Thermodynamic Imbalance, Surface Energy, and Segregation Reveal the True Origin of Nanotube Synthesis

Abstract

Extensive analyses of thermodynamic imbalance, surface energy, and segregation of nanotubes on nanoparticle surfaces are performed. A model for surface energy i developed. In addition, nanotube growth both by vapor-phase and solid-phase mechanisms is described. Segregation of the nanotube species to the periphery of the nanoparticle, the creation of an amorphous shell at this periphery, a droplet created in this shell, and the mediation of this droplet for supersaturation and nucleation of the nanotube species may be the true causes of nanotube growth.

Employing ZnS as a capping material for PbS quantum dots and bulk heterojunction solar cells

“…The underlying ZnS layer facilitates the deposition of PbS with a cation exchange process, while the overlaying ZnS layer tends to fully cover the PbS QDs in light of much smaller surface energy (i.e., 0.65 vs. 2.45 J m −2 for ZnS film [39,40] and PbS nanocrystals [41], respectively). Meanwhile, in such manners the ZnS performs as a bridging layer between two adjacent ensembles of PbS QDs, and dominates the electronic coupling by controlling the deposition cycle and hence the distance between QDs.…”

Unified platform for the chemical reactivity and catalytic potential of catalyst nanoparticles of even very diverse structures and characteristics for nanotube (including carbon nanotube) syntheses

“…There are other problems of the existing models, as well. In an attempt to alleviate these problems, we recently proposed a new model called the shell model [21][22][23][24] for nanotube syntheses. This model demonstrates that the R S species are segregated from the FECA bulk to or near the FECA periphery creating peripheral shell(s) (see Fig.…”

“…The shell dimension, as compared to the bulk dimension, is small and the shell is molten (semimolten) by size-dependent mesoscopic effects 26 even when the bulk is solid, both at the same temperature. 22 Also the shell may be amorphous with the R S species segregated to the FECA periphery (see Fig. 1); its R L content may compose of FECA and X as the primary components, and may be called an FECA/X alloy (for example, FECA/X^Fe 3 C for CNT synthesis) or a solid solution of FECA and X.…”

“…All of these R S species should participate in creating one or more concentric shell(s) at (or near) the FECA surface periphery. 21,22 A single graphene layer of finite size may have many dangling bonds, and these dangling bonds may correspond to high-energy states. 35 Unlike graphite, defect-free CNTs may benefit from a very low concentration of chemically active dangling bonds and it would be materialized by unsaturated dangling bonds, if any, of carbon atoms at the CNT open edges stabilized by shell(s).…”

“…Due to thermodynamic imbalance, even if T nanob is the melting temperature everywhere in a nanoparticle, including the shell, and the temperature in the entire nanoparticle is identical, the shell at (or near) the surface periphery of this nanoparticle senses 22 this temperature to be higher than the temperature of the central core, and acts accordingly. In other words, the same function may be performed 22 at a temperature T nanob in the core, but at a temperature T nano at the shell, even though T nano < T nanob (see eqn (5)). Recall that thermodynamic imbalance leads to fluctuations 22 and yields structural disturbance and disorder giving rise to porosity.…”