Hydrated Electron Research Articles

Kinetic Control of [AuCl4]- Photochemical Reduction and Gold Nanoparticle Size with Hydroxyl Radical Scavengers.

Laser-induced photochemical reduction of aqueous [AuCl4]- is a green synthesis approach requiring no chemical reducing agents or stabilizers; but size control over the resulting gold nanoparticles remains a challenge. Under optical breakdown conditions producing hydrated electrons (eaq-) and hydroxyl radicals (OH•) through decomposition of water, [AuCl4]- reduction kinetics follow an autocatalytic rate law, which is governed by rate constants: nucleation rate k1, dependent on eaq-; and growth rate k2, dependent on the OH• recombination product, H2O2. In this work, we add the hydroxyl radical scavengers isopropyl alcohol and sodium acetate to limit H2O2 formation. Higher scavenger concentrations both lowered k2 values and produced smaller gold nanoparticles with Gaussian size distributions and remarkably narrow mass-weighted size distributions. With sufficiently high scavenger concentrations, the mean nanoparticle size could be tuned from 3.8 to 6.1 nm with polydispersity indices below 0.08. Both the higher surface area-normalized catalytic activity of the gold nanoparticles synthesized in the presence of scavengers, and FTIR measurements, indicate no capping ligands on the nanoparticle surfaces. These results demonstrate that the size distributions of "naked" gold nanoparticles produced by photochemical [AuCl4]- reduction can be effectively tuned by controlling the reaction kinetics.

Enhanced photoreductive degradation of perfluorooctanesulfonate by UV irradiation in the presence of ethylenediaminetetraacetic acid

Perfluorooctanesulfonate (PFOS) is a persistent organic pollutant (POP) that is globally distributed. Hydrated electrons (eaq−) are known to effectively initiate the decomposition of PFOS. In this study, we explore an alternative photolytic approach employing aquated electrons, eaq− in the presence of ethylenediaminetetraacetic acid (EDTA) in order to enhance the photo-induced degradation of PFOS. EDTA, in this case, serves primarily as a hydroxyl radical scavenger, which inhibits the recombination of eaq− with ·OH resulting in an increased average photolytic lifetime for eaq−. The net effect is to enhance the degradation of PFOS. UV/EDTA irradiation is shown to increase the overall decomposition percentages of PFOS. The empirical pseudo first-order rate constant for the loss of PFOS is 0.113 h−1. In addition, we used laser flash photolysis kinetics to show that the eaq− is the dominant species responsible for the decomposition of PFOS. EDTA also allows for the photolytically-produced hydrated electrons to be used in the presence of air over a wide range of pH. Furthermore, perfluoroalkyl sulfonates with longer chain lengths have higher overall decomposition percentages and increased defluorination percentages. The observed kinetic enhancements appear to be due primarily to the impact of the amine and methylene groups of EDTA with respect of hydroxyl radical scavenging.

Ultrafast Internal Conversion and Solvation of Electrons in Water, Methanol, and Ethanol.

Ultrafast internal conversion from the first excited state of a solvated electron in water, methanol, and ethanol is investigated using time-resolved photoelectron spectroscopy of liquid microjets and a spectral retrieval method. Photoelectron spectra corrected for inelastic scattering clearly reveal well-separated signals from the excited and ground states, and the latter enables us to analyze the solvation dynamics in the ground state after internal conversion. Measurements with 25 fs time resolution identify a rapid increase in the vertical electron binding energy of the solvated electron owing to nuclear wave packet motions in the excited state and allow us to precisely determine the internal conversion time.

Application of surfactant modified montmorillonite with different conformation for photo-treatment of perfluorooctanoic acid by hydrated electrons

Perfluoroalkyl and polyfluoroalkyl substances (PFASs) are a class of highly persistent contaminants with high bioaccumulation and toxicity. Our previous studies showed that perfluorooctanoic acid (PFOA) can be completely defluorinated under UV irradiation in organo-montmorillonite/indole acetic acid (IAA) system. However, there is still lack of information for the degradation mechanism and the test for wastewater treatment. Here, we systematically investigated the defluorination reaction in the presence of different organo-montmorillonites and found that the degradation process was apparently controlled by the configuration of surfactants. In hexadecyltrimethyl ammonium (HDTMA)-modified montmorillonite, HDTMA exists as a tilt conformation and isolated clay interlayer from the aqueous solution, protecting hydrated electrons generated by photo-irradiation of IAA from quenching by oxygen. Defluorination hydrogenation process was the dominant degradation pathway. While in poly-4-vinylpyridine-co-styrene (PVPcoS)-modified montmorillonite, due to the multiple charges of PVPcoS, a flat conformation parallel to clay surface was expected. Hydroxyl radicals, which were generated by the reaction of hydrated electrons with oxygen molecules diffused into clay interlayer, are also involved in the degradation process. Our results further demonstrate that mixture modified montmorillonite could combine the advantages of both modifications, thus showing superior reactivity even for actual industrial wastewater without any pretreatment. This technique would have great potential for treatment of actual wastewater.

Solvent effects on the geometry, electronic structure, and bonding style of Zn(N5)2: A theoretical study

AbstractNitrogen‐rich compounds involving the cyclo‐pentazole anion (cyclo‐N5−) have attracted extensive attention due to higher energy release and environmental friendliness than traditional high energy density materials (HEDMs). However, the synthesis of stable HEDMs with cyclo‐N5− is still a challenge. In this study, the effect of nine solvents on the geometrical and electronic structures and solvation energies of Zn(N5)2, one of the recently synthesized nitrogen‐rich compounds, was studied using the density functional theory and the polarized continuum model. The results indicated an increase in the stability of Zn(N5)2 in the solution phase compared to the vacuum phase, and the stability of Zn(N5)2 increases with increasing dielectric constants. The energy gap of frontier molecular orbitals and the absolute value of total energy in water are the largest, revealing that Zn(N5)2 is more stable in water than in other solvents. To understand the stabilization mechanism of Zn(N5)2 by water, further studies were performed with the natural bond orbital (NBO) analysis and the quantum theory of atoms in molecules (QTAIM) analysis using the explicit solvent model. The charge transfer and the hydrogen bonds are observed between Zn(N5)2 and water, which are beneficial to improvement of the stability of Zn(N5)2. This may indicate the solvents that have strong interactions with the cyclo‐N5− candidate may improve the possibility of success of synthesis.

Laser-Induced Wurtz-Type Syntheses with a Metal-Free Photoredox Catalytic Source of Hydrated Electrons.

Upon irradiation with ns laser pulses at 355 nm, 2-aminoanthracene in SDS micelles readily produces hydrated electrons. These "super-reductants" rapidly attack substrates such as chloro-organics and convert them into carbon-centred radicals through dissociative electron transfer. For a catalytic cycle, the aminoanthracene needs to be restored from its photoionization by-product, the radical cation, by a sacrificial donor. The ascorbate monoanion can only achieve this across the micelle-water interface, but the monoanion of ascorbyl palmitate results in a fully micelle-contained regenerative electron source. The shielding by the micelle in the latter case not only increases the life of the catalyst but also strongly suppresses the interception of the carbon-centred radicals by the hydrogen-donating ascorbate moiety; and in conjunction with the high local concentrations effected by the pulsed laser, termination by radical dimerization thus dominates. We have obtained a complete and consistent picture through monitoring the individual steps and the assembled system by flash photolysis on fast and slow timescales, from microseconds to minutes; and in preparative studies on a variety of substrates, we have achieved up to quantitative dimerization with a turnover on the order of 1 mmol per hour.

Confinement induced formation of silver nanoparticles in self-assembled micro-granules

Effect of nanoscale confinement on the formation of Ag nanoparticles in porous matrix of nanostructured silica micro-granules has been investigated. One step rapid drying of tiny aerosol droplets has been employed for incorporation of metal precursor in the porous micro-granules. The formation of nanoparticles in the confined geometry of interstitial granular pores has been revealed by small-angle scattering and field-emission scanning electron microscopy. It has been observed that nanometric confinement accelerates the chemical reduction of precursor as compared to that in a bulk solution and the formation yield gets further enhanced by irradiation of X-ray. The growth kinetics of nanoparticles has been probed by time-resolved scattering experiments. It is revealed that X-ray-induced dissociation of bound water in the granular interstices produces hydrated electrons favouring the reduction reaction.

Ligation Kinetics as a Probe for Non-Covalent Electrostatic Bonding and Electron Solvation of Alkali and Alkaline Earth Cations with Ammonia.

Mono-ligation kinetics were measured for ammonia reacting with atomic cations in the first two groups of the periodic table (K+, Rb+, Cs+ and Ca+, Sr+, Ba+). Also, mono-ligation energies were computed using density functional theory (DFT) in an attempt to assess the role of non-covalent electrostatic interactions in these chemical reactions. The measurements were performed at room temperature in helium bath gas at 0.35Torr using an inductively coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer. Rate coefficients are reported for ammonia addition, the only reaction channel that was observed with all these cations. A systematic decrease in the rate of addition of NH3 was observed for both group 1 and 2 cations going down the periodic table. The computational studies predict a decrease in the adduct binding energy and an increase in the bond separation going down groups 1 and 2 of the periodic table and provide some insight into the role of the extra selectron in the group 2 radical cations in ligand bonding. A correlation is seen between the efficiency of ligation and the binding energy of the adduct ion and attributed to the lifetime of the intermediate encounter complex against back dissociation which is dependent on its well depth. Higher-order additions of ammonia were also observed. Remarkable differences in the extent and kinetics were seen between the group 1 and 2 cations, and these were attributed to the occurrence of ammonia solvation of the extra s electron in the higher-order adducts of the alkaline earth cations.

Diamond photochemistry with visible light

This project demonstrates diamond-based photochemistry using visible light. Diamond has the unique property of having a widely tunable electrochemical surface, with hydrogen terminated diamond known to have a negative electron affinity. This property enables the emission of electrons from the surface of the diamond. In this work, electrons are emitted into an aqueous solution, creating a reservoir of free, solvated electrons, that can be used to initiate energy intensive chemical reactions. We demonstrate that visible light incident on thin films of diamond on molybdenum substrates can be used to reduce nitrogen gas to ammonia via the photogenerated electrons.

Photophysics of Acyl- and Ester-DBD Dyes: Quadrupole-Induced Solvent Relaxation Investigated by Transient Absorption Spectroscopy.

A new generation of wavelength-tunable, fluorescent dyes, so-called DBD ([1,3]dioxolo[4,5- f][1,3]benzodioxole) dyes, were developed a few years ago, and they showed great potential as probes, for example, for fluorescence microscopy. However, their photophysics is not fully explored and leaves open questions regarding their large fluorescence Stokes shifts and sensitivity to solvent conditions of differently substituted DBD dyes. To improve the understanding of the influence of the substitution pattern of the DBD dyes on their respective photophysics, transient absorption spectroscopy (TAS) was used, that is, a pump-probe experiment on the femtosecond timescale. TAS allows measurements of excited states, ground state recovery, solvent relaxation, and fluorescence properties on time scales of up to several nanoseconds. Two different DBD dye samples were investigated: acyl- and ester-substituted DBD dyes. Experiments were carried out in solvents with different polarities using different excitation energies and at different viscosities. Based on the experimental data and theoretical calculations, we were able to determine the conformational changes of the molecule due to electronic excitation and were able to investigate solvent relaxation processes for both types of DBD dyes. By generalizing the theory for quadrupole-induced solvent relaxation developed by Togashi et al., we derived quadrupole moments of both molecules in the ground and excited state. Our data showed differences in the binding of polar solvent molecules to the dyes depending on the substituent on the DBD dye. In the case of water as the solvent, an additional efficient quenching process in the electronically excited state was revealed, which was indicated by the observation of solvated electrons in the TAS signals.

Ultrafast Electron Solvation in Room Temperature Ionic Liquid Observed by Femtosecond Time-resolved Near-IR Spectroscopy

The solvation process of electrons reflects the solvent structure and its fluctuation sensitively. The solvation process of electrons in room-temperature ionic liquid N,N,N-trimethyl-N-propylammoni...

(Invited) Electrolytic Synthesis of Ammonia at Ambient Conditions from Nitrogen and Acidic Water By a Catalyst-Free, Plasma Process

Industrially, nitrogen reduction to ammonia is performed via the Haber-Bosch process at high pressure and high temperature which makes it difficult to conduct at small scales, and uses hydrogen gas as a reactant that currently comes from steam methane reforming which results in significant carbon dioxide emission. Electrolytic synthesis of ammonia is attractive because it can take place at or near ambient conditions, enables remote operation and integration with renewable sources of energy such as solar and wind, and can rely on water (typically acidic) as the source of hydrogen atoms. However, to date, the selectivity for electrolytic ammonia formation has been low, mostly because of issues associated with the catalyst materials. Specifically, large overpotentials are required to reduce the stable nitrogen molecule, and at these potentials hydrogen preferentially adsorbs on the catalyst surface and leads to hydrogen gas evolution instead of ammonia formation. In this talk, we will present a new electrolytic approach to ammonia synthesis in which the solid cathode is replaced by a gas discharge (plasma). Similar to the more typical electrochemical systems, ammonia is formed at ambient conditions from nitrogen and water using only electricity. However, nitrogen is reduced without a catalyst by the unique interactions of the plasma and water. Specifically, electrons in the plasma potentially excite or even dissociate the nitrogen molecule in the gas phase, and solvate to produce one of the strongest reducing agents known. Remarkably, we find that at certain processing conditions, ammonia is synthesized with 100% charge-transfer (faradaic) efficiency. Scavenger experiments show that solvated electrons and hydrogen radicals are key intermediates. The dependence of ammonia formation on current, time, and pH, as well as potential reasons for the highly selective synthesis of ammonia in our process will also be discussed.

Inception of electronic damage of matter by photon-driven post-ionization mechanisms.

“Probe-before-destroy” methodology permitted diffraction and imaging measurements of intact specimens using ultrabright but highly destructive X-ray free-electron laser (XFEL) pulses. The methodology takes advantage of XFEL pulses ultrashort duration to outrun the destructive nature of the X-rays. Atomic movement, generally on the order of >50 fs, regulates the maximum pulse duration for intact specimen measurements. In this contribution, we report the electronic structure damage of a molecule with ultrashort X-ray pulses under preservation of the atoms' positions. A detailed investigation of the X-ray induced processes revealed that X-ray absorption events in the solvent produce a significant number of solvated electrons within attosecond and femtosecond timescales that are capable of coulombic interactions with the probed molecules. The presented findings show a strong influence on the experimental spectra coming from ionization of the probed atoms' surroundings leading to electronic structure modification much faster than direct absorption of photons. This work calls for consideration of this phenomenon in cases focused on samples embedded in, e.g., solutions or in matrices, which in fact concerns most of the experimental studies.

Solvated Electrons for Photochemistry Syntheses Using Conjugated Carbon Nitride Polymers

Solvated electrons have attracted increasing research interest due to their strong reducing ability. However, the generation and utilization of solvated electrons in photocatalytic systems are rare...

Pulse Radiolysis Studies for Mechanism in Biochemical Redox Reactions.

Pulse radiolysis is a powerful method for generating highly reduced or oxidized species and free radicals. Combined with fast time-resolved spectroscopic measurement, we can monitor the reactions of intermediate species on time scales ranging from picoseconds to seconds. The application of pulse radiolysis to water generates hydrated electrons (eaq-) and specific radicals, rendering this technique useful for investigating a number of biological redox processes. The first pulse radiolysis redox investigations explored in this review involved intramolecular electron transfer processes in protein with multiple electron-accepting sites. Pulse radiolysis enabled direct monitoring of the internal electron transfer rates and the distribution of electrons within proteins. Structural information from X-ray data has allowed analysis of the rate constants and their activation parameters in relation to the mechanisms with current theoretical treatments. The second set of pulse radiolysis redox investigations explored here concerned the intermediates of enzyme reactions after redox reactions. Pulse radiolysis allowed the extremely rapid donation of electrons to a redox center in a protein. It makes it possible to observe the unstable intermediates after the reduction and the following subsequent steps. For example, the intermediates generated through the one-electron reduction of oxygenated hemoproteins, such as cytochrome P450 and nitric oxide synthase, were characterized. Interestingly, ligand exchange can occur upon the reduction of heme iron, in which different amino acid residues bind to heme in the ferrous and ferric states, respectively. We directly observed the ligand-switching intermediates of bacterial CooA, a CO sensor, and bacterial iron response regulator protein. These ligand exchange processes are physiologically important for regulating the electrode potential and effective formation of superoxide anion or HO•. The third set of pulse radiolysis redox investigations explored in this review concerns free-radical processes in biological systems. Free radicals are produced in cells and organisms in a variety of processes. The cell has developed special and very effective machinery for controlling and detoxifying reactive radicals. Radiation-generated radicals allow studies of the reactions between specific radicals and solutes, often revealing the mechanisms underlying the initial and subsequent reactions. The crucial contribution was made using pulse radiolysis techniques and knowledge of the identities, properties, and reactions of radicals. These radicals include superoxide (O2•-), nitric monoxide (NO•), ascorbate, urate, and protein radicals. This review focuses on the reactions of these radicals and their physiological functions.

Anharmonic Vibrational Analysis of Biomolecules and Solvated Molecules Using Hybrid QM/MM Computations.

Quantum mechanics/molecular mechanics (QM/MM) calculations are applied for anharmonic vibrational analyses of biomolecules and solvated molecules. The QM/MM method is implemented into a molecular dynamics (MD) program, GENESIS, by interfacing with external electronic structure programs. Following the geometry optimization and the harmonic normal-mode analysis based on a partial Hessian, the anharmonic potential energy surface (PES) is generated from QM/MM energies and gradients calculated at grid points. The PES is used for vibrational self-consistent field (VSCF) and post-VSCF calculations to compute the vibrational spectrum. The method is first applied to a phosphate ion in solution. With both the ion and neighboring water molecules taken as a QM region, IR spectra of representative hydration structures are calculated by the second-order vibrational quasi-degenerate perturbation theory (VQDPT2) at the level of B3LYP/cc-pVTZ and TIP3P force field. A weight-average of IR spectra over the structures reproduces the experimental spectrum with a mean absolute deviation of 16 cm–1. Then, the method is applied to an enzyme, P450 nitric oxide reductase (P450nor), with the NO molecule bound to a ferric (FeIII) heme. Starting from snapshot structures obtained from MD simulations of P450nor in solution, QM/MM calculations have been carried out at the level of B3LYP-D3/def2-SVP(D). The spin state of FeIII(NO) is likely a closed-shell singlet state based on a ratio of N–O and Fe–NO stretching frequencies (νN–O and νFe–NO) calculated for closed- and open-shell singlet states. The calculated νN–O and νFe–NO overestimate the experimental ones by 120 and 75 cm–1, respectively. The electronic structure and solvation of FeIII(NO) affect the structure around the heme of P450nor leading to an increase in νN–O and νFe–NO.

Detailed study of water radiolysis-based degradation of chloroorganic pollutants in aqueous solutions.

Water radiolysis-induced destruction, dechlorination and mineralization of harmful chlorophenols, i.e., 2,4,6-trichlorophenol (2,4,6-tCPH), 4-chlorophenol (4-CPH) and 4-chlorocatechol (4-CC), using radioactive Co-60 have been investigated as individual and combination methods (2,4,6-tCPH+4-CPH+4-CC) with an initial concentration of 100 μM of each pollutant. The kinetic efficiencies of chlorophenol destruction were compared. The individual destruction percentages of 2,4,6-tCPH, 4-CPH and 4-CC reached at least 99% with absorbed doses (D99%) of 1.44, 1.73 and 1.85 kGy, respectively. Substantially higher absorbed doses were required to destroy each chlorophenol when they were all present in the treated. HCO3- anions inhibit the elimination efficiency of chlorophenols. The effects of S2O82- anions, N2O and N2 on destruction and mineralization were elaborated. O2 was crucial for the mineralization. Except for the γ-ray/N2 system, full mineralization was achieved for both individual and combined chlorophenols. The results indicate that hydrated electrons (eaq-) do not have a direct effect on the destruction of these chlorophenols. The study main goals were to show the successful application of ionizing radiation as a useful tool for environmental remediation, to continue scientific research on ionizing radiation as an advanced oxidation technology (AOT) and to provide a new, economic, practical and efficient solution to remove pollutants from aqueous media.

Ultrafast dynamics of solvated electrons at anatase TiO2/H2O interface

Solvated electrons are known to be the lowest energy charge transfer pathways at oxide/aqueous interface and the understanding of the electron transfer dynamics at the interface is fundamental for photochemical and photocatalytic processes. Taking anatase TiO2/H2O interface as a prototypical system, we perform time-dependent ab initio nonadiabatic molecular dynamics calculations to study the charge transfer dynamics of solvated electrons. For the static electronic properties, we find that the dangling H atoms can stabilize solvated electrons. A solvated electron band can be formed with one monolayer H2O adsorption. The energies of the solvated electron band minimum (SEBM) decrease when H2O adsorbs dissociatively. Moreover, the surface oxygen vacancies are also helpful for stabilizing the solvated electron band. For the dynamics behaviour, we find that the ultrafast charge transfer from SEBM to anatase TiO2 (1 0 1) surface at 100 K is mainly contributed by nonadiabatic mechanism. Comparing with rutile TiO2 (1 1 0) surface, the lifetime of solvated electron on anatase TiO2 (1 0 1) surface is longer, suggesting a better photocatalytic properties. Our results provide essential insights into the understanding of the charge transfer dynamics and the possible photocatalytic mechanism at oxide/aqueous interface.

Unveiled correlations between electron affinity and solvation in redox potential of quinone-based sodium-ion batteries

First-principles density functional theory method is employed with experimental techniques to investigate the redox properties and charge storage performance of seven quinone derivatives and to assess their potential as cathodes in sodium-ion batteries. The computed redox properties are comprehensively correlated with other properties, namely, electron affinity (EA), solvation energy, charge storage capacity, and energy density. The correlations are further verified to be applied not only to quinones but also to other organic molecules. The established universal correlations highlight three main conclusions. First, EA and solvation energy need to be cooperatively tuned to achieve a specific redox potential. Second, the exceptionally high performance of anthraquinone-2,6-dicarboxylic acid can be explained by the correlation of the redox potential with EA and solvation energy. Third, the differences in the performance between the calculated and experimental values for the other six quinone derivatives mainly result from the Na binding configurations, highlighting the experimental charge capacity is extraordinarily enhanced by metastable Na binding scenarios.

Solvent reorganization triggers photo-induced solvated electron generation in phenol

The analysis of the absorption spectrum and density of states of a cluster of phenol solvated with 15 water molecules indicates that the reorganization of the water molecules, facilitating the formation of solvated electrons, is a plausible mechanism in the photodissociation of phenol. Using quantitative wavefunction analysis, we demonstrate that while charge-transfer states involving electron transfer from phenol to water are mainly dark, a considerable number of them exists below the maximum of the ππ* absorption band and could be populated by internal conversion. These low-lying charge-transfer states do not show extended O-H distances, but are found for large electron-hole separations at which several water molecules can solvate and stabilize the transferred electron. Thus, charge-transfer states in solvated phenol can be stabilized by two factors: (i) elongation of the O-H bond, as was extensively discussed in the past, and (ii) reorganization of solvent molecules, as it is shown here.