Interfacial Electron Transfer Rate Constant Research Articles

Aspect Ratio-Dependent Charge Carrier Dynamics in Matchstick-like Ag2S-ZnS Nanorods for Solar Hydrogen Generation

Matchstick-like Ag2S-ZnS nanorods (NRs) with a tunable aspect ratio (AR) were synthesized using one-pot thermal decomposition. The ultraviolet photoelectron spectra and time-resolved photoluminescence spectra of the Ag2S-ZnS NRs were collected to study their electronic band structures and charge carrier dynamics. The energy difference (ΔE) at the interface between the ZnS stem and Ag2S tip was altered as the AR of Ag2S-ZnS NRs increased from 11.9 to 18.4, resulting in an enlarged driving force for the delocalized electrons along the conduction band of ZnS being injected into that of Ag2S. The interfacial electron transfer rate constant (ket) from ZnS to Ag2S could be enhanced by ∼2 orders of magnitude from 5.27 × 106 to 3.24 × 108 s-1, leading to a significant improvement in the efficiency of solar hydrogen generation. This investigation provides new physical insights into the manipulation of charge carrier dynamics by means of AR adjustment in semiconductor nanoheterostructures for photoelectric conversions.

Interfacial bridge-mediated electron transfer: mechanistic analysis based on electrochemical kinetics and theoretical modelling

Understanding the physical and chemical factors that control the kinetics of interfacial electron-transfer (ET) reactions is important for a large number of technological applications. The present article describes electrochemical kinetic studies of these factors, in which standard interfacial ET rate constants (k(0)(l)) have been measured for ET between substrate Au electrodes and various redox couples attached to the electrode surfaces by variable lengths (l) of oligomethylene (OM), oligophenylenevinylene (OPV) and oligophenyleneethynylene (OPE) bridges, which were constituents of mixed self-assembled monolayers (SAMs). The k(0)(l) measurements employed the indirect laser-induced temperature jump (ILIT) technique, which permits the measurement of interfacial ET rates that are orders of magnitude faster than those measurable by conventional techniques using the macroelectrodes that are the most convenient substrates for the mixed SAMs. The robustness of the measured rate constants (k(0)(l)), together with the Arrhenius activation energies (E(a)(l)) and preexponential factors (A(l)), is demonstrated by their invariance with respect to several experimental system parameters (including the chemical nature and length of the diluent component of the mixed SAM). Analysis of the kinetic results demonstrates that all of the observed interfacial ET processes proceed through a common type of transition state (predominantly associated with solvent reorganization around the redox moiety) and that the actual ET step involves direct electronic tunnelling between the Au electrode and the redox moiety. However, for the full range of l investigated, a global exponential decay of A(l) is not found for any of the three types of bridges. Possible reasons for this behavior, including the role of rate determining steps associated with adiabatic mechanisms within or beyond the transition state theory framework, are discussed, and comparisons with related conductance measurements are presented.

Comparison of the Self-Exchange and Interfacial Charge-Transfer Rate Constants for Methyl- versus tert-Butyl-Substituted Os(III) Polypyridyl Complexes

Differences in the self-exchange and interfacial electron-transfer rate constants have been evaluated for a relatively unhindered Os(III/II) redox system, osmium(III/II) tris(4,4'-di-methyl-2,2'-bipyridyl), [Os(Me2bpy)3]3+/2+, relative to those of a relatively hindered system, osmium(III/II) tris(4,4'-di-tert-butyl-2,2'-bipyridyl), [Os(t-Bu2bpy)3]3+/2+. In contrast to the predicted increase in rate constant by a factor of 2-3 due to the difference in reorganization energy of the two complexes, introduction of the tert-butyl functionality decreased the self-exchange rate constant, as measured by NMR line-broadening techniques, by a factor of approximately 50 as compared to that of the analogous methyl-substituted osmium complex. Steady-state current density versus potential measurements, in conjunction with differential capacitance versus potential measurements, were used to compare the interfacial electron-transfer rate constants at n-type ZnO electrodes of [Os(t-Bu2bpy)3]3+/2+ and [Os(Me2bpy)3]3+/2+. The interfacial electron-transfer rate constant for the reduction of [Os(t-Bu2bpy)3]3+ was 100 times smaller than that for [Os(Me2bpy)3]3+. The results indicate that the tert-butyl group can act as a spacer on an outer-sphere redox couple and significantly decrease the electronic coupling of the electron-transfer reaction in both self-exchange and interfacial electron-transfer processes.

A Comparison between Interfacial Electron-Transfer Rate Constants at Metallic and Graphite Electrodes

The Fermi golden rule formalism has been used to derive the rate constant for interfacial electron transfer from a semimetallic electrode, such as highly ordered pyrolytic graphite (HOPG), to a redox couple in solution. A simple expression is presented that semiquantitatively relates the electron-transfer rate constant at a semimetallic electrode to that at a metallic electrode. The approach allows for the estimation of the value of the rate constant for interfacial charge transfer to nonadsorbing outer-sphere redox species at semimetallic electrodes. Rate constants for interfacial electron transfer for a variety of one-electron redox couples at semimetallic electrodes have been calculated relative to the rate constant of the ferrocenium/ferrocene redox couple at a gold electrode. Good agreement is found, in general, between the calculated and observed rate constants.

In situscanning tunnelling spectroscopy of inorganic transition metal complexes

Redox molecules with equilibrium potentials suitable for electrochemical control offer perspectives in nanoscale and single-molecule electronics. This applies to molecular but also towards higher sophistication such as transistor or diode function. Most recent nanoscale or single-molecule functional systems are, however, fraught with operational limitations such as cryogenic temperatures and ultra-high vacuum, or lack of electrochemical potential control. We report here cyclic voltammetry (CV) using single-crystal Au(111)- and Pt(111)-electrodes and electrochemical in situ scanning tunnelling microscopy (STM) of a class of Os(II)/(III)- and Co(II)/(III)-complexes, the former novel molecular electronics. The complexes are robust, with ligand groups suitable for linking the complexes to the Au(111)- and Pt(111)-surfaces via N- and S-donor atoms. The data reflect monolayer behaviour. Interfacial ET of the Os-complexes is fast, kET(0) > or = 10(6) s(-1), while the Co-complex reacts much more slowly, kET(0) approximately (1-3) x 10(3) s(-1). In STM of the Os-complexes shows a maximum in the tunnelling current/overpotential relation at constant bias voltage with up to 50-fold current rise. The peak position systematically the bias voltage and equilibrium potential, in keeping with theoretical frames for two-step electron transfer (ET) of in situ STM of redox molecules. The molecular conductivity behaves broadly similarly. The Co-complex also shows a tunnelling spectroscopic feature but much weaker than the Os-complexes. This can be ascribed much smaller interfacial ET rate constant, again caused by large intramolecular nuclear reorganization and weak electronic coupling to the substrate electrode. Overall the has mapped the properties of target molecules needed for stable electronic switching, possible importance in molecular electronics towards the single-molecule level, in room temperature condensed matter environment.

Enzyme-immobilized SiO2–Si electrode: Fast interfacial electron transfer with preserved enzymatic activity

The enzyme, glucose oxidase (GOx), is immobilized using electrostatic interaction on the native oxide of heavily doped n-type silicon. Voltammetric measurement shows that the immobilized GOx gives rise to a very fast enzyme-silicon interfacial electron transfer rate constant of 7.9s−1. The measurement also suggests that the enzyme retains its native conformation when immobilized on the silicon surface. The preserved native conformation of GOx is further confirmed by testing the enzymatic activity of the immobilized GOx using glucose. The GOx-immobilized silicon is shown to behave as a glucose sensor that detects glucose with concentrations as low as 50μM.

Measurement of the Dependence of Interfacial Charge-Transfer Rate Constants on the Reorganization Energy of Redox Species at n-ZnO/H2O Interfaces

The interfacial energetic and kinetics behavior of n-ZnO/H2O contacts have been determined for a series of compounds, cobalt trisbipyridine (Co(bpy)3(3+/2+)), ruthenium pentaamine pyridine (Ru(NH3)5 py(3+/2+)), cobalt bis-1,4,7-trithiacyclononane (Co(TTCN)2(3+/2+)), and osmium bis-dimethyl bipyridine bis-imidazole (Os(Me2bpy)2(Im)2(3+/2+)), which have similar formal reduction potentials yet which have reorganization energies that span approximately 1 eV. Differential capacitance vs potential and current density vs potential measurements were used to measure the interfacial electron-transfer rate constants for this series of one-electron outer-sphere redox couples. Each interface displayed a first-order dependence on the concentration of redox acceptor species and a first-order dependence on the concentration of electrons in the conduction band at the semiconductor surface, in accord with expectations for the ideal model of a semiconductor/liquid contact. Rate constants varied from 1 x 10(-19) to 6 x 10(-17) cm4 s(-1). The interfacial electron-transfer rate constant decreased as the reorganization energy, lambda, of the acceptor species increased, and a plot of the logarithm of the electron-transfer rate constant vs (lambda + deltaG(o)')(2)/4lambda k(B)T (where deltaG(o)' is the driving force for interfacial charge transfer) was linear with a slope of approximately -1. The rate constant at optimal exoergicity was found to be approximately 5 x 10(-17) cm4 s(-1) for this system. These results show that interfacial electron-transfer rate constants at semiconductor electrodes are in good agreement with the predictions of a Marcus-type model of interfacial electron-transfer reactions.

Immobilization and Electrochemistry of Negatively Charged Proteins on Modified Nanocrystalline Metal Oxide Electrodes

The immobilization of two acidic, low isoelectric point proteins, green fluorescence protein and ferredoxin (FRD) is investigated on nanocrystalline, mesoporous TiO2 and SnO2 electrodes. Modification of these electrodes with a cationic polypeptide (poly-L-lysine) or an aminosilane prior to protein immobilization is found to enhance protein binding at least ten fold, attributed to more favorable protein/electrode electrostatic interactions. Cyclic voltammetry studies of FRD-modified SnO2 electrodes indicate reversible protein electrochemistry with a midpoint potential of −0.59 V (vs. Ag/AgCl) and an interfacial electron transfer rate constant of 0.45 s−1.

Voltammetry of native and recombinant Pseudomonas aeruginosa azurin on polycrystalline Au- and single-crystal Au(111)-surfaces modified by decanethiol monolayers

The native blue single-copper protein azurin ( Pseudomonas aeruginosa) was recently shown to adsorb in close to monolayer coverage and well-defined stable orientations on alkanethiol monolayers self-assembled on Au(111)-surfaces. Adsorption is caused by hydrophobic interactions between the alkanethiol and the hydrophobic protein surface around the copper centre, orienting the latter towards the electrode surface in a way favourable for electron exchange. In this report we show that similar stable adsorption of functional azurin on polycrystalline electrodes can be achieved, represented by azurin adsorption on a decanethiol monolayer. This facilitates significantly the use of this approach to protein immobilization. Reversible monolayer voltammetry is observed for scan rates up to about 1 V s −1. The peaks separate at higher rates. Equilibrium potentials and interfacial electron transfer rate constants are indistinguishable from those at single-crystal Au(111)-electrodes. The sensitivity of azurin monolayer voltammetry on self-assembled alkanethiols to hydrophobic interactions was also used to address possible voltammetric differences between native and recombinant azurin. Voltammetric patterns, equilibrium reduction potentials, and electrochemical rate constants were, however, indistinguishable for the two proteins.

A photoactivated ‘molecular train’ for optoelectronic applications: light-stimulated translocation of a β-cyclodextrin receptor within a stoppered azobenzene-alkyl chain supramolecular monolayer assembly on a Au-electrode

Abstract A light-driven molecular shuttle is organized on a gold electrode surface. The assembly consists of a ferrocene-functionalized β-cyclodextrin, Fc-β-CD, molecule threaded on a monolayer-immobilized long alkyl component containing a photoisomerizable azobenzene unit, and terminated with a bulky anthracene group. The Fc-β-CD resides preferentially on the trans-azobenzene component, and photoisomerization of the trans-azobenzene to the cis-azobenzene state causes the translocation of the Fc-β-CD to the alkyl-chain component of the assembly. The state of the molecular shuttle is electronically transduced by chronoamperometry. The interfacial electron transfer rate constants for the oxidation of the ferrocene units of Fc-β-CD in the respective positions are kt=65 s−1 and kc=15 s−1. The light-driven translocation of Fc-β-CD is reversible, and proceeds by the cyclic isomerization of the azobenzene component between the trans and cis states.

Biomaterial engineered electrodes for bioelectronics.

A series of single-cysteine-containing cytochrome c, Cyt c, heme proteins including the wild-type Cyt c (from Saccharomyces cerevisiae) and the mutants (V33C, Q21C, R18C, G1C, K9C and K4C) exhibit direct electrical contact with Au-electrodes upon covalent attachment to a maleimide monolayer associated with the electrode. With the G1C-Cyt c mutant, which includes the cysteine residue in the polypeptide chain at position 1, the potential-induced switchable control of the interfacial electron transfer was observed. This heme protein includes a positively charged protein periphery that surrounds the attachment site and faces the electrode surface. Biasing of the electrode at a negative potential (-0.3 V vs. SCE) attracts the reduced Fe(II)-Cyt c heme protein to the electrode surface. Upon the application of a double-potential-step chronoamperometric signal onto the electrode, where the electrode potential is switched to +0.3 V and back to -0.3 V, the kinetics of the transient cathodic current, corresponding to the re-reduction of the Fe(III)-Cyt c, is controlled by the time interval between the oxidative and reductive potential steps. While a short time interval results in a rapid interfacial electron-transfer, ket1 = 20 s-1, long time intervals lead to a slow interfacial electron transfer to the Fe(III)-Cyt c, ket2 = 1.5 s-1. The fast interfacial electron-transfer rate-constant is attributed to the reduction of the surface-attracted Fe(III)-Cyt c. The slow interfacial electron-transfer rate constant is attributed to the electrostatic repulsion of the positively charged Cyt c from the electrode surface, resulting in long-range electron transfer exhibiting a lower rate constant. At intermediate time intervals between the oxidative and reductive steps, two populations of Cyt c, consisting of surface-attracted and surface-repelled heme proteins, are observed. Crosslinking of a layered affinity complex between the Cyt c and cytochrome oxidase, COx, on an Au-electrode yields an electrically-contacted, integrated, electrode for the four-electron reduction of O2 to water. Kinetic analysis reveals that the rate-limiting step in the bioelectrocatalytic reduction of O2 by the integrated Cyt c/COx electrode is the primary electron transfer from the electrode support to the Cyt c units.

Characteristic Behavior of an Electron-Transfer Reaction across a Tributyl Phosphate Droplet/Water Interface: Micrometer Droplet-Size Effect

Abstract An electron-transfer (ET) reaction between decamethylferrocene in a single tributyl phosphate (TBP) droplet and hexacyanoferrate(III) in the surrounding water phase was investigated by laser trapping, microspectroscopic, and electrochemical techniques. The interfacial ET rate constant (kobs) was inversely proportional to the droplet radius (r) at > 5 μm, but deviated from a linear relationship between kobs and r−1 at r < 5 μm. The characteristic droplet-size effect was independent of the Galvani potential between the TBP and water phases. The results are discussed in terms of the physical properties of the micrometer-sized TBP droplet/water interface.

Measurement of Interfacial Charge-Transfer Rate Constants at n-Type InP/CH3OH Junctions

Steady-state current density vs potential methods have been used to measure interfacial electron-transfer rate constants at n-type indium phosphide/liquid junctions. n-InP/CH3OH-1,1‘-dimethylferrocene+/0, n-InP/CH3OH-ferrocene+/0, n-InP/CH3OH-tetrahydrofuran-decamethylferrocene+/0, and n-InP/CH3OH-1,1‘-diphenyl-4,4‘-dipyridinium2+/+• contacts displayed bimolecular kinetic behavior in which the observed current density was first order in the concentration of electrons at the semiconductor surface and in the concentration of acceptors in the solution. Differential capacitance potential measurements were used to determine the energetics for the charge-transfer process as well as to determine the concentration of electrons at the semiconductor surface as a function of applied potential. These measurements indicated that the voltage dropped across the semiconductor space charge region varied linearly with changes in the Nernst potential of the solution, as expected for an ideally behaving semiconductor/liquid junction. The measured charge-transfer rate constants, ket, for these systems were ≈10-16 cm4 s-1, in excellent agreement with previous theoretical predictions.

Time-resolved microwave conductivity. Part 1.—TiO2photoreactivity and size quantization

Charge-carrier recombination dynamics after laser excitation are investigated by time-resolved microwave conductivity (TRMC) measurements of quantum-sized (Q-) TiO_2, Fe^(III)-doped Q-TiO_2, ZnO and CdS, and several commercial bulk-sized TiO2 samples. After pulsed laser excitation of charge carriers, holes that escape recombination react with sorbed trans-decalin within ns while the measured conductivity signal is due to conduction-band electrons remaining in the semiconductor lattice. The charge-carrier recombination lifetime and the interfacial electron-transfer rate constant that are derived from the TRMC measurements correlate with the CW photo-oxidation quantum efficiency obtained for aqueous chloroform in the presence of TiO_2. The quantum efficiencies are 0. 4 % for Q-TiO_2, 1. 6 % for Degussa P25, and 2. 0 % for Fe^(III)-doped Q-TiO_2. The lower quantum efficiencies for Q-TiO_2 are consistent with the relative interfacial electron-transfer rates observed by TRMC for Q-TiO_2 and Degussa P25. The increased quantum efficiencies of Fe^(III)-doped Q-TiO_2 and the observed TRMC decays are consistent with a mechanism involving fast trapping of valence-band holes as Fe^(IV) and inhibition of charge-order recombination.

Reduction of acceptor relay species by conduction band electrons of colloidal titanium dioxide; light-induced charge separation in the picosecond time domain

Photoinduced interfacial electron transfer from the conduction band of colloidal TiO 2 semiconductor particles to the cofacial dimeric viologen DV 4+, an electron acceptor, occurs in the picosecond time domain. The interfacial electron-transfer rate constant is about 2 × 10 10 s −1 at pH 7.8. The reaction involves consecutive one-electron transfer to give the monoreduced DV 3+ initially, followed by formation of DV 2+. In acidic aqueous media ( pH 3.5) transient picosecond absorption spectra show the formation of the monoreduced species only.

Electronic processes in small semiconductor particles

An intense nanosecond pulse of light is used to create electron-hole pairs in small CdS crystallites (containing ca. 10 2 atoms) suspended in a liquid. Electronic processes within the crystallite are monitored by measuring the changes of oxidation state of a redox indicator partitioned between the crystallite surface and the bulk liquid. The observed time course of these changes is analysed by the method of competition kinetics, and numerical values of key microscopic parameters (recombination rate constant, trap concentration and depth, interfacial electron transfer rate constant, etc.) are determined. An unexpected result is that the trapping cross section for the shallow electron traps is much greater than in bulk CdS.

Estimation of the rate of electron transfers between two contacting polymer surfaces

Abstract : A bilayer of redox polymer films, Pt/poly-Os(bpy)2(vpy)2(2+)/polyRu(vbpy)3(2+), is coated on a Pt electrode. Reduction of the poly-Ru(3+) outer film of a pre-oxidized bilayer, Pt/poly-Os(3+)/poly-Ru(3+) is controlled by the rate of electron diffusion through the inner, polyOs(2+)/(3+) film. When oxidizing the outer poly-Ru(2+) film of a reduced Pt/poly-Os(2+)/poly-Ru(2+) bilayer, however, the rate of oxidation depends largely on the kinetics of electron transfer between the ca. monolayer of poly-Os(3+) and poly-Ru(2+) sites in contact at the interface between the two polymer films. The electron diffusion kinetics in the two polymer films perturb the interfacial reaction rate only slightly, so that a lower limit to the interfacial electron transfer rate constant is available.