In view of the potential of transition metal (TM) oxides for photocatalytic applications, the adsorption and dissociation of water on their surfaces has been widely studied. Surface chemistry can strongly influence the adsorption mechanism. For example, the SrO-terminated perovskite oxide SrTiO3 (STO) favors dissociative adsorption, whereas on the TiO2-terminated surface, molecular adsorption is more stable. Chemisorption can be enhanced by precursor-mediated adsorption, where the molecule is first trapped in a weakly bound physisorbed state and then migrates to encounter a more active site, for example, an oxygen vacancy (VO), at which chemical bonds are formed. The interaction with ferroelectric oxide surfaces is less well-known, yet it is of considerable importance because hydrogen yield can be enhanced by an order of magnitude at the surface of a ferroelectric. The static charge on the surface changes the depth of the physisorption well, which determines the average residence time of the precursor on the surface, leading to a greater chance of finding a chemisorption site.
Another important aspect of dissociative adsorption is the production of H+ capable of bonding to lattice oxygen. The presence of hydrogen in ferroelectrics is known to be a factor that increases imprint, potentially inhibiting switching of the polarization. In PbTiO3, OH− adsorption favors an upward pointing polarization state, whereas H+ adsorption stabilizes downward pointing polarization.
Maj : 14/03/2014 (2080)