A first-principles study of H_2O adsorption and dissociation on the SrTiO_3(100) surface

摘要

We use density functional theory (DFT) to examine the adsorption of H _2O on the SrO- and TiO_2-terminated SrTiO_3(100) surface. At coverages of 0.5 monolayer (ML), we find that water preferentially binds associatively on the TiO2 termination by 0.2eV/H_2O and negligible differences in energy between associated and dissociated H _2O on the SrO termination. These results are in agreement with an earlier hybrid Hartree-Fock-DFT study and the general conclusions from several experimental studies. But at coverages below 3/8 ML, our DFT calculations predict a crossover to preference for dissociative H_2O on the TiO_2 termination. On the SrO termination, the use of larger surface supercells (22) allows for the stabilisation of mixed adsorption configuration at 1/2 ML and subsequent decrease in coverage results in dissociative water configurations. We explored dimer configurations of molecular H_2O and HO-H_2O complexes at 1/4 ML coverage but these configurations are less stable than the isolated dissociated state. The energy barrier for dissociation of H_2O on the TiO_2 termination at 1/8 ML coverage is found to be 0.08eV, which suggests that the dissociated state is both favoured energetically and kinetically at low coverages and temperatures. These low coverage results from our DFT study conflict with existing experimental studies and we present scenarios that can be explored both experimentally and theoretically to resolve this discrepancy.