Multi-Walled Carbon Nanotubes Synthesized by Methane Pyrolysis: Structure and Magnetic Properties

Abstract

The structure and magnetic properties of multi-walled carbon nanotubes produced by catalytic pyrolysis of methane have been investigated by means of mutually complementary physical methods. The average sizes and number of carbon layers forming nanotubes, smearing of the density of states near the Fermi level, degeneracy temperature of gas of extrinsic current carriers, concentrations of localized spins and extrinsic two-dimensional current carriers have been determined. The conclusion has been drawn that ferromagnetic nanoparticles are present in the inner regions of nanotubes, including their tubular cavities. The difference in electronic structure near the Fermi level for carbon nanotubes and ordered graphite has been revealed. The possible reason is that the electronic states near zigzag-type sites of ends as well as edges of linear structural defects in nanotubes make greater contribution to the spectrum than that from similar sites of graphite.

Molecular Reactive Force-Field Simulations on the Carbon Nanocavities from Methane Pyrolysis

“…As pointed out above, six ReaxFF MD simulations were first conducted separately on both CH 4 cells (their d are of 0.01 and 0.1 g/cm 3 , respectively) with 1200 CH 4 molecules at 2500, 3000, and 3500 K, and subsequently on three cells of 240, 600, and 2400 CH 4 with a density of 0.1 g/cm 3 at 3500 K. Because the subsequent simulations were carried out to clarify the size effect, the related results will only be discussed later in Section 3.5.…”

“…In addition, we compare snapshots in Figures S1−S5 of the SI with those in Figure 2 and find a similar evolution mechanism exhibited in all six simulations, only with a difference in reaction rate. That is, to more comprehensively and deeply understand the underlying mechanism of CNC formation, we mainly pay attention to the results corresponding to the highest d of 0.1 g/cm 3 and the highest temperature of 3500 K, unless otherwise specified.…”

“…Two factors including structure and condition were concerned for simulating CH 4 pyrolysis. In our simulations, first, 1200 CH 4 molecules were included in two cubic cells with lengths of 68.38 and 147.3 Å, corresponding to densities (d) of 0.1 and 0.01 g/cm 3 , respectively. Relative to the density at standard state of 0.0007 g/cm 3 , these CH 4 molecules in the cell of 0.1 g/ cm 3 were significantly compressed to a d increased ∼143 times.…”

“…In our simulations, first, 1200 CH 4 molecules were included in two cubic cells with lengths of 68.38 and 147.3 Å, corresponding to densities (d) of 0.1 and 0.01 g/cm 3 , respectively. Relative to the density at standard state of 0.0007 g/cm 3 , these CH 4 molecules in the cell of 0.1 g/ cm 3 were significantly compressed to a d increased ∼143 times. This compression that causes a higher pressure and a higher CH 4 concentration will increase reaction rates and shorten equilibrium time at the same temperature, according to chemical kinetic theory.…”

“…With respect to the simulations on pure CH 4 pyrolysis, only one study by molecular reactive force field (ReaxFF) MD simulations was reported according to our knowledge. The simulations were conducted on a system of 150 CH 4 molecules for 1 ns with various temperatures of 2500 to 3500 K (in 250 K intervals) and densities of 0.05, 0.1, and 0.2 g/cm 3 . As a result, H 2 , an extensive range of hydrocarbons, and C dimers were observed in the simulations, and the basic reactions agreed with existing models of thermal CH 4 decomposition.…”