oxidation.adsorption isotherms, increasing the synthesis temperature renders MWCNTs assemblies with lower BET surface Area and total pore volume because of the progressively larger interstitial spaces between neighbouring nanotubes, which contribute predominantly to N2 adsorption for assemblies built of relatively large inner diameter CNTs as the present ones. Sequential post-treatments of a batch of MWCNTs synthesized for conditions deliberately selected to emphasize impurities mainly include oxidation with O2 (10% v/v) at 375 C, acid treatment of the oxidized assemblies, followed by dispersion in ethanol and further ultrasonication. As judged from TEM images and thermal analysis, the sequence of post-treatments applied to the raw assemblies is effective for removal of cobalt nanoparticles and C based impurities, without extensively damaging the nanotubes and keeping their dimensions almost unchanged. It also leads to improve thermal stability and N2 adsorption capacity of the MWCNTs assemblies. The latter is mainly attributed to the additional contribution of CNTs endohedral spaces as a result of impurities oxidation.