ONIOM DFT study of the adsorption of cytosine on the Au/Ag and Ag/Au bimetallic nanosurfaces: The effect of sublayer

In this work, the adsorption of cytosine (CYT) on the Au/Ag and Ag/Au bimetallic nanosurfaces has been studied using density functional theory (DFT) in the scheme of the "our own n-layered integrated molecular orbital and molecular mechanics" (ONIOM) and compared with its adsorption on pure Au and Ag nanosurfaces, respectively. It was found the arrangement of Au and Ag layers on each other in the nanosurface has considerable effect on the adsorption energy (E-ad), adsorption mode and the charge transfer between the CYT and nanosurface. The presence of Ag as sublayer in the Au/Ag nanosurface decreases the E-ad of the CYT, increases the vertical distance of the CYT from the surface and changes its tilting direction from its oxygen atom to nitrogen atom of ring C=N compared to the adsorption of the CYT on pure Au nanosurface. The opposite trend has been seen for the adsorption of the CYT on the Ag/Au nanosurface which Au is as sublayer compared to the adsorption of the CYT on pure Ag surface. The tilting of the CYT from its oxygen atom on the Ag/Au nanosurface increases compared to that on pure Ag surface. Also, it has been shown that the Ag sublayers increase the charge transfer from the CYT to the Au/Ag nanosurface while the Au sublayers decrease the charge transfer from the CYT to the Ag/Au nanosurface, significantly. The quantum theory of atoms in molecules (QTAIM) has been also used to determine the interatomic interaction lines between the CYT and nanosurfaces to determine which atoms of the CYT and first layer of nanosurfaces are in local interactions with each other. The interaction energies of the CYT with the selected nanosurfaces have also been calculated at the Hartree-Eock (HF) level of theory and compared with those obtained using the DFT method to understand the reason of the change of E-int due to the sublayers.