Energy Sciences

[No abstract]

Molecular beam scattering experiments are carried out to study collisions between Ne atoms (E _i = 24.3 kJ mol^-1) and the surface of a cold salty water (8 m LiBr_(aq), 230 K) flat jet. Translational energy distributions are collected as a function of scattering angle using a rotatable mass spectrometer. Impulsive scattering and thermal desorption contribute to the overall scattering distributions, but impulsive scattering dominates at all three incidence angles explored. Highly super-specular scattering is observed in the impulsive scattering channel that is attributed to anisotropic momentum transfer to the liquid surface. The thermal desorption channel exhibits a cos θ angular distribution. Compared to Ne scattering from dodecane, fractional energy loss in the impulsive scattering channel is much larger across a wide range of deflection angles. A soft-sphere model is applied to investigate the kinematics of energy transfer between the scatterer and liquid surface. Fitting to this model yields an effective surface mass of 250_-60 ⁺¹⁰⁰ amu and internal excitation of 11.8 ± 1.6 kJ mol^-1, both of which are considerably larger than for Ne/dodecane. It thus appears that energy transfer to cold salty water is more efficient than to a dodecane liquid surface, a result attributed to the extensive hydrogen-bonded network of liquid water and roughness of the liquid surface.

Water treatment technologies separate relevant solutes from water resources for water reuse, valuable resource recovery, and increasing the equity and availability of clean water worldwide. Although a variety of treatment methods exist, their performance needs to be improved to enable selective separation with increased durability and fouling resistance. To achieve this, we need to gain a better understanding of how molecular-level physics and chemistry impact integrated systems. Regarding current research on water treatment techniques, there is a clear need to study such systems under realistic environmental conditions. In this review, we aim to show that X-ray spectroscopic techniques are uniquely positioned to provide such information by obtaining detailed molecular insight into phenomena relevant to water research. By doing so, we hope to accelerate the rational design of novel treatment materials and processes. Specifically, a deeper understanding of the complex and interconnected phenomena that impact multilevel water treatment processes will lead to the successful development of next-generation water purification technologies.

Photoinduced biological and chemical reactions are often based on key structural transformations of a molecule driven across multiple electronic states. Acetylacetone (AcAc) is a prototypical system for complex chemical pathways involving several conical intersections (CI) and singlet-triplet intersystem crossings (ISC) characterized by distinct geometries. In the gas phase, AcAc is predominantly in a planar ring-like enolic form stabilized by a strong intramolecular O-H···O hydrogen bond. Following excitation into the S₂ (ππ*) state at 266 nm, acetylacetone undergoes rapid internal conversion followed by intersystem crossing. Such relaxation pathways are associated with structural changes including ring opening, deplanarization, and bond elongation. In this work, ultrafast electron diffraction (UED) at the SLAC MeV-UED setup is employed as a direct structural probe with a time resolution of 160 fs. Together with trajectory surface hopping simulations, analysis of the UED data provides a new perspective on the early time nuclear dynamics in acetylacetone. Specifically, AcAc is observed to undergo ring opening, deplanarization, and bond elongation all within the first 700 fs after photoexcitation. The monitored dynamics is associated mainly with the nuclear motion on the S₁ potential energy surface, formed after very rapid transfer from S₂ to S₁, allowing AcAc to reach the conical intersection to intersystem crossing. Such time scales of nuclear motion are contrasted with the time scales of electronic transitions in AcAc that were previously characterized with spectroscopic methods, specifically internal conversion (<100 fs) and intersystem crossing (∼1.5 ps).

Metal-mediated electrochemical synthesis of ammonia (NH₃) is a promising method to activate N₂ at room temperature. While a Li-mediated approach has been optimized to produce NH₃ at high current density and selectivity, Li's scarcity and its highly negative plating potential limit scalability and energy efficiency. Alternative mediators have been proposed, but only Ca has shown some promise, achieving ≈50% Faradaic efficiency (FE), though requiring voltages beyond -3 V. Here, we report a Mg-mediated nitrogen reduction reaction (Mg-NRR), where N₂ is activated on Mg to form Mg₃N₂, followed by protolysis to release NH₃ and regenerate Mg. A notable NH₃ FE of 25.28 ± 3.80% is achieved at a current density of -45 mA cm^-2, corresponding to an NH₃ partial current density of -11.30 ± 1.77 mA cm^-2 under 6 bar N₂. Isotope-labeled experiments confirm that NH₃ originates from N₂, with similar FE (25.15 ± 1.01%). Importantly, NH₃ production is demonstrated at a total cell potential as low as -3 V. This Li-free Mg-NRR system offers key advantages, including lower energy input and use of earth-abundant materials, making it a scalable route for sustainable NH₃ synthesis.

Supramolecular hosts create unique microenvironments which enable the tuning of reactions via steric confinement and electrostatics. It has been shown that "solvent shaping inside hydrophobic cavities" is an important thermodynamic driving force for guest encapsulation in the nanocage host. Here, we show that even small (5%) changes in the solvent composition can have a profound impact on the free energy of encapsulation. In a combined THz, NMR and ab initio MD study, we reveal that the preferential residing of a single DMSO molecule in the cavity upon addition of ≥5% DMSO results in a considerable change of ΔS from 63-76 cal mol^-1 K^-1 to 23-24 cal mol^-1 K^-1. This can be rationalized by reduction of the cavity volume due to the DMSO molecule which resides preferentially in the cavity. These results provide novel insights into the guest-binding interactions, emphasizing that the entropic driving force is notably influenced by even small changes in the solvent composition, irrespective of changes in metal ligand vertices. Having demonstrated that the local solvent composition within the cage is essential for regulating catalytic efficiency, solvent tuning might enable novel applications in supramolecular chemistry in catalysis and chemical separation.

Diffuse atomic orbital basis sets have proven to be essential to obtain accurate interaction energies, especially in regard to non-covalent interactions. However, they also have a detrimental impact on the sparsity of the one-particle density matrix (1-PDM), to a degree stronger than the spatial extent of the basis functions alone could explain. This is despite the fact that the matrix elements of the 1-PDM of insulators (systems with significant highest occupied molecular orbital-lowest unoccupied molecular orbital gaps) are expected to decay exponentially with increasing real-space distance from the diagonal. The observed low sparsity of the 1-PDM appears to be independent of representation and even persists after projecting the 1-PDM onto a real-space grid, leading to the conclusion that this "curse of sparsity" is solely a basis set artifact, which, counterintuitively, becomes worse for larger basis sets, seemingly contradicting the notion of a well-defined basis set limit. We show that this is a consequence of the low locality of the contra-variant basis functions as quantified by the inverse overlap matrix S-1 being significantly less sparse than its co-variant dual. Introducing the model system of an infinite non-interacting chain of helium atoms, we are able to quantify the exponential decay rate to be proportional to the diffuseness as well as local incompleteness of the basis set, meaning small and diffuse basis sets are affected the most. Finally, we propose one solution to the conundrum in the form of the complementary auxiliary basis set singles correction in combination with compact, low l-quantum-number basis sets, showing promising results for non-covalent interactions.

Chiral hybrid metal-halide perovskites show low-symmetry crystal structures, large Rashba splitting, spin-filtering, and strong chiroptical activity. Circular dichroism and circularly polarized photoluminescence have been investigated in chiral perovskites with increasingly distorted chiral structures. Here, we report the fabrication of chiral (R/S)-EBAPbI₃ (EBA = α-ethylbenzylamine) single crystals, which possess highly distorted octahedral structures with a high angle variance value of ∼68 degree². Using control in the fabrication conditions, we transfer chiral single crystals to thin films and achieve different crystal orientation preferences that induce tunable chiroptical properties to their heterostructures with PbI₂ nanodomains, which we characterize with in situ X-ray diffraction and grazing-incidence wide-angle X-ray scattering measurements. Using transient chiroptical spectroscopies, we resolve photoexcited charge carrier dynamics and chirality transfer processes in such heterostructures down to cryogenic temperatures. We observe rapid carrier transfer along the in-plane (002) facets in chiral perovskite phases to PbI₂ nanostructures within the initial few picoseconds, while carrier transfer along the out-of-plane (002) facets occurs at a slower rate. This fast transfer process leads to high photoluminescence intensities and large degrees of circular polarization in the emission from PbI₂ nanodomains at cryogenic temperatures. Our findings report a multidimensional chiral-achiral heterostructure which takes advantage of controllable chirality transfer and offers new routes for future spintronic and chiroptical applications.

Although cross sections for double photoionization (DPI) are much smaller than single photoionization cross sections, DPI by a single photon is a sensitive means of probing correlated electron dynamics. We extend a rigorous method for computing double ionization amplitudes in both time-independent and time-dependent computational formalisms by eliminating the requirement that the one-electron testing functions used to extract DPI amplitudes are continuum eigenfunctions that are orthogonal to the singly ionized states of the target. It is demonstrated that simple Coulomb testing functions can be used in an integral for the DPI amplitude restricted to the interaction region if few low-energy bound states of the singly charged ion are projected out of the full-scattered wave solution, resulting in surprisingly accurate triply and singly differential cross sections. These findings will simplify calculations of DPI amplitudes in more complicated polyatomic molecular targets than the benchmark two-electron systems considered here.

The requirement that photogenerated holes accumulate to drive the rate-limiting step is thought to cause slow water oxidation by TiO₂ to form O₂; however, detailed kinetic studies that directly establish the connection between photoabsorption and surface reactions have not been reported. In this work, we use physically realistic kinetic models of photo-driven water oxidation on TiO₂ to evaluate how hole generation, bulk diffusion, surface mobility, and reaction are coupled. The calculations show that hole formation and diffusion in the bulk crystal dominate O₂ formation at low light intensity, resulting in an apparent high-order dependence of the O₂ production rate on holes. As the light intensity increases, the water-splitting reaction becomes nearly independent of hole concentrations because of a buildup of intermediates that can only react thermally. Although it is believed that high hole mobility is a requirement for hole accumulation, a comparison of predicted to observed surface species indicates that immobilized holes dominate the surface reactivity. The primary surface reaction sites are predicted to involve oxygen atoms that bridge two Ti atoms, supplied with OH formed by water dissociation on the Ti sites. Because of the similarity among photocatalytic water oxidation mechanisms on diverse metal oxide semiconductors, which generally have low hole mobilities, the findings from this work may be relevant to them as well. If so, manipulations of hole mobility and acceleration of the rate of thermal steps may provide a general pathway for improving water oxidation efficiency.