Electric-Field Effects on Adsorbed-Water Structural and Dynamical Properties at Rutile- and Anatase-TiO₂ Surfaces

We have investigated the effects of external static electric fields applied to a wide variety of TiO₂/water interfaces using nonequilibrium molecular-dynamics techniques. The externally applied electric fields were found to be relatively weak vis-à-vis intrinsic electric fields computed in the interfacial regions, the magnitude of which varied from 1.8 V/Å toward bulklike water up to 4.5 V/Å at the interface. The molecular arrangement of the first hydration layer is determined fully by the surface structure of TiO₂, where water is coordinated to unsaturated titanium atoms and/or interacting with exposed surface oxygen atoms. Moreover, the water dipoles tend to align with the strong intrinsic field. As a result, diffusion of water in this region was found to be by 1 order of magnitude lower than that of bulk water; application of an external electric field did not lead to a considerable change. In contrast to unperturbed diffusivity, a rather strong response of hydrogen-bond lifetime to the applied field was observed. The interfacial water is heavily confined, although the extent to which shows marked variation with specific surfaces; indeed, this environmental interplay has considerable effect on corresponding IR spectra in the interfacial-water region, and is affected by applied static fields.