Correlating Cu dopant concentration, optoelectronic properties, and photocatalytic activity of ZnO nanostructures: experimental and theoretical insights.

The photocatalytic activity of photocatalysts can be enhanced by cation doping, and the dopant concentration plays a key role in achieving high efficiency. This study explores the impact of copper (Cu) doping at concentrations ranging from 0% to 10% on the microstructural, optical, electronic, and photocatalytic properties of zinc oxide (ZnO) nanostructures. The x-ray diffraction analysis shows a non-linear alteration in the lattice parameters with increasing the Cu content and the formation of CuO as a secondary phase at the Cu concentration of >3%. Density functional theory calculations provide insights into the change in the electronic structures of ZnO induced by Cu doping, leading to the formation of localizeddelectronic levels above the valence band maximum. The modulation of the electronic structure of ZnO by Cu doping facilitates the visible light absorption via O 2p → Cu 3d and Cu 3d → Zn 2p transitions. Photoluminescence spectroscopy reveals a quenching of the defect-related emission peak at approximately 570 nm for all Cu-doped ZnO nanostructures, indicating a reduction in the structural and other defects. The photocatalytic activity tests confirm that the ZnO nanostructures doped with 3% Cu exhibit the highest efficiency compared to other samples due to the suitable band-edge position and visible light absorption.

Two-Dimensional High-Entropy Selenides for Boosting Visible-Light-Driven Photocatalytic Performance.

High-entropy materials (HEMs) have garnered extensive attention owing to their diverse and captivating physicochemical properties. Yet, fine-tuning morphological properties of HEMs remains a formidable challenge, constraining their potential applications. To address this, we present a rapid, low-energy consumption diethylenetriamine (DETA)-assisted microwave hydrothermal method for synthesizing a series of two-dimensional high-entropy selenides (HESes). Subsequently, the obtained HESes are harnessed for photocatalytic water splitting. Noteworthy is the optimized HESes, Cd0.9Zn1.2Mn0.4Cu1.8Cr1.2Se4.5, showcasing an output rate of hydrogen of 16.08 mmol h-1 g-1 and a quantum efficiency of ca. 30% under 420 nm monochromatic LED irradiation. It is revealed that the photocatalytic performance of these HESes stems not only from the enlarged specific surface area and enhanced photogenerated charge carrier utilization efficiency but also from the promoted formation of the Cd-Hads bond, influenced by multiple principal elements on the Cd. These findings provide a guide for the design of HEMs tailored for various applications.

Unveiling the Borohydride Ion through Force-Field Development.

The borohydride ion, BH4-, is an essential reducing agent in many technological processes, yet its full understanding has been elusive, because of at least two significant challenges. One challenge arises from its marginal stability in aqueous solutions outside of basic pH conditions, which considerably limits the experimental thermodynamic data. The other challenge comes from its unique and atypical hydration shell, stemming from the negative excess charge on its hydrogen atoms, which complicates the accurate modeling in classical atomistic simulations. In this study, we combine experimental and computer simulation techniques to devise a classical force field for NaBH4 and deepen our understanding of its characteristics. We report the first measurement of the ion's activity coefficient and extrapolate it to neutral pH conditions. Given the difficulties in directly measuring its solvation free energies, owing to its instability, we resort to quantum chemistry calculations. This combined strategy allows us to derive a set of nonpolarizable force-field parameters for the borohydride ion for classical molecular dynamics simulations. The derived force field simultaneously captures the solvation free energy, the hydration structure, as well as the activity coefficient of NaBH4 salt across a broad concentration range. The obtained insights into the hydration shell of the BH4- ion are crucial for accurately modeling and understanding its interactions with other molecules, ions, materials, and interfaces.

Short-Range Cooperative Slow-down of Water Solvation Dynamics Around SO4 2--Mg2+ Ion Pairs.

The presence of ions affects the structure and dynamics of water on a multitude of length and time scales. In this context, pairs of Mg2+ and SO4 2- ions in water constitute a prototypical system for which conflicting pictures of hydration geometries and dynamics have been reported. Key issues are the molecular pair and solvation shell geometries, the spatial range of electric interactions, and their impact on solvation dynamics. Here, we introduce asymmetric SO4 2- stretching vibrations as new and most specific local probes of solvation dynamics that allow to access ion hydration dynamics at the dilute concentration (0.2 M) of a native electrolyte environment. Highly sensitive heterodyne 2D-IR spectroscopy in the fingerprint region of the SO4 2- ions around 1100 cm-1 reveals a specific slow-down of solvation dynamics for hydrated MgSO4 and for Na2SO4 in the presence of Mg2+ ions, which manifests as a retardation of spectral diffusion compared to aqueous Na2SO4 solutions in the absence of Mg2+ ions. Extensive molecular dynamics and density functional theory QM/MM simulations provide a microscopic view of the observed ultrafast dephasing and hydration dynamics. They suggest a molecular picture where the slow-down of hydration dynamics arises from the structural peculiarities of solvent-shared SO4 2--Mg2+ ion pairs.

Water-separated ion pairs cause the slow dielectric mode of magnesium sulfate solutions.

We compare the dielectric spectra of aqueous MgSO4 and Na2SO4 solutions calculated from classical molecular dynamics simulations with experimental data, using an optimized thermodynamically consistent sulfate force field. Both the concentration-dependent shift of the static dielectric constant and the spectral shape match the experimental results very well for Na2SO4 solutions. For MgSO4 solutions, the simulations qualitatively reproduce the experimental observation of a slow mode, the origin of which we trace back to the ion-pair relaxation contribution via spectral decomposition. The radial distribution functions show that Mg2+ and SO42- ions form extensive water-separated-and thus strongly dipolar-ion pairs, the orientational relaxation of which provides a simple physical explanation for the prominent slow dielectric mode in MgSO4 solutions. Remarkably, the Mg2+-SO42- ion-pair relaxation extends all the way into the THz range, which we rationalize by the vibrational relaxation of tightly bound water-separated ion pairs. Thus, the relaxation of divalent ion pairs can give rise to widely separated orientational and vibrational spectroscopic features.

Force fields for monovalent and divalent metal cations in TIP3P water based on thermodynamic and kinetic properties.

Metal cations are essential in many vital processes. In order to capture the role of different cations in all-atom molecular dynamics simulations of biological processes, an accurate parametrization is crucial. Here, we develop force field parameters for the metal cations Li+, Na+, K+, Cs+, Mg2+, Ca2+, Sr2+, and Ba2+ in combination with the TIP3P water model that is frequently used in biomolecular simulations. In progressing toward improved force fields, the approach presented here is an extension of previous efforts and allows us to simultaneously reproduce thermodynamic and kinetic properties of aqueous solutions. We systematically derive the parameters of the 12-6 Lennard-Jones potential which accurately reproduces the experimental solvation free energy, the activity derivative, and the characteristics of water exchange from the first hydration shell of the metal cations. In order to reproduce all experimental properties, a modification of the Lorentz-Berthelot combination rule is required for Mg2+. Using a balanced set of solution properties, the optimized force field parameters aim to capture the fine differences between distinct metal cations including specific ion binding affinities and the kinetics of cation binding to biologically important anionic groups.

Orientation-Induced Adsorption of Hydrated Protons at the Air-Water Interface.

The surface tension of the air-water interface increases upon addition of inorganic salts, implying a negative surface excess of ionic species. Most acids, however, induce a decrease in surface tension, indicating a positive surface excess of hydrated protons. In combination with the apparent negative charge at pure air-water interfaces derived from electrokinetic experiments, this experimental observation has been a source of intense debate since the mid-19th century. Herein, we calculate surface tensions and ionic surface propensities at air-water interfaces from classical, thermodynamically consistent molecular dynamics simulations. The surface tensions of NaOH, HCl, and NaCl solutions show outstanding quantitative agreement with experiment. Of the studied ions, only H3 O+ adsorbs to the air-water interface. The adsorption is explained by the deep potential well caused by the orientation of the H3 O+ dipole in the interfacial electric field, which is confirmed by ab initio simulations.

Optimization of classical nonpolarizable force fields for OH(-) and H3O(+).

We optimize force fields for H3O(+) and OH(-) that reproduce the experimental solvation free energies and the activities of H3O(+) Cl(-) and Na(+) OH(-) solutions up to concentrations of 1.5 mol/l. The force fields are optimized with respect to the partial charge on the hydrogen atoms and the Lennard-Jones parameters of the oxygen atoms. Remarkably, the partial charge on the hydrogen atom of the optimized H3O(+) force field is 0.8 ± 0.1|e|--significantly higher than the value typically used for nonpolarizable water models and H3O(+) force fields. In contrast, the optimal partial charge on the hydrogen atom of OH(-) turns out to be zero. Standard combination rules can be used for H3O(+) Cl(-) solutions, while for Na(+) OH(-) solutions, we need to significantly increase the effective anion-cation Lennard-Jones radius. While highlighting the importance of intramolecular electrostatics, our results show that it is possible to generate thermodynamically consistent force fields without using atomic polarizability.

Force fields for divalent cations based on single-ion and ion-pair properties.

We develop force field parameters for the divalent cations Mg(2+), Ca(2+), Sr(2+), and Ba(2+) for molecular dynamics simulations with the simple point charge-extended (SPC/E) water model. We follow an approach introduced recently for the optimization of monovalent ions, based on the simultaneous optimization of single-ion and ion-pair properties. We consider the solvation free energy of the divalent cations as the relevant single-ion property. As a probe for ion-pair properties we compute the activity derivatives of the salt solutions. The optimization of the ionic force fields is done in two consecutive steps. First, the cation solvation free energy is determined as a function of the Lennard-Jones (LJ) parameters. The peak in the ion-water radial distribution function (RDF) is used as a check of the structural properties of the ions. Second, the activity derivatives of the electrolytes MgY(2), CaY(2), BaY(2), SrY(2) are determined through Kirkwood-Buff solution theory, where Y = Cl(-), Br(-), I(-). The activity derivatives are determined for the restricted set of LJ parameters which reproduce the exact solvation free energy of the divalent cations. The optimal ion parameters are those that match the experimental activity data and therefore simultaneously reproduce single-ion and ion-pair thermodynamic properties. For Ca(2+), Ba(2+), and Sr(2+) such LJ parameters exist. On the other hand, for Mg(2+) the experimental activity derivatives can only be reproduced if we generalize the combination rule for the anion-cation LJ interaction and rescale the effective cation-anion LJ radius, which is a modification that leaves the cation solvation free energy invariant. The divalent cation force fields are transferable within acceptable accuracy, meaning the same cation force field is valid for all halide ions Cl(-), Br(-), I(-) tested in this study.

Rational design of ion force fields based on thermodynamic solvation properties.

Most aqueous biological and technological systems contain solvated ions. Atomistic explicit-water simulations of ionic solutions rely crucially on accurate ionic force fields, which contain most commonly two adjustable parameters: the Lennard-Jones diameter and the interaction strength. Assuming these parameters to be properly optimized, the plethora of parameters one finds in the literature for one and the same ion is surprising. In principle, the two parameters should be uniquely determined by matching two ionic properties obtained for a particular water model and within a given simulation protocol with the corresponding experimental observables. Traditionally, ion parameters were chosen in a somewhat unsystematic way to reproduce the solvation free energy and to give the correct ion size when compared with scattering results. Which experimental observable one chooses to reproduce should in principle depend on the context within which the ionic force field is going to be used. In the present work we suggest to use the solvation free energy in conjunction with the solvation entropy to construct thermodynamically sound force fields for the alkali and halide ions for the simulation of ion-specific effects in aqueous environment. To that end we determine the solvation free energy and entropy of both cations and anions in the entire relevant parameter space. As an independent check on the quality of the resulting force fields we also determine the effective ionic radius from the first peak of the radial ion-water distribution function. Several difficulties during parameter optimization are discussed in detail. (i) Single-ion solvation depends decisively on water-air surface properties, which experimentally becomes relevant when introducing extrathermodynamic assumptions on the hydronium (H(3)O(+)) solvation energy. Fitting ion pairs circumvents this problem but leaves the parameters of one reference ion (here we choose chloride) undetermined. (ii) For the halides the problem is almost underdetermined, i.e., there is a whole set of degenerate parameters that equally well describe, e.g., chloride and bromide ions. (iii) For the heavy cations the problem is overdetermined, i.e., no combination of Lennard-Jones parameters is able to reproduce simultaneously energy and entropy of solvation. We discuss various possibilities to deal with these problems and finally present an optimized force field for the halide anions that reproduces the free energy and the entropy of solvation. For the alkali metal cations there is no unambiguous choice of parameters. Therefore, we give three different parameter sets for every ion with a small, intermediate, or large Lennard-Jones interaction strength, where the Lennard-Jones diameters are optimized to reproduce the solvation free energy. The ionic radius is reproduced with acceptable accuracy by this optimization strategy, meaning that the proposed force fields are reliable beyond the target observables (i.e., free energy and entropy of solvation).

Water at hydrophobic substrates: curvature, pressure, and temperature effects.

We studied the water density profile close to spherical and planar hydrophobic objects using molecular dynamics (MD) simulations. For normal pressure and room temperature, the depletion layer thickness of a planar substrate is approximately 2.5 Angstroms. Even for quite large spherical solutes with a radius of R = 18 Angstroms, the depletion layer thickness is reduced by 30%, which shows that substrate curvature and roughness is an experimentally important factor. Rising temperature leads to a substantial increase of the depletion layer thickness. The compressibility of the depletion layer is found to be surprisingly small and only approximately 5 times higher than that of bulk water. A high electrostatic surface potential of 0.5 V is found, which presumably plays an important role in the presence of charged solutes, since it can promote adsorption into the interfacial layer.