Ultrathin Films Of Nafion Research Articles

Functional electrochemically-active ultra-thin Nafion films

Two methods for adding electrochemical functionality to ultra-thin Nafion films are described. First, a novel “one-step procedure” to incorporate cationic redox mediators such as methyltrimethylammonioferrocene (FA +), within ultra-thin Nafion films is proposed. Ultra-thin films of Nafion containing FA +, were prepared on indium tin oxide (ITO) electrodes using the Langmuir–Schaefer (LS) technique. The morphology and thickness of the films were evaluated with atomic force microscopy (AFM). Film thicknesses were 1.9 ± 0.3 nm per LS layer and voltammetric data confirmed the successful incorporation of the mediator into the films. Key parameters such as surface coverage, concentration of the mediator within the films and apparent diffusion coefficients were extracted. Second, Pd nanoparticles were formed within Nafion LS films by loading pristine films with Pd 2+ and then reducing the ions to form Pd. The catalytic properties of this novel nanocomposite for hydrogen oxidation were examined using cyclic voltammetry.

One-step formation of ultra-thin chemically functionalized redox-active Langmuir-Schaefer Nafion films.

A novel "one-step procedure" to incorporate different cationic redox-active species within ultra-thin Nafion films is reported. Ultra-thin films of Nafion containing trimethylammonioferrocene (FA), tris(2,2'-bipyridyl)ruthenium(), Ru(bpy), and hexaaminoruthenium(), [Ru(NH)], were prepared using the Langmuir-Schaefer (LS) technique. The morphology and thickness of the films were evaluated with atomic force microscopy (AFM). Film thicknesses were 1.9 ± 0.7, 1.8 ± 0.9, 1.5 ± 0.9 nm per layer for Nafion-FA, Nafion-Ru(bpy)and Nafion-[Ru(NH)], respectively. Electrochemical data confirmed the successful incorporation of the mediators into the films and proved that the redox activity was maintained during extensive voltammetric cycling. Key parameters such as surface coverage, concentration of the mediator within the films and apparent diffusion coefficients were extracted using cyclic voltammetry. To demonstrate the utility of the redox-active films for sensing applications, films with Ru(bpy) incorporated were utilized for the electrocatalysis of oxalate in aqueous solution.

Formation and evaluation of electrochemically-active ultra-thin palladium–Nafion nanocomposite films

A simple method for producing electrochemically-active palladium nanoparticles within ultra-thin Nafion films is described.

Kinetics of Solute Partitioning into Ultrathin Nafion Films on Electrode Surfaces: Theory and Experimental Measurement

A simple voltammetric procedure for measuring the kinetics of slow partitioning processes into thin films is described and applied to determine the transport of trimethylammonium ferrocinium, FA(+), into ultrathin Nafion Langmuir-Schaefer films. By recording cyclic voltammograms for the oxidation of FA(+) as a function of time of a Nafion modified electrode placed in a dilute solution containing FA(+), one can readily measure the uptake of FA(+) into a film. An analytical model for the charge accumulated in the film has been derived for the cases of (i) transport-controlled partitioning; (ii) transport with a kinetic barrier at the film/solution (f/s) interface; and (iii) pure kinetic control. Analysis of experimental data indicates that, in contrast to dropcast Nafion films, there is a significant kinetic barrier at the f/s interface to FA(+) partitioning. The methodology and theory described are generic and could be applied to other chemical systems and detection methods.

Measurement of Apparent Diffusion Coefficients within Ultrathin Nafion Langmuir−Schaefer Films: Comparison of a Novel Scanning Electrochemical Microscopy Approach with Cyclic Voltammetry

The use of scanning electrochemical microscopy (SECM) to evaluate the apparent diffusion coefficient, Dapp, of redox-active species in ultrathin Nafion films is described. In this technique, an ultramicroelectrode (UME) tip, positioned close to a film on a macroscopic electrode, is used to oxidize (or reduce) a species in bulk solution, causing the tip-generated oxidant (reductant) to diffuse to the film/solution interface. The oxidation (reduction) of film-confined species regenerates the reductant (oxidant) in solution, leading to feedback to the UME. A numerical model is developed that allows Dapp to be determined. For these studies, ultrathin films of Nafion were prepared using the Langmuir-Schaefer (LS) technique and loaded with an electroactive species, either the ferrocene derivative ferrocenyltrimethylammonium cation, FA+, or tris(2,2'-bipyridyl)ruthenium(II), Ru(bpy)32+. The morphology and the thickness of the Nafion LS films (1.5 +/- 0.2 nm per layer deposited) were evaluated using atomic force microscopy (AFM). For comparison with the SECM measurements, cyclic voltammetry (CV) was employed to evaluate the concentration of electroactive species within the Nafion LS films and to determine Dapp. The latter was found to be essentially invariant with film thickness, but the value for Ru(bpy)32+ was 1 order of magnitude larger than for FA+. CV and SECM measurements yield different values of Dapp, and the underlying reasons are discussed. In general, the Dapp values for these films are considerably smaller than for recast Nafion films, which can be attributed to the compactness of Nafion LS films. Nonetheless, the ultrathin nature of the films leads to fast response times, and we thus expect that these modified electrodes could find applications in sensing, electroanalysis, and electrocatalysis.

Langmuir−Schaefer Films of Nafion with Incorporated TiO2 Nanoparticles

An easy method of incorporating TiO(2) nanoparticles into Nafion perfluorinated ionomer is proposed. Ultrathin films of Nafion were prepared by employing the Langmuir-Schaefer (LS) technique. The pressure-area isotherm study of a Langmuir monolayer of Nafion at the air-water interface on different concentrations of NaCl as the subphase allowed us to find the best experimental conditions for the deposition of stable Langmuir-Schaefer films. Incorporation of TiO(2) nanoparticles was performed by dipping Nafion LS films in a solution of TiO(2) nanoparticles. The uniformity of the TiO(2) incorporation was detected by UV-visible spectroscopy. The morphology of the Nafion, Nafion/TiO(2) nanoparticles thin films, and the changes due to the annealing procedure were investigated by atomic force microscopy. Interestingly, the AFM investigation showed that Nafion and Nafion/TiO(2) LS films have thermal stability up to 600 degrees C.

Electrochemistry of cytochrome c incorporated in Langmuir–Blodgett films of Nafion® and Eastman AQ 55®

Ultrathin films of Nafion® and Eastman-AQ 55® loaded with cytochrome c (cyt c) were obtained and transferred on indium tin oxide (ITO) electrodes via the Langmuir–Blodgett (LB) technique. The pressure-area isotherms for mixed ionomer-protein films indicate that the miscibility of cyt c in the interfacial layer is better for Nafion® than for AQ 55. Interestingly, these composite films maintain the electroactivity of cyt c without requiring the addition of promoters or mediators. Both for AQ 55-cyt c and Nafion-cyt c films, the half-wave potential for the reversible reduction of ferricytochrome c corresponds to the value expected for the weakly adsorbed protein. The modified electrodes show electrocatalytic reaction with ascorbate anion. Comparison with previous literature reports indicate that for Nafion the LB coating procedure is unique in keeping the electroactivity of cyt c.

Preparation and voltammetric characterization of electrodes coated with Langmuir-Schaefer ultrathin films of Nafion®

Ultrathin films of Nafion® perfluorinated polymer were deposited on indium-tin oxide electrodes (ITO) by using Langmuir-Schaefer (LS) technique, after optimization of the subphase composition conditions. Morphological characteristics of these coatings were obtained by Atomic Force Microscopy (AFM). Nafion® LS films showed a good uniformity and complete coverage of the electrode surface, however a different organization degree of the polymer layer was evidenced with respect to thin films deposited by spin-coating. ITO electrodes modified with Nafion® LS coatings preconcentrate by ion-exchange electroactive cations, such as Ru[(NH3)6]3+, dissolved in diluted solutions. The electroactive species is retained by the Nafion® LS coated ITO also after transfer of the modified electrode into pure supporting electrolyte. This allowed the use of the ruthenium complex as voltammetric probe to test diffusion phenomena within the Nafion® LS films. Apparent diffusion coefficients (Dapp) of Ru[(NH3)6]3+ incorporated in Nafion® LS films were obtained by voltammetric measurements. Dapp values decrease slightly by increasing the amount of ruthenium complex incorporated in the ultrathin film. They are significantly lower than values typical for recasted Nafion® films, in agreement with the highly condensed nature of the Nafion® LS fims.