Strong Enhancement of Nanoconfined Water Mobility by a Structure Breaking Salt

Backscattering silicon spectrometer (BASIS): sixteen years in advanced materials characterization

Quasielastic neutron scattering (QENS) is an experimental technique that can measure parameters of mobility, such as diffusion jump rate and jump length, as well as localized relaxations of chemical species (molecules, ions, and segments) at atomic and nanometer length scales. Due to the high penetrative power of neutrons and their sensitivity to neutron scattering cross-section of chemical species, QENS can effectively probe mobility inside most bulk materials. This review focuses on QENS experiments performed using a neutron backscattering silicon spectrometer (BASIS) to explore the dynamics in various materials and understand their structure-property relationship. BASIS is a time-of-flight near-backscattering inverted geometry spectrometer with very high energy resolution (approximately 0.0035 meV of full width at half maximum), allowing measurements of dynamics on nano to picosecond timescales. The science areas studied with BASIS are diverse, with a focus on soft matter topics, including traditional biological and polymer science experiments, as well as measurements of fluids ranging from simple hydrocarbons and aqueous solutions to relatively complex room-temperature ionic liquids and deep-eutectic solvents, either in the bulk state or confined. Additionally, hydrogen confined in various materials is routinely measured on BASIS. Other topics successfully investigated at BASIS include quantum fluids, spin glasses, and magnetism. BASIS has been in the user program since 2007 at the Spallation Neutron Source of the Oak Ridge National Laboratory, an Office of Science User Facility supported by the U.S. Department of Energy. Over the past sixteen years, BASIS has contributed to various scientific disciplines, exploring the structure and dynamics of many chemical species and their fabrication for practical applications. A comprehensive review of BASIS contributions and capabilities would be an asset to the materials science community, providing insights into employing the neutron backscattering technique for advanced materials characterization.

Effects of Nanoconfinement on Dynamics in Concentrated Aqueous Magnesium Chloride Solutions

Water behavior in various natural and manufactured settings is influenced by confinement in organic or inorganic frameworks and the presence of solutes. Here, the effects on dynamics from both confinement and the addition of solutes are examined. Specifically, water and ion dynamics in concentrated (2.5-4.2 m) aqueous magnesium chloride solutions confined in mesoporous silica (2.8 nm pore diameter) were investigated using polarization selective pump-probe and 2D infrared spectroscopies. Fitting the rotational and spectral diffusion dynamics measured by the vibrational probe, selenocyanate, with a previously developed two-state model revealed distinct behaviors at the interior of the silica pores (core state) and near the wall of the confining framework (shell state). The shell dynamics are noticeably slower than the bulk, or core, dynamics. The concentration-dependent slowing of the dynamics aligns with behavior in the bulk solutions, but the spectrally separated water-associated and Mg2+-associated forms of the selenocyanate probe exhibit different responses to confinement. The disparity in the complete reorientation times is larger upon confinement, but the spectral diffusion dynamics become more similar near the silica surface. The length scales that characterize the transition from surface-influenced to bulk-like behavior for the salt solutions in the pores are discussed and compared to those of pure water and an organic solvent confined in the same pores. These comparisons offer insights into how confinement modulates the properties of different liquids.

Wet Etching of Silicon in Planar Nanochannels

Silicon (Si) alkaline etching constitutes a fundamental process in the semiconductor industry. Although its etching kinetics on plain substrates have been thoroughly investigated, the kinetics of Si wet etching in nanoconfinements have yet to be fully explored despite its practical importance in three-dimensional (3-D) semiconductor manufacturing. Herein, we report the systematic study of potassium hydroxide (KOH) wet etching kinetics of amorphous silicon (a-Si)-filled two-dimensional (2-D) planar nanochannels. Our findings reveal that the etching rate would increase with the increase in nanochannel height before reaching a plateau, indicating a strong nonlinear confinement effect. Through investigation using etching solutions with different ionic strengths and/or different temperatures, we further find that both electrostatic interactions and the hydration layer inside the nanoconfinement contribute to the confinement-dependent etching kinetics. Our results offer fresh perspectives into the kinetic study of reactions in nanoconfinements and will shed light on the optimization of etching processes in the semiconductor industry.

Advanced Bacterial Cellulose Ionic Conductors with Gigantic Thermopower for Low-Grade Heat Harvesting

Ionic conductors such as polymer electrolytes and ionic liquids have high thermoelectric voltages several orders of magnitude higher than electronic thermoelectric materials, while their conductivity is much lower than the latter. This work reports a novel approach to achieve high-performance ionic conductors using calcium ion (Ca²⁺) coordinated bacterial cellulose (CaBC) through molecular channel engineering. Through the coordination of Ca²⁺ with cellulose molecular chain, the distance between the cellulose molecular chains is widened, so that ions can transport along the cellulose molecular chain. Therefore, we reported ionic thermoelectric (i-TE) material based on CaBC/NaCl with a relatively high ionic Seebeck coefficient of -27.2 mV K^-1 and high ionic conductivity of 204.2 mS cm^-1. This ionic hydrogel is promising in the design of high-thermopower i-TE materials for low-grade heat energy harvesting.

Quantum chemical investigation of the effect of alkali metal ions on the dynamic structure of water in aqueous solutions

We report quantum chemical molecular dynamics (MD) simulations based on the density-functional tight-binding (DFTB) method to investigate the effect of K+, Na+, and Mg2+ ions in aqueous solutions on the static and dynamic structure of bulk water at room temperature and with various concentrations. The DFTB/MD simulations were validated for the description of ion solvation in aqueous ionic solutions by comparing static pair distribution functions (PDFs) as well as the cation solvation shell between experimental and available ab initio DFT data. The effect of the cations on the water structure, as well as relative differences between K+, Na+, and Mg2+ cations, were analyzed in terms of atomically resolved PDFs as well as time-dependent Van Hove correlation functions (VHFs). The investigation of the VHFs reveals that salt ions generally slow down the dynamic decay of the pair correlations in the water solvation sphere, irrespective of the cation size or charge. The analysis of partial metal-oxygen VHFs indicates that there are long-lived correlations between water and Na+ over long distances, in contrast to K+ and Mg2+.

ICE-MAN the Integrated Computational Environment for Modeling and Analysis for Neutrons at ORNL

ICE-MAN is a modeling and analysis workbench for multi-modal studies, designed with neutron science in mind. It streamlines the workflow between different experimental techniques, computer modeling, and databases and reduces the time and learning curve needed to access them thus making a holistic approach to data interpretation more amenable and efficient.