Organisation and electrochemical reactivity at molecular interfaces

Organisation and heterogeneous photoreactivity of water-soluble zinc meso-tetrakis(4-carboxyphenyl) porphyrin (ZnTPPC) were investigated at polarisable liquid / liquid interfaces by systematically varying the organic phase solvent. The remarkable affinity of ZnTPPC for the molecular interfacial boundary manifested itself by modulations in the surface excess charge. Potential dependence of interfacial coverage of the porphyrin appeared to be affected by the nature of the organic phase solvent. Heterogeneous photoinduced ET between the specifically adsorbed porphyrin molecules and the redox species present in the organic solvent manifests itself as photocurrent responses. Photocurrent responses are also found to be strongly dependent on the nature of the organic phase solvent. Variations in the photocurrent magnitude were rationalised in terms of the interfacial organisation of the porphyrin and the phenomenological electron transfer rate constant using a modified Marcus model. Formation of a self-assembled monolayer of 11-mercaptoundecanoic acid (MUA) on gold electrode was investigated by Kelvin probe technique and electrochemical impedance spectroscopy as a function of immersion time in thiol solution. Kelvin probe measurements exhibited a monotonic increase in surface potential with immersion time. The increase in surface potential was attributed to the formation of gold-thiolate bonding. Maximum coverage of thiol molecules were obtained after an immersion time of 10 h. Double-layer capacitance (Cdl) values of thiol modified electrodes were extracted from the impedance measurements as a function of immersion time. Furthermore, Cdl values dropped significantly and reached a constant value within a short immersion time in thiol. Multilayer film of pLys and pGlu was fabricated on MUA modified gold electrodes. Polyelectrolyte modified electrodes displayed characteristics electrochemical features of diffusion through pinholes. Photocurrent responses originating from the porphyrin sensitised ultrathin polyelectrolyte multilayer assembly is investigated as a function of bias potential and illumination light intensity. Redox couples incorporated within the multilayer assembly act as carriers of the photogenerated charges across the film. The incident photon to current conversion efficiency (IPCE) of porphyrin sensitised multilayer assemblies is estimated to be 0.02%. Finally, electrochemical and optical properties of metal nanoparticle architectures on pLys modified gold electrodes were investigated. Monodispersed randomly distributed arrays of Au nanoparticles were formed upon electrostatic deposition from solution. Kelvin probe measurements exhibited a steady increase in compensation potential difference (CPD) with deposition time which reached a plateau after 4 h. From the scanning electron micrographs, we estimated maximum particle coverage of 8.6 × l010cm-2 separated by an average edge-to-edge separation of 25 nm from the nearest neighbour. The plasmon resonance band of Au nanoparticles in the ensemble featured a shift to longer wavelengths and were broader compared to that of colloidal particles in aqueous solution. Cyclic voltammetry and impedance measurements indicated strong electronic coupling between the nanoparticles in the ensemble and the electrode. Furthermore, fabrication of a three-dimensional assembly of metal nanoparticles and pLys using electrostatic layer-by-layer method is also illustrated.

Girault, Hubert
Lausanne, EPFL

Note: The status of this file is: EPFL only

 Record created 2005-03-16, last modified 2018-01-27

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