First Principle Calculation of Polar and Nonpolar Molecule Adsorption on ZnO (0001) and (10(1)over-bar0) Surface

ZnO is one of the most effective metal oxide semiconductors for chemical sensors. In this study, the absorption behaviors of various small molecules on the polar (0001) and nonpolar (10 (1) over bar0) ZnO surfaces have been systematically studied based on first-principle calculation, which include polar molecules of ethanol and methanol, and nonpolar molecules of oxygen and carbon dioxide. It demonstrates that ethanol and methanol are easier to be adsorbed on nonpolar ZnO surface. This because the hydroxyl in ethanol and methanol which interact with both Zn and O atoms in the surface. The O atom in the hydroxyl forms a chemical bond with Zn on the nonpolar ZnO surface, and the H atom in the hydroxyl forms a hydrogen bond with O atom. While for the polar surface, only O atoms which come from adsorbed molecules interact with Zn atoms on the top layer. The adsorption energy on polar ZnO surface is higher than that on nonpolar surface for nonpolar molecule oxygen and carbon dioxide. The calculation by Milliken population reveals that all charges transfer from ZnO surface to molecules except ethanol and methanol adsorbed on ZnO nonpolar surface, which is on the contrary. The analysis of density state shows that the charge transfers mainly originate from Zn 3p and O 2p orbit.