Photostimulated generation of defects and surface reactions on a series of wide band gap metal-oxide solids

The interconnection between photostimulated formation and destruction of defect centers (F- and V-type color centers) and surface photochemical reactions taking place on the surface has been examined for a series of 21 wide band gap metal-oxide specimens, which comprise insulators and semiconductors whose band-gap energies span the range from ca. 3 eV (ZnO, TiO2) to ca. 11 eV (BeO). Photostimulated post-adsorption of O-2 was seen for 9 of the 21 metal oxides tested. Three of these specimens, namely scandia (Sc2O3), zirconia (ZrO2), and normal spinel (MgAl2O4), were chosen for detailed study to establish that spectral sensitization of metal oxides by UV illumination is a generally occurring phenomenon which is carried over to photostimulated adsorption of molecules and to surface photochemical reactions. Results demonstrate that irradiation into the absorption bands of the color centers of the photocolored metal oxides leads to a red shift of the spectral limit of surface photoreactions. A simple mechanism is described quantitatively for the photocoloration and surface reactions. Quantum yields of photobleaching of color centers in vacuo and in the presence of H-2 and O-2 are reported for scandia and zirconia, together with the quantum yields of photoadsorption of H-2 and O-2 molecules.