Session S2: Focus Session: Solvation, Dynamics, and Reactivity in Complex Environments III

Time-dependent density-functional theory for real-time electronic dynamics on material surfaces

The real-time electronic dynamics on material surfaces is critically important to a variety of applications. However, numerical simulations are rather challenging for conventional first-principles methods such as the time-dependent density-functional theory (TDDFT). To solve this problem, we extend the applicability of TDDFT to open electronic systems[\newblock Phys. Rev. B {\bf 75}, 195127 (2007)]. The dissipative system-environment interactions are treated by a hierarchical equations of motion (HEOM) approach. The combined TDDFT--HEOM method, along with a \emph{k}-sampling scheme [J. Chem. Phys. {\bf 132}, 114703 (2010)] for calculating the spectral function of a two-dimensional system, is applied to simulate real-time electronic dynamics on material surfaces. Two prototypical scenarios are exemplified [Phys. Rev. B (accepted) (2013)]: the relaxation of an excess electron on a graphene surface, and the electron transfer across the molecule-graphene interface. These two examples accentuate the fundamental importance and usefulness of an open-system TDDFT approach, and they also provide some insights into the characteristic features of temporal electron evolution and dissipation on surfaces of bulk materials.   

The electrostatic double layer of Pt/water interfaces from first principles molecular dynamics

The formation of the electrostatic double layer is the most basic phenomenon taking place at electrified interfaces. However, even in the relatively simple case of a Pt/water interface, none of the current theoretical approaches provides a realistic microscopic view of this double layer, accounting for electronic, polarization and solvent re-organization effects. Here we provide for the first time a comprehensive description of the electrostatic double layer of a Pt-water interface, based on ab initio computations, including charge polarization effects at both sides of the interface, explicit solvent and its rearrangements upon changing the electrode polarization. This interface has been modeled with up to 1000 atoms. A simple, fully dissociated salt in solution has been explicitly included. Varying the relative number of cations and anions provides a way to control the charge on the electrode, controlling, in turn, the applied potential. The proposed approach allows to provide a detailed description of the structure of the Pt/water double layer reproducing the localization of electric field and potential energy drop within a microscopic distance from the metal surface. An a posteriori calibration of the relation between charge and potential is performed, analyzing the potential energy profile vs. the distance from the electrode for any given charge, providing for the first time a realistic ab initio determination of the interface capacitance and the point of zero charge.   

Venturing into the kinetics and mechanism of nanoconfined solid-state reactions: Trimerization of sodium dicyanamide in nanopores

This study represents the first attempt to determine the effect of nanoconfinement on the kinetics and mechanism of solid-state reactions. FTIR, NMR, and DSC were employed to analyze the thermally initiated trimerization of sodium dicyanamide (NaC$_{2}$N$_{3}$) to sodium tricyanomelaminate (Na$_{3}$C$_{6}$N$_{9}$) in bulk and organically modified nanopores. The trimerization occurred at a decelerated rate as evidenced by an increase in reaction temperature as measured by DSC. Nanoconfinement did not cause apparent changes in the reaction mechanism as the products of the reaction were the same in bulk and in nanopores. Kinetic analysis linked the deceleration to a dramatic decrease (several orders of magnitude) in the pre-exponential factor. This effect is especially significant in view of previous studies on nanoconfined liquid state reactions in which the effect is opposite: considerable acceleration due to an increase in the pre-exponential factor. We propose that the difference arises respectively from disordering of the solid and ordering of the liquid reaction media.   

Water adsorption and proton conduction in metal-organic frameworks: Insights from molecular simulations

Metal-organic frameworks (MOFs) are a relatively new class of porous materials that hold great potential for a wide range of applications in chemistry, materials science, and nanoengineering. Compared to other porous materials such as zeolites, MOF properties are highly tunable. In particular, it has been shown that both size and shape of the MOF pores can be rationally designed for specific applications. For example, the ability to modify the framework properties with respect to hydrophilicity/hydrophobicity and acidity/basicity can enable the direct control of proton conduction through carrier molecules adsorbed inside the pores. Here, I report on our current efforts aimed at providing a molecular-level characterization of water-mediated proton conduction through the MOF pores. Particular emphasis will be put on correlation between proton conduction and both structural and chemical properties of the frameworks as well as on the dynamical behavior of water confined in the MOF pores.   

Simulating and Modeling Transport Through Atomically Thin Membranes

The world is running out of clean portable water. The efficacy of water desalination technologies using porous materials is a balance between membrane selectivity and solute throughput. These properties are just starting to be understood on the nanoscale, but in the limit of atomically thin membranes it is unclear whether one can apply typical continuous time random walk models. Depending on the size of the pore and thickness of the membrane, mass transport can range from single stochastic passage events to continuous flow describable by the usual hydrodynamic equations. We present a study of mass transport through membranes of various pore geometries using reverse nonequilibrium simulations, and analyze transport rates using stochastic master equations.   

Theoretical analysis on ion transport through polymer networks in electrochemical capacitors

The development of predictive methods for deformable electronics calls for an equally composite theoretical foundation that unites traditionally separated fields. We are pioneering theoretical methods that unite polymer physics with liquid state theory, and develop a dynamical algorithm for inhomogeneous polarizable media between capacitor plates. By a quantitative study of the local molecular correlations we can explain the macroscopic behavior and the induced (non-equilibrium) potentials of mean force between the ions, the supporting medium, and the electrodes. Several timescales are found that correspond to different relaxation processes, related to ion diffusion, double layer formation, and the elastic response of the network. The application of an alternating current reveals a complex frequency-dependent response, by which the relative importance of the different underlying processes can be tuned. Typical non-equilibrium forces, generated by the applied field, are found to arise between regions with sharp gradients in the molecular structure or supporting background. The results may inform experimental efforts on noise reduction in soft capacitors, and suggest new functionality based on frequency-dependent non-equilibrium forces.   

Direct simulation of proton-coupled electron transfer reaction dynamics and mechanisms

Proton-coupled electron transfer (PCET) reactions, in which both an electron and an associated proton undergo reactive transfer, play an important role in many chemical and biological systems. Due to the complexity of this class of reactions, a variety of different mechanisms fall under the umbrella of PCET. However, the physical driving forces that determine the preferred mechanism in a given system still remain poorly understood. Towards this end, we extend ring polymer molecular dynamics (RPMD), a path-integral quantum dynamics method, to enable the direct simulation and characterization of PCET reaction dynamics in both fully atomistic and system-bath models of organometallic catalysts. In addition to providing validation for the simulation method via extensive comparison with existing PCET rate theories, we analyze the RPMD trajectories to investigate the competition between the concerted and sequential reaction mechanisms for PCET, elucidating the large role of the solvent in controlling the preferred mechanism. We further employ RPMD to determine the kinetics and mechanistic features of concerted PCET reactions across different regimes of electronic and vibrational coupling, providing evidence for a new and distinct PCET reaction mechanism.   

Wetting of hydrophobic and nanostructured surfaces

Understanding wetting phenomena on nanostructured and nanopatterned surfaces is important in materials science and biology. The talk will highlight some of our recent progress on nanowetting of surfaces with topological and chemical heterogeneities using molecular modeling. Examples will include electric, pressure, or chemistry induced dynamic transitions of water on superhydrophobic surfaces to achieve reversible switching between contrasting wetting states.   

Memory in 2D FT Spectra of Quantum Dots

We have used the first femtosecond 2DFT spectrometer in the short-wave infrared to record 2DFT spectra of the polar dye IR26 in dichloroethane. The 2DFT spectra of IR26 at early mixing times shows a diagonally elongated positive peak, which reflects the strong correlation between excitation and detection frequencies. The peak also has a slight shift above the diagonal and an off-diagonal negative region that is indicative of vibrational and solvent frequency memory (the finite timescale for frequency shifts from inertial solvation). Nearly all correlation is gone at long mixing times and the 2D spectra approach a product lineshape. We also measured the first 2DFT spectra of oleate-capped colloidal PbSe quantum dots in tetrachloroethylene in the short-wave infrared region. These measure a bi-exciton binding energy that is consistent with prior spectrally resolved pump-probe experiments. Most interestingly, certain similarities between the 2DFT spectra of IR26 and quantum dots at early mixing times points towards coherent phonon/solvent memory; calculations by Prezhdo and co-workers both predicted phonon memory in quantum dots and indicated that it affects carrier relaxation. The analysis and implications of these results will be discussed.   

First-principles study of single water interactions with theLaMnO$_3$ surface in the presence of defects, Sr substitution and varied surface morphologies

Lanthanum manganite (LaMnO$_3$) has been shown to have tremendous catalytic activity for the oxygen reduction reaction (OER) and oxygen evolution reaction (ORR) and is cheaper than other catalytic materials (Suntivich, Jin et al. 2011 Nature Chemistry 3, 546). Previous work studying ORR and OER indicates that water plays an important role in the intermediate reactions, however very little research has been done on the interaction between water and the LaMnO$_3$ surface (Wang Yan, et al. 2013. Journal Phys Chem C 5, 2106). Using density functional theory calculations, we examine the details of water adsorption and dissociation on a perfect and defective LaMnO$_3$ surfaces. We find that oxygen vacancies cause a strong preference for water dissociation on the surface but that the interaction is largely robust in the presence of strontium (Sr) substitutions. We also explore the dependence of interaction on structural parameters with a few different surface morphologies. Our results provide insights to the catalytic function of LaMnO$_3$ in both ORR and OER applications.   

Nucleus-Coupled Electron Transfer Mechanism for TiO2-Catalyzed Water Splitting

Using first-principles calculations employing explicit interface of TiO$_2$ crystal and liquid water, we reveal the microscopic mechanism of the oxygen evolution reaction (OER). It is found that, during the formation of an O--O species, such as HO--OH and O--OH, an occupied molecular orbital with anti-bonding character evolves from the valence band and pops up all the way into the conduction band of TiO$_2$. This occupied high-energy orbital results in a high reaction barrier making the OER forbidden in the dark. The presence of photoholes depletes this anti-bonding orbital, which significantly reduces the reaction energy and determines the reaction barrier in the rate-limting step. A novel reaction mechanism, termed necleus-coupled electron transfer (NCET), emerges from this study. In this mechanism, the oxidation of a pair of hydroxyl groups, which is an electron transfer reaction, is enabled by the movement of the nuclei (i.e., the two O atoms moving towards O-O bond formation) that pushes the $reactive$ orbital (the $\sigma^{*}_{2p}$ orbital in the present case) to become the $frontier$ orbital (i.e., above the valence band maximum of TiO). Based on the NCET mechanism, we identify a reaction pathway of the OER that exhibits a kinetic barrier surmountable at room temperature.