Ab initio molecular dynamics study of water dissociation and proton dynamics on rutile and cassiterite surfaces

Metal oxides have numerous applications in our everyday lives and in most of these applications their interaction with water is inevitable. Hence, it is important to understand the interactions that occur at the water/metal oxide interfaces. In this dissertation, we have used ab initio molecular dynamics simulations to study these interactions. In particular, we will study the (110) surfaces of the commonly occurring natural polymorphs of titanium dioxide (TiO2) and tin dioxide (SnO2) known as rutile and cassiterite, respectively.;A detailed analysis is presented for the structure of water above the rutile (110) surface. We find three distinct layers of water that are identified as L1, L2 and L3. The water molecules in L1 layer are covalently bonded with the metal atoms in the surface. The L2 layer is highly structured, which is in good agreement with the X-ray studies. The water molecules in this layer adsorb at two well-defined sites. The most prominent location is above and around the bridging oxygen. The other site is located above and between the two terminal oxygens from the L1 layer. The water molecules in the L3 layer do not show any specific adsorption preference. We show that the preferential adsorption sites, in L2 water layer, are produced by strong hydrogen bonds (H-bonds) between the surface and the water layer above the surface. The strength of these H-bonds is estimated from the broadening of the corresponding stretching mode of the vibrational band. The total vibrational spectrum from our ab initio simulations is in excellent agreement with those obtained from the inelastic neutron scattering experiments.;We compare the proton jump processes in the hydration layer on both rutile (110) and cassiterite (110) surfaces. A set of five simulations is done, for both surfaces, to remove the effect of initial configuration on the simulation results. These simulations differ in the dissociation level of the adsorbed water molecules in the initial configurations. We find that all five simulations equilibrate to ~25% and ~60% water dissociation level on rutile and cassiterite, respectively. This dissociation level is dynamical and both surfaces exhibit proton jumping events for the whole time of the simulation and throughout the whole surface. The cassiterite surface has three times higher proton jumping events than rutile surface. The higher proton jump activity on cassiterite is produced by stronger H-bond between the surface and the water layer above the surface. We find that the stronger covalent and ionic interaction, due to the difference in the electronic structure, leads to stronger H-bond on cassiterite. In general, we find that the H-bonds are stronger on cassiterite than on rutile surface. Among these, the bridging oxygen atoms form the strongest H-bonds between the surface and the hydration layer. Thus, the cassiterite surface has the highest proton jump activity, around 10 times greater than the rutile surface.;Another surface property that provides insight into the interaction of the water with the metal oxide surface is the length of the bond between the bridging or terminal oxygen and the five fold coordinated metal atom on the surface. This metal oxygen (M-O) bond length is sensitive to its local environment. We find that the M-O bond length increases as the number of covalently bonded hydrogen atoms increase on either the bridging oxygen or the terminal oxygen, on both surfaces. The histogram of the M-O bond length is narrower on cassiterite surface indicating stronger M-O bond. The terminal water forms higher average number of H-bonds on rutile surface resulting in an additional broadening of the corresponding M-O bond length histogram. We also find that, along with the number of H-bonds, the species forming the H-bond plays an important role in determining the M-O bond length. The M-O bond length histogram for the dissociated water molecule, on rutile surface, exhibits an interestingly broad peak. A closer look at the H-bonding environment shows that the broad M-O histogram contains two distinct peaks. We show that one of these peaks is completely due to a specific set of species that forms the H-bond with the dissociated water molecule.;Finally, we study the dependence of dissociation preference of the adsorbed water molecules on the thickness of the slab that is used to model the rutile surface. A three-layer slab shows preference for partial dissociation. But this preference changes as thicker slabs are used in the simulation. Moreover, the adsorption energy difference between the associated and partially dissociated configurations increases monotonically with the slab thickness. We find that this energy difference converges when the atoms below the first layer of the slab are kept fixed at the bulk positions. This indicates that the bulk plays an important role in determining the dissociation state of the water molecules adsorbed on the surface. The H-bond between the bridging oxygen and the water molecule weakens with the increasing slab thickness. We show that the weakening of the H-bond affects the proton transfer that is necessary for obtaining a dissociated configuration. Along with the slab thickness, we also investigate the effect of lateral size of the simulation cell on the dissociation preference. This study shows that a larger simulation cell, with sufficient slab thickness, may eventually prefer fully associated configuration for the adsorbed water molecules.