Interaction between chloride ions mediated by carbon nanotubes: a chemical attraction
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ORIGINAL PAPER
Interaction between chloride ions mediated by carbon nanotubes: a chemical attraction Fabiola Dominguez-Flores1 · Elizabeth Santos1 · Wolfgang Schmickler1
· Fernanda Juarez1
Received: 11 July 2020 / Revised: 6 August 2020 / Accepted: 7 August 2020 © The Author(s) 2020
Abstract The interaction between two Cl− ions separated by the wall of a narrow carbon nanotube has been investigated by density functional theory (DFT) and by DFT-based tight binding (DFTB+). The direct Coulomb interaction between the ions is screened by the nanotube, no matter if the latter is conducting or semiconducting. The presence of the ions induces changes in the electronic density of states of the nanotube, which results in an effective attraction between the ions of the order of 0.2–0.3 eV. The interaction of the outside ions with the tube has a covalent component, when the two ions are near there is even a direct chemical attraction between the ions. In contrast to the effective attraction between two Li+ ions reported before (Juarez et al., Phys Chem Chem Phys 22:10,603, 2020), the effect cannot be explained in terms of physical concepts alone. DFTB+ performs well when compared with DFT, and lends itself to fast calculations for large systems.
Introduction Carbon is the atom of life and of organic chemistry with its myriads of compounds. Even the elemental forms of carbon have intriguing properties: There are the extremely hard but metastable sp3 bonded diamond, and a variety of sp2 bonded forms, where one p electron forms delocalized π bonds. The latter materials are characterized by a low electronic density of states (DOS) at the Fermi level, or in the case of semiconducting carbon nanotubes (CNTs) by a small band gap of the order of 1 eV [2, 3]. Therefore, their electronic properties are easily modified by doping, by adsorption, even by an external electric field. In particular, doping can convert graphene and CNTs, which in their pure form are not very reactive, to catalysts for a variety of reactions such as oxygen reduction or hydrogen evolution [4, 5]. In our group, we have been concerned with the interaction of CNTs with ions. Triggered by the observation of anomalously high capacitances of CNTs [6–8], and the
Wolfgang Schmickler
[email protected] Fernanda Juarez
[email protected] 1
Institute of Theoretical Chemistry, Ulm University, D-89069, Ulm, Germany
concept of a superionic state [9], we have investigated the insertion of ions into narrow tubes and their resulting effects such as the formation of image charges on the walls of the tubes, and the shift of the Fermi level, which can locally convert semiconducting into conducting tubes [10–12]. Recently, we discovered an unexpected effect: The virtual attraction between two lithium ions separated by the wall of a nanotube, which we explained by the shift in the Fermi level of the tube and the concomitant work required—the direct Coulomb interaction between the ions is negligible since it is screened by the tube [14]. Perhaps less
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