Hydrated Electron Research Articles

Real-Time Observation of Solvation Dynamics of Electron in Actinide Extraction Binary Solutions of Water and n-Tributyl Phosphate.

The excess electron in solution is a highly reactive radical involved in various radiation-induced reactions. Its solvation state critically determines the subsequent pathway and rate of transfer. For instance, water plays a dominating role in the electron-induced dealkylation of n-tributyl phosphate in actinide extraction processing. However, the underlying electron solvation processes in such systems are lacking. Herein, we directly observed the solvation dynamics of electrons in H-bonded water and n-tributyl phosphate (TBP) binary solutions with a mole fraction of water (Xw) varying from 0.05 to 0.51 under ambient conditions. Following the evolution of the absorption spectrum of trapped electrons (not fully solvated) with picosecond resolution, we show that electrons statistically distributed would undergo preferential solvation within water molecules extracted in TBP. We determine the time scale of excess electron full solvation from the deconvoluted transient absorption-kinetical data. The process of solvent reorganization accelerates by increasing the water molar fraction, and the rate of this process is 2 orders of magnitude slower compared to bulk water. We assigned the solvation process to hydrogen network reorientation induced by a negative charge of the excess electron that strongly depends on the local water environment. Our findings suggest that water significantly stabilizes the electron in a deeper potential than the pure TBP case. In its new state, the electron is likely to inhibit the dealkylation of extractants in actinide separation.

Experimental confirmation of solvated electron concentration and penetration scaling at a plasma–liquid interface

In this work, the transport of the plasma injected solvated electron is experimentally studied using total internal reflection absorption spectroscopy (TIRAS). A recently derived a theoretical model predicts power-law scalings between the interfacial concentration n 0 and penetration depth l with plasma current density j, namely and . Here, we extend this model to show that the optical absorption intensity should follow a power law behavior with current density, and we perform TIRAS measurements to confirm this behavior. By altering the ionic strength (salt concentration) of our electrolyte solution to control the current density, we find that at higher concentrations a scaling of approximately power is observed. However, the scaling is linear at lower concentrations, which we show is due to the transient response of the TIRAS experiment operating in a modulated mode. Ultimately, the experimentally-confirmed scaling law predicts approximate upper limits of penetration depth and interfacial concentration for solvated electrons, findings essential for tailoring plasma-liquid systems for specific applications.

Modulation of photo-generated solvated electrons for ammonia synthesis via facet-dependent engineering of heterojunctions

Photocatalytic reduction of N2 to NH3 is kinetically hindered because of the formation of high-energy reaction intermediates and intrinsically low efficiency of hot-carrier dynamics at typical reactant/photocatalyst interface. Direct injection of energized solvated electrons into a reactant medium has the potential to effectively overcome these barriers. Here, we demonstrate that when the two semiconducting materials, CdS and LDHs, are structurally arranged by forming a heterojunction, solvated electrons can be effectively generated, transported, and eventually promote the photocatalytic activity of ammonia synthesis in the form of solvated electrons in an aqueous solution. In particular, when 003 or 012 crystal face of LDHs and 002 facet of CdS form a heterojunction, the built-in electric field (BIEF) promotes the bulk-charge separation (BCS) of hot carriers from LDHs and CdS to the reaction media efficiently. As demonstrated in our theoretical calculations, solvated electrons form at the H2O/photocatalyst interface and drive the reduction of N2 and the formation of -N2H in the rate-determining step. Using in-situ DRIFTS, we were able to capture the formation of imine (-NH=NH) as one of the reaction intermediates that sheds light on the atomistic mechanism of the photocatalyzed reaction. DFT calculation also indicates that CdS@LDHs-3 (003 crystal face of LDHs and 002 facet of CdS) has the minimum relative energy for the rate-determining step and is therefore the most favorable configuration for the ammonia synthesis reaction, as confirmed by the experimental results.

Modification and Validation of the DREIDING Force Field for Molecular Liquid Simulations (DREIDING-UT)

In this study, we modified Lennard-Jones (LJ) parameters and point-charge parameters of the DREIDING force field (the modified force-field model is named DREIDING-UT). While the original LJ parameters of DREIDING were derived through an analytical formula to reproduce the potential depths and the equilibrium lengths of the Buckingham potentials of DREIDING/X6, the modified LJ parameters were derived through the least square fitting of the Buckingham potentials. Because the Gasteiger-Marsili (GM) charges of the original DREIDING underestimated electrostatic interactions, we replaced it with the restrained electrostatic potential (RESP) charges calculated from the ab initio wavefunctions, taking the dynamic electron correlation and solvation effects into account. To confirm how the modified force field works, we conducted molecular dynamics (MD) simulations of typical liquids. It was found that the densities and self-diffusion coefficients of the DREIDING-UT model agreed with the experimental ones much better than those of the original model.

Solvents forming redox mediators

Surface Chemistry The role of solvents participating directly in thermal catalytic reactions is clearer for homogeneous catalysis than for heterogeneous catalysis. Adams et al. studied the formation of hydrogen peroxide from hydrogen and oxygen on palladium nanoparticles by measuring the kinetic isotope effect and performing density functional theory simulations in aqueous and organic solvents. Methanol formed chemisorbed hydroxymethyl intermediates. These surface redox mediators transferred electrons and protons to adsorbed oxygen species and were regenerated by oxidizing chemisorbed hydrogen atoms. However, water molecules heterolytically oxidized hydrogen to generate solvated protons and surface electrons that reduced oxygen. Science , this issue p. [626][1] [1]: /lookup/doi/10.1126/science.abc1339

Transient evolution of quasifree electrons of plasma in liquid water revealed by optical-pump terahertz-probe spectroscopy

The fundamental properties of laser-induced plasma in liquid water, such as the ultrafast electron migration and solvation, have not yet been clarified. We use 1650-nm femtosecond laser pulses to induce the plasma in a stable free-flowing water film under the strong field ionization mechanism. Moreover, we adopt intense terahertz (THz) pulses to probe the ultrafast temporal evolution of quasifree electrons of the laser-induced plasma in water on the subpicosecond scale. For the first time, the THz wave absorption signal with a unique two-step decay characteristic in time domain is demonstrated, indicating the significance of electron solvation in water. We employ the Drude model combined with the multilevel intermediate model and particle-in-a-box model to simulate and analyze the key information of quasifree electrons, such as the frequency-domain absorption characteristics and solvation ratio. In particular, we observe that the solvation capacity of liquid water decreases with the increase of pumping energy. Up to ∼50 % of quasifree electrons cannot be captured by traps associated with the bound states as the pumping energy increases to 90 μJ / pulse. The ultrafast electron evolution in liquid water revealed by the optical-pump/THz-probe experiment provides further insights into the formation and evolution mechanisms of liquid plasma.

Comparative study of the dynamics of laser breakdown in water and hexane using interference microscopy

The changes in the optical properties of water and hexane under femtosecond irradiation (Ti : Al2O3 laser, wavelength 800 nm, intensity ∼1013 W cm−2) are investigated in the cavitation (bubble-formation) regime using interference microscopy in a time interval of ∼1.5 ns since the impact onset. A comparison of the dynamics of radiation-induced processes (solvation of excess electrons, pair recombination, and development of precavitation processes) is performed. The excited-carrier concentration is estimated, and these estimates are found to be inconsistent with the amount of energy that must be transferred to the liquid during a pulse to implement its heating and subsequent cavitation. This inconsistency is especially pronounced for hexane, where ionisation processes can barely be detected by interferometry. The experimental results put new questions about the mechanisms of energy transfer in both polar and nonpolar liquids subjected to intense laser irradiation.

Confined water radiolysis in aluminosilicate nanotubes: the importance of charge separation effects.

Imogolite nanotubes are potentially promising co-photocatalysts because they are predicted to have curvature-induced, efficient electron-hole pair separation. This prediction has however not yet been experimentally proven. Here, we investigated the behavior upon irradiation of these inorganic nanotubes as a function of their water content to understand the fate of the generated electrons and holes. Two types of aluminosilicate nanotubes were studied: one was hydrophilic on its external and internal surfaces (IMO-OH) and the other had a hydrophobic internal cavity due to Si-CH3 bonds (IMO-CH3), with the external surface remaining hydrophilic. Picosecond pulse radiolysis experiments demonstrated that the electrons are efficiently driven outward. For imogolite samples with very few external water molecules (around 1% of the total mass), quasi-free electrons were formed. They were able to attach to a water molecule, generating a water radical anion, which ultimately led to dihydrogen. When more external water molecules were present, solvated electrons, precursors of dihydrogen, were formed. In contrast, holes moved towards the internal surface of the tubes. They mainly led to the formation of dihydrogen and of methane in irradiated IMO-CH3. The attachment of the quasi-free electron to water was a very efficient process and accounted for the high dihydrogen production at low relative humidity values. When the water content increased, electron solvation dominated over attachment to water molecules. Electron solvation led to dihydrogen production, albeit to a lesser extent than quasi-free electrons. Our experiments demonstrated the spontaneous curvature-induced charge separation in these inorganic nanotubes, making them very interesting potential co-photocatalysts.

Spectroscopic Measurement of Solvated Electrons for Analysis of Plasma Reaction Field in Liquid

Spectroscopic Measurement of Solvated Electrons for Analysis of Plasma Reaction Field in Liquid

Radiation-Assisted Formation of Metal Nanoparticles in Molten Salts.

Knowledge of structural and thermal properties of molten salts is crucial for understanding and predicting their stability in many applications such as thermal energy storage and nuclear energy systems. Probing the behavior of metal contaminants in molten salts is presently limited to either foreign ionic species or metal nanocrystals added to the melt. To bridge the gap between these two end states and follow the nucleation and growth of metal species in molten salt environment in situ, we use synchrotron X-rays as both a source of solvated electrons for reducing Ni2+ ions added to ZnCl2 melt and as an atomic-level probe for detecting formation of zerovalent Ni nanoparticles. By combining extended X-ray absorption fine structure analysis with X-ray absorption near edge structure modeling, we obtained the average size and structure of the nanoparticles and proposed a radiation-induced reduction mechanism of metal ions in molten salts.

On the Primary Water Radicals' Production in the Presence of Gold Nanoparticles: Electron Pulse Radiolysis Study.

Gold nanoparticles are known to cause a radiosensitizing effect, which is a promising way to improve radiation therapy. However, the radiosensitization mechanism is not yet fully understood. It is currently assumed that gold nanoparticles can influence various physical, chemical, and biological processes. Pulse radiolysis is a powerful tool that can examine one of the proposed effects of gold nanoparticles, such as increased free radical production. In this work, we shed light on the consequence of ionizing radiation interaction with gold nanoparticles by direct measurements of solvated electrons using the pulse radiolysis technique. We found that at a therapeutically relevant gold concentration (<3 mM atomic gold, <600 μg × cm−3), the presence of gold nanoparticles in solution does not induce higher primary radicals’ formation. This result contradicts some hypotheses about free radical formation in the presence of gold nanoparticles under ionizing radiation previously reported in the literature.

(Invited) Electrolyte and Interphase Revealed By Cryogenic Electron Microscopy and Solvation Measurement

Obtaining atomic and molecular level understanding of electrolyte and electrode-electrolyte interphase is critical to the battery developments. In this talk, I will present a multimodal approach combining the powerful cryogenic electron microscopy technique, electrochemical technique and solvation entropy and free energy measurements. I will discover including: 1) atom-resolved images of solid electrolyte interphase for Li metal and Si anodes; 2) Insight on the debate on the existence of cathode electrolyte interphase; 3) the solvation structure and related thermodynamic parameters and their correlation with electrochemical performance.

Dynamics of solvated electrons during femtosecond laser-induced plasma generation in water.

We studied the dynamics of solvated electrons in the early stage of plasma generation in water induced with an intense femtosecond laser pulse. According to the decay kinetics of solvated electrons, a fast recombination process of solvated electrons (geminate recombination) occurred with a more prolonged lifetime (500 ps to 1 ns) than that observed in previous pulse photolysis studies (10-100 ps). This unusually longer lifetime is attributed to additional production of solvated electrons due to abundant free electrons generated with the laser-induced plasma, implying significant influence of free electrons on the dynamics of solvated electrons.

Mechanism of Aqueous Carbon Dioxide Reduction by the Solvated Electron.

Aqueous solvated electron (eaq-), a key species in radiation and plasma chemistry, can efficiently reduce CO2 in a potential green chemistry application. Here, the mechanism of this reaction is unravelled by condensed-phase molecular dynamics based on the correlated wave function and an accurate density functional theory (DFT) approximation. Here, we design and apply the holistic protocol for solvated electron's reactions encompassing all relevant reaction stages starting from diffusion. The carbon dioxide reduction proceeds via a cavity intermediate, which is separated from the product (CO2-) by an energy barrier due to the bending of CO2 and the corresponding solvent reorganization energy. The formation of the intermediate is caused by solvated electron's diffusion, whereas the intermediate transformation to CO2- is triggered by hydrogen bond breaking in the second solvation shell of the solvated electron. This picture of an activation-controlled eaq- reaction is very different from both rapid barrierless electron transfer and proton-coupled electron transfer, where key transformations are caused by proton migration.

Modeling of an atmospheric pressure plasma-liquid anodic interface: Solvated electrons and silver reduction

Solvated electrons (eaq−) generated by atmospheric pressure plasmas in contact with liquids are a key source of plasma-induced liquid chemistry that enable applications in biotechnology and nanoparticle synthesis. In this paper, we report liquid phase reactive species concentrations near an anodic plasma-liquid interface as described by a fluid model. In particular, the interfacial structures and plasma-induced reactive species in NaCl and AgNO3 solutions as generated by a pulsed plasma are highlighted. The results show that the magnitude and the penetration depth of the eaq− concentration in AgNO3 solution are smaller than that in the NaCl solution due to the scavenger reactions of eaq− by Ag+ and NO3−. The early products of the plasma-induced Ag+ reduction are also presented, and the impact of the current density, the pulse width, and the AgNO3 concentration on the silver reduction is analyzed. It is further shown that a typical OH radical flux present in such plasmas can highly impact the eaq− concentration and the Ag+ reduction while the impact of vacuum ultraviolet radiation, H, and H2O2 is less pronounced.

Ratio Oxalate to Formate Tuned by pH During CO2 Reduction Driven by Solvated Electron at the Electrified Plasma/Liquid Interface

The electrified plasma/liquid interface (PLI) promotes the CO2 reduction (CO2R) with a spectrum of products distinct from that of other electrochemical platforms. However, the lack of fundamental understanding greatly disables the preconize of its industrial potential. In particular, the inaccurate reaction mechanism of CO2R via electrified PLI brings imprecision on the theoretical predictions of selectivities in acid electrolytes. For this reason, the present work categorically considers the proton as a reactant and the proton concentration as an essential parameter for the dynamical model describing the CO2RR via electrified PLI. Two new routes toward formic acid production are proposed, namely, the disproportionation-like and radical cross-combination reactions. Both were capable of significantly reproducing the feature observed with real-life experiments for the final product proportions; however, the cross-combination reaction adjusts the most. With these reasonably assertive mechanisms of CO2RR in acid media, it was possible to predict that an extremely acid pH (~ 1) is required to attain concentration ratio oxalate to formate equal to 50 to 50 and 10 to 90 when considering the disproportionation-like and cross-combination reaction, respectively.

Probing the Birth and Ultrafast Dynamics of Hydrated Electrons at the Gold/Liquid Water Interface via an Optoelectronic Approach.

The hydrated electron has fundamental and practical significance in radiation and radical chemistry, catalysis, and radiobiology. While its bulk properties have been extensively studied, its behavior at solid/liquid interfaces is still unclear due to the lack of effective tools to characterize this short-lived species in between two condensed matter layers. In this study, we develop a novel optoelectronic technique for the characterization of the birth and structural evolution of solvated electrons at the metal/liquid interface with a femtosecond time resolution. Using this tool, we record for the first time the transient spectra (in a photon energy range from 0.31 to 1.85 eV) in situ with a time resolution of 50 fs revealing several novel aspects of their properties at the interface. Especially the transient species show state-dependent optical transition behaviors from being isotropic in the hot state to perpendicular to the surface in the trapped and solvated states. The technique will enable a better understanding of hot electron driven reactions at electrochemical interfaces.

Tunneling matrix element and tunneling pathways of protein electron transfer calculated with a fragment molecular orbital method

Practical ways to calculate the tunneling matrix elements and analyze the tunneling pathways for protein electron-transfer (ET) reactions with a fragment molecular orbital (FMO) method are presented. The straightforward use of minimal basis sets only for the atoms involved in the covalent bond detachment in FMO can properly describe the ETs through the protein main-chains with the cost-effective two-body corrections (FMO2) without losing the quality of double-zeta basis sets. The current FMO codes have been interfaced with density functional theory, polarizable continuum model, and model core potentials, with which the FMO-based protein ET calculations can consider the effects of electron correlation, solvation, and transition-metal redox centers. The reasonable performance of the FMO-based ET calculations is demonstrated for three different sets of protein-ET model molecules: (1) hole transfer between two tryptophans covalently bridged by a polyalanine linker in the ideal α-helix and β-strand conformations, (2) ET between two plastoquinones covalently bridged by a polyalanine linker in the ideal α-helix and β-strand conformations, and (3) hole transfer between ruthenium (Ru) and copper (Cu) complexes covalently bridged by a stretch of a polyglycine linker as a model for Ru-modified derivatives of azurin.

Below Band Gap Formation of Solvated Electrons in Neutral Water Clusters?

Below band gap formation of solvated electrons in neutral water clusters using pump–probe photoelectron imaging is compared with recent data for liquid water and with above band gap excitation studies in liquid and clusters. Similar relaxation times on the order of 200 fs and 1–2 ps are retrieved for below and above band gap excitation, in both clusters and liquid. The independence of the relaxation times from the generation process indicates that these times are dominated by the solvent response, which is significantly slower than the various solvated electron formation processes. The analysis of the temporal evolution of the vertical electron binding energy and the electron binding energy at half-maximum suggests a dependence of the solvation time on the binding energy.

New sonochemiluminescence involving solvated electron in Ce(III)/Ce(IV) solutions

The moving single-bubble sonoluminescence of Ce3+ in water and ethylene glycol solutions of CeCl3 and (NH4)2Ce(NO3)6 was studied. As found, a significant part of intensity of the luminescence (100% with cerium concentration less than 10–4 M) is due to the sonochemiluminescence. A key reaction of sonochemiluminescence is the Ce4+ reduction by a solvated (or hydrated in water) electron: Ce4+ + es (eaq) → *Ce3+. Solvated electrons are formed in a solution via electrons ejection from a low-temperature plasma periodically generated in deformable moving bubble at acoustic vibrations. Reactions of heterolytic dissociation of solvents make up the source of electrons in the plasma. In aqueous CeCl3 solutions, the Ce4+ ion is formed at the oxidation of Ce3+ by OH radical. The latter species originates from homolytic dissociation of water in the plasma of the bubble, also penetrating from the moving bubble into the solution. The sonochemiluminescence in cerium trichloride solutions are quenched by the Br− (acceptor of OH) and H+ ions (acceptor of eaq). In water and ethylene glycol solutions of (NH4)2Ce(NO3)6, the sonochemiluminescence also quenched by the H+ ion. The sonochemiluminescence in CeCl3 solutions is registered at [Ce3+] ≥ 10–5 M. Then the sonochemiluminescence intensity increases with the cerium ion concentration and reaches the saturation plateau at 10–2 M. It was shown that sonophotoluminescence (re-emission of light of bubble plasma emitters by cerium ions) also contributes to the luminescence of Ce3+ in solutions with [Ce3+] ≥ 10–4 M. If the cerium concentration is more than 10–2 M, a third source contributes to luminescence, viz., the collisional excitation of Ce3+ ions penetrating into the moving bubble.