| Summary: | The relationship between structure and reactivity plays a dominant role in water dissociation on the various TiO<sub>2</sub> crystallines. To observe the adsorption and dissociation behavior of H<sub>2</sub>O, the reaction force field (ReaxFF) is used to investigate the dynamic behavior of H<sub>2</sub>O on rutile (110) and anatase (101) surfaces in an aqueous environment. Simulation results show that there is a direct proton transfer between the adsorbed H<sub>2</sub>O (H<sub>2</sub>O<sub>ad</sub>) and the bridging oxygen (O<sub>br</sub>) on the rutile (110) surface. Compared with that on the rutile (110) surface, an indirect proton transfer occurs on the anatase (101) surface along the H-bond network from the second layer of water. This different mechanism of water dissociation is determined by the distance between the 5-fold coordinated Ti (Ti<sub>5c</sub>) and O<sub>br</sub> of the rutile and anatase TiO<sub>2</sub> surfaces, resulting in the direct or indirect proton transfer. Additionally, the hydrogen bond (H-bond) network plays a crucial role in the adsorption and dissociation of H<sub>2</sub>O on the TiO<sub>2</sub> surface. To describe interfacial water structures between TiO<sub>2</sub> and bulk water, the double-layer model is proposed. The first layer is the dissociated H<sub>2</sub>O on the rutile (110) and anatase (101) surfaces. The second layer forms an ordered water structure adsorbed to the surface O<sub>br</sub> or terminal OH group through strong hydrogen bonding (H-bonding). Affected by the H-bond network, the H<sub>2</sub>O dissociation on the rutile (110) surface is inhibited but that on the anatase (101) surface is promoted.
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