Reactivity of organogermanes with ZnO substrate

Abstract

Germanium (Ge)-based thin film through sol-gel coating technology can be a suitable alternative like their silane analogue to develop metal-free and solvent-free corrosion protective coating with additional functional properties for galvanized (GI) steel. In this work, adsorption mechanism and reactivity of functionalized (by functional groups such as epoxy, amine, and mercapto) and non-functionalized - organogermane alkoxides towards GI steel substrate were investigated using density functional theory (DFT), where (ZnO)12 cluster was adopted as model of GI steel and other ZnO substrate. Calculated global reactivity parameters of organogermanes suggested that functionalized organogermanes were more efficient for corrosion protection of ZnO substrate. Adsorption process was exothermic and free energy change (ΔG) in adsorption process lied between 27.4 kcal/mol and 51.9 kcal/mol which confirmed adsorption potentiality of organogermanes. Adsorptions of functionalized organogermanes were much more exothermic (∼50 kcal/mol) than that of non-functionalized one (∼27 kcal/mol) which suggested chemisorption of functional organogermane molecules. Geometry, MOs, and charge analyses of adsorbed structures further supported that all functional organogermanes chemisorbed on the GI steel. Multiple bonds were formed at the interface. Presence of heteroatoms, like O/N/S in organogermane molecules assisted in formations of chemical bonds with Zn atoms of the substrate. Ge center took hypervalency and formed non-classical Ge-O bond with O atom of the substrate in all functional organogermanes. Formation of OH—O hydrogen bonds were also observed at the interface. These Ge-O and OH—O bond formation played key roles in the interface and manifested stronger interaction, making functional organogermane alkoxides suitable for coating applications. Stronger adsorptions of functional organogermanes could be interpreted in terms of higher highest occupied molecular orbital (HOMO) energies and lower HOMO-lowest occupied molecular orbital (LUMO) gap values for functionalized organogermanes which favored the electron donation from the HOMOs of functionalized organogermanes to the LUMOs of (ZnO)12 cluster.