Toward Advanced QM/MM MD Simulations of Solid-Liquid Interfaces-Adsorption and Proton Transfer of Multilayer Water on R-TiO2(001)

Surface phenomena such as proton transfer occurring near solid-liquid or solid-solid interfaces represent key events in catalysis as well as electrochemical processes. In the case of TiO2, this event is related to water splitting processes, which are often utilized in hydrogen generation or photocatalysis in wastewater management. Recent investigations of R-TiO2(001) showed that this facet has the potential to increase the photocatalytic activity. However, due to its highly reactive character, this surface is known to undergo rapid reconstruction. This makes the measurement and characterization of the respective surface structures challenging, not only in experimental, but also in theoretical investigations. In previous work, a novel 2d-periodic QM/MM protocol was applied to investigate proton transfer reactions occurring at the R-TiO2(001) facet in contact with a H2O monolayer. It was concluded that to evaluate the established protonation, long MD simulations are required, which are impossible to perform via conventional DFT. However, the application of SCC DFTB provides an efficient alternative to reduce the computational cost. Nonetheless, the implementation of a solvent monolayer seemingly represents a solid-gas environment. In the current work, an enhanced QM/MM framework is present, enabling the inclusion of additional solvent molecules to study surface protonation events in more detail. For comparison, an alternative setup including all solvent molecules in the QM zone is conducted. The result showed that the structure and dynamics of the solvent close to the interfaces of both the QM/MM and all-QM models are almost identical, which verifies the applicability of the suggested 2d-periodic QM/MM protocol as a general framework for theoretical investigations of surface structures.