Violating Coulomb's Law: an appartent attraction between Li+ ions mediated by carbon nanotubes

JUAREZ, FERNANDA; SANTOS, E.; SCHMICKLER, W.; MOHAMMADZADEH, LEILA; QUAINO, PAOLA; DOMINGUEZ-FLORES, FABIOLA; GODULJAN, ALEKSEJ

Durban

Congreso; The 70th Annual Meeting of the International Society of Electrochemistry; 2019

ISE

Normally ions with the same charge repel each other, but in very rare cases an intervening medium canproduce an apparent attraction, superconductivity and the formation of Cooper pairs being the bestexample [1]. We have studied the adsorption of Li-ions on semiconducting (8,0) carbon nanotubes (CNT)by density functional theory, one ion being inside, the other outside. The tube shields the direct Coulombinteraction between the ions, but the presence of the ion inside facilitates the adsorption of the ion outside,thus producing an apparent attraction.We repeated the calculations for the conducting (5,5) CNT. The attraction persists, but it isweaker than on the (8,0) CNT. In addition, the magnitude of the effect depends on the position of the Li-ion inside.We offer a semi-quantitative explanation based on the unique band structure of CNTs /HW¶Vconsider the case of the semiconducting (8,0) tube [2]. The presence of the Li-ion inside induces apolarization or image charge, which makes the tube locally conductive ± see picture. This transformationcosts a certain amount of energy. When the second ion is adsorbed in the vicinity, the tube is alreadyconducting, and the energy required for the second polarization charge is lower. This argument can bysupported by explicit formulae. In the case of the already conducting (5,5) charge the adsorption of thefirst ion enhances the electronic density of state at the Fermi level, which also favors the adsorption of thesecond ion. But since the tube is already conducting, the effect is smaller.The adsorption of halide ions inside or outside the tube is complicated by the formation of achemical bond. Our first results suggest, that a Li-ion inside the tube enhances the adsorption of a Cl-ionoutside.