Chemisorption of ${\mathrm{H}}_2$O on the surface of ${\mathrm{Ti}}_2$${\mathrm{O}}_3$: Role of $d$ electrons and ligand geometry

The interaction of ${\mathrm{H}}_2$O with both nearly perfect and defect ${\mathrm{Ti}}_2$ ${\mathrm{O}}_3$ surfaces has been studied by ultraviolet photoemission spectroscopy. Perfect ${\mathrm{Ti}}_2$ ${\mathrm{O}}_3$ (047) surfaces, which contain fivefold O-coordinated ${\mathrm{Ti}}^{3+}$ cations, rapidly chemisorb molecular ${\mathrm{H}}_2$O, with an accompanying transfer of electrons from the Ti $a_{1g}$ band to the molecule-surface complex. Saturation coverage of adsorbed ${\mathrm{H}}_2$O is less than one-half monolayer. The extramolecular relaxation-polarization shift for molecularly adsorbed ${\mathrm{H}}_2$O is 1.0 eV, and the O-lone-pair orbital is dominant in bonding to the surface. There is no evidence for dissociative chemisorption of ${\mathrm{H}}_2$O on ${\mathrm{Ti}}_2$ ${\mathrm{O}}_3$ (047). On ${\mathrm{Ti}}_2$ ${\mathrm{O}}_3$ surfaces containing a high density of defects, dissociative adsorption of ${\mathrm{H}}_2$O is observed; the chemisorbed species is believed to be O${\mathrm{H}}^{-}$ radicals. Some molecular adsorption of ${\mathrm{H}}_2$O may also occur for larger exposures. These results indicate that the presence of ${\mathrm{Ti}}^{3+}$ surface ions alone is not sufficient to catalyze the dissociation of ${\mathrm{H}}_2$O, in contrast to previous interpretations of data on Ti${\mathrm{O}}_2$ and SrTi${\mathrm{O}}_3$. Presumably other Ti valence states or the more complicated ligand structure of defect sites are required for dissociation of ${\mathrm{H}}_2$O.