Niall J. English

Photo-induced Charge Separation across the Graphene–TiO2 Interface Is Faster than Energy Losses: A Time-Domain ab Initio Analysis

Graphene-TiO(2) composites exhibit excellent potential for photovoltaic applications, provided that efficient photoinduced charge separation can be achieved at the interface. Once charges are separated, TiO(2) acts as an electron carrier, while graphene is an excellent hole conductor. However, charge separation competes with energy losses that can result in rapid electron-hole annihilation inside metallic graphene. Bearing this in mind, we investigate the mechanisms and, crucially, time scales of electron transfer and energy relaxation processes. Using nonadiabatic molecular dynamics formulated within the framework of time-domain density functional theory, we establish that the photoinduced electron transfer occurs several times faster than the electron-phonon energy relaxation (i.e., charge separation is efficient in the presence of electron-phonon relaxation), thereby showing that graphene-TiO(2) interfaces can form the basis for photovoltaic and photocatalytic devices using visible light. We identify the mechanisms for charge separation and energy losses, both of which proceed by rapid, phonon-induced nonadiabatic transitions within the manifold of the electronic states. Electron injection is ultrafast, owing to strong electronic coupling between graphene and TiO(2). Injection is promoted by both out-of-plane graphene motions, which modulate the graphene-TiO(2) distance and interaction, and high-frequency bond stretching and bending vibrations, which generate large nonadiabatic coupling. Both electron injection and energy transfer, injection in particular, accelerate for photoexcited states that are delocalized between the two subsystems. The theoretical results show excellent agreement with the available experimental data [Adv. Funct. Mater. 2009, 19, 3638]. The state-of-the-art simulation generates a detailed time-domain atomistic description of the interfacial charge separation and relaxation processes that are fundamental to a wide variety of applications, including catalysis, electrolysis, and photovoltaics.

Molecular dynamics simulations of microwave heating of water

Nonequilibrium molecular dynamics simulations of water in an intense external microwave field have been performed using a rigid/polarizable and a flexible/nonpolarizable potential model, from ambient conditions to supercriticality. The heating profiles were compared to that predicted from a macroscopic energy balance, and the polarizable model was found to be superior in this regard.

First-principles calculation of nitrogen-tungsten codoping effects on the band structure of anatase-titania

The electronic properties and photocatalytic activity of nitrogen (N) and/or tungsten (W)-doped anatase are calculated using density functional theory. For N-doping, isolated N 2p states above the top of the valence band are responsible for experimentally observed redshifts in the optical absorption edge. For W-doping, W 5d states below the conduction band lead to band gap narrowing; the transition energy is reduced by 0.2 eV. Addition of W to the N-doped system yields significant band gap narrowing gap by 0.5 eV. This rationalizes recent experimental data which showed that N/W-doped titania exhibits higher visible-light photocatalytic efficiency than either N- or W-doping alone.

Molecular-dynamics simulations of methane hydrate dissociation

Nonequilibrium molecular-dynamics simulations have been carried out at 276.65 K and 68 bar for the dissolution of spherical methane hydrate crystallites surrounded by a liquid phase. The liquid was composed of pure water or a water-methane mixture ranging in methane composition from 50% to 100% of the corresponding theoretical maximum for the hydrate and ranged in size from about 1600 to 2200 water molecules. Four different crystallites ranging in size from 115 to 230 water molecules were used in the two-phase systems; the nanocrystals were either empty or had a methane occupation from 80% to 100% of the theoretical maximum. The crystal-liquid systems were prepared in two distinct ways, involving constrained melting of a bulk hydrate system or implantation of the crystallite into a separate liquid phase. The breakup rates were very similar for the four different crystal sizes investigated. The method of system preparation was not found to affect the eventual dissociation rates, despite a lag time of approximately 70 ps associated with relaxation of the liquid interfacial layer in the constrained melting approach. The dissolution rates were not affected substantially by methane occupation of the hydrate phase in the 80%-100% range. In contrast, empty hydrate clusters were found to break up significantly more quickly. Our simulations indicate that the diffusion of methane molecules to the surrounding liquid layer from the crystal surface appears to be the rate-controlling step in hydrate breakup. Increasing the size of the liquid phase was found to reduce the initial delay in breakup. We have compared breakup rates computed using different long-range electrostatic methods. Use of the Ewald, minimum image, and spherical cut-off techniques led to more rapid dissociation relative to the Lekner method.

Hydrogen bonding and molecular mobility in liquid water in external electromagnetic fields

Nonequilibrium molecular dynamics simulations of water have been performed in the isothermal–isobaric ensemble in the presence of external electromagnetic fields of varying intensity in the microwave to far-infrared frequency range, using a rigid/polarizable and a flexible/nonpolarizable potential model, from 260 to 400 K. Significant alterations in molecular mobility and hydrogen bonding patterns were found vis-a-vis zero-field conditions. In addition, the influence of the isothermal–isobaric ensemble on these observations was gauged by means of comparison with pure Newtonian simulation findings in the presence of external fields, and the former results were in reasonable accord with the latter.

Structural and dynamical properties of methane clathrate hydrates

Equilibrium molecular dynamics (MD) simulations have been performed in both the NVT and NPT ensembles to study the structural and dynamical properties of fully occupied methane clathrate hydrates at 50, 125, and 200 K. Five atomistic potential models were used for water, ranging from fully flexible to rigid polarizable and nonpolarizable. A flexible and a rigid model were utilized for methane. The phonon densities of states were evaluated and the localized rattling modes for the methane molecules were found to couple to the acoustic phonons of the host lattice. The calculated methane density of states was found to be in reasonable agreement with available experimental data.

Controlled semiconductor nanorod assembly from solution: influence of concentration, charge and solvent nature

Spontaneous supercrystal organisation of semiconductor nanorods (CdS and CdSe) of different aspect ratios into ordered superstructures was obtained by controlled evaporation of a nanorod solution. The rods either align into two dimensional close packed perpendicular arrays or into one dimensional rail tracks depending on the total interaction energy between the rods in solution. A detailed study has identified critical factors that affect this interaction energy such as nanorod concentration, surface charge, dipole moment and solvent nature (polarity and volatility), thereby allowing a general approach to control the nature of nanorod assembly (1D or 2D). Molecular dynamics (MD) of small charged nanorods showed that opposite dipolar alignment (antiferromagnetic) was the preferred rod orientation during self-assembly.

Determining the appropriate exchange-correlation functional for time-dependent density functional theory studies of charge-transfer excitations in organic dyes

UV-Vis spectra are calculated using time-dependent density functional theory for several organic dyes--4-(N, N-dimethylamino) benzonitrile, alizarin, squaraine, polyene-linker dyes, oligothiophene-containing coumarin dyes (NKX series) and triphenylamine-donor dyes. Most of these dyes (except, for the first two) or their derivatives are considered to be promising organic dyes for dye-sensitized solar cells. An accurate description of the photophysics of such dyes is imperative for understanding and creating better dyes. To this end, we studied the dyes within several approximations to the exchange-correlation functional. The chosen functionals--PBE, M06L, B3LYP, M06, CAM-B3LYP, and wB97--represent the various classes of approximations that are currently being used to study material properties. From amongst the six approximations studied here, CAM-B3LYP outperformed the others in its description of charge-transfer excitations in most (though, not all) of the dyes. This study shows why it is difficult to choose a particular functional a priori, especially when starting out with a new dye for solar cell application. A possible way to judge the fitness of an approximation is used in this work and it is shown to provide a good quantitative guideline for subsequent research in this field.

Denaturation of hen egg white lysozyme in electromagnetic fields: A molecular dynamics study

Nonequilibrium molecular dynamics simulations of hen egg white lysozyme have been performed in the canonical ensemble at 298 K in the presence of external electromagnetic fields of varying intensity in the microwave to far-infrared frequency range. Significant nonthermal field effects were noted, such as marked changes in the proteins secondary structure which led to accelerated incipient local denaturation relative to zero-field conditions. This occurred primarily as a consequence of alignment of the proteins total dipole moment with the external field, although the enhanced molecular mobility and dipolar alignment of water molecules is influential on sidechain motion in solvent-exposed regions. The applied field intensity was found to be highly influential on the extent of denaturation in the frequency range studied, and 0.25-0.5 V Arms-1 fields were found to induce initial denaturation to a comparable extent to thermal denaturation in the 400 to 500 K range. In subsequent zero-field simulations following exposure to the e/m field, the extent of perturbation from the native fold and the degree of residual dipolar alignment were found to be influential on incipient folding.

Nonequilibrium molecular dynamics study of electric and low-frequency microwave fields on hen egg white lysozyme

Nonequilibrium molecular dynamics simulations of various mutants of hen egg white lysozyme have been performed at 300 K and 1 bar in the presence of both external static electric and low-frequency microwave (2.45 GHz) fields of varying intensity. Significant nonthermal field effects were noted, such as marked changes in the proteins secondary structure relative to the zero-field state, depending on the field conditions, mutation, and orientation with respect to the applied field. This occurred primarily as a consequence of alignment of the proteins total dipole moment with the external field, although the dipolar alignment of water molecules in both the solvation layer and the bulk was also found to be influential. Substantial differences in behavior were found for proteins with and without overall net charges, particularly with respect to translational motion. Localized motion and perturbation of hydrogen bonds were also found to be evident for charged residues.