Maria Hepel

The Electrocatalytic Oxidation of Methanol at Finely Dispersed Platinum Nanoparticles in Polypyrrole Films

A new method of the formation of composite polypyrrole films containing a highly dispersed three-dimensional array of platinum catalyst particles is presented. PtCl{sub 4}{sup 2{minus}} anions were trapped inside the polypyrrole matrix during the electropolymerization of pyrrole. In the next step followed by solution exchange, PtCl{sub 4}{sup 2{minus}} anions were reduced to Pt{sup 0} particles with an average size of 10 nm. Metallic particles were incorporated in electrically conducting polypyrrole films in order to achieve multielectron-transfer processes in a three-dimensional matrix. These films were characterized using the electrochemical quartz crystal microbalance technique. The use of this technique allowed evaluation of the Pt{sup 0} loading inside the polymer film. The electropolymerization process was controlled by measuring frequency changes of piezoelectrodes. The presence of Pt{sup 0} particles in composite polypyrrole films and their uniform distribution were confirmed by energy-dispersive x-ray spectroscopy and x-ray diffraction. The size of the Pt{sup 0} particles was evaluated from transmission electron microscopy experiments. The electrocatalytic effect toward the methanol oxidation was observed. Larger surface area and higher catalytic activity were found for electrodes with dispersed Pt{sup 0} nanoparticles in the polymer evaluation of matrix than electrodes with electrodeposited Pt{sup 0} on the surface of the conductive polymer.

Bicomponent WO{sub 3}/TiO{sub 2} films as photoelectrodes

An analysis of the capability of porous tungsten trioxide films as a material for photocurrent generation was performed. The photogeneration properties of single-component WO{sub 3} and TiO{sub 2} films and bicomponent WO{sub 3}/TiO{sub 2} films were compared. The morphology, structure, fundamental absorption edge, flatband potential, vibration spectra, and photocurrent response of the amorphous WO{sub 3} films and nanocrystalline TiO{sub 2} films were measured. The authors found that in bicomponent WO{sub 3}/TiO{sub 2} films, the porous films of the tungsten trioxide with a high open surface area can serve as substrates for nanocrystalline TiO{sub 2} films to increase the efficiency of photocurrent generation at bandgap excitation.

Photoelectrochemical mineralization of textile diazo dye pollutants using nanocrystalline WO3 electrodes

Photoelectrochemical degradation processes of Naphthol Blue Black (NBB) dye in aqueous solutions were investigated seeking an effective method of degrading textile diazo dyes. Nanocrystalline WO3 film on a Pt mesh electrode was used as the photoanode for photoelectrochemical degradation of NBB dye. Higher rates of NBB degradation were observed in chloride-containing environments than in sulfate solutions. A strong pH dependence of the NBB dye degradation rate has been found in chloride media. The pH dependence follows the trend of decreasing oxidizing power of electron holes, hVB,s, trapped at the surface of WO3 particles, with increasing solution pH. In the low pH range (1.0–4.5), photooxidation of the medium favors the formation of Cl and Cl2− radicals which are powerful oxidants for the NBB dye. In alkaline solutions, the formation of OH radicals is predominant over Cl and Cl2− radicals but the kinetics of NBB degradation is much slower than in acidic media. In sulfate solutions, the rate of NBB photodegradation is slower than in chloride media. Evidently, the lower oxidizing power of the dipersulfate ion S2O8−2 being the oxidant in this environment, is the limiting factor. The intermediates of the NBB dye degradation were detected using HPLC analysis. Possible dye degradation mechanism is proposed.

Effect of the Composition of Polypyrrole Substrate on the Electrodeposition of Copper and Nickel

Electrodeposition of copper and nickel on a variety of substrates including conductive polymer films with two different compositions has been studied. The enhancement of the copper and nickel electrodeposition rate was observed for composite polypyrrole films with cation-exchange properties in comparison to undoped polypyrrole films with anion-exchange properties. Significant enhancement of the nickel electrodeposition rate on both types of conductive polymer vs. the bare gold electrode was also found. An explanation of these effects is presented. An electrochemical quartz crystal microbalance (EQCM) with 10 MHz AT-cut piezoelectrodes was used in this study. Use of the EQCM technique allowed for simultaneous monitoring of voltamperometric and resonance frequency vs. potential or time characteristics. The thickness of electropolymerized polypyrrole films as well as the amounts of electrodeposited metals were controlled by monitoring the EQCM resonant frequency. The nucleation density has been determined from scanning electron microscope experiments.

Decrease of Recombination Losses in Bicomponent WO 3 / TiO2 Films Photosensitized with Cresyl Violet and Thionine

The recombination of photoinjected electrons in semiconducting metal oxide films, photosensitized with cresyl violet and thionine, has been investigated. A comparative analysis of photoresponse characteristics for single-component photoelectrodes, WO 3 and TiO 2 , and bicomponent heterogeneous WO 3 /TiO 2 photoelectrodes, was performed for front and back side illumination of the films. A considerably higher photoresponse was observed for bicomponent WO 3 /TiO 2 films in comparison to single-component films. Employing a porous WO 3 matrix layer with high open surface area as the substrate or a very thin nanoparticulate TiO 2 overlayer is beneficial for efficient photogeneration for two reasons. First, a lower lying conduction band of the WO 3 matrix layer provides the escape path for photoinjected electrons from the conduction band of the TiO 2 film to the external circuit and prevents the recombination at the TiO 2 /electrolyte interface. Second, the recombination losses during the electron transport through grain boundaries in TiO 2 film decrease strongly due to the small film thickness. It has been shown that by employing the back side illumination, further decrease in recombination losses of photoinjected electrons can be achieved

Study of the Initial Stages of Anodic Oxidation of Polycrystalline Silver in KOH Solutions

Abstract : Growth of the anodic AG2O film on a polycrystalline silver disk electrode in 1 mol/cu. dm KOH solution at ambient temperature has been examined using a variety of electrochemical techniques. On the basis of the experimental results, a solid-state model of silver electrode covered by a thin semiconductor film with a finite ionic conductivity has been proposed. It has been found, that under potentiostatic control, thickness of the oxide film adjusts very quickly to the hydrodynamic conditions by a deposition/dissolution process and the steady-state oxidation currents fulfill the Levich equation for RDE despite the fact that the electrode is covered by a thin AG2O film. In the case of thicker film, after completion of nucleation and growth, changes in the hydrodynamic conditions do not influence the oxidation current and adjustment of the thickness of the oxide film to new hydrodynamic conditions proceeds at slower rate because of the slowness of the solid-state diffusion step. The Ag2O nucleation and growth peak has been observed in chronoampermetric transients above potential of + 240 mV(vs. Ag/AgCl). It has been observed that the nucleation and growth peak decreases as the convective diffusion is accelerated by the rotation speed of the RDE.

Ligand exchange effects in gold nanoparticle assembly induced by oxidative stress biomarkers: Homocysteine and cysteine

The interactions of oxidative stress biomarkers: homocysteine (Hcys) and cysteine (Cys) with the multifunctional gold nanoparticles, important in view of novel biomedical applications in diagnostics and therapy, have been investigated using resonance elastic light scattering (RELS), UV-Vis plasmonic spectroscopy, and high-resolution TEM imaging. The Hcys-induced assembly of gold nanoparticles has been observed for non-ionic surfactant-capped gold nanoparticles as well as for negatively-charged citrate-capped gold nanoparticles. We have observed for the first time the de-aggregation of citrate-capped gold nanoparticle ensembles followed by their conversion to citrate-linked Hcys-capped nanoparticle assemblies. The Cys molecules, which are smaller than Hcys by only one CH(2) group, show much less activity. The mechanisms leading to this intriguing disparity in the abilities of these two thioaminoacids to ligand exchange with surfactant- or citrate-capping molecules of the gold nanoparticle shells are proposed on the basis of the experimental evidence, molecular dynamics simulations, and quantum mechanical calculations. For citrate-capped gold nanoparticles, we postulate the formation of surface complexes facilitated by electrostatic attractions and formation of double hydrogen bonds for both Hcys and Cys. The conformational differences between these two kinds of complexes result in marked differences in the distance between -SH groups of the biomarkers to the gold surface and different abilities to induce nanoparticle assembly. Analytical implications of these mechanistic differences are discussed.

Catalytic activation of core-shell assembled gold nanoparticles as catalyst for methanol electrooxidation

This paper describes the results of an investigation of the evolution and the reconstitution of core-shell assembled gold nanoparticles in electrocatalytic activation for methanol electrooxidation. The aim is to probe the structural and morphological reconstitution upon the catalytic activation. Gold nanoparticles of 2-nm core size are linked by 1,9-nonanedithiolates into a network thin film on planar substrate, and are explored as a model system of core-shell nanostructured catalysts. This system is probed using three characterization techniques: electrochemical quartz-crystal nanobalance (EQCN), infrared reflection spectroscopy (IRS), and atomic force microscopy (AFM). The EQCN detected two types of mass changes across the nanostructured catalysts. One corresponds to shell desorption upon the oxidative potential-driven activation, and the other relates to the formation of surface oxygenated species during the catalytic oxidation of methanol. IRS provided two pieces of evidence for the shell reconstruction upon the activation. One is indicative of the desorption of the shell thiolates, and the other relates to the interparticle electronic effect. AFM revealed morphological changes of the nanoparticle assemblies in terms of the film smoothness and the particle size that are dependent on the thickness of the nanoparticle assembly. While thick films displayed enlarged nanoparticle features, thinner films exhibited a relatively smaller evolution. The catalytic activity is associated with the partial or even complete desorption of network shell components accompanied by the formation of surface oxygenated species, a reconstitution process that may have important implications to the delineation of design and preparation parameters of nanoporous and highly active nanoscale catalysts.

Resonance elastic light scattering (RELS) spectroscopy of fast non-Langmuirian ligand-exchange in glutathione-induced gold nanoparticle assembly

The interactions of a biomolecule glutathione (GSH) with citrate-capped gold nanoparticles (AuNP) have been investigated to evaluate the viability of a rapid GSH-capture by gold nanoparticle carriers, as a model system for applications ranging from designing nanoparticle-enhanced functional biosensor interfaces to nanomedicine. The measurements, performed using resonance elastic light scattering (RELS) spectroscopy, have shown a strong dependence of GSH-induced scattering cross-section on gold nanoparticle size. A large increase in RELS intensity after injection of GSH, in a short reaction time (tau=60 s), has been observed for small AuNP (5nm dia.) and ascribed to the fast ligand-exchange followed by AuNP assembly. The unexpected non-Langmuirian concentration dependence of scattering intensity for AuNP (5 nm) indicates on a 2D nucleation and growth mechanism of the ligand-exchange process. The ligand-exchange and small nanoparticle ensemble formation followed by relaxation have been observed in long term (10 h) monitoring of GSH-AuNP interactions by RELS. The results of molecular dynamics and quantum mechanical calculations corroborate the mechanism of the formation of hydrogen-bonded GSH-linkages and interparticle interactions and show that the assembly is driven by multiple H-bonding between GSH-capped AuNP and electrostatic zwitterionic interactions. The RELS spectroscopy has been found as a very sensitive tool for studying interparticle interactions. The application of RELS can be expanded to monitor reactivities and assembly of other monolayer-protected metal clusters, especially in very fast processes which cannot be followed by other techniques.

Composite polypyrrole films switchable between the anion- and cation-exchanger states

Abstract Catechol, hydroquinone, anthraquinone-2-sulfonate and melanin were incorporated into a polypyrrole matrix during electropolymerization of pyrrole. The concentration of quinones was adjusted such that composite films with mixed ion dynamics were formed. Part of the polypyrrole cationic groups was charge compensated by the negative charge of incorporated quinones, and part by anions from the electrolyte. These composite polypyrrole films can be “switched” between a cation-exchanger state of the electrode at negative electrode potentials, and an anion-exchanger state at more positive potentials by varying the potential. The same films can be also switched between a cation-exchanger state and an anion-exchanger state by varying the pH of the solution. The type of ion dynamics in these films has been investigated by the EQCM technique. Use of this technique allowed us to control the composition of prepared films and to evaluate their relative doping level.