Simulation of the Interfaces of Anatase TiO2 (001), (100), (101) with KOH Solution By Molecular Dynamics

Hydrogen can be produced by the photoelectrochemical water splitting system with KOH solution as electrolyte and TiO2 as photoanode. In this work, the interaction between anatase TiO2 (001), (100), (101) surfaces and KOH solution is studied by molecular dynamics simulations at room temperature. The dynamic interfacial structures and properties of K+ on anatase TiO2 surfaces are obtained by analyzing the radial distribution function, the diffusion coefficient, the surface charge distribution, electric field and electrostatic potential distribution. The results have revealed that the coordination number of K+ is reduced because it adsorbed on the anatase TiO2 surface, resulting in the weakening of hydration. And there are three kinds of K+ adsorption sites on (001) and (101) surfaces and two adsorption sites on (100) surface. To obtain more information about the effect of KOH on the interface, the electrostatic properties of KOH solution and aqueous solution were qualitatively compared and analyzed. The maxima of the charge density and electric field profiles are increased in KOH solution but remains more or less at the same position as for the aqueous solution. There is a significant difference in electrostatic potential comparison. While the electrostatic potential of aqueous solution are smooth and well converged, that of KOH solution are rather noisy. This can be explained by the asymmetrical distribution of K+ and OH-. Therefore, the KOH solvent contributes substantially to the electrostatic behavior of the liquid near the interface. Based on Grahame double layer model, the zeta potential of the three surfaces (001), (100), (101) in inner Helmholtz layer is observed as 6.42V, 4.76V and 5.18V, respectively, corresponding to the width of Stern layer 3.99Å, 4.40 Å, 3.72 Å. The thickness is in full agreement with already mentioned compact layer (3.4 Å). We also approximately estimate that the surface depletion layer width of anatase (001), (100), (101) is about 7.16Å, 10.23Å and 8.23Å, respectively. All above results clearly illustrate the enormous improvement in our quantitative understanding of the molecular-scale structure and diffusional dynamics of K+ at semiconductor-electrolyte interfaces, as compared with continuum-based Grahame model of the electrical double layer. And the differences between aqueous solution and KOH solution qualitatively explain the effect of KOH solution on the electrolyte, which can increase the conductivity of the solution and provide a chemical reaction bias. Investigations of the double layer structure, which are presently under way in our group, are expected to provide further insight into semiconductor/electrolyte interface. Figure 1