Exploring Rutile (110) and Anatase (101) TiO₂ Water Interfaces by Reactive Force-Field Simulations

We have investigated static/structural as well as dynamical properties of anatase (101) and rutile (110) TiO₂ interfaces with liquid bulk water by reactive force fields (ReaxFF). Layered, well-organized structure of water in the interface region was clearly observed within 6.5 Å of the surfaces. The first-hydration layer molecules adsorbed to unsaturated surface Ti atoms undergo spontaneous dissociation leading, rather controversially, to full coverage of O_2c/O_b by H⁺ and partial coverage of Ti_5c by OH^–. Expected large variations of intrinsic electric field on the interfaces, and drop of electrostatic potential, were detected. Interfacial water was found to be heavily confined with a self-diffusion constant of 2 orders of magnitude lower than 2.28 × 10^–9 m²/s measured in the bulk water region. Moreover, the rotational movement of adsorbed water molecules was found to be considerably hindered as well. On the other hand, the calculated hydrogen-bond lifetime on the interface was shorter than in bulk water for both surface types. Finally, the IR spectra obtained from collective-water-dipole variations in the interfacial region revealed stronger effects on stretching vibrations on anatase (101) than on rutile (110); however, description of liquid-water bond-stretching vibrations generally suffers from lack of accuracy in the applied reactive potential

Free-Energy Calculations of the Intercage Hopping Barriers of Hydrogen Molecules in Clathrate Hydrates

Clathrate hydrates are nonstoichiometric crystalline inclusion compounds in which a water host lattice encages small guest atoms or molecules in cavities, and they have potential utility as a hydrogen-storage vector. In spite of the anomalous mechanistic nature of guest-diffusivity in clathrate hydrates, characterizing the precise mechanisms of intercage diffusive migration therein remains an elusive challenge. Also, nuclear quantum effects are particularly important for small guests such as H₂, and cannot realistically be neglected in the host lattice in any rigorous dynamical treatment of H₂ intercage diffusivity. Here we compute free-energy profiles and barriers, showing that quantal delocalization increases these barriers dramatically vis-à-vis classical dynamics for intercage H₂ diffusion, by combining umbrella sampling with path-integral molecular dynamics in the extended solid. Results are compared to earlier DFT ab initio molecular dynamics calculations of Trinh et al, who found that the free-energy barriers decrease with increasing temperature

Role of Hydration Layer in Dynamical Transition in Proteins: Insights from Translational Self-Diffusivity

Elucidation of the role of hydration water underpinning dynamical crossover in proteins has proven challenging. Indeed, many contradictory findings in the literature seek to establish either causal or correlative links between water and protein behavior. Here, via molecular dynamics, we compute the temperature dependence of mean-square displacement and translational self-diffusivities for both hen egg white lysozyme and its hydration layer from 190 to 300 K. We find that the protein’s mobility increases sharply at ∼230 K, indicating dynamical onset; concerted motion with hydration-water molecules is evident up to ∼285 K, confirming dynamical correlation between them. Exploring underlying mechanisms of such concerted motion, we scrutinize the water–protein hydrogen-bonding network as a function of temperature, noting sharp deviation from linearity of the hydrogen bond number’s profile with temperature originating near the protein dynamical transition. Our studies reveal a common temperature profile/dependence of self-diffusivity values of the protein, hydration water, and the bulk solvent, originating from a common dependence on the bulk solvent viscosity, η_S. The key mechanistic role adopted by the protein–water hydrogen bond network in relation to the onset of proteins’ dynamical transition is also discussed

Minimizing Electron–Hole Recombination on TiO₂ Sensitized with PbSe Quantum Dots: Time-Domain Ab Initio Analysis

TiO₂ sensitized with quantum dots (QDs) gives efficient photovoltaic and photocatalytic systems due to high stability and large absorption cross sections of QDs and rapid photoinduced charge separation at the interface. The yields of the light-induced processes are limited by electron–hole recombination that also occurs at the interface. We combine ab initio nonadiabatic molecular dynamics with analytic theory to investigate the experimentally studied charge recombination at the PbSe QD–TiO₂ interface. The time-domain atomistic simulation directly mimics the laser experiment and generates important details of the recombination mechanism. The process occurs due to coupling of the electronic subsystem to polar optical modes of the TiO₂ surface. The inelastic electron–phonon scattering happens on a picosecond time scale, while the elastic scattering takes 40 fs. Counter to expectations, the donor–acceptor bonding strengthens at an elevated temperature. An analytic theory extends the simulation results to larger QDs and longer QD–TiO₂ bridges. It shows that the electron–hole recombination rate decreases significantly for longer bridges and larger dots and that the main effect arises due to reduced donor–acceptor coupling rather than changes in the donor–acceptor energy gap. The study indicates that by varying QD size or ligands one can reduce charge losses while still maintaining efficient charge separation, providing design principles for optimizing solar cell design and increasing photon-to-electron conversion efficiencies

Dispersion and Solvation Effects on the Structure and Dynamics of N719 Adsorbed to Anatase Titania (101) Surfaces in Room-Temperature Ionic Liquids: An <i>ab Initio</i> Molecular Simulation Study

<i>Ab initio</i>, density functional theory (DFT)-based molecular dynamics (MD) has been carried out to investigate the effect of explicit solvation on the dynamical and structural properties of a [bmim]­[NTf₂] room-temperature ionic liquid (RTIL), solvating a N719 sensitizing dye adsorbed onto an anatase titania (101) surface. The effect of explicit dispersion on the properties of this dye-sensitized solar cell (DSC) interface has also been studied. Upon inclusion of dispersion interactions in simulations of the solvated system, the average separation between the cations and anions decreases by 0.6 Å; the mean distance between the cations and the surface decreases by about 0.5 Å; and the layering of the RTIL is significantly altered in the first layer surrounding the dye, with the cation being on average 1.5 Å further from the center of the dye. Inclusion of dispersion effects when a solvent is not explicitly included (to dampen longer-range interactions) can result in unphysical “kinking” of the adsorbed dye’s configuration. The inclusion of solvent shifts the HOMO and LUMO levels of the titania surface by +3 eV. At this interface, the interplay between the effects of dispersion and solvation combines in ways that are often subtle, such as enhancement or inhibition of specific vibrational modes

Ice-Amorphization of Supercooled Water Nanodroplets in No Man’s Land

Elucidating freezing mechanisms of liquid water into ice, especially in “No Man’s Land” (150 K< <i>T</i> < 235 K), carries scientific and technological importance. Indeed, superior predictions of upper-troposphere cirrus-cloud formation and surface-bound ice-fog formation constitute powerful motivations in addition to unravelling long-standing puzzles such as persistent liquid fogs well below frost point and understanding interstellar-space water states, together with advancing cryopreservation technology. Unlocking the secrets of water’s anomalous deep-cooling complexities, such as structural ordering and microscopic nucleation mechanisms, are the subject of lively debate. Exploring nucleation mechanism in No Man’s Land (NML) is technically demanding, owing to rapid nucleation rates with, unsurprisingly, very few reported experimental studies. However, amorphization is a key intermediate stage in NML-based nucleation, and it is also not particularly well understood. In this microsecond long molecular dynamics study, we have explored microstructural processes involved in the amorphization of aggressively quenched supercooled water nanodroplets in the gas phase where surface effects are non-negligible. A dynamically arrested state is observed in these droplets that resembles structurally low-density amorphous polymorphs of ice. Importantly, the curved geometry of the nanodroplets themselves is seen to inhibit amorphization relative to bulk systems under identical thermodynamic conditions

Elastic Characterization of <i>S</i>- and <i>P</i>‑Wave Velocities in Marinelike Silica: The Role of Nonequilibrium Molecular Dynamics

The α-quartz polymorph of SiO₂ forms the basis of mineral sands stable down to 100 km depths below the surface, making it of central geoscientific relevance. The characterization of the nanoscale properties of these materials is of importance, especially for elastic properties governing phonon and sound propagation, and is of very high industrial relevance for oil exploration. Here, for the first time, we apply non-equilibrium molecular dynamics simulation to analyze the propagation of an artificial velocity perturbation in silica systems and, in so doing, determine <i>S</i>- and <i>P</i>-wave velocities in a manner redolent of concept to seismic-based oil-exploration approaches. This propagation has been analyzed systematically by means of different metrics in terms of spatiotemporal system response; these produce consistent results, by and large. In particular, we find excellent quantitative agreement with experimental <i>S</i>- and <i>P</i>-wave velocities, in many cases

Methane Clathrate Hydrate Nucleation Mechanism by Advanced Molecular Simulations

The nucleation mechanisms of methane hydrates are studied using well-tempered metadynamics and restrained molecular dynamics. The collective variables we used to follow the process are the methane–methane and methane–water coordination numbers, from which we computed the corresponding Landau free energy surface. This surface is characterized by two minima, corresponding to the two-phase methane bubble/water solution and clathrate crystal, and a transition state. The clathrate crystal is of type II, while in the simulation conditions (<i>T</i> = 273 K and <i>P</i> = 500 atm) the most stable phase should be type I. We constructed the steepest ascent/descent path connecting the two-phase methane bubble/water solution to the clathrate state and passing through the transition state. We interpret this path as the nucleation path, which shows four phases. First, the concentration of solvated methane increases in the aqueous domain via diffusion through the methane–water interface. Second, units of methane molecules solvated in water meet to form an unstructured cluster. Third, the water content of the nucleus decreases to a value compatible with the type II methane clathrate hydrate composition. Finally, a reordering process of solvated methane and water molecules occurs in a manner consistent with the “blob” hypothesis (Jacobson, L. C.; Hujo, W.; Molinero, V. <i>J. Am. Chem. Soc.</i> <b>2010</b>, <i>132</i>, 11806–11811)

Vibrational Modes of Hydrogen Hydrates: A First-Principles Molecular Dynamics and Raman Spectra Study

We have employed classically propagated molecular dynamics (MD), within the framework of density functional theory (DFT), to calculate vibrational spectral band of molecular hydrogen trapped in clathrate hydrate, with large-cage occupancy from 1 to 4, at ∼260 K and ∼2 kbar. The predicted vibrations, obtained by applying a state-of-the-art generalized gradient approximation (GGA) functional with nonlocal correlation (VdW-DF), reproduce satisfactorily our own accurate Raman spectra (at the same temperature and pressure conditions). We decomposed the MD-sampled vibrational band to individual peaks and assigned them to the vibration of H₂ molecules enclosed in small and large cages of SII hydrate. By summing the resulting spectral bands, we have demonstrated that the measured spectral response is a complex composition of signals originating from H₂ molecules experiencing different local, intracage environments

Electrophoretic deposition of poly(3-decylthiophene) onto gold-mounted cadmium selenide nanorods

Molecular mechanisms of electrophoretic deposition (EPD) of P3DT poly(3-decylthiophene) molecules onto vertically aligned cadmium selenide arrays have been studied using large-scale, nonequilibrium molecular dynamics (MD), in the absence and presence of static external electric fields. The field application and larger polymer charges accelerated EPD. Placement of multiple polymers at the same lateral displacement from the surface reduced average deposition times due to ‘crowding’, giving monolayer coverage. These findings were used to develop and validate Brownian dynamics simulations of multi-layer polymer EPD in scaled-up systems with larger inter-rod spacings, presenting a generalised picture in qualitative agreement with random sequential adsorption