ZnO(10(1)over-bar0) Surface Hydroxylation under Ambient Water Vapor

The interaction of water vapor with a single crystal ZnO(10 (1) over bar0) surface was investigated using synchrotron-based ambient pressure X-ray photoelectron spectroscopy (APXPS). Two isobaric experiments were performed at 0.3 and 0.07 Torr water vapor pressure at sample temperatures ranging from 750 to 295 K up to a maximum of 2% relative humidity (RH). Below 10(-4) % RH the ZnO(10 (1) over bar0) interface is covered with similar to 0.25 monolayers of OH groups attributed to dissociation at nonstoichiometric defect sites. At similar to 10(-4) % RH there is a sharp onset in increased surface hydroxylation attributed to reaction at stoichiometric terrace sites. The surface saturates with an OH monolayer similar to 0.26 nm thick and occurs in the absence of any observable molecularly bound water, suggesting the formation of a 1 x 1 dissociated monolayer structure. This is in stark contrast to ultrahigh vacuum experiments and molecular simulations that show the optimum structure is a 2 x 1 partially dissociated H2O/OH monolayer. The sharp onset to terrace site hydroxylation at similar to 10(-4) % RH for ZnO(10 (1) over bar0) contrasts with APXPS observations for MgO(100) which show a sharp onset at 10(-2) % RH. A surface thermodynamic analysis reveals that this shift to lower RH for ZnO(10 (1) over bar0) compared to MgO(100) is due to a more favorable Gibbs free energy for terrace site hydroxylation.