Electron Hopping Process in SWCNT-Mediated Redox Reaction: An Evidence Observed by DFT Theory
T. Nongnual, S. Choomwattana, S. Nokbin, P. Khongpracha, J. Limtrakul
Laboratory for Computational and Applied Chemistry, Physical Chemistry Division, TH
Keywords: electron hopping process, SWCNT-mediated redox reaction, anthraquinonnyl (AQH2-) group, 4-arylhydroxyl amine (4AHA-) group, generalized gradient approximation (GGA), Perdew-Burke-Ernzerhof (PBE) method, carbon nanotube
Abstract:The electron hopping in SWCNT-mediated redox reaction of anthraquinonnyl (AQH2-) and 4-arylhydroxyl amine (4AHA-) groups is systematically studied by DFT for the first time. The spin-unrestricted DFT calculations were performed with generalized gradient approximation (GGA) as in the Perdew-Burke-Ernzerhof (PBE) functional and an all-electron double numerical basis set with polarized function (DNP). The SWCNT-mediated reaction was proposed earlier to be more favorable than the tunneling process through reagents because the electron transfer distance is shorter. In this work, we then investigate the reaction mechanism of electron hopping involved with the reversible oxidation and irreversible reduction reactions through AQH2-SWCNT-4AHA (substrate), [AQH-SWCNT-4AHA]1- (intermediate-1), [AQ-SWCNT-4AHA]2- (intermediate-2), [AQ-SWCNT-Ph-NH]1- (intermediate-3), and AQ-SWCNT-4AA (product) (AQ = anthraquinonyl, 4AHA = 4-arylhydroxyl amine, 4AA = 4-arylamine, SWCNT = (8,0) zigzag SWCNT). It was found that electrons from the oxidized AQH2 group can transfer to the oxidizing 4AHA group at the other end of the nanotube by the hopping process through the mediating SWCNT. The electron density and Hirshfeld partial charges analysis shows that SWCNT can hold 87% of the extra electron density of the hypothetical negative intermediate forming by the oxidation of AQH2 process. The Hirshfeld charge differences, electron density differences, and Fukui function plots also clearly show the connection of the hopping electrons at the intermediate states. Chemical attachments of these two redox reagents to the SWCNT also caused new impurity states within the band gap, thereby introducing more metallic characteristics to the system. These findings provide a detailed understanding of the electron hopping process and agree well with the previous experimental study. Such phenomenon is unique for both oxidizing and reducing groups confined on the same CNT. This work is not only complementing experimental study by giving a fundamental interpretation but also demonstrating one another promising application of the CNT materials in the nanotechnology field.