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ABSTRACT: Multilayer films with anionic phosphomolybidic acid (PMo(12)) clusters have been fabricated via the electrostatic layer-by-layer (LbL) method. The charged mass transport phenomena of these thin films have been studied using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) with [Fe(CN)(6)](3-/4-) and [Ru(NH(3))(6)](3+/2+) as the redox probes. By adding a film resistance and a film capacitance to the conventional Randles equivalent circuit, we can calculate the diffusion coefficient values that help understand the microscopic nature of the thin films. When the negatively charged probe [Fe(CN)(6)](3-/4-) was used, lower diffusion coefficients were obtained for multilayers deposited from higher ionic strength solutions. This relationship was less obvious when the positively charged probe [Ru(NH(3))(6)](3+/2+) was used, in which the electrostatic attraction between PMo(12) clusters and the probe ions complicates the mass-transfer process. It is believed that the addition of salt to dipping solutions increases the tortuosity of the films so the mass transport takes longer paths, inducing lower diffusion coefficients. Higher PMo(12) loading causes lower diffusion coefficients due to the polyoxometalate clusters blocking the paths for charged probe ions.
The Journal of Physical Chemistry B 11/2010; 114(48):15818-24. · 3.70 Impact Factor
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ABSTRACT: Results obtained from this study contain information for mass transfer through thin membranes. A model developed for membrane with particle components has broader applications. The conditions considered in this study can be easily applied to many situations in both industry and basic science. Here, we are able to develop a modified Randle's circuit equivalent to calculate redox species diffusion coefficients through layer-bylayer thin films deposited on electrodes. The films generally show Nyquist plots with a Warburg line slope ~ 1. From the diffusion coefficient calculations, it appears that using high ionic strength solutions would not help greatly in achieving higher ionic diffusion in the case of POM films. However, the ionic strength and concentrations of the dipping
04/2010; , ISBN: 978-953-307-059-9
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ABSTRACT: Polyelectrolyte multilayers built via the layer-by-layer (LbL) method has been one of the most promising systems in the field of materials science. Layered structures can be constructed by the adsorption of various polyelectrolyte species onto the surface of a solid or liquid material by means of electrostatic interaction. The thickness of the adsorbed layers can be tuned precisely in the nanometer range. Stable, semiconducting thin films are interesting research subjects. We use a conducting polymer, poly(p-phenylene vinylene) (PPV), in the preparation of a stable thin film via the LbL method. Cyclic voltammetry and electrochemical impedance spectroscopy have been used to characterize the ionic conductivity of the PPV multilayer films. The ionic conductivity of the films has been found to be dependent on the polymerization temperature. The film conductivity can be fitted to a modified Randle's circuit. The circuit equivalent calculations are performed to provide the diffusion coefficient values.
International Journal of Molecular Sciences 01/2010; 11(4):1956-72. · 2.60 Impact Factor
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ABSTRACT: Poly( p -phenylene vinylene) mulilayers have been prepared from its cationic precursor via the layer-by-layer deposition. The photoluminescence (PL) and film thickness of the multilayers have been examined via fluorimetry and atomic force microscopy. The PL of the multilayers has been observed that is consistent with the literature results. When phosphomolybdate P Mo <sub>12</sub> is incorporated into the multilayer structure, PL quenching is detected that is proportional to the amount of P Mo <sub>12</sub> used. The quenching is interpreted as exciton diffusion through the polymer multilayers, followed by exciton dissociation at the polymer/ P Mo <sub>12</sub> interface. We show that the modeling used for calculating the PL intensities derived from inorganic semiconductors is also applicable to conjugated polymers. According to the model, an exciton diffusion length is found to be 11.5±0.4 nm .
Journal of Applied Physics 12/2007; · 2.17 Impact Factor
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ABSTRACT: This study investigates the products, kinetics, and reactor design of atrazine photolysis under 254-nm ultraviolet-C (UVC) irradiation. With an initial atrazine concentration of 60 microg/L (60 ppbm), only two products remain in detectable levels. Up to 77% of decomposed atrazine becomes hydroxyatrazine, the major product. Both atrazine and hydroxyatrazine photodecompose following the first-order rate equation, but the hydroxyatrazine photodecomposition rate is significantly slower than that of atrazine. For atrazine photodecomposition, the rate constant is proportional to the square of UVC output, but inversely proportional to the reactor volume. For a photochemical reactor design, a series of equations are proposed to calculate the needed UVC output power, water treatment capacity, and atrazine outlet concentration.
Water Environment Research 09/2007; 79(8):851-7. · 0.88 Impact Factor
