Charge Transfer Reaction Mechanism Research Articles

A critical review on core/shell-based nanostructured photocatalysts for improved hydrogen generation

Photocatalytic water splitting into gaseous hydrogen and oxygen in the presence of semiconductor photocatalysts under a visible spectrum of solar irradiation is one of the most promising processes for plummeting energy demands and environmental pollution. Among the successful photocatalytic materials, the core/shell nanostructures show promising results owing to their fascinating morphology that protects the surface features of the core besides the effective separation of photo-excitons resulting in an enhanced rate of hydrogen production up to 162 mmol h−1g−1cat, which is a notable highest value reported in the literature. In this review, we have focused on the basic characteristics of the core-shell structure-based semiconductor photocatalytic systems and their efficient water-splitting reactions under light irradiation. Comprehensive detail on various synthesis methods of core-shell nanostructures, shell thickness-dependent properties, charge-transfer reaction mechanisms, and photocatalytic stability are highlighted in this review. Core-shell nanostructured materials have been extensively used as a photocatalyst, co-catalyst, and by coupling with supporting materials to improve the apparent quantum efficiency up to 45.6%. Besides, important photocatalytic properties that influence the redox reactions i.e., effective exciton separation, the effect of different light sources/wavelengths, surface charge modeling, photocatalytic active sites, and turnover frequency (TOF) have also been focused on and extensively described. Finally, the present and future prospects of the core-shell nanostructured photocatalysts for solar energy conversion into green hydrogen production have been expounded.

Innovative wastewater treatment based on electrodeposited thin film: Systematic studies of interfacial processes between birnessite and Mn(II) for a better efficiency

Electrodeposited birnessite thin films have remarkable properties for spontaneous degradation of organic pollutants until mineralization via a complex mechanism of charge transfer reactions. But, birnessite may be transformed into passivated material by reaction with Mn(II) species generated during the reduction of birnessite due to oxidation of organic pollutants. Such modification could imply a possible inhibition of the degradation process, a depreciated condition for future applications. This paper focuses on the determination of the real robustness of electrodeposited birnessite thin films and their quality for innovative wastewater treatment. Systematical studies were performed with solutions containing different Mn(II) concentrations in presence and in absence of oxygen to have unfavorable conditions. Samples were characterized at macroscopic scales (SEM and XRD) in addition to a focused study on the chemical modification at the extreme near surface by XPS. Open circuit potential (OCP) was also followed to complete the study. Similar characterizations were performed after interaction with organic pollutants solutions. As a result, nature and structure of the bulk and the near surface of birnessite stay unchanged after long times even in absence of oxygen or in presence of organic pollutants at high concentrations, proving their very good robustness and real interests for future applications.

Electrochemical Characterization of Li2s-P2S5 Glass-Ceramic Solid Electrolyte/Li Metal Electrode Interface

Introduction All solid-state Li rechargeable batteries are considered as promising candidates for post Li-ion batteries because non-flammability of inorganic solid electrolyte enhances their safety and reliability and Li metal can be used for a negative electrode. Sulfide-based solid electrolytes like Li2S-P2S5 showed high lithium ion conductivity of about 10-4 S cm-1 at 25 °C. In general, glass crystallization results in lower conductivities. However, 70Li2S-30P2S5 glass-ceramic obtained by crystallization of the glass showed conductivity of about 10-3 S cm-1 at room temperature because enhancement of the conductivity of the glass-ceramic came from the precipitation of superionic Li7P3S11 crystal in the glass-ceramic.1 Previously, we investigated the charge transfer reaction at the sulfide-based solid electrolyte/Li metal electrode interface and organic liquid electrolyte/Li interface by electrochemical methods with microelectrodes to evaluate activation energy (Ea ) of each process in charging. For the liquid electrolyte, the Ea for charge transfer reaction was much higher than that for ionic conduction. This suggests that the desolvation step of solvated Li+ has a large barrier. On the other hand, for the sulfide-based glass electrolyte, the Ea for charge transfer was comparable to that for ionic conduction due to the lacking of the desolvation step.2 In this study, the Ea for charge transfer reaction at the 70Li2S-30P2S5 glass-ceramic/Li metal electrode interface was evaluated, and it was compared with the Ea for the charge transfer at the Li2S-P2S5glass/Li metal electrode interface. Experimental 70Li2S-30P2S5 glass was prepared by mechanical milling of Li2S and P2S5 mixed powders for 12h. 70Li2S-30P2S5 glass-ceramics were prepared using one-step heat treatment 280 °C for 2h. 1 For electrochemical measurements, a two-electrode cell was fabricated. The glass-ceramic electrolyte powder was pelletized and sandwiched with Ni microelectrode (Φ50 μm) as working electrode and Li disk (Φ8 mm) attached on the stainless steel current collector as counter and reference electrode. All procedure was executed in Ar-filled glove box. Li was deposited on the working electrode at -150 mV (vs. Li/Li+) and the potential was stepped to -10, +10, -20, +20 mV (vs. OCV) until it reached ±150 mV. Each potential was held for 2 s. All measurements were performed at various temperatures. Results and Discussion To evaluate i0 , the steady-state current (i) measured at each overpotential (η) by potential step method was inserted into Allen-Hickling equation.Fig. 1 shows Allen-Hickling plots by the potential step method at various temperatures. All plots exhibited a linear relationship in the η range from -80 to +80 mV. From the intercept of each plot, i0 was evaluated and then i0 was plotted against the reciprocal of absolute temperature to calculate Ea using Arrhenius equation.Fig. 2 shows the Arrhenius plot of i0 obtained from potential step method. We successfully measured the Ea of charge transfer reaction at the interface between glass ceramic electrolyte and Li metal electrode. The Ea for the charge transfer was larger than that for ionic conduction, suggesting that the mechanism of charge transfer reaction at glass-ceramic/Li metal electrode interface is different from that for glass/Li metal electrode interface. Reference 1) K. Minami, A. Hayashi, M. Tatsumisago, J. Am. Ceram. Soc., 94, 1779 (2011).2) M. Chiku, W. Tsujiwaki, E. Higuchi, H. Inoue Electrochemistry, 80, 740 (2012).

Redox State Sensitive Spectroscopy of the Model Compound [(H-dcbpy)2RuII(NCS)2]2– (dcbpy = 2,2′-Bipyridine-4,4′-dicarboxylato)

The charge transfer reaction mechanism in a ruthenium polypyridine model complex with isothiocyanato ligands, i.e., [(H-dcbpy)2Ru(NCS)2]2– 2Bu4N+ (Ru2H) (dcbpy = 2,2′-bipyridine-4,4′-dicarboxylato), has been investigated by combining UV–vis absorption, resonance Raman spectroscopy, and electrochemical methods. Understanding the photophysics of light-harvesting complexes of this class is an indispensable prerequisite to improve the efficiency and durability of photocatalysts for hydrogen production, a possible basis of future energy sources. In this study, light-induced excitation of Ru2H has been mimicked by electrochemical reduction, and the intermediate redox state has been extensively characterized by UV–vis absorption and resonance Raman spectroscopy. The reduction occurs at the dcbpy ligand, and the spectra of reduced Ru2H resemble the excited state spectra. The reduced Ru2H exhibits a new absorption band at 440 nm localized at the bipyridine framework. This assignment is consistent with the observed enhancement of the bipyridine bands with respect to nonreduced Ru2H in the resonance Raman spectra of the reduced Ru2H excited at 458 nm. The detected shifts in Raman band positions and changes in the relative band intensities between nonreduced and reduced Ru2H are rationalized on an orbital level by quantum chemical calculations. The results indicate that the reduction is accompanied by deprotonation, i.e., an intramolecular electron-transfer-induced cleavage of the O–H bond splitting off hydrogen, of the carboxylic acid groups of the dcbpy ligand.

Long-range electron and charge transfers in peptide bridging phthalimide and methyl aminoacetate radical systems: Super-exchange and hopping mechanisms

Diabatic potential energy profiles and ET mechanisms for pa - - ( gly ) n = 3 -aa → pa- ( gly ) n = 3 - aa - and pa- ( gly ) n = 3 - aa - → pa - - ( gly ) n = 3 -aa where the (gly) n =3 in β-strand conformation. Curves P (or p ′), B, B1, B2, B3 and S refer to the pa − -(gly) n =3 -aa, pa- ( gly ) n = 3 - -aa , pa-(gly) − -(gly) n =2 -aa, pa-(gly)-(gly) − -(gly)-aa, pa-(gly) n =2 -(gly) − -aa and pa-(gly) n =3 -aa − states, respectively. Super-exchange and hopping mechanisms and distance dependence of long-range electron transfer (ET) and charge transfer (CT) in pa-(gly) n =0–6 -aa (pa = C 6 H 4 (CO) 2 N-(CH 2 CO), gly = glycine, aa = HNCH 2 COOCH 3 ) ionic molecules have been explored by computational studies of ET matrix element V PS , ET rate constant k ET and reorganization energies using two-state variational method (TSVM) and classical and semi-classical rate models. For comparison electron correlation effects are examined in the calculations. Calculated distance dependences of k ET are in good agreement with the experimental predictions. These results are then used to elucidate ET and CT reaction mechanisms.

Camptothecins–guanine interactions: mechanism of charge transfer reaction upon photoactivation

The potent activity exhibited by the antitumoral camptothecin (CPT) and its analog irinotecan (CPT-11) is known to be related to a close contact between the drug and the nucleic acid base guanine. This specificity of interaction between these two chromophores was examined by following changes in the photophysical properties of the drug using steady-state as well as time-resolved absorption and fluorescence methods. The observed effects on absorption, fluorescence emission and singlet excited state lifetimes give evidence for the occurrence of a stacking complex formation restricted to the quinoline part of CPT or CPT-11 and the guanine base but also with the adenine base. The triplet excited state properties of the drugs have been also characterized in absence and in presence of guanosine monophosphate and reveal the occurrence of an electron transfer from the guanine base to the drug. Support for this conclusion was obtained from the studies of a set of biological targets of various oxido-reduction potentials, adenosine monophosphate, cytidine, cytosine, tryptophan, tyrosine and phenylalanine. This finding gives an interpretation of the CPT-induced guanine photolesions previously reported in the literature. These data taken together are discussed in connection with the drug activity. The stacking complex CPT/guanine is necessary but not sufficient to explain the role of the chirality and of the lactone structure in the function of the drug. A stereospecific interaction with the enzyme topoisomerase I seems necessary to stabilize the stacking complex. The first experiments using time-resolved fluorescence by two-photon excitation confirms that CPT does not bind to the isolated enzyme.

Flow injection potentiometric determination of phosphate in waste waters and fertilisers using a cobalt wire ion-selective electrode

Flow injection potentiometric (FIP) determinations of dihydrogenphosphate (H2PO4–) in fertilisers and waste waters were undertaken using a cobalt wire phosphate ion selective electrode (ISE). The cobalt wire electrode was used in the FIP determination of phosphate in a carrier containing 4 × 10–2 mol l–1 potassium hydrogenphthalate at pH 5. The FIP technique was validated for fertiliser and waste water samples against standard spectrophotometric methods of analysis. The experimental data demonstrated that FIP is a highly selective and accurate technique for the determination of phosphate in fertilisers [i.e., the FIP and spectrophotometric data compare to within a few per cent (relative)]. A significant chloride interference occurs in waste water samples; however, this problem can be eradicated by using the chloride selectivity coefficient, and chloride concentrations determined independently using ion-selective electrode potentiometry. The discrepancy between corrected FIP and spectrophotometric data for phosphate in waste waters is generally ±5% (relative). X-ray photoelectron spectroscopic and electrochemical impedance spectroscopic (EIS) results suggest that the cobalt oxide surface film of the electrode is dissolved in phosphate media, facilitating the corrosion process that regulates the response of the cobalt wire electrode. It has been found that the EIS response of the cobalt wire electrode is dependent on both the pH and phosphate content of the solution. A charge-transfer reaction mechanism, and accompanying Nernst equation, have been proposed to explain the phosphate response of the cobalt wire electrode.

Dispersion of crystallites of the metal substrate during the electropolymerization of 2-hydroxy-3-aminophenazine (HAPh) on Au: effect of Au crystallites on the kinetics and mechanism of charge-transfer reactions between the polymer and redox species in electrolyte

The electrochemical polymerization of 2-hydroxy-3-aminophenazine (HAPh) has been studied in aqueous perchloric acid solutions. Under potential cycling the polymerization reaction is accompanied by electrodissolution/electrodeposition of the Au substrate onto which the polymer is deposited. TEM observations have revealed two kinds of crystalline grains in the pHAPh polymer film. The first kind consists of spherical grains of pure gold which are randomly oriented in the polymer matrix. The second kind consists of oriented well-shaped crystals of unknown composition, but most probably crystals of an intermediate Au(III) complex. The dispersed Au crystallites affect the kinetics and mechanism of charge-transfer reactions between the polymer and redox species in electrolyte, such as dihydroxybenzenes and hydrazine, as well as their permeability and incorporation into the polymer film.

Kinetics and mechanism of charge-transfer reactions between conducting polymers and redox ions in electrolytes

Charge-transfer reactions of selected ionic redox couples were studied on rotating glassy carbon electrodes covered with conducting polymers of different membrane properties: poly-N-methylpyrrole as the anion exchanger or poly-N-methylpyrrole with immobilized poly(4-styrenesulfonate) ions as the cation exchanging matrix. The electrochemical and spectroscopic (EDAX) results obtained with the redox systems: Fe(CN) 3− 4− 6, Ru(NH 3) 3+ 2+ , Eu 3+/Eu 2+, Co(en) 3+ 2+ 3 and Fe(C 2O 4) 3− 4− 3 pointed to reactions proceeding at the polymer-solution interface. The data were analysed on the basis of a model in which the charge transfer was regarded as a redox reaction between polymeric sites in the film and the redox species in the solution. The rate of electron transfer was found to be: (i) proportional to the concentration of the oxidized or reduced polymeric sites; (ii) dependent to some extent on the Donnan potential prevailing at the interface; and (iii) correlated with the thermodynamic driving force of the reaction between the polymer and redox species.

Mechanism of Charge Transfer Reactions: Reactions of Rare Gas Ions with the trans-, cis-, and 1,1-Difluoroethylene Geometric Isomers

Mechanism of Charge Transfer Reactions: Reactions of Rare Gas Ions with the <i>trans-, cis-</i>, and 1,1-Difluoroethylene Geometric Isomers