A Study of Adsorption Behavior of Single Water Molecule on the Surface of Polyhedral Oligomeric Silsesquioxanes

Abstract

The adsorption of a water molecule on the surface of polyhedral oligomeric silsesquioxanes (POSS, (HSiO3/2) n , n = 4, 6, 8, 10 and 12) with cage structures was studied using the density functional tight-binding method with the inclusion of an empirical dispersion term in total energy. The results revealed that water molecule is favor to adsorb on the ring surfaces which are formed by Si–O–Si bonds of various cage structures. With the increase of the ring size in the same cage, both the adsorption energy and charge transfer increase, while the hydrogen bond length decreases. For all the cage structures, the presence of a single water molecule causes only negligible charge transfer (no more than 0.01 e) from the cages to the water molecule, indicating that the adsorption of a single water molecule has no significant effects on the electronic properties of POSS. Comparing to the large energy gaps (about 8.27–8.63 eV) between the highest occupied molecular orbital and the lowest unoccupied molecular orbital, the water adsorption can only cause slight reductions (within 0.15 eV) in the energy gap. Hence, the energy gap of the systems are primarily determined by the characteristics of POSS, instead of the energy transfer.