292693 Development of a Reaxff Parameterization for the Ti-O-H System
The aims of molecular simulation studies are to provide structural information of the mineral surface atoms and their interaction with water in the interface region. Specifically, bonding distributions for surface metal-oxygen, oxygen-hydrogen and hydrogen bonding will be predicted. In addition, the energetics and structures of hydrated nanoparticles and the adsorption of ions will be investigated as a function of nanoparticle size. A primary objective of the molecular simulation component of this project is to provide structural and energetic information that can be used to help constrain surface complexation modeling (SCM) efforts. The former has been successful for bulk surfaces as interatomic distances predicted via density functional theory (DFT) were used as input to MUSIC model calculations that predicted the pKa’s of surface sites on the TiO2(110) surface (Fitts et al., 2005). However, little work of this type has been performed on model nanoparticles (rather than 2‑D periodic surfaces) or on charged surfaces. This work will break ground on both areas and use the experimental data to test its validity. Simulation results will then be used to test the ability of the MUSIC model as well as other modeling approaches that can incorporate molecular scale detail, to make macroscopic predictions of adsorption behavior based on atomic structure.
Model nanocrystals of anatase ((100) and (001) faces) were constructed of approximately 1, 2, and 3 nm in diameter (Wulff construction). The surfaces of these nanocrystals were initially be hydrated based on results of periodic planewave density functional theory calculations of these surfaces. Each nanocrystal will be solvated by at least three H2O layers.
The calculated energies are dependent on the H2O configuration. The conformational space will therefore be explored by classical molecular dynamics simulations using the ReaxFF method. By performing nanosecond scale simulations of the nanocrystal-water system, we can test a wide range of possible water structures. The most stable of these structures can be found via single point and energy minimization DFT calculations. Energy minimization of anatase nanoparticles surrounded by H2O molecules will be used to predict surface Ti-O, O-H and H-bonding distributions for use in the SCM component of this study. These results can be compared directly to real-time X-ray diffraction results of TiO2 nanoparticle crystallization (Hummer et al., 2009) to verify that model structures are reproducing observed TiO2nanocrystal structures.
A procedure similar to the one described above for the anatase-water interactions has been used to simulate the behavior of ions surrounding the anatase nanoparticles . Using the models generated above for the pure anatase-water system, ions will be added at random positions within the solvent. Classical MD simulations using ReaxFF will be used to predict distributions of inner-sphere, outer-sphere and solvated ions. Selected configurations will be subjected to DFT single-point energy and energy minimizations using VASP to determine if the energetics predicted by the force field calculations is accurate. The combination of classical and quantum mechanical results will be compared to adsorption isotherms and SCM modeling of the same ions as a function of anatase particle size.
Fitts J. P., Machesky M. L., Wesolowski D. J., Shang X., Kubicki J. D., Flynn G. W., Heinz T. F. and Eisenthal K. B. (2005) Chem. Phys. Lett. 411: 399-403.
Hummer D. R., Kubicki J. D., Kent P. R. C., Post J. E. and Heaney P. J. (2009) J. Phys. Chem. C, 113, 4240-4245.
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