Larry R. Faulkner

Polyoxometallate‐Based Layered Composite Films on Electrodes Preparation Through Alternate Immersions in Modification Solutions

An approach for the modification of electrode surfaces with thin films composed of polyoxometallate anions and large water-soluble cationic species is described. In the procedure, a ca. monolayer of the iso- or heteropolyanion is first adsorbed onto the electrode surface. By immersing the resulting system into a solution containing a large monovalent, multivalent, or polyvalent cation, a composite layer is formed due to the interaction between the adsorbed polyanion and the solution cation. After rinsing, this electrode is reimmersed into the solution of the polyanion, and immobilization of an additional quantity of the polyanion takes place. By the repeated and alternate immersions into the anionic and cationic modification solutions, the amount of material on the electrode can be increased systematically in a controlled fashion leading to stable three-dimensional multilayered molecular assemblies. The immobilized polyanions (isopolymolybdate, phosphotungstate, or silicotungstate) have redox characteristics similar to those of their solution counterparts. The precipitate-forming cationic species include tetrabutylammonium and tris(1,10-phenanthroline)-iron(II) ions, as well as protonated poly(4-vinylpyridine). Composite films of heteropoly-12-tungstate anions with protonated poly(4-vinylpyridine) are the most robust. The approach permits introduction of multiple redox centers into the thin films on electrodes, formation of bilayer-type systems, and, in some cases, even insulating coatings. Detailsmorexa0» of the preparation and physicochemical, particularly electrochemical, properties of the produced systems are described.«xa0less

Photovoltaic effects of metalfree and zinc phthalocyanines. I. Dark electrical properties of rectifying cells

The electrical properties of thin sandwich cells (M/Pc/Au) have been studied in a quantitative fashion. Here, M is either aluminum or indium (150–400 A thick), and Pc represents metalfree phthalocyanine (H2Pc) or its zinc complex (ZnPc). The phthalocyanines were ∼3000 A thick. These devices were assembled by deposition in vacuo, but were studied by recording current–voltage curves in dry air. All showed rectification. Forward bias corresponded to a negative voltage at M. Cells with In contacts adhered quantitatively to expectations for Schottky junctions at the In/Pc boundaries. Cells with Al contacts passed much smaller currents under a given bias, and failed to behave quantitatively like Schottky junction devices. Various pieces of evidence indicate that the Al/Pc junction is spoiled by an interposed zone of Al2O3.

Photovoltaic effects of metalfree and zinc phthalocyanines. II. Properties of illuminated thin‐film cells

Rather strong rectification and photovoltaic effects are observed for sandwich cells in which a thin film (∼3000 A) of metalfree or zinc phthalocyanine is interposed between an ohmic contact (Au) and a blocking contact (Al or In). The action spectra of the short‐circuit photocurrent suggest that only light absorbed near the blocking contact is effective in producing carriers for the external circuit. Cells with indium contacts show performance that is quantitatively consistent with the presence of a Schottky junction at that contact. The quantum efficiency for light incident on the ZnPc phase of an (In/ZnPc/Au) cell can be as high as 14% at 6328 A. For light actually absorbed in the barrier region, the yield approaches unity. The efficiencies of cells with aluminum contacts are 2–3 orders smaller, apparently because an insulating layer (∼65 A) of Al2O3 occupies the Al/ZnPc junction. This interfacial insulating film also affects the relation between the short‐circuit photocurrent and the light intensity. I...

Electron transport dynamics in partially quaternized poly(4-vinylpyridine) thin films containing ferri/ferrocyanide

Electron transport dynamics were studied for the Fe(CN)63−4− couple incorporated in partially-quaternized polyvinylpyridine (QPVP) modified electrodes. The goal was to identify the major factors controlling electron diffusion in the electroactive polymer films. Variable-temperature (0–30° C) chronocoulometry was employed to measure the electron diffusion coefficients (DE) and activation energies (Ea) for the electron transport. The Ea values were in the range of 24–80 kJ/mol (6–20 kcal/mol); Ea increased steadily as the concentration of the ferri/ferrocyanide couple was increased in the films; Ea was also influenced strongly by the anionic species (nitrate, p-toluenesulfonate, and perchlorate) in the aqueous electrolyte adjacent to the film. The DE values in the presence of nitrate were compared to those measured by a form of steady-state voltammetry. The electron diffusion coefficients were essentially equal by both methods. The permeability of Fe(dmbpy)32+ ion was measured in QPVP films loaded with Fe(CN)63−4− and exposed to nitrate; the observed permeation rates (DS · κ) were normally much larger than the electron diffusion coefficients. The results suggest that counterion motion is not a factor limiting the electron motion. The polymer lattice clearly becomes more crosslinked upon taking up the redox ions, an effect that induces a loss in short-range ion mobility within the film and causes a decline in DE as the film is more heavily loaded. The anion dependence of Ea was explained on the basis of changes in internal structure reflecting interactions between the cationic polymer lattice, the anionic species in the background electrolyte, and the solvent. Perchlorate produces nearly dehydrated, rather impermeable films. The anion dependence is shown to be a structural, not a dynamic, effect.

Electrochemical preparation of electrodes modified with non-stoichiometric mixed-valent tungsten(VI, V) oxides

Abstract Non-stoichiometric tungsten(VI, V) oxide films were grown on carbon substrates by cycling the potential between −0.40 and 0.80 V for 0.5–3 h in a colloidal suspension of Na2SO4 · 2H2O in 2M H2SO4. Different thicknesses and extents of hydration were obtained by varying the experimental parameters during the modification step. Longer cycling times in the freshly prepared modification solution resulted in thicker films. With a gradual increase of temperature from 15 to 90° C, the film converted from dihydrate to monohydrate. Redox transitions appeared to be faster in more fully hydrated films but the stability was poorer. The cathodic processes are believed to yield the hydrogen tungsten oxide bronzes, HxWO3 (0

Intercalation of metal complex cations in polyoxometallates : formation of composite films with distinct electrocatalytic centers

In this preliminary note, we propose and characterize briefly two novel composite films produced by: (1) precipitation of Os(bpy) 3 2+ (bpy=2,2-bipyridine) with SiW 12 O 40 4− (Os-bpy/Si-W-O) and and (2) electrodeposition of Ru(III, IV)-oxo species together with silicotungstates (Ru-O/Si-W-O)

Electrochemical behavior of tris(2,2'-bipyridine)ruthenium complexes in films of poly(styrenesulfonate) on electrodes

Abstract Poly(styrenesulfonate, sodium salt) was spincoated onto Pt electrodes at dry thicknesses near 100 nm. These films were then loaded with Ru(bpy)2+3 by exposing them to solutions of the complex in the mM range in CH3CN containing 0.1 M tetra-n-butylammonium fluoborate (TBABF4), 5–16% MeOH, and 1 mM dicyclohexyl-18-crown-6. Exchange was encouraged by voltammetric cycling over the Ru(III/II) couple. Loaded films contained quantities of Ru(II) high enough to account for 70% neutralization of sulfonate groups in the films. Cyclic voltammetry of the films in Ru-free CH3CN with 0.01 M TBABF4 revealed that the complex exists in two distinct, interconvertible sites. Galvanostatic transients showed evidence for nucleation of Ru(III) centers into very tight, almost electroinactive domains. A structural model is proposed on the basis of the results.

Luminescence as an indicator of electron exchange dynamics in poly(stryrenesulfonate) films containing tris(2,2'-bipyridine)ruthenium complexes

The dynamics of electron diffusion among Ru(bpy)2+3+3 centers in poly(styrenesulfonate) films have been studied by a novel method based on luminescence quenching. The films were supported on Pt surfaces and were on the order of 100 nm thick in the dry state. The method involves observations of the intensity of luminescence vs. the state of oxidation of the film. When a small fraction (<10%) of Ru(III) centers is introduced by oxidation, there is random motion of these centers by electron exchange with neighboring Ru(II) species. These mobile holes can quench luminescence from Ru(bpy)2+3 excited states produced by photoexcitation. The rate constant for quenching can be related to the diffusion coefficient for hole migration. This diffusion coefficient seems to be 40–80 times larger than the diffusion coefficient for charge movement as measured by chronocoulometry. The difference is discussed in terms of the counter ion dynamics operating in the two different experiments. At high concentration of Ru(bpy)2+3 in the films there were very unusual increases of luminescence intensity, after an initial falloff, with increased degrees of oxidation. They are believed to manifest a nucleation of Ru(III) centers into compact domains.

A microscopic model for diffusion of electrons by successive hopping among redox centers in networks

Abstract A microscopic model has been developed to describe electron (or hole) diffusion coefficients, D E , in a rigid three-dimensional network as a function of redox species concentration, c . The model takes excluded volume into consideration and is based on simple probability distribution arguments and a random walk. The following conditions are assumed: (1) there is a homogeneous random distribution of hard-sphere redox centers; (2) each step corresponds to an electron transfer across the average nearest neighbor distance between redox centers; and (3) the electron transfer rate constant depends exponentially on edge-to-edge separation between a reacting pair. As the molecular diameter approaches zero, the hard-sphere case approaches the point-molecule case. In general, the point-molecule case affords a simpler relationship between D E and c , but is inadequate for networks with experimentally realistic concentrations. In dilute systems, D E rises sharply with concentration; however, at larger concentrations, the hard sphere D E reaches a limiting value due to packing. Applying the model to rate-distance data in the literature for metalloprotein and steroid-spacer systems yielded semi-empirical D E values as a function of concentration. Hard-sphere D E values for metalloprotein kinetics and a redox molecular diameter of 1.3 nm (corresponding to Ru(bpy) 3 2+ ) are 1.9 × 10 −10 and 5.0 × 10 −9 cm 2 s −1 at 0.10 M and 1.07 M , respectively. At a ferrocene-like diameter (0.6 nm), D E values are 2.8 × 10 −12 and 4.6 × 10 −10 cm 2 s −1 , respectively. The corresponding D E values for the steroid-spacer kinetics are 1.1 × 10 −3 , 3.0 × 10 −2 , 1.7 × 10 −5 , and 2.7 × 10 −3 cm 2 s −1 , respectively. Also derived is a two-dimensional version of the model that may be applied to systems such as Langmuir monolayers. Plots of the two-dimensional hard-sphere electron diffusion coefficient vs. area concentration yield curve shapes and magnitudes that are similar to those of the three-dimensional model.

Electron transport dynamics in thin polymer films containing tris(2,2'-bipyridine)osmium(III/II) complexes

Rotating carbon disk electrodes were modified by a random copolymer (PSS-PVP) of styrenesulfonate and 4-vinylpyridine. The polymer was spincoated onto the electrodes, crosslinked by reaction with 1,2-dibromoethane, then loaded with Os(bpy)2+3 by ion exchange. The oxidation of Fe(CN)4−6 contained in an aqueous contacting solution was effectively mediated by the osmium centers in the film. A study of the dynamics was carried out, and the results were analyzed via the theory of Saveant and Andrieux. When chronocoulometry was used to measure the electron diffusion rate in the film, the overall dynamics remained quantitatively inconsistent. The problem with self-consistency seems to arise from an intrinsic inadequacy of chronocoulometry for evaluating electron diffusion coefficients. A steady-state method was developed as an alternative. The polymer film was deposited across the face of a ring-disk assembly, then lateral current flow from disk to ring was measured by a modified form of square-wave voltammetry. The results suggest that the true rates of electron diffusion among osmium centers in PSS-PVP and in Nafion are almost two orders of magnitude larger than projected from chronocoulometry. As the ferrocyanide concentration is raised from 0.021 mM to 13.49 mM the dynamics of mediated oxidation change from SR to SR + E, then perhaps to S + E.