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High-resolution photoemission spectroscopy and scanning tunneling microscopy (STM) have been used to investigate defects on Cu 2 O(111) and their interaction with water and sulfur dioxide (SO 2 ). Two types of point defects, i.e., oxygen and copper vacancies, are identified. Copper vacancies are believed to be the most important defects in both water and SO 2 surface chemistry. Multiply coordinatively unsaturated oxygen anions (O MCUS ) such as oxygen anions adjacent to copper vacancies are believed to be adsorption sites for both water and SO 2 reaction products. Water adsorption at 150 K results in both molecular and dissociated water. Molecular water leaves the surface at 180 K. At 300 K and even more at 150 K, SO 2 interacts with oxygen sites at the surface forming SO 3 species. However, thermal treatment up to 280 K of Cu 2 O(111)/SO 2 prepared at 150 K renders only SO 4 on the surface.

We present room temperature scanning tunneling microscopy and photoemission spectroscopy studies of water adsorption on the Zn-terminated ZnO(0001) surface. Data indicates that the initial adsorption is dissociative leaving hydroxyl groups on the surface. At low water coverage, the adsorption occurs next to the oxygen-terminated step edges, where water is believed to bind to zinc cations leaving off hydrogen atoms to under-coordinated oxygen anions. When increasing the water dose, triangular terraces grow in size and pits diminish until the surface is covered with wide irregular terraces and a large number of small pits. Higher water exposure (20 Langmuir) results in a much more irregular surface. Hydrogen, which is produced in the dissociation reaction is believed to have an important role in the changed surface structure at high exposures. The fact that adsorbed water completely changes the structure of ZnO (0001) is an important finding toward the understanding of this surface at atmospheric conditions.