Interfacial energy transfer during gamma radiolysis of water on the surface of ZrO2 and some other oxides

The effect of an oxide interface on Co-60 gamma radiolysis of water molecules was studied. On the basis of the molecular hydrogen yield when compared with the radiolysis of the control ampules without oxides, all the tested materials can be generally classified into three groups: I. Oxides that lower the H-2 yield (MnO2, Co3O4, CuO, and Fe2O3); II. Oxides with H-2 yields that are close to C values obtained in control experiments (MgO, CaO, SrO, BaO, ZnO, CdO, Cu2O, NiO, Cr2O3, Al2O3, CeO2, SiO2, TiO2, Nb2O5 and WO3); and III. Oxides that can increase the H-2 yield as compared with the radiolysis of water without oxides (Ga2O3, Y2O3, La2O3, Nd2O3, Sm2O3, Eu2O3, Gd2O3, Yb2O3, Er2O3, HfO2, and ZrO2). For the third group, the H-2 yield can be much greater than for the homogeneous process due to effective energy transfer at the oxide/water interface. There are several parameters, such as the oxide band-gap, water adsorption form, and energy migration distance that can collectively contribute to enhanced radiolysis yields at interfaces. We present data of effective H-2 yield vs band-gap (Eg) energy that show a resonant maximum at Eg = 5 eV. This maximum corresponds to the energy of H-OH bond in the water molecule (5.1 eV). By varying the ZrO2 specific area within a 0.02-12 m(2) g(-1) range, we also demonstrated that the H-2 yield per unit area of the crystal surface is constant. Doping the ZrO2 bulk with Nb5+ and Li+ ions has a remarkable effect on the H-2 yield from the surface. We estimated the energy migration distance to be similar to5 nm. Thermostimulated luminescence was used to probe the nature of the migrating electronic excitations in ZrO2. Finally, we propose a mechanism for the adsorbed water radiolysis based on the migration of excitons to the surface and their resonant coupling with the H2O/oxide adsorption complex.