Origin of enhanced water adsorption at step edge on rutile TiO2(110) surface

Water adsorption behavior at <110> and <001> step edges on rutile TiO(2) (110) surface has been investigated using density-functional theory calculations. We find that the <110> edge exhibits significantly enhanced water adsorption, especially dissociative adsorption, as compared to the pristine (110) surface and <001> step edge. The origin for the enhanced water adsorption is found to be due to the existence of fourfold coordinated Ti atoms (Ti(4c)) at the <110> step edge, which lead to charge transfer to adsorbates more easily than fivefold coordinated Ti atoms on the (110) surface and <001> step edge. Furthermore, due to the local configuration of the dissociative mode, the charge transfer occurs more easily from Ti(4c) to the hydroxyl group 1π orbital, which considerably increases the adsorbate-substrate interaction. As a result, the <110> step edge favors dissociative water adsorption, making it more reactive to water than the (110) surface and <001> edge.