H-2 cracking at SiO2 defect centers

The interaction of H-2 With the defect sites of the SiO2 surface has been s tudied by means of gradient-corrected density functional theory calculation s on cluster models. The mechanism of hydrogen dissociation, the energy of reactants and products, and the corresponding activation energies and trans ition states have been determined for the following defect sites: Si singly occupied sp(3) dangling bonds (E' centers), =Si-.; nonbridging oxygen cent ers (NBO), =Si-O-.; divalent Si, =Si:; and neutral oxygen vacancies, =Si-Si =. H-2 cracking on the NBO sites is exothermic by similar to 0.4 eV and has an energy barrier of similar to 0.1 eV (or less considering nonadiabatic e ffects) which suggest the occurrence of the process even at low temperature . On Si dangling bonds the formation of =Si-H and neutral H atom is endothe rmic and occurs with an activation energy of less than 0.5 eV; the reaction can occur at room temperature. The interaction of molecular hydrogen with the diamagnetic oxygen deficient centers, =Si: and =Si-Si=, leads to the fo rmation of stable =Si-H groups with exothermic processes and relatively hig h activation energies of about 2 eV. Thus, H-2 cracking is predicted to occ ur at room temperature on paramagnetic defects and only at high temperature s on the diamagnetic centers.