Assembly and reactivity of nanoparticles at liquid / liquid interfaces

The interface between two immiscible electrolyte solutions (ITIES), or liquid|liquid interface, provides a defect-free junction for fundamental studies of adsorption phenomena, heterogeneous charge transfer reactions, and phase-formation processes. Recent research interests include the organization and reactivity of metallic and semiconducting nanoparticles (NPs) at ITIES. In a general sense, the present thesis is devoted to addressing this issue, including the voltage-induced assembly of charged NPs, the photoreactivity of semiconducting NPs and the redox properties of alkanethiolate monolayer-protected gold nanoclusters. The adsorption of ionic species at the liquid|liquid interface was first modeled theoretically. The aim of the simulations was to envisage the effect of the adsorption of charged NPs on the interfacial charge distribution across the liquid|liquid interface. For simplicity, in the calculations the charged NPs were considered as large ions, which were further taken as point charges. Various adsorption isotherms were considered, including potential independent, Langmuir and Frumkin models. The simulation indicates that the ionic adsorption has a strong influence on the potential distribution across the interface. Under certain conditions, it results in a non-monotonous potential profile with a trap at the interface. The voltage induced assembly of mercaptosuccinic acid (MSA) stabilized gold and cadmium selenide (CdSe) NPs were investigated at the polarizable water|1,2-dichloroethane (DCE) interface. The studies reveal that the surface concentration of NPs at the liquid|liquid boundary is reversibly controlled by the applied bias potential. There was no evidence of irreversible aggregation or deposition of the particles at the interface. Furthermore, the adsorbed CdSe NPs at the water|DCE interface demonstrated the photoreactivity characteristic of a self-assembled ultrathin p-type semiconductor photoelectrode. The surface structure also has significant effect on the optical properties of CdSe NPs. The redox properties of hexanethiolate monolayer-protected gold nanoclusters (MPCs) were studied in detail at electrochemical interfaces. First, the absolute standard redox potential of MPCs in solutions was theoretically derived from electrostatic considerations. The significant effect of the solvent polarity was verified experimentally by studies in various organic solvents. Second, the redox properties of self-assembled MPCs on a gold electrode were studied in organic electrolytes and room temperature ionic liquids (RTILS). The effect of the electrostatic interaction between MPCs and electrode on the redox properties of MPCs was theoretically considered in terms of the method of images in classical electrostatics and justified experimentally. In RTILS, the rectification of the successive oxidation of self-assembled MPCs by the anionic component of RTILS was observed. Third, MPCs were further used as redox quenchers at the polarizable water|DCE interface. Photocurrent responses originating from the heterogeneous quenching of photoexcited porphyrins by MPCs dissolved in the DCE phase were observed. As MPCs can function as both electron acceptors and electron donors, the photocurrent results from the superposition of two simultaneous processes, which correspond to the oxidation and reduction of MPCs. The magnitude of the net photocurrent is essentially determined by the balance of the kinetics of these two processes, which can be controlled by tuning the Galvani potential difference between the two phases. We show that, within the available potential window, the apparent electron transfer rate constants follow the classical Butler-Volmer dependence on the applied potential difference.

Photoelectrochemical properties of sensitized thin films

Mohamad Hojeij

In recent years, the fabrication of novel ultrathin film systems featuring multi-functionalities has been extensively studied due to their potential applications in photonics, sensors and solar cells. In a general sense, the primary goal of the present thesis was to develop dye-sensitized ultrathin film-modified electrodes based on the layer-by-layer methodology for future light energy conversion applications. In this work, we show that stable and uniform ultrathin multilayer films were successfully assembled by the electrostatic adsorption of alternating monolayer of poly-L-lysine (pLys) and poly-L-glutamic acid onto modified gold electrodes. The multilayer films become electroactive when ferri/ferrocyanide ions are loaded into the film and behave as photoactive when sensitized by Cadmium selenide (CdSe) quantum dots. Photocurrent responses originating from the CdSe sensitized ultrathin multilayer film are observed and were dependent on the applied potential, the thickness of the film and the presence of quenchers in the organic phase. From the obtained photocurrent and the kinetic analysis, two different regimes were observed for the potential distribution in the film: (i) the strong potential dependence regime observed in case of thin films where the system reacts as a modified electrode and (ii) the weak potential dependence regime observed in case of thick films where the potential drop occurs mainly in the first 5 layers and the system behave as supported Two Immiscible Electrolyte Solutions (ITIES). Thereafter, three-dimensional assembly of nanoparticles and pLys was found to be an efficient way to increase the photocurrent efficiency of the system. Large photocathodic and photoanodic currents were recorded in the case of CdSe and CdSe/CdS sensitized films, respectively. The kinetic analysis of photoinduced processes and coupled reactions revealed that the multilayer films behave as one electronic unit, i.e. homogeneous nanoporous semiconducting photoelectrodes. When sensitizing the multilayer film by porphyrin molecules instead of CdSe quantum dots, mainly cathodic photocurrent is observed and is dependent on the applied potential and the thickness of the film. Depending its thickness, the films vary from that of a modified electrode to that of a supported liquid film. Under illumination, the maximum photocurrent responses were reached only after five bilayers and the photocurrent efficiency was weaker than in case of CdSe nanoparticles. Furthermore, another way to increase the photocurrent efficiency is by increasing the specific surface area of the substrate. Reticulated Vitreous Carbon (RVC) foam with properties of high porosity was used as the substrate and the photocurrent efficiency is increased by an order of magnitude in comparison with a planar electrode. The photostability of CdSe nanoparticles is studied in different solvent and under different environment. In presence of light and oxygen, the photo-oxidation of CdSe particles occurs. When the particles are synthesized in water and present lattice defects, a luminescence photo-enhancement is observed due to the photo-oxidation. On the other hand, the photo-oxidation of organic soluble CdSe particles with high quantum yield and with no structure defects is translated by a photobleaching. Consequently, in both cases, the size of particles is decreased and a large absorption and emission blue shift is observed. The size decreases and the blue-shift can be precisely controlled by controlling the irradiation time by a pulsed nano-laser. The presence of ZnS shell around the particles can decrease the photo-oxidation processes. Moreover, adding a sacrificial holes acceptor (n-butylamine) is shown to be an efficient way to prevent the photo-oxidation of the particles even in presence of oxygen. The photophysical properties of cadmium telluride (CdTe) and gold particles are studied at liquid|liquid interfaces. First, time-resolved fluorescence of CdTe nanoparticles adsorbed at the liquid/liquid interface were probed by Time Correlated Single Photon Counting (TCSPC), which was compared with CdTe nanoparticles originally synthesized in aqueous solution and those transferred from the aqueous to organic solvents by using tetraoctylammonium bromide (TOAB) as a phase transfer reagent. Finally, the formation of metal-like liquid films using gold nanoparticles at liquid|liquid interfaces was successfully obtained. The optical properties of the meltal-like liquid film are studied by Surface Plasmon Resonance (SPR) technique at liquid|liquid interface. General experiments and simulations were carried out in order to determine the best parameters to optimize the SPR technique at liquid|liquid interface.

EPFL2009

Dynamic photoelectrochemistry at porphyrin-sensitised liquid/liquid interfaces

Nicolas Eugster

The interface between two immiscible electrolyte solutions provides a well-defined framework for fundamental studies of heterogeneous charge transfer processes. Recent developments in experimental methodology have allowed addressing the kinetics of electron transfer across these molecular junctions. Among these advances, photoinduced heterogeneous electron transfer has provided interesting insights into the elementary processes at dye-sensitised liquid|liquid interfaces. In the present work, we show that the self-organisation of a variety of water-soluble dyes at the water|1,2-dichloroethane interface can be followed by admittance and photocurrent measurements in the presence of a redox couple in the adjacent phase. The specific adsorption manifests itself by changes in the surface excess charge as well as in the dielectric properties of the interface. Photocurrent data allow extracting Gibbs energies of adsorption in the range of -30 to -45 kJ mol-1. The average molecular orientation of the adsorbed photoactive species is estimated from the photocurrent dependence on the angle of light polarisation in total internal reflection. The results show a clear correlation between the orientation of the transition dipole and the distribution of the peripheral charged groups responsible for the hydrophilic nature of the dyes. The anionic zinc meso-tetrakis(p-sulphonatophenyl)porphyrin and the cationic zinc meso-tetrakis(N-methylpyridyl)porphyrin undergo spontaneous association in solution, leading to electrically neutral surface active heterodimer species of particular interest in the investigation of the kinetics of heterogeneous electron transfer processes. Indeed, the surface coverage of heterodimer appears independent of the Galvani potential difference between the two phases. Hence the photocurrent dependence on the applied potential can be directly related to changes in the kinetics of electron transfer. While the depopulation of the S1 state of the monomers via fluorescence and intersystem crossing occurs in the nanosecond range, the excited state of the heterodimer features a fast decay within 100 ps. This decay leads to the formation of an intermolecular charge transfer state with lifetime extending into the microsecond time scale. The dynamics of photoinduced heterogeneous electron transfer between ferrocene or quinone derivatives and the heterodimer are studied at the polarised water|1,2-dichloroethane interface. The photocurrent responses originating from the heterogeneous quenching of the porphyrin complex show a well-defined dependence on the formal Gibbs energy of electron transfer (ΔGo'et). The photocurrent as a function of ΔGo'et can be unambiguously described in terms of a Marcus-type behaviour of the phenomenological bimolecular electron transfer rate constant (kIIet). The solvent reorganisation energy is estimated to be 1.05 eV, from which an average distance of 0.8 nm between the redox species can be evaluated within the framework of the Marcus model for sharp liquid|liquid boundary. These studies also provide an estimate of the activation-less limit of kIIet of 3·10-19 cm4 s-1, which reflects a rather non-adiabatic behaviour of the charge transfer process. In the presence of ferrocene derivatives, back electron transfer from the reduced porphyrin complex introduces a relaxation of the photocurrent in the millisecond time scale. In the case of electron acceptors such as 1,4-benzoquinone, the photocurrent responses exhibit a strong decay due to back electron transfer to the oxidised porphyrin complex as well as interfacial protonation of the semiquinone radical anion. The dynamics of charge separation, back electron transfer, coupled ion transfer and diffusion of redox species to the liquid|liquid interface can be resolved in the time and frequency domains by photocurrent transient measurements as well as intensity modulated photocurrent spectroscopy.

EPFL2004

Controlled Reversible Adsorption of Core-Shell Metallic Nanoparticles at the Polarized Water/1,2-Dichloroethane Interface Investigated by Optical Second-Harmonic Generation

Jean-Pierre Abid, Christophe Bauer, Pierre-François Marie Brevet, Hubert Girault

We report the observation of the reversible adsorption of core-shell gold-silver nanoparticles at the polarized water/1,2-dichloroethane interface using the nonlinear optical technique of surface second-harmonic generation. This study unambiguously demonstrates the excellent stability against aggregation of these core-shell nanoparticles, namely, gold core nanoparticles coated with silver layers of variable thickness, in the presence of an electrolyte salt like lithium chloride. Furthermore, it is also demonstrated that the adsorption of the nanoparticles is reversible by modulating the applied potential at water/1,2-dichloroethane interface. The analysis of these results is performed within the Debye-Hückel approximation of the electrostatic interactions between the nanoparticles. This approach shows that the stability of core-shell nanoparticles can be attributed to the formation of a silver oxide layer at the surface of the particles.