Photo-recycling the Sacrificial Electron Donor: Towards Sustainable Hydrogen Evolution in a Biphasic System.

H2 may be evolved biphasically using a polarised liquid|liquid interface, acting as a "proton pump", in combination with organic soluble metallocenes as electron donors. Sustainable H2 production requires methodologies to recycle the oxidised donor. Herein, the photo-recycling of decamethylferrocenium cations (DcMFc+ ) using aqueous core-shell semiconductor CdSe@CdS nanoparticles is presented. Negative polarisation of the liquid|liquid interface is required to extract DcMFc+ to the aqueous phase. This facilitates the efficient capture of electrons by DcMFc+ on the surface of the photo-excited CdSe@CdS nanoparticles, with hydrophobic DcMFc subsequently partitioning back to the organic phase and resetting the system. TiO2 (P25) and CdSe semiconductor nanoparticles failed to recycle DcMFc+ due to their lower conduction band energy levels. During photo-recycling, CdS (on CdSe) may be self-oxidised and photo-corrode, instead of water acting as the hole scavenger.

Photocurrents at polarized liquid|liquid interfaces enhanced by a gold nanoparticle film.

Photocurrent responses associated with the interfacial quenching of the photo-excited water-soluble zinc meso-tetra(4-carboxyphenyl)porphyrin (ZnTPPC) by ferrocene have been studied at a water|1,2-dichloroethane interface in the absence and in the presence of adsorbed gold nanoparticles. Upon addition of methanol, a mirror-like gold film is formed and an important enhancement of the photocurrent response can be observed. Intensity modulated photocurrent spectroscopy experiments (IMPS) have been performed, in order to deconvolute in the frequency domain the contribution from the competition between the recombination and the product separation arising after the electron transfer, and the attenuation associated with the resistance and interfacial capacitance (RC(int)) time constant of the cell.

Molecular electrocatalysis at soft interfaces.

The fundamental aspects of electrochemistry at liquid-liquid interfaces are introduced to present the concept of molecular electrocatalysis. Here, a molecular catalyst is adsorbed at the interface to promote a proton coupled electron transfer reaction such as hydrogen evolution or oxygen reduction using lipophilic electron donors.

Surface plasmon resonance of gold nanoparticles assemblies at liquid | liquid interfaces.

Surface plasmon resonance (SPR) was observed when a planar close-packed assembly of gold nanoparticles (Au NPs) is adsorbed at the water|1,2-dichloroethane interface. Aqueous gold nanoparticles, 13 or 16 nm in diameter, are deposited at the interface by adding methanol to form a close-packed film with a visible gold mirror reflectance. By total internal reflection of a light beam on the interface, the angular dependence of the interfacial reflectivity was measured in a pseudo-Kretschmann configuration and compared to Fresnel simulations for a homogeneous gold film. The experimental angles for minimum reflectivity were found to match the simulated values. Then, the fluorescence of dye molecules co-adsorbed within 13 and 16 nm gold nanoparticles assemblies at the liquid|liquid interface was measured. The fluorescence intensity under SPR is revealed to be much greater than under total internal reflection conditions, yielding an enhancement factor of approximately 30 and 50 for 13 and 16 nm Au NPs assemblies, respectively. Also, the fluorescence lifetime was found to decrease under SPR conditions.

Soft stylus probes for scanning electrochemical microscopy.

A soft stylus microelectrode probe has been developed to carry out scanning electrochemical microscopy (SECM) of rough, tilted, and large substrates in contact mode. It is fabricated by first ablating a microchannel in a polyethylene terephthalate thin film and filling it with a conductive carbon ink. After curing the carbon track and lamination with a polymer film, the V-shaped stylus was cut thereby forming a probe, with the cross section of the carbon track at the tip being exposed either by UV-photoablation machining or by blade cutting followed by polishing to produce a crescent moon-shaped carbon microelectrode. The probe properties have been assessed by cyclic voltammetry, approach curves, and line scans over electrochemically active and inactive substrates of different roughness. The influence of probe bending on contact mode imaging was then characterized using simple patterns. Boundary element method simulations were employed to rationalize the distance-dependent electrochemical response of the soft stylus probes.

Nanoporous photocathode and photoanode made by multilayer assembly of quantum dots.

We perform in this paper a kinetic study of the photoelectrochemical responses of nanoporous thin photoactive films. The films were fabricated by by a layer-by-layer assembly of positively charged polyelectrolytes (poly-l-Lysine, pLys) and negatively charged semiconductor nanoparticles (NPs) on a carboxylic acid terminated alkanethiol-modified gold electrode. Two types of NPs were used to build uniform films: cadmium selenide (CdSe) and cadmium selenide/cadmium sulfide core/shell (CdSe@CdS). Large photocathodic and photoanodic currents were recorded for CdSe and CdSe@CdS sensitized films, respectively. A theoretical model of the photocurrent responses was developed to analyze the kinetics of photoinduced processes and coupled reactions, showing that the multilayer films behave as homogeneous nanoporous semiconducting photoelectrodes.

CdSe sensitized thin aqueous films: probing the potential distribution inside multilayer assemblies.

Ultrathin polypeptide multilayer films are assembled by the sequential electrostatic adsorption of monolayers of poly-l-lysine and poly-l-glutamic acid onto carboxylic acid terminated alkanethiol-modified gold surfaces. The polypeptide multilayer films are hydrophilic, can incorporate electroactive species such as ferri/ferrocyanide, and are stable when immersed in organic solvents such as 1,2-dichloroethane. Cadmium selenide quantum dots stabilized by negatively charged citrate groups are electrostatically attached to the multilayer film assembly in order to act as photoactive species. Photocurrent responses originating from the CdSe sensitized ultrathin multilayer film are investigated as functions of the applied potential, the thickness of the film and the presence of quenchers in the organic phase. A theoretical model is proposed in order to analyze the kinetics of the photoinduced electron-transfer reactions and to probe the potential distribution within the film.