In recent years, the interaction between single-walled carbon nanotubes (SWNTs) and small particles has raised great attention in the areas of controllable growth of SWNTs, gas sensing as well as the functionalization of SWNTs. Thus it becomes one of hot topics for both experimental and theoretical research on carbon nanotubes. However, the mechanisms of sulfur-containing promoter and SWNT sensors for sulfur oxide molecules (SO2/SO3) are still not clear. In this work, we studied the interactions between sulfur atom, SO2/SO3 and SWNTs based on first-principles method and put forward possible mechanisms to explain the phenomenon observed in experiments.
Firstly, we studied the changes of binding energy, bond length and charge transfer when sulfur atoms are adsorbed on the pristine or defective SWNTs. We obtained the following results: (1) The adsorption of sulfur atoms on the caps and sidewalls of SWNTs are not stable and therefore the weak interaction between sulfur atoms and SWNTs is not directly affect the growth process of SWNTs. (2) The adsorption energy of sulfur atoms on Stone-Wales defects is still not large enough to form stable chemical bond to affect the growth of SWNTs. However, the adsorption energy and charge transfer on 5-1DB defects are notably larger than that on the caps and sidewalls of SWNTs. Detailed NBO analysis shows that the adsorbed sulfur atoms stabilize single vacancy defects because of the formation of S-C bonds. On one hand, sulfur atoms can give rise to relatively unstable sites on sidewall of SWNTs and may cause the formation of Y-shaped SWNTs. On the other hand, sulfur atoms impede the self-healing process of SWNTs by occupying the positions of carbon atoms, and reduce the growth of SWNTs with small diameters, which often contain many 5-1DB defects.
Secondly, we studied the interactions between SWNTs and SO2/SO3, including stable configurations, adsorption energy and charge transfer. The following results were obtained: (1) The interaction between SO2 molecules and SWNTs is too weak to be responsible for the high sensitivity of SWNTs to SO2 observed in experiments. Some adsorption configurations of SO3 molecules can form automatically at room temperature, which indicates that SWNTs can be used as sensors for detecting sulfur-containing gas. However, the interaction mechanism of these configurations can not explain the high sensitivity in experiments. (2) Nitrogen doping can introduce extra electrons into the network of SWNTs, change the stable C-C π conjugated structure on the surface, reduce the non-planar distortion during the interaction and enhance the chemical activity of neighboring carbon atoms, all of which can promote the interaction intensity between SWNTs and SO2 /SO3 molecules and thus help to improve the performance of SWNTs as gas sensor for SO2/SO3.
In summary, our results deepen the understanding of interaction between sulfur atoms, SO2 /SO3 and SWNTs, which provides meaningful guidance for the controllable synthesis of SWNTs using sulfur-containing promoter and application of SWNTs as sensors for gas molecules containing sulfur.
修改评论