Charge Transfer Diffusion Coefficient Research Articles

Relationship between Charge Transfer Diffusion Coefficients and Doping Level for electro generated thin PEDOT films on ITO

Thin films of poly-3,4-ethylene dioxythiophene (PEDOT) were electrodeposited on transparent electrodes of indium tin oxide (ITO) using potentiostatic regime. These films had thicknesses ranging from 15 nm to 60 nm and were studied using UV-vis absorption and chronoamperometric techniques in monomer-free tetrabutylammonium perchlorate/acetonitrile solutions. The charge transfer diffusion coefficient (D) of the films were calculated using Cottrell model for a wide range of potential steps from −1.60 to 1.60 V vs. Ag°/AgNO3. For p-doped films, the highest diffusion coefficients were obtained when a potential of 0.70 V was applied. Moreover, a direct relationship between film thickness and their diffusion coefficients was found for thin PEDOT films showing D values up to 1.4 × 10−9 cm2 s−1 for 60 nm thickness films. These values are remarkably higher than the D of 1.8 × 10−11 cm2 s−1 obtained for spin-coated PEDOT: PSS films of similar thickness.

Deactivation of the ruthenium excited state by enhanced homogeneous charge transport: Implications for electrochemiluminescent thin film sensors

The electrochemiluminescent (ECL) performance of three ruthenium-based metallopolymer platforms with different homogeneous charge transfer diffusion coefficients (DCT) is reported. Significantly, simultaneous detection of light and current in tandem with steady-state photoluminescence studies demonstrate that increasing the rate of Ru3+ production via enhanced charge transport results in a decrease in ECL intensity of up to 82% when the concentration of the co-reactant, sodium oxalate, is low, i.e., sub-mM. Spectroelectrochemical studies demonstrate that for maximum sensitivity to be obtained, the electroactive properties of the polymeric support matrix need to be considered in tandem with luminophore, analyte and co-reactant concentrations.

Electrochemical properties of ruthenium metallopolymer: Monolayer-protected gold cluster nanocomposites

Nanocomposite polymer films of [Ru(bpy) 2(PVP) 10] 2+ metallopolymer and gold monolayer-protected clusters (MPCs) have been formed. Cyclic voltammetry of the films has been performed in both acidic (0.1 M H 2SO 4) and neutral (0.1 M Na 2SO 4) pH electrolyte solution. In acidic electrolyte, the protecting monolayers is desorbed as the gold surface is oxidized. Transmission electron microscopy of the nanocomposite following electrochemical cycling reveals aggregates of “naked” MPCs. In sharp contrast, in neutral pH electrolyte, the MPCs are stable towards voltammetric cycling and no gold oxide formation is evident in the voltammetry. The homogeneous charge-transfer diffusion coefficient, D CT , was determined for a range of nanocomposite films as the MPC mole fraction is systematically varied. When acidic electrolyte is used, D CT decreases from 2.8 ± 0.5 × 10 −10 to 1.5 ± 0.1 × 10 −11 cm 2 s −1 as the MPC mole fraction is increased from 0 to 0.45. In contrast, the D CT determined in neutral electrolyte is universally lower than that determined in acidic electrolyte, it increases by over two orders of magnitude from 2.6 ± 0.9 × 10 −15 to 4.0 ± 0.5 × 10 −13 cm 2 s −1 as MPC mole fraction increases from 0.05 to 0.5.

Electrochemical behaviors of nanoporous coordination polymer multilayers in hexacyanoferrate solution

Coordination polymers (CPs) can encapsulate small molecules within their nanosized coordination cages to form hybrid supramolecular materials. We reported here assembly of Pd(II)-mediated CP multilayers on the substrate surfaces by layer-by-layer method with six kinds of multidentate ligands as linkers, which included 4,4′-bipyridyl (BPy), binaphthylbis(amidopyridyl) (BNBAPy), tris(4-pyridyl)-1,3,5-triazine (TPyTa), tetra(4-pyridyl)porphyrin (TPyP), ligand-like complex of Fe(pyterpy) 2 (pyterpy: 4′-(4-pyridyl)-2,2′:6′,2″-terpyridine), and poly(4-vinylpyridine) (PVP). The multilayer assembly process was characterized by using UV–vis absorption spectra and X-ray photoelectron spectra. Electrochemical behaviors of the as-prepared CP-modified electrodes were investigated in hexacyanoferrate solution, which revealed well-reversible redox waves of Fe(CN) 6 3−/4− ions for the electrode covered with the Pd–TPyTa or Pd–Fe(pyterpy) 2, while irreversible ones for the electrode covered with the Pd–BPy or Pd–PVP. The results indicated that Fe(CN) 6 3−/4− ions could reach electrode surface across the CP multilayers of TPyTa or Pd–Fe(pyterpy) 2, which was attributed to the fact that their coordination cages were large enough for the Fe(CN) 6 3−/4− ions to penetrate through. Charge-transfer diffusion coefficient ( D) was calculated based on chronocoulomograms of the CP multilayer-modified electrodes. D was in the range from 0.34 × 10 −10 to 1.3 × 10 −10 cm 2/s for the electrodes modified by four layers of Pd–TPyP, Pd–BNBAPy, Pd–TPyTa and Pd–Fe(pyterpy) 2, respectively. The present methodology provides a facile way to construct three-dimensional supramolecular materials on the solid surfaces as well as to study the charge-transfer behaviors in artificial membranes.

Charge Transport Performance of Nafion-based Redox Polymer in Atmosphere

The charge transport performance of the Nafion-based redox polymer films prepared under different conditions was investigated by cyclic voltammetry in the sandwiched cells. The results showed that the apparent charge transfer diffusion coefficient(Dct) of the Nafion[M(bpy) 3 , PEG (polyethylene glycol)] (M=Ru or Fe) films is about 1010 cm·s and the mobility of the electron or hole ( μ) was about 1010 cm·V·s. In order to decrease the resistance of the interface, a polyaniline (PANI) layer was added to the interface of indium tin oxide coated conducting glass (ITO) electrode and the redox polymer. Consequentially , the Dct could be increased to about 10 10 6 cm·s 1 and μto about 1010 cm·V·s, and the operating current of the cells under 600 mV bias could be increased by almost 100 times . In addition, the Dct and μof this redox polymer film had just reduced slightly after 30 d kept in atmosphere. Therefore, the redox polymer film is sufficiently stable.

Electrochemical properties of hydrogenase on glass carbon electrodes modified with carbon nanotubes

Hydrogenase (H2 ase) purified from phototropic bacteriumThiocapsa roseopersicina was coassembled with carbon nanotubes (CNTs) on glass carbon electrodes. Both oxidized CNTs and Nafion-CNT composites were used to modify the electrodes. The pure H2 are formed dot-like domains, while the oxidized CNT-H2 ase and Nafion-CNT-H2 ase composites formed wire-like and large closely packed aggregates, respectively. The reductive potentials for the [4Fe-4S]2+/1+ clusters of H2ase were at about −500, −650, and −700 mV (vs Ag/AgCl) for the electrodes modified with pure H2ase, Nafion-SWNT-H2ase, and Nafion-MWNT-H2ase composites, respectively. Potential step chronocoulometry measurements indicated a larger charge-transfer diffusion coefficient between the H2ase and electrodes when the CNTs were co-assembled with H2ase, suggesting that the CNTs can not only act as a supporting layer to immobilize enzymes, but also act as a highly conductive wire throughout the films.

Coordination Polymers Based on Bis(bipyridyl)alkane Ligands: Film Preparation and Charge-Transport Characteristics

Electroactive coordination polymers have been prepared by reacting Fe2+ and Co2+ with bis(bipyridyl)alkane (bba) ligand films that have been spin coated onto glassy carbon electrodes. The resulting films exhibit redox characteristics similar to those of electropolymerized vinyl(bipyridyl) metal complexes. The stability of the electrode-bound films increases as the length of the linking alkyl chain decreases. Better stability is also obtained with Fe2+ films than Co2+ films and with the alkyl-linking group attached at the 4-position rather than the 5-position. Charge-transport measurements using chronoamperometry have been conducted on poly-FeII/bba films in which the alkyl linker consists of 2, 4, and 6 methylene groups. Values of D1/2C (D = charge-transfer diffusion coefficient; C = concentration of redox sites) range from 9.5 × 10-8 to 2.3 × 10-8 mol/cm2 s1/2 at 298 K and follow a trend of decreasing D1/2C with increasing alkyl chain length for the 4-position−linked FeII/bba polymers. No such trend is observed for the analogous 5-position−linked systems. Activation parameters Ea and ΔH⧧ range from 22 to 31 kJ/mol, whereas ΔS⧧ ranges from −30 to −75 J/mol K. The results are consistent with a mechanism in which the increase in methylene chain length causes a decrease in the rate of charge transport due to the lower frequency of electron-transfer collisions between neighboring redox sites. However, polymer segmental motion is also believed to play a secondary role in this mechanism.

Diffusional Mediation of Surface Electron Transfer on TiO2

The results of a kinetic study are reported for cross-surface electron transfer in the reversible oxidation of [OsII(bpy)2(4,4‘-(CO2H)2bpy)](PF6)2 to OsIII surface adsorbed to nanocrystalline TiO2 films on optically transparent ITO (tin-doped indium oxide) electrodes. The kinetics are sensitive to the extent of surface loading. From monolayers adsorbed from CH3CN, a percolation threshold of ∼60% was measured. Adsorption isotherms for [OsII(bpy)2(4,4‘-(CO2H)2bpy)](PF6)2 in EtOH and CH3CN show that surface coverage from EtOH is ∼60% of that from CH3CN at similar adsorbate concentrations. Apparent charge-transfer diffusion coefficients in 0.1 M [N(n-C4H9)4](PF6) in CH3CN measured by chronoabsorptometry are Dapp = 1.4 × 10-9 cm2/s (Γ = 1.1 × 10-10 mol/cm2, CH3CN) and 1.4 × 10-11 cm2/s (Γ = 7 × 10-11 mol/cm2, EtOH). Electron-transfer mediation of the oxidative component of the OsIII/II surface couple occurs in the presence of added Ru(bpy)32+ by stepping the potential past the Ru(bpy)33+/2+ wave. Mediation of ...

Electrochemical behavior of 5-amino-1,10-phenanthroline and oxidative electropolymerization of tris[5-amino-1,10-phenanthroline]iron(II)

The electrochemical behavior of 5-amino-1,10-phenanthroline and tris[5-amino-1,10-phenanthroline]-iron(II) at carbon paste, glassy carbon, and platinum electrodes is reported. The iron complex undergoes electrochemically induced oxidative polymerization from acetonitrile solutions and the resulting polymers are very stable. Charge transport through the polymer films occurs with a charge transfer diffusion coefficient, Dct, equal to 3.1 × 10−8 cm2 s−1 corresponding to an electron self-exchange rate of 5.2×107M−1 s−1. The activation energy and the entropy change for the charge transfer diffusion process are (approximate values) 32.0 ± 0.12 kJ mol−1 and −24.7 ± 0.4 J K−1 mol−1, respectively.

Non-stationary processes at polymer coated rotating disk electrodes: Part I. Model and simulation

A time dependent model for the mediated oxidation of a substrate in bulk solution at a polymer modified rotating disk electrode is proposed and a numerical solution is found by orthogonal collocation. The mediation system consists of a surface bound redox species which can accept electrons from a substrate in solution. In the model considered here the substrate can enter the film freely but does not react at the electrode surface. The effect of the variation of substrate concentration, mediator concentration, effective charge transfer diffusion coefficient of the bound species and homogeneous rate constant on the electrochemical behaviour of the modified electrode is considered. Good qualitative agreement was found between simulated current-potential curves and experimental curves using [Fe(CN) 6] 4− a substrate and [Ru(bpy) 2Cl(PVP) 5]Cl as the polymer coating.

Researches on chemically modified electrodes: Part XVI. Electron diffusion coefficient in vanadium hexacyanoferrate film

The charge-transfer diffusion coefficient D ct for the new electrochromic mixed-valence-compound film of vanadium hexacyanoferrate(VHF), KV 3+ Fe II(CN) 6, was determined for both its oxidation and reduction processes, in H 2SO 4 + M 2SO 4 aqueous solutions, using short-time transient chronoamperometry (CA), chronocoulometry (CC), and conventional chronopotentiometry (CP). It was revealed that the overall charge transfer process in a VHF film was limited by the electron diffusion rate between adjacent redox sites within the film. So D ct is identified as an electron diffusion coefficient D e, which has an average value of 1.4 × 10 −9 cm 2 s −1. Interaction between redox sites in the film was suggested by its constant-current doubly-stepped chronopotentiometric behavior. The calculated E− t data based on the interaction parameter theory, agree very well with the experimental curve.