Diameter dependence of oxidative stability in multiwalled carbon nanotubes: Role of defects and effect of vacuum annealing

Abstract

While the oxidative stability of single walled carbon nanotubes has been studied extensively, very little is known about the diameter dependence of oxidative stability in multiwalled carbon nanotubes (MWNTs) and the effect of vacuum annealing on such stability. We have investigated six sets of samples with different diameters in the range of 5–100 nm systematically by thermogravimetric analysis (TGA) before and after vacuum annealing. High resolution transmission electron microscopy studies provide evidence for structural defects in the nanotube walls. Further, it reveals that with increasing diameter, interlayer d-spacing between concentric tubes decreases. Experimental TGA profile is found to constitute multiple components of oxidation as revealed from reverse Sigmoidal fitting. Analysis of the TGA profile shows that the oxidation temperature follows an exponential recovery function with increasing diameter, while width of the differentiated TGA spectra decreases. It is shown that oxidative stability of lower diameter MWNTs are primarily controlled by lattice strain, while that of large diameter MWNTs is decided by the defects in the nanotube wall. High vacuum annealing studies in the temperature range 300–1000 °C show major improvement in the structure and oxidative stability for MWNTs annealed at 500 °C. Results are compared with those of the single walled nanotubes. The observed diameter dependence is explained on the basis of strain in the nanotube lattice and defects in the nanotube walls.

Dry reforming of methane over CeO2 supported Ni, Co and Ni–Co catalysts

“…In order to reveal the morphology of deposited carbon and especially for the one that oxidized at 470°C, TEM images could be used ( Figure 12). The differences observed in the oxidation temperature of carbon nanotubes depends on several parameters such as diameter of the tubes [66][67], length of the tubes [68], percentage of metal loading used in the production of the nanotube [69][70]. The diversity of the deposited carbon structures in terms morphology (straight long filaments, highly entangled and curly shaped filaments, filaments with knuckle-like structure and carbon onions) can be another reason for the presence of two distinctive peaks in the derivative weight loss of spent Ni-Co/CeO 2 -700.…”

Laser irradiation of carbon nanotube films: Effects and heat dissipation probed by Raman spectroscopy

“…This ratio is usually considered as an indication of the degree of order of the CNTs. 4,5,8,19,20,24,25 Indeed, besides intrinsic defects such pentagon-based structures that are formed at the CNTs caps, as-grown and purified CNTs usually present other sp 2 defects such as distorted hexagons and 5-7 carbon rings that contribute to the D-band. However, the D-band can also arise from disordered and amorphous carbon materials that are typically observed in CNT samples in the form of particles and amorphous layers externally coating the tubes.…”

“…Hightemperature annealing in vacuum has been used as a method to improve the purity and the structural quality of CNTs. [4][5][6] It has also been shown that thermal treatments in air can be used to remove amorphous carbon contaminants by selective oxidation. [7][8][9][10][11] The resistance against oxidation increases with the degree of graphitization.…”

An efficient method for the carboxylation of few-wall carbon nanotubes with little damage to their sidewalls

“…There are several proposed methods to recover the oxidation activation energy from non-isothermal thermograms [45,46,49,51,52]. Chang's method [52] recovers DE from the slope of ln(q$da/dT/(1 À a) n ) vs. (1/T) plot, where the conversion degree is defined as a ¼ (m 0 À m t )/(m 0 À m N ), q is the heating rate and…”