Can uranyl complexes encapsulate to carbon nanotubes? A periodic DFT study

Abstract

Periodic density functional theory (DFT)-based calculations were carried out on a series of uranyl complexes encapsulated within single walled (SW)-CNT to understand their encapsulation affinities. We find that uranyl-aqua complex $$([\hbox {UO}_{2}(\hbox {H}_{2}\hbox {O})_{5}]^{2+}$$ ) binds stronger as compared to uranyl-hydroxo-complex $$([\hbox {UO}_{2}(\hbox {OH})_{4}]^{2-}$$ ) due to the variable overall charge of the complex. Further, binding affinities of uranyl formate complexes with different formate stoichiometries (1:1, 1:2 and 1:3) with SW-CNT are considered. Here again, due to variable charges, cationic mono-‘formate-uranyl $$([\hbox {UO}_{2}(\hbox {FM})(\hbox {H}_{2}\hbox {O})_{3}]^{1+})$$ complex binds stronger as compared to anionic tri-formate uranyl $$([\hbox {UO}_{2}(\hbox {FM})_{3}]^{1-})$$ . Further, due to the very weak binding commonly found in $$[\hbox {UO}_{2}(\hbox {FM})_{3}]^{1-}$$ to CNT, the tubular ends of SW-CNT are sealed with functionalized $$\hbox {C}_{36}$$ fullerene. The binding affinity of uranyl complex is not improved when $$\hbox {C}_{36}$$ fullerene is used to seal to the SW-CNT as compared to its unsealed counterpart. However, upon functionalizing (at the hub carbon) the $$\hbox {C}_{36}$$ cork, the binding affinity of $$[\hbox {UO}_{2}(\hbox {FM})_{3}]^{1-}$$ is larger inside the CNT due to favorable hydrogen bonding interactions with the uranyl oxygens. Our findings are consistent with the experimental observations which will help to design novel nanomaterials for nuclear waste management processes. The use of functionalized fullerene as removable corks which can seal single walled carbon nanotubes has the ability to bind uranyl complexes very efficiently is revealed through density functional theory based calculations.