Hydrated Electron Research Articles

Charge-transfer-to-solvent reactions from I(-) to water, methanol, and ethanol studied by time-resolved photoelectron spectroscopy of liquids.

The charge-transfer-to-solvent (CTTS) reactions from iodide (I(-)) to H2O, D2O, methanol, and ethanol were studied by time-resolved photoelectron spectroscopy of liquid microjets using a magnetic bottle time-of-flight spectrometer with variable pass energy. Photoexcited iodide dissociates into a weak complex (a contact pair) of a solvated electron and an iodine atom in similar reaction times, 0.3 ps in H2O and D2O and 0.5 ps in methanol and ethanol, which are much shorter than their dielectric relaxation times. The results indicate that solvated electrons are formed with minimal solvent reorganization in the long-range solvent polarization field created for I(-). The photoelectron spectra for CTTS in H2O and D2O-measured with higher accuracy than in our previous study [Y. I. Suzuki et al., Chem. Sci. 2, 1094 (2011)]-indicate that internal conversion yields from the photoexcited I(-*) (CTTS) state are less than 10%, while alcohols provide 2-3 times greater yields of internal conversion from I(-*). The overall geminate recombination yields are found to be in the order of H2O > D2O > methanol > ethanol, which is opposite to the order of the mutual diffusion rates of an iodine atom and a solvated electron. This result is consistent with the transition state theory for an adiabatic outer-sphere electron transfer process, which predicts that the recombination reaction rate has a pre-exponential factor inversely proportional to a longitudinal solvent relaxation time.

Ultrafast Scavenging of the Precursor of H(•) Atom, (e(-), H3O(+)), in Aqueous Solutions.

Picosecond pulse radiolysis measurements have been performed in several highly concentrated HClO4 and H3PO4 aqueous solutions containing silver ions at different concentrations. Silver ion reduction is used to unravel the ultrafast reduction reactions observed at the end of a 7 ps electron pulse. Solvated electrons and silver atoms are observed by the pulse (electron beam)-probe (supercontinuum light) method. In highly acidic solutions, ultrafast reduction of silver ions is observed, a finding that is not compatible with a reaction between the H(•) atom and silver ions, which is known to be thermally activated. In addition, silver ion reduction is found to be even more efficient in phosphoric acid solution than that in neutral solution. In the acidic solutions investigated here, the species responsible for the reduction of silver atoms is considered to be the precursor of the H(•) atom. This precursor, denoted (e(-), H3O(+)), is a pair constituting an electron (not fully solvated) and H3O(+). Its structure differs from that of the pair of a solvated electron and a hydronium ion (es(-), H3O(+)), which absorbs in the visible region. The (e(-), H3O(+)) pair , called the pre-H(•) atom here, undergoes ultrafast electron transfer and can, like the presolvated electron, reduce silver ions much faster than the H(•) atom. Moreover, it is found that with the same concentration of H3O(+) the reduction reaction is favored in the phosphoric acid solution compared to that in the perchloric acid solution because of the less-efficient electron solvation process. The kinetics show that among the three reducing species, (e(-), H3O(+)), (es(-), H3O(+)), and H(•) atom, the first one is the most efficient.

Free Energies of Cavity and Noncavity Hydrated Electrons Near the Instantaneous Air/Water Interface.

The properties of the hydrated electron at the air/water interface are computed for both a cavity and a noncavity model using mixed quantum/classical molecular dynamics simulation. We take advantage of our recently developed formalism for umbrella sampling with a restrained quantum expectation value to calculate free-energy profiles of the hydrated electron's position relative to the water surface. We show that it is critical to use an instantaneous description of the air/water interface rather than the Gibbs' dividing surface to obtain accurate potentials of mean force. We find that noncavity electrons, which prefer to encompass several water molecules, avoid the interface where water molecules are scarce. In contrast, cavity models of the hydrated electron, which prefer to expel water, have a local free-energy minimum near the interface. When the cavity electron occupies this minimum, its absorption spectrum is quite red-shifted, its binding energy is significantly lowered, and its dynamics speed up quite a bit compared with the bulk, features that have not been found by experiment. The surface activity of the electron therefore serves as a useful test of cavity versus noncavity electron solvation.

Solvated electrons at the atmospheric pressure plasma–water anodic interface

We present results from a particle-in-cell/Monte Carlo model of a dc discharge in argon at atmospheric pressure coupled with a fluid model of an aqueous electrolyte acting as anode to the plasma. The coupled models reveal the structure of the plasma–electrolyte interface and near-surface region, with a special emphasis on solvated or hydrated electrons. Results from the coupled models are in generally good agreement with the experimental results of Rumbach et al ( Nat. Commun. 6 7248). Electrons injected from the plasma into the water are solvated, then lost by reaction with water within about 10–20 nm from the surface. The major reaction products are OH− and H2. The solvated electron density profile is controlled by the injected electron current density and subsequent reactions with water, and is relatively independent of the external plasma electric field and the salt concentration in the aqueous electrolyte. Simulations of the effects of added scavenger compounds (H2O2, , and H+) on near-surface solvated electron density generally match the experimental results. The generation of near-surface OH− following electron-water decomposition in the presence of bulk acid creates a highly basic region (pH ~ 11) very near the surface. In the presence of bulk solution acidity, pH can vary from a very acidic pH 2 away from the surface to a very basic pH 11 over a distance of ~200 nm. High near-surface gradients in aqueous solution properties could strongly affect plasma-liquid applications and challenge theoretical understanding of this complex region.

Serial crystallography captures enzyme catalysis in copper nitrite reductase at atomic resolution from one crystal.

Relating individual protein crystal structures to an enzyme mechanism remains a major and challenging goal for structural biology. Serial crystallography using multiple crystals has recently been reported in both synchrotron-radiation and X-ray free-electron laser experiments. In this work, serial crystallography was used to obtain multiple structures serially from one crystal (MSOX) to study in crystallo enzyme catalysis. Rapid, shutterless X-ray detector technology on a synchrotron MX beamline was exploited to perform low-dose serial crystallography on a single copper nitrite reductase crystal, which survived long enough for 45 consecutive 100 K X-ray structures to be collected at 1.07-1.62 Å resolution, all sampled from the same crystal volume. This serial crystallography approach revealed the gradual conversion of the substrate bound at the catalytic type 2 Cu centre from nitrite to nitric oxide, following reduction of the type 1 Cu electron-transfer centre by X-ray-generated solvated electrons. Significant, well defined structural rearrangements in the active site are evident in the series as the enzyme moves through its catalytic cycle, namely nitrite reduction, which is a vital step in the global denitrification process. It is proposed that such a serial crystallography approach is widely applicable for studying any redox or electron-driven enzyme reactions from a single protein crystal. It can provide a 'catalytic reaction movie' highlighting the structural changes that occur during enzyme catalysis. The anticipated developments in the automation of data analysis and modelling are likely to allow seamless and near-real-time analysis of such data on-site at some of the powerful synchrotron crystallographic beamlines.

Corrigendum: The solvation of electrons by an atmospheric-pressure plasma.

Nature Communications 6 Article number:7248 (2015); Published: 19 June 2015; Updated 6 June 2016. We have discovered an error in the original data analysis of our publication, which significantly impacts the model parameters derived from experiment. Specifically, the correction changes our estimations of the average penetration depth l and scavenging rate constants, but these new estimates do not change the overall physics conveyed in the Article.

A pulsed electron beam synthesis of PEDOT conducting polymers by using sulfate radicals as oxidizing species

In this study, an original radiolytic method, based on pulsed electron beam irradiation, is used for the synthesis of conducting PEDOT in an aqueous solution containing EDOT monomers in the presence of potassium persulfate, K2S2O8, at 0°C. At this low temperature, EDOT monomers are not chemically oxidized by S2O82− anions, initiating PEDOT polymerization, but are rather oxidized by sulfate radicals, SO4•−, which are radiolytically generated by the reaction of solvated electrons, produced by water radiolysis, with persulfate anions. Successfully, as demonstrated by UV–vis absorption spectrophotometry and ATR-FTIR spectroscopy, irradiating the aqueous solution, by using a series of accumulated electron pulses, enables complete EDOT oxidation and quantitative in situ PEDOT polymerization through a step-by-step oxidation mechanism. The morphology of PEDOT polymers, mixed with unreacted K2S2O8 salt, is characterized by Cryo-TEM microscopy in aqueous solution and by SEM after deposition. Successfully, in the absence of any washing step, high resolution AFM microscopy, coupled with infrared nanospectroscopy, is used to discriminate between the organic polymers and the inorganic salt and to probe the local chemical composition of PEDOT nanostructures. The results demonstrate that PEDOT polymers form globular self-assembled nanostructures which preferentially adsorb onto unreacted K2S2O8 solid nanoplates. The present results first demonstrate the efficiency of sulfate radicals as oxidizing species for the preparation of nanostructured PEDOT polymers and second highlight the promising potentiality of electron accelerators in the field of conducting polymers synthesis.

Gamma and ion-beam irradiation of DNA: Free radical mechanisms, electron effects, and radiation chemical track structure

The focus of our laboratory's investigation is to study the direct-type DNA damage mechanisms resulting from γ-ray and ion-beam radiation-induced free radical processes in DNA which lead to molecular damage important to cellular survival. This work compares the results of low LET (γ-) and high LET (ion-beam) radiation to develop a chemical track structure model for ion-beam radiation damage to DNA. Recent studies on protonation states of cytosine cation radicals in the N1-substituted cytosine derivatives in their ground state and 5-methylcytosine cation radicals in ground as well as in excited state are described. Our results exhibit a radical signature of excitations in 5-methylcytosine cation radical. Moreover, our recent theoretical studies elucidate the role of electron-induced reactions (low energy electrons (LEE), presolvated electrons (epre−), and aqueous (or, solvated) electrons (eaq−)). Finally DFT calculations of the ionization potentials of various sugar radicals show the relative reactivity of these species.

(ECS Toyota Young Investigator Fellowship Program Address) Electrochemical Reduction of CO2(aq) By Solvated Electrons at a Plasma-Liquid Interface

While electron transfer reactions typically occur at the interface between a reactant and a metal in an electrolyte solution, it is also possible to initiate these reactions at the interface between a plasma (gas discharge) and a solution. In this case, a free, gaseous electron is injected into the solution from the plasma phase, solvating before reacting away. In our previous work, we have shown that an atmospheric pressure plasma in argon can be used as a cathode in an electrolytic cell to produce solvated electrons[1]. In this work, we show that the solvated electrons produced by the plasma cathode can be used to reduce dissolved carbon dioxide, CO2(aq), to form the carboxyl radical anion, CO2 – (aq). The CO2 – (aq) radical can either recombine to form oxalate under basic conditions or react with H+ (aq) to produce formate under acidic conditions. We measure the Faradaic yield of both oxalate and formate formation using liquid ion chromatography and show that it is dictated by the well-known reaction kinetics of solvated electrons and CO2 – (aq). Importantly, these 3-dimensional reaction kinetics are distinct from the 2-dimensional kinetics found in traditional electrochemistry and catalysis, and may open up new opportunities to explore electrochemical carbon dioxide reforming. [1]P. Rumbach, D.M. Bartels, R.M. Sankaran, and D.B. Go, Nature Comm. 6, 7248 (2015).

(Invited) Plasma-Liquid Interactions for Nanoparticle Synthesis, Functionalization and Biomedical Interactions

Non-equilibrium atmospheric pressure plasmas interacting with liquids offer a unique source of highly reactive chemistry beneficial for many applications in biology, medicine and advanced materials manufacturing. Plasma-liquid interactions can involve synthesis of nanoparticles (NPs) and inactivation of pathogens. While the underlying mechanisms of these processes are poorly understood, the application potential is enormous. The presentation will highlight some examples of reaction pathways responsible for the synthesis of AgNPs at the plasma-liquid interface. This will include a detailed analysis of the reduction mechanism of Ag+, the initial step for the formation of NPs in solution, through various mechanisms proposed in literature including solvated electrons, UV photons and hydrogen atoms. Plasmas are typically oxidizing and not reducing environments as they produce an abundant source of reactive oxygen species (ROS). We will present the possibility of using these ROS for the functionalization of NPs and in particular the generation of luminescent and hydrophobic water-soluble silicon quantum dots. In addition, we will discuss the importance of these ROS in the inactivation of pathogens in solutions. We will conclude with an overview of the challenges and opportunities ahead.

Monochromatic X-Ray Induced Novel Synthesis of Plasmonic Nanostructure for Photovoltaic Application.

It has been universally delineated that the plasmonic metal nanoparticles can enhance the efficiency of photovoltaic cell by increasing the probability of energetic solar photons capturing phenomena using localized surface plasmonic resonance response. In this paper, we developed a novel in-situ simple approach to synthesize noble plasmonic silver nanoparticles (AgNP) from aqueous poly-vinyl-pyrrolidone solution of metal salt using radiolysis of water via synchrotron monochromatic X-ray irradiation without any chemical reducing agent. X-ray irradiation of water produces hydrated electrons , superoxide and atom radicals , which triggers the reaction and reduces metal salt. X-ray radiolysis based synthesis provides the control over the reaction and prevent the formation of secondary products as occurs in case of chemical reduction route. In the previous studies, synchrotron “white” X-rays had been examined for the synthesis of metal nanoparticles, but that technique limits only upto the material synthesis while in this work we explored the role of “monochromatic” X-rays for the production of bulk amount of nanoparticles which would also provide the feasibility of in-situ characterization. Transmission electron micrographs show that the synthesized AgNP appears spherical with diameter of 2–6 nm and is in agreement with the size estimation from uv-vis spectra by “Mie theory”.

Radiolysis of Supercritical Water at 400°C: A Sensitivity Study of the Density Dependence of the Yield of Hydrated Electrons on the (eaq−+eaq−) Reaction Rate Constant

The temperature dependence of the rate constant (k) of the bimolecular reaction of two hydrated electrons (eaq−) measured in alkaline water exhibits an abrupt drop between 150°C and 200°C; above 250°C, it is too small to be measured reliably. Although this result is well established, the applicability of this sudden drop in k(eaq−+eaq−)) above ∼150°C to neutral or slightly acidic solution, as recommended by some authors, still remains uncertain. In fact, the recent work suggested that in near-neutral water the abrupt change in k above ∼150°C does not occur and that k should increase, rather than decrease, at temperatures greater than 150°C with roughly the same Arrhenius dependence of the data below 150°C. In view of this uncertainty of k, Monte Carlo simulations were used in this study to examine the sensitivity of the density dependence of the yield of eaq− in the low–linear energy transfer (LET) radiolysis of supercritical water (H2O) at 400°C on variations in the temperature dependence of k. Two different values of the eaq− self-reaction rate constant at 400°C were used: one was based on the temperature dependence of k above 150°C as measured in alkaline water (4.2×108 M−1 s−1), and the other was based on an Arrhenius extrapolation of the values below 150°C (2.5×1011 M−1 s−1). In both cases, the density dependences of our calculated eaq− yields at ∼60 ps and 1 ns were found to compare fairly well with the available picosecond pulse radiolysis experimental data (for D2O) for the entire water density range studied (∼0.15–0.6 g/cm3). Only a small effect of k on the variation of G(eaq−)) as a function of density at 60 ps and 1 ns could be observed. In conclusion, our present calculations did not allow us to unambiguously confirm (or deny) the applicability of the predicted sudden drop of k(eaq−+eaq−) at ∼150°C in near-neutral water.

Electron Solvation in Liquid Ammonia: Lithium, Sodium, Magnesium, and Calcium as Electron Sources.

A free electron in solution, known as a solvated electron, is the smallest possible anion. Alkali and alkaline earth atoms serve as electron donors in solvents that mediate outer-sphere electron transfer. We report herein ab initio molecular dynamics simulations of lithium, sodium, magnesium, and calcium in liquid ammonia at 250 K. By analyzing the electronic properties and the ionic and solvation structures and dynamics, we systematically characterize these metals as electron donors and ammonia molecules as electron acceptors. We show that the solvated metal strongly modifies the properties of its solvation shells and that the observed effect is metal-specific. Specifically, the radius and charge exhibit major impacts. The single solvated electron present in the alkali metal systems is distributed more uniformly among the solvent molecules of each metal's two solvation shells. In contrast, alkaline earth metals favor a less uniform distribution of the electron density. Alkali and alkaline earth atoms are coordinated by four and six NH3 molecules, respectively. The smaller atoms, Li and Mg, are stronger electron donors than Na and Ca. This result is surprising, as smaller atoms in a column of the periodic table have higher ionization potentials. However, it can be explained by stronger electron donor-acceptor interactions between the smaller atoms and the solvent molecules. The structure of the first solvation shell is sharpest for Mg, which has a large charge and a small radius. Solvation is weakest for Na, which has a small charge and a large radius. Weak solvation leads to rapid dynamics, as reflected in the diffusion coefficients of NH3 molecules of the first two solvation shells and the Na atom. The properties of the solvated electrons established in the present study are important for radiation chemistry, synthetic chemistry, condensed-matter charge transfer, and energy sources.

Examination of the formation process of pre-solvated and solvated electron in n-alcohol using femtosecond pulse radiolysis

Abstract The formation process of pre-solvated and solvated electron in methanol (MeOH), ethanol (EtOH), n -butanol (BuOH), and n -octanol (OcOH) were investigated using a fs-pulse radiolysis technique by observing the pre-solvated electron at 1400 nm. The formation time constants of the pre-solvated electrons were determined to be 1.2, 2.2, 3.1, and 6.3 ps for MeOH, EtOH, BuOH, and OcOH, respectively. The formation time constants of the solvated electrons were determined to be 6.7, 13.6, 22.2, and 32.9 ps for MeOH, EtOH, BuOH, and OcOH, respectively. The formation dynamics and structure of the pre-solvated and solvated electrons in n -alcohols were discussed based on relation between the obtained time constant and dielectric relaxation time constant from the view point of kinetics. The observed formation time constants of the solvated electrons seemed to be strongly correlated with the second component of the dielectric relaxation time constants, which are related to single molecule motion. On the other hand, the observed formation time constants of the pre-solvated electrons seemed to be strongly correlated with the third component of the dielectric relaxation time constants, which are related to dynamics of hydrogen bonds.

Wavelength Dependence of UV Photoemission from Solvated Electrons in Bulk Water, Methanol, and Ethanol.

We have measured the wavelength dependence (340-215 nm) of one-photon photoemission from the ground electronic state of solvated electrons in bulk water, methanol, and ethanol. In every case, the vertical electron binding energy (VBE) gradually increased with photon energy, indicating that the photoelectron kinetic energy diminishes as a result of electron-vibration inelastic scattering prior to emission from the liquid surface. In contrast, the VBE of the Rydberg electron in DABCO (1,4-diazabicyclo[2,2,2]octane), which has a surface-excess density, revealed no clear wavelength dependence. These results suggest that the solvated electrons are created predominantly in the bulk and that VBEs measured using UV photoemission spectroscopy of liquids generally require energy corrections to account for inelastic scattering effects. From the wavelength dependence, we have re-estimated the VBEs of solvated electrons in bulk water, methanol, and ethanol to be 3.3, 3.1, and 3.1 eV, respectively. Hydrated electrons were also identified by photoemission spectroscopy using 90 nm radiation.

Electrogenerated chemiluminescence induced by sequential hot electron and hole injection into aqueous electrolyte solution

Hole injection into aqueous electrolyte solution is proposed to occur when oxide-coated aluminum electrode is anodically pulse-polarized by a voltage pulse train containing sufficiently high-voltage anodic pulses. The effects of anodic pulses are studied by using an aromatic Tb(III) chelate as a probe known to produce intensive hot electron-induced electrochemiluminescence (HECL) with plain cathodic pulses and preoxidized electrodes. The presently studied system allows injection of hot electrons and holes successively into aqueous electrolyte solutions and can be utilized in detecting electrochemiluminescent labels in fully aqueous solutions, and actually, the system is suggested to be quite close to a pulse radiolysis system providing hydrated electrons and hydroxyl radicals as the primary radicals in aqueous solution without the problems and hazards of ionizing radiation. The analytical power of the present excitation waveforms are that they allow detection of electrochemiluminescent labels at very low detection limits in bioaffinity assays such as in immunoassays or DNA probe assays. The two important properties of the present waveforms are: (i) they provide in situ oxidation of the electrode surface resulting in the desired oxide film thickness and (ii) they can provide one-electron oxidants for the system by hole injection either via F- and F+-center band of the oxide or by direct hole injection to valence band of water at highly anodic pulse amplitudes.

Photocatalytic reduction of nitrogen to ammonia on diamond thin films grown on metallic substrates

Recent studies have demonstrated that boron-doped diamond can act as a solid-state source of electrons in water when illuminated with above-bandgap light. Excitation of electrons to diamond's conduction band leads to facile emission of electrons into water; the resulting solvated electrons are potent reducing agents able to initiate many chemical reactions such as the reduction of N2 to NH3 and the reduction of CO2 to CO. Here, we report investigations of the photocatalytic activity of diamond thin films grown on Mo, Ni, and Ti substrates. Our results show that in each case, there is a maximum in the photocatalytic activity that occurs just when the diamond coalesces into a contiguous film, then decreasing as the film becomes thicker. These results suggest that electron emission arises in part from excitation of electrons in the metal substrate, followed by injection into the conduction band of the diamond film. More detailed studies on films grown on niobium show that at the open-circuit potential the films have a downward bend-bending of ~0.3V, which is favorable for barrier-free electron emission. Our result suggests that metal–diamond heterostructures may provide a scalable approach to achieving difficult photocatalytic reactions.

Metal ion reductions by femtosecond laser pulses with micro-Joule energy and their efficiencies

Abstract Chemical reactions of metal ions in solution were studied using 800 nm, 90 femtosecond laser pulses with a laser intensity range beginning at sub-micro-Joules/pulse, where the intensities were at or just below supercontinuum generation. Products were observed as surface resonance absorptions assignable to Ag, Au nanoparticles (NPs) and light scattering, indicating the formation of Pd NPs. The consumption efficiencies of metal ions from the Au and Pd ions were estimated to be 10−3 per incident photon. The major absorbing species of the laser energy was water, which led to the formation of solvated electrons that acted as a reducing agent of metal ions, while direct multi-photon dissociation to neutral atoms was unlikely. Metal ion Fe3+and Yb3+ systems are also discussed, where Fe3+ to Fe2+ was ascribed to two-photon absorption and Yb3+ to Yb2+ to a reduction followed by solvent ionization.

Electrochemical Production of Oxalate and Formate from CO2 by Solvated Electrons Produced Using an Atmospheric-Pressure Plasma

In this work, we use an atmospheric-pressure plasma in argon as a cathode to electrochemically reduce carbon dioxide in aqueous solution. Using optical absorption spectroscopy, we directly show that solvated electrons reduce CO2(aq) to form the carboxyl radical anion CO2−(aq), and the reaction obeys 3D bulk reaction kinetics similar to those measured in radiolysis experiments. We then use liquid ion chromatography to show that the CO2− (aq) intermediate ultimately reacts to produce oxalate and formate. The overall faradaic efficiency of the reaction is close to 10% for a CO2(aq) concentration of 34 mM. However, given the known reaction kinetics of solvated electrons, this efficiency should approach 100% as the concentration of CO2(aq) is increased.

Combining energy and electron transfer in a supramolecular environment for the "green" generation and utilization of hydrated electrons through photoredox catalysis.

We present a new mechanism that sustainably produces hydrated electrons, i.e., extremely strong reductants, yet consumes only green photons (532 nm) and the bioavailable ascorbate as sacrificial donor. The mechanism couples an energy-transfer cycle, in which a light-harvesting ruthenium polypyridine complex absorbs a first photon and passes the excitation energy on to a pyrene-based redox catalyst, with an electron-transfer cycle, in which the resulting triplet is reductively quenched and the energy-rich aryl radical anion is finally ionized by a second photon. Thus separating the roles of primary and secondary absorber permitted choosing a redox catalyst with a nonabsorbing ground state but efficiently ionizable radical anion; the quantum yield of the ionization step in our complex mechanism surpasses that in a simple photoredox cycle featuring only the metal complex by a factor of four. We suppressed undesired cross reactions through the noncovalent interactions of an anionic micelle with the charges of the reactants, intermediates, and products: the cationic light-harvesting complex remains affixed to the micelle surface, which blocks the access of the negatively charged sacrificial donor, aryl radical anion and hydrated electron, but allows the pyrene ground-state almost unhindered entry into the Stern layer despite a carboxylate substituent by virtue of its large dipole moment. We demonstrate the applicability of the mechanism to the reductive detoxification of halogenated organic waste, which hitherto required UV-C for electron generation, by decomposing the typical model compound chloroacetate.