Deep vacancy induced low-density fluxional interfacial water

Interfacial water on transition metal oxides such as ${\mathrm{TiO}}_2$ has been widely studied because of its structural complexity and scientific relevance in, e.g., photocatalysis and ice growth. Using ab initio molecular dynamics, we find that interfacial water on the anatase (101) surface features an unconventional fluxional structure with reduced contact layer density. The density reduction and flexibility of interfacial water are induced by oxygen vacancy defects located deep below the surface. Our study proposes a fresh perspective of the anatase-water interface, raising the importance of nontrivial long-range effects caused by deep defects. These often-neglected effects highlight the necessity and challenges of the state-of-the-art simulation and experimental probing of solid-liquid interfaces.

Figure 1

Comparison of the low-density and the high-density interfacial water. (a) Side view and (b) top view of the high-density interfacial water. The oxygen and titanium atoms are represented by red and blue, respectively. Orange and green spheres highlight water molecules in the contact layer on ${\mathrm{Ti}}_{5c}$ and ${\mathrm{O}}_{2c}$ sites, respectively. (c) Side view and (d) top view of the low-density interfacial water. (e) Number of hydrogen bonds formed in two density models. Green bars correspond to hydrogen bonds between water molecules within the contact layer and blue bars correspond to hydrogen bonds formed between ${\mathrm{O}}_{2c}$ and water. (f) Radial distribution function of ${\mathrm{O}}_{2c}\text{−}{\mathrm{H}}_W$ pairs, where ${\mathrm{H}}_W$ is the hydrogen atom in liquid water.

Figure 2

Structural model and $g\left(z\right)$ profiles of ten systems. (a) Side view of a snapshot from the simulation using the pristine substrate. The nine oxygen vacancy sites of defective systems are labeled with numbers 1 to 9 corresponding to the first to the ninth oxygen layer under the surface. The $g\left(z\right)$ profiles for all systems are classified as (b) high-density interfacial water and (c) low-density interfacial water. The colors and labels are consistent with those in (a). Here $g\left(z\right)=\frac{N_{z,z+\mathrm{\Delta }\left(z\right)}}{N_{\mathrm{total}}}$, where $N_{z,z+\mathrm{\Delta }\left(z\right)}$ is the number of water molecules within a $\mathrm{\Delta }\left(z\right)$ thick slab and $N_{\mathrm{total}}$ is the total number of water molecules. The reference point $z=0$ is chosen as the average $z$ value of all ${\mathrm{Ti}}_{5c}$ sites of the whole trajectory. In each $g\left(z\right)$ profile, the reference point $z=0$ is chosen as the average $z$ value of all ${\mathrm{Ti}}_{5c}$ sites of the whole trajectory. The vertical gray line indicates the position of the second peak on $g\left(z\right)$ profiles of the conventional high-density interfacial water. The interfacial water density integrated from $g\left(z\right)$ profiles is shown in Fig. 3. The convergence tests are discussed in Figs. S1 and S2 in [41].

Figure 3

Fluxional properties. Projected trajectories for (a) the pristine system and (b) $V_{\text{O}}8$ systems. Figure S7 in [41] plots projected trajectories for all ten systems. The position of water is identified by the oxygen atom. A uniform sequential colormap of viridis represents the lateral distribution density of a water molecule adsorbed on ${\mathrm{O}}_{2c}$. Water molecules on ${\mathrm{Ti}}_{5c}$ are plotted with black lines. Water molecules within the first shell, with a cutoff radius of 3 Å from ${\mathrm{O}}_{2c}$ and 2.5 Å from ${\mathrm{Ti}}_{5c}$, are taken into account. (c) Number density (water molecules bonded to ${\mathrm{O}}_{2c}$) and order parameter of the ten systems labeled. The gray line is a guide to the eye, showing a positive relation between the density and the ordering parameter. The distribution of water orientation $\varphi$ is plotted in (d) the pristine substrate and (e) the $V_{\text{O}}8$ system. The inset shows the definition of $\varphi$, namely, the angle between the surface normal and the orientation of a water molecule.

Figure 4

Substrate atomic position distribution, substrate charge distribution, and VDOS of interfacial water. The average partial charge of every (a) oxygen and (b) titanium substrate layer is plotted. Error bars and shadow regions indicate the standard deviation and the interval between the minimum and maximum, respectively. The average is calculated every 1000 steps and every six atoms in a substrate layer (10 layers for Ti and 20 layers for O). The reference zero point of the $z$ coordinate is chosen to be the average $z$ value of atoms in the lowermost interfacial layer. (c) The red line is the distribution density of oxygen atoms (20 layers) except for the dangling oxygen atom in the $V_{\text{O}}8$ system. The blue line is the difference from the same distribution density in the pristine system, showing that the difference in two substrates is extended across the whole substrate. Here $\mathrm{\Delta }Z$ is defined as the difference between the $z$ value of an oxygen atom and the average $z$ value of all six oxygen atoms in the ninth layer. The ninth oxygen layer is chosen as the reference for the plot, since its average oxygen position is unaffected by the existence of $V_{\text{O}}$. (d) Snapshots A1–D1 show the structures concerned in the VDOS of (e) the pristine system and (f) the $V_{\text{O}}8$ system. Black lines in the VDOS represent the liquid reference in the systems.