Room temperature bilayer water structures on a rutile TiO2(110) surface: hydrophobic or hydrophilic?

The lack of understanding of the molecular-scale water adsorbed on TiO2 surfaces under ambient conditions has become a major obstacle for solving the long-time scientific and applications issues, such as the photo-induced wetting phenomenon and designing novel advanced TiO2-based materials. Here, with the molecular dynamics simulation, we identified an ordered water bilayer structure with a two-dimensional hydrogen bonding network on a rutile TiO2(110) surface at ambient temperature, corroborated by vibrational sum-frequency generation spectroscopy. The reduced number of hydrogen bonds between the water bilayer and water droplet results in a notable water contact angle (25 ± 5°) of the pristine TiO2 surface. This surface hydrophobicity can be enhanced by the adsorption of the formate/acetate molecules, and diminishes with dissociated H2O molecules. Our new physical framework well explained the long-time controversy on the origin of the hydrophobicity/hydrophilicity of the TiO2 surface, thus help understanding the efficiency of TiO2 devices in producing electrical energy of solar cells and the photo-oxidation of organic pollutants.

where is the frequency of the IR beam; and are respectively the components of the total polarizability tensor and dipole moment; the dot stands for the time derivative; and stands for a statistical average. A computationally efficient 〈…〉 algorithm based on the velocity-velocity correlation function was used to obtain the time-correlation function of the dipole moment and polarizability. In this method, 1, 2 the total polarizability and dipole moment are first decomposed into their molecular components, , ( {̇= ∑̇, ̇= 3 We finally got the simplified by substituting Eq. 2 (2, ) and Eq. 3 to Eq. 1 and neglecting the inter-molecular couplings. We have also performed VSFG simulations of the rutile TiO 2 (110)/water interface using a flexible SPC/E water model 4 and NN potential to confirm the robustness of the coverage-dependent ordered water bilayer structure. For each set of the simulation, the mode intensity first increases with the D 2 O coverage ranging from 0 to 2 MLs but decreases at even higher coverage (see Fig. S1), which is quite consistent with the simulation results with the flexible SPC water model (Fig 1b in main text).

PS 4. Calculation of contact angles of water droplets
To obtain a contact angle, the density distribution profile of the droplet is firstly    All of the simulations support the main idea of our work that the ordered water bilayer structure results in an unexpected water droplet that does not completely wet the bilayer water.

PS 7. Water droplets on the water monolayer on the rutile TiO 2 (100)
We have also performed wetting simulations for the rutile TiO 2 (100) surface using NN-MD, which also resulted in a water droplet coexisting with a water monolayer spreading all over the surface as shown in Fig. S7.

Fig. S7. Side view snapshot of rutile TiO 2 (100) solid with a water droplet (red and white balls) coexisting with the water monolayer (cyan and white balls) based on the NN-MD simulation
The VSFG experiments and simulations were also performed for a rutile TiO 2 (100) surface. Fig. S8 displayed the experimental and theoretical calculated VSFG spectra dependent on the D 2 O coverage. In the experiment, the mode intensity increases with the relative humidity (RH) increasing from ~0% to ~30% but drops at even higher D 2 O coverage (see Fig. S8a). The nonmonotonic change of the intensity with D 2 O coverage, same as the previous results on the rutile TiO 2 (110) surface, was also observed in the theoretical calculation where the mode intensity first increases with the D 2 O coverage ranging from 0 to 1 ML but decreases at even higher coverage (see Fig. S8b). The experiment and simulation show an ordered structure of the interfacial water on rutile TiO 2 (100), consistent with our previous measurements on the rutile TiO 2 (110) surface.  We have performed the wetting behavior simulation of rutile TiO 2 (110) surface with 5% and 10% covering ratios of -OH groups, by randomly planting the -OH groups on the solid surfaces. The contact angle of the water droplet on the water bilayer decreased to 19° at a covering ratio of 5% and disappeared at a covering ratio of 10%, as shown in Fig. S10. We have also calculated the solid-water interaction energies per water molecule, the absolute value interaction energy decreases from 137 kJ/mol to 87kJ/mol as the increase of the ratio. Despite of the strong interaction, the hydrogen bond number between the bilayer and water droplet molecules is more important for this wetting transformation. We have calculated the hydrogen bond numbers per bilayer water molecule that formed between bilayer and droplet for different covering ratios of -OH groups. As the covering ratio increases to 5% and 10%, the hydrogen bond number per bilayer water molecule that formed between bilayer and droplet increases from 0.85 to 0.95 and 1.18 respectively. The increasing hydrogen bond number between bilayer and droplet indicates that the water bilayer hydrogen network is gradually disrupted.

PS 12. Calculation of the solid-water interaction energy
The interaction energy between two atoms i, j is described as, , = + where stands the van der Waals interaction and stands the Coulomb interaction. For the van der Waals interaction, we use the Buckingham potential in the simulations, which is described as, (1), where, i and j stand different atoms, means the distance between the atoms, , and are Buckingham parameters between the atoms. The Coulomb interaction is described as, where and stand the charges of atom i and j respectively. The solid-water interaction energy is calculated with the average interaction energy per bilayer water molecule in contact with the surface.

PS 13. Thickness comparison between water bilayers obtained from MD and NN-MD
We have investigated the distribution of the water molecules near the rutile (110)