Molecular dynamics simulations of the adsorption of industrial relevant silane molecules at a zinc oxide surface

Abstract

The physical behavior of different adsorbed silane molecules (octyltrihydroxysilane, aminopropyltrihydroxysilane, and thiolpropyltrihydroxysilane) at a ZnO surface (0001̄) dissolved in isopropanol are studied via constant temperature (298 K) molecular dynamics simulations. The adsorbed silane molecules exhibit a different behavior depending on the chemical nature of their tail. Octyltrihydroxysilane molecules with their rather unpolar tail show two distinct, energetic different orientations at the polar metal oxide surface. Mostly the three polar hydroxy groups of the head are in contact with ZnO the unpolar tail remaining in the isopropanol phase. Occasionally only two hydroxy groups interact with the surface the whole tail simultaneously being attached. On the contrary, due to their highly polar tail aminopropyltrihydroxysilane molecules have only one favorite orientation at the surface: Apart from some minor fluctuations two hydroxy groups as well as the amino group of the tail are in contact with the surface. The behavior of the thiolpropyltrihydroxysilane molecules is somehow located in between—the thiol group is not polar enough to exhibit such a strong attraction to the polar surface to force permanent contact. Therefore, the molecules show—like octyltrihydroxysilane—two different orientations though the one similar to that of aminopropyltrihydroxysilane is slightly preferred. From adsorption energies of preferred orientations it follows that the strength of adsorption decreases from aminopropyltrihydroxysilane over thiolpropyltrihydroxysilane to octyltrihydroxysilane.