Applications of Molecular Dynamics, Monte Carlo and Metadynamics Simulations Using ReaxFF Reactive Force Fields to Fluid/Solid Interfaces PDF Download

Author: Muralikrishna Raju
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Languages : en
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The interaction of dense fluids (water, polar organic solvents, room temperature ionic liquids, etc.) with solid substrates controls many chemical processes encountered in nature and industry. The key features of fluid-solid interfaces (FSIs) are the high mobility and often reactivity of the fluid phase, and the structural control provided by the solid phase. In this dissertation we apply molecular modeling methods to study FSIs in the following systems:1. Dissociation of water on titania surfacesWe studied the adsorption and dissociation of water at 300 K on the following TiO2 surfaces: anatase (101), (100), (112), (001) and rutile (110) at various water coverages, using a recently developed ReaxFF reactive force field. The molecular and dissociative adsorption configurations predicted by ReaxFF for various water coverages agree with previous theoretical studies and experiment. ReaxFF predicts a complex distribution of water on these surfaces depending on an intricate balance between the spacing of the adsorption sites (under-coordinated Ti and O surface atoms), water-surface interactions, and water-water interactions. Using molecular dynamics simulations to quantify water dissociation over the TiO2 surfaces at various water coverages, we find that the extent of water dissociation predicted by the ReaxFF reactive force field is in general agreement with previous density-functional theory studies and experiments. We demonstrate a correlation between the extent of water dissociation on different TiO2 surfaces and the strength of hydrogen bonding between adsorbed water molecules and water outside the adsorbed layer, as evidenced by the red shift of the O-H vibrational stretching mode of adsorbed water.2. Mechanisms of Oriented Attachment in TiO2 nanocrystalsOriented attachment (OA) of nanocrystals is now widely recognized as a key process in the solution-phase growth of hierarchical nanostructures. However, the microscopic origins of OA remain unclear. Using the same ReaxFF Ti/O/H reactive force field employed in the previous study, we perform molecular dynamics simulations to study the aggregation of various titanium dioxide (anatase) nanocrystals in vacuum and humid environments. In vacuum, the nanocrystals merge along their direction of approach, resulting in a polycrystalline material. By contrast, in the presence of water vapor, the nanocrystals reorient themselves and aggregate via the OA mechanism to form a single or twinned crystal. They accomplish this by creating a dynamic network of hydrogen bonds between surface hydroxyls and surface oxygens of aggregating nanocrystals. We determine that OA is dominant on surfaces that have the greatest propensity to dissociate water. Our results are consistent with experiment, are likely to be general for aqueous oxide systems, and demonstrate the critical role of solvent in nanocrystal aggregation. This work opens up new possibilities for directing nanocrystal growth to fabricate nanomaterials with desired shapes and sizes.3. Li interactions in carbon based materialsGraphitic carbon is still the most ubiquitously used anode material in Li-ion batteries. In spite of its ubiquity, there are few theoretical studies that fully capture the energetics and kinetics of Li in graphite and related nanostructures at experimentally relevant length/time-scales and Li-ion concentrations. In this study we describe development and application of a ReaxFF reactive force field to describe Li interactions in perfect and defective carbon based materials using atomistic simulations. We develop force-field parameters for Li-C systems using van der Waals-corrected density-functional theory (DFT). Grand canonical Monte Carlo simulations of Li intercalation in perfect graphite with this new force-field not only gives a voltage profile in good agreement with known experimental and DFT results but also captures the in-plane Li ordering as well as the interlayer separations for stage I and II compounds. In defective graphite, the ratio of Li/C i.e. the capacitance increases and the voltage shifts, both in proportion to the concentration of vacancy defects and metallic lithium is observed explaining lithium plating seen in recent experiments. We also demonstrate the robustness of the force-field by simulating model carbon nanostructures i.e. both 0D and 1D structures, that can be potentially used as battery electrode materials. While a 0D defective onion-like carbon facilitates fast charging/discharging rates by surface Li-adsorption, a 1D defect-free carbon nanorod requires a critical density of Li for intercalation to occur at the edges. Our force-field approach opens up the opportunity for studying energetics and kinetics of perfect and defective Li/C structures containing thousands of atoms as a function of intercalation. This is a key step towards modeling of realistic carbon materials for energy applications.4. Transfer of aqueous protons across single layer grapheneProton transfer across single layer graphene is associated with large computed energy barriers and is therefore thought to be unfavorable at room temperature. Experiments, however, have not yet been performed to test this prediction. Here, we subject a single layer of graphene on fused silica to cycles of high and low pH and show that protons transfer reversibly through the graphene to undergo acid-base chemistry with silica surface hydroxyl groups. After ruling out diffusion through macroscopic pinholes, the protons are found to transfer through rare, naturally occurring atomic defect sites. Computer simulations reveal low energy processes for water-mediated proton transfer across hydroxyl-terminated atomic defect sites that participate in a Grotthuss-type relay mechanism, while defects terminated by pyrylium-like ether bridges shut down proton exchange. Given the unfavorable energy barriers to transfer of helium and H2, the calculations show that single layer graphene is selectively permeable to aqueous protons.5. Capacitive mixingThe amount of salinity-gradient energy that can be obtained through capacitive--mixing based on double layer expansion depends on the extent the electric double layer (EDL) is altered in a low salt concentration (LC) electrolyte (e.g., river water). We showed that the electrode-rise potential, which is a measure of the EDL perturbation process, was significantly (P = 10--5) correlated to the concentration of strong acid surface functional groups using five types of activated carbon. Electrodes with the lowest concentration of strong acids (0.05 mmol g--1) had a positive rise potential of 59 ±4 mV in the LC solution, whereas the carbon with the highest concentration (0.36 mmol g--1) had a negative rise potential (--31 ± 5 mV). Chemical oxidation of a carbon (YP50) using nitric acid decreased the electrode rise potential from 46 ± 2 mV (unaltered) to --6 ± 0.5 mV (oxidized), producing a whole cell potential (53 ± 1.7 mV) that was 4.4x larger than that obtained with identical electrode materials (from 12 ± 1 mV). Changes in the EDL were linked to the behavior of specific ions in a LC solution using molecular dynamics and metadynamics simulations. The EDL expanded in the LC solution when a carbon surface (pristine graphene) lacked strong acid functional groups, producing a positive-rise potential at the electrode. In contrast, the EDL was compressed for an oxidized surface (graphene oxide), producing a negative-rise electrode potential. These results established the linkage between rise potentials and specific surface functional groups (strong acids), and demonstrated on a molecular scale changes in the EDL using oxidized or pristine carbons.Our applications to these five systems clearly demonstrate the capability of ReaxFF-based molecular dynamics simulations to provide a bridge between ab-initio results and experiments.