Nitrate adsorption by carbon nanotubes in the vacuum and aqueous phase

We performed a computational study (density functional theory, DFT) on the adsorption of the nitrate ion on single-walled carbon nanotubes in both the gas phase and aqueous medium. The nitrate ion adsorption is energetically favorable in the gas phase with a large adsorption energy of -1.30 eV, but it is endothermic in solvent medium. The electronic properties analysis reveals that both solvent and chemisorbed NO3 (-) significantly change the highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) energy gap of the nanotubes (by about 31 %). The present results indicate that the HOMO level of NO3 (-) in solvent is much more stable than that in gas phase. It suggests that this phenomenon in addition to the solvent molecules (covering the ions) effectively prevents the adsorption of the NO3 (-) ion on carbon nanotubes.