(393aq) Ion Adsorption On Nanocrystalline TiO2 Surfaces by Using a Reactive Force Field

The reactive force field, ReaxFF, developed by van Duin and co-workers make feasible for the simulations of nanoparticles for nanoseconds with bond-making and bond-breaking reactions which lack in classical MD simulations. Previously, a new ReaxFF reactive force field has been developed to describe reactions in the Ti-O-H system. The ReaxFF force field parameters have been fitted to a quantum mechanical (QM) training set containing structures and energies related to bond dissociations, angle and dihedral distortions, and reactions between water and titanium dioxide as well as experimental crystal structure, heat of formation and bulk modulus data. Model configurations for the training set were based on DFT calculations on molecular clusters and periodic systems (both bulk crystals and surfaces). ReaxFF reproduces accurately the QM training set for structures and energetics of small clusters. Most important is that ReaxFF also describes the energetics for the rutile, brookite and anatase. The results of ReaxFF match reasonably well with those of QM for water binding energies, surface energies and H₂O dissociation energy barriers. To validate this ReaxFF description, we have compared its performance against DFT/MD simulations for 1 and 3 monolayer water interaction with a rutile (110) surface. We found agreement within 10% error between the DFT/MD and ReaxFF water dissociation levels for both coverages.

Apart from that, a new Ti-Cl force field parameter has been developed to describe reactions in Ti/O/H/Cl materials. The ReaxFF force field parameters are fitted against a quantum mechanical (QM) training set containing structures and energies related to bond dissociations, angle and dihedral distortions, and reactions between titanium and chlorine gases as well as heats of formation of TiCl_x crystals. These newly developed Ti-Cl force field parameters were combined with the recently developed Ti-O-H force field. ReaxFF accurately reproduces the QM training set for structures and energetics of small clusters and TiCl_x crystals. By adding Ca parameters with the previously developed Ti/O/H/Cl force field, the simulations of TiO₂ nanocrystalline surfaces in aqueous solutions with Ca and Cl ions were conducted. Before that, the force field was validated by comparing bond length between Ca and O with experimental results*. The energetic and structures of hydrated ions will be investigated. Total numbers of H₂O molecules, OH and Ca ions attached to the surfaces with respect to time will be shown. Also, the mechanism of ion adsorption caused by proton induced charge transfer and Ca ions diffusion behavior will be discussed. In addition to these, the ion adsorption characteristic behaviors of the 5 different specific faces for anatase and rutile will be presented. For another ion simulation, Na and Cl ions were added to the H₂O and the characteristic behavior will be discussed and compared with the result of Ti/O/H/Ca/Cl system.

* Markus, Y. Chemical Reviews, 1988, 88,1475