J. R. Schmidt

Efficient hydrogen evolution catalysis using ternary pyrite-type cobalt phosphosulphide

The scalable and sustainable production of hydrogen fuel through water splitting demands efficient and robust Earth-abundant catalysts for the hydrogen evolution reaction (HER). Building on promising metal compounds with high HER catalytic activity, such as pyrite structure cobalt disulphide (CoS2), and substituting non-metal elements to tune the hydrogen adsorption free energy could lead to further improvements in catalytic activity. Here we present a combined theoretical and experimental study to establish ternary pyrite-type cobalt phosphosulphide (CoPS) as a high-performance Earth-abundant catalyst for electrochemical and photoelectrochemical hydrogen production. Nanostructured CoPS electrodes achieved a geometrical catalytic current density of 10 mA cm(-2) at overpotentials as low as 48 mV, with outstanding long-term operational stability. Integrated photocathodes of CoPS on n(+)-p-p(+) silicon micropyramids achieved photocurrents up to 35 mA cm(-2) at 0 V versus the reversible hydrogen electrode (RHE), onset photovoltages as high as 450 mV versus RHE, and the most efficient solar-driven hydrogen generation from Earth-abundant systems.

Hydrogen bonding definitions and dynamics in liquid water

X-ray and neutron diffractions, vibrational spectroscopy, and x-ray Raman scattering and absorption experiments on water are often interpreted in terms of hydrogen bonding. To this end a number of geometric definitions of hydrogen bonding in water have been developed. While all definitions of hydrogen bonding are to some extent arbitrary, those involving one distance and one angle for a given water dimer are unnecessarily so. In this paper the authors develop a systematic procedure based on two-dimensional potentials of mean force for defining cutoffs for a given pair of distance and angular coordinates. They also develop an electronic structure-based definition of hydrogen bonding in liquid water, related to the electronic occupancy of the antibonding OH orbitals. This definition turns out to be reasonably compatible with one of the distance-angle geometric definitions. These two definitions lead to an estimate of the number of hydrogen bonds per molecule in liquid simple point charge/extended (SPC/E) water of between 3.2 and 3.4. They also used these and other hydrogen-bond definitions to examine the dynamics of local hydrogen-bond number fluctuations, finding an approximate long-time decay constant for SPC/E water of between 0.8 and 0.9 ps, which corresponds to the time scale for local structural relaxation.

Hydrogen bonding and Raman, IR, and 2D-IR spectroscopy of dilute HOD in liquid D2O

We present improvements on our previous approaches for calculating vibrational spectroscopy observables for the OH stretch region of dilute HOD in liquid D2O. These revised approaches are implemented to calculate IR and isotropic Raman spectra, using the SPC/E simulation model, and the results are in good agreement with experiment. We also calculate observables associated with three-pulse IR echoes: the peak shift and 2D-IR spectrum. The agreement with experiment for the former is improved over our previous calculations, but discrepancies between theory and experiment still exist. Using our proposed definition for hydrogen bonding in liquid water, we decompose the distribution of frequencies in the OH stretch region in terms of subensembles of HOD molecules with different local hydrogen-bonding environments. Such a decomposition allows us to make the connection with experiments and calculations on water clusters and more generally to understand the extent of the relationship between transition frequency and local structure in the liquid.

Ultrafast vibrational spectroscopy of water and aqueous N-methylacetamide: Comparison of different electronic structure/molecular dynamics approaches.

Kwac and Cho [J. Chem. Phys. 119, 2247 (2003)] have recently developed a combined electronic structure/molecular dynamics approach to vibrational spectroscopy in liquids. The method involves fitting ab initio vibrational frequencies for a solute in a cluster of solvent molecules to a linear combination of the electrostatic potentials on the solute atoms due to the charges on the solvent molecules. These authors applied their method to the N-methylacetamide-D/D(2)O system. We (S. A. Corcelli, C. P. Lawrence, and J. L. Skinner, [J. Chem. Phys. 120, 8107 (2004)]) have recently explored a closely related method, where instead of the electrostatic potential, the solute vibrational frequencies are fit to the components of the electric fields on the solute atoms due to the solvent molecules. We applied our method to the HOD/D(2)O and HOD/H(2)O systems. In order to make a direct comparison of these two approaches, in this paper we apply their method to the water system, and our method to the N-methylacetamide system. For the water system we find that the electric field method is superior to the potential approach, as judged by comparison with experiments for the absorption line shape. For the N-methylacetamide system the two methods are comparable.

Mixed quantum-classical equilibrium: Surface hopping

We re-examine the analysis of the equilibrium limits of the fewest switches surface hopping algorithm for mixed quantum-classical dynamics. In contrast with previously reported results, we show that surface hopping does not, in general, exactly yield Boltzmann equilibrium, but that in practice the observed deviations are quite small. We also demonstrate that surface hopping does approach the exact equilibrium distribution in both the limits of small adiabatic splitting and/or strong nonadiabatic coupling. We verify these analytical results with numerical simulations for a simple two-level quantum system connected to a bath of classical particles.

Vertical Heterostructures of Layered Metal Chalcogenides by van der Waals Epitaxy

We report a facile chemical vapor deposition (CVD) growth of vertical heterostructures of layered metal dichalcogenides (MX2) enabled by van der Waals epitaxy. Few layers of MoS2, WS2, and WSe2 were grown uniformly onto microplates of SnS2 under mild CVD reaction conditions (<500 °C) and the heteroepitaxy between them was confirmed using cross-sectional transmission electron microscopy (TEM) and unequivocally characterized by resolving the large-area Moiré patterns that appeared on the basal planes of microplates in conventional TEM (nonsectioned). Additional photoluminescence peaks were observed in heterostructures of MoS2-SnS2, which can be understood with electronic structure calculations to likely result from electronic coupling and charge separation between MoS2 and SnS2 layers. This work opens up the exploration of large-area heterostructures of diverse MX2 nanomaterials as the material platform for electronic structure engineering of atomically thin two-dimensional (2D) semiconducting heterostructures and device applications.

Hydrodynamic boundary conditions, the Stokes–Einstein law, and long-time tails in the Brownian limit

Using molecular dynamics computer simulation, we have calculated the velocity autocorrelation function and diffusion constant for a spherical solute in a dense fluid of spherical solvent particles. The size and mass of the solute particle are related in such a way that we can naturally approach the Brownian limit (when the solute becomes much larger and more massive than the solvent particles). We find that as long as the solute radius is interpreted as an effective hydrodynamic radius, the Stokes–Einstein law with slip boundary conditions is satisfied as the Brownian limit is approached (specifically, when the solute is roughly 100 times more massive than the solvent particles). In contrast, the Stokes–Einstein law is not satisfied for a tagged particle of the neat solvent. We also find that in the Brownian limit the amplitude of the long-time tail of the solute’s velocity autocorrelation function is in good agreement with theoretical hydrodynamic predictions. When the solvent density is substantially lo...

Approaches for the calculation of vibrational frequencies in liquids: Comparison to benchmarks for azide/water clusters

Ultrafast vibrational spectroscopy experiments, together with molecular-level theoretical interpretation, can provide important information about the structure and dynamics of complex condensed phase systems, including liquids. The theoretical challenge is to calculate the instantaneous vibrational frequencies of a molecule in contact with a molecular environment, accurately and quickly, and to this end a number of different methods have been developed. In this paper we critically analyze these different methods by comparing their results to accurate benchmark calculations on azide/water clusters. We also propose an optimized quantum mechanics/molecular mechanics method, which for this problem is superior to the other methods.

Physically Motivated, Robust, ab Initio Force Fields for CO2 and N2

We present a novel methodology for developing physically motivated, first-principles polarizable force fields and apply these techniques to the specific cases of CO(2) and N(2). Exchange, electrostatic, induction, and dispersion interaction parameters were fit to symmetry adapted perturbation theory (SAPT) dimer energy calculations, with explicit terms to account for each of the dominant fundamental interactions between molecules. Each term is represented by a physically appropriate functional form and fitted individually based on the results of the SAPT decomposition. The resulting CO(2) and N(2) force field was benchmarked against a diverse set of experimental data, including the second virial coefficient, density, scattering structure factor, heat capacity, enthalpy of vaporization, and vapor-liquid coexistence curves. In general, excellent agreement with experimental data is obtained with our model. Due to the physical nature of their construction, these force fields are robust and transferable to environments for which they were not specifically parametrized, including gas mixtures, and we anticipate applications in modeling CO(2)/N(2) adsorption in polar and/or heterogeneous media.

Line shapes and photon echoes within a generalized Kubo model

Kubo introduced a stochastic model of frequency fluctuations that naturally leads to an understanding of homogeneous and inhomogeneous broadening in condensed phase spectroscopy. In this Letter we discuss a generalized Kubo model that has two different timescales. The model is relevant for a variety of different physical situations, and in certain limits describes combined homogeneous and inhomogeneous broadening. We explore the limits of applicability of this homogeneous/inhomogeneous phenomenology for both the line shape and the integrated two-pulse photon echo intensity. Our results have implications for non-linear ultrafast liquid-state spectroscopy.