Electrochemical and optical properties of two dimensional electrostatic assembly of Au nanocrystals

The spectroscopic and electrochemical properties of two-dimensional electrostatic assembly of Au nanocrystals are examined on poly-L-lysine (pLys) modified gold electrodes. The surface preparation for the nanoparticle deposition involved the self-assembly of a monolayer of 11-mercaptoundecanoic acid on the electrode surface, followed by the electrostatic deposition of pLys from aqueous solution. The polyelectrolyte layer acts as the electrostatic anchor for the Au particles. Electrostatically stabilised Au particles were prepared by homogeneous reduction in the presence of citrate, yielding monodispersed colloidal suspension with an average diameter of 18 ± 2 nm. After 4 h of deposition, the citrate-stabilised particles reach a maximum surface density of (8.2 ± 0.1) × 1010 particles cm−2, with an average edge-to-edge distance of 25 nm. The particle surface density was estimated from scanning electron micrographs. Kelvin probe measurements were employed for examining changes in surface dipole introduced by the 2D array of nanocrystals. From simple electrostatic arguments, the apparent static dipole moment per particle was estimated of the order of 2700 D. The strong interaction between the nanocrystals and the pLys layer is responsible for the surface charge displacement, leading to changes in the surface dipole of 0.35 eV. These electrostatic interactions also manifest itself by the red shift of the plasmon resonance of the assembly with respect to the aqueous colloidal suspension. Analysis of the spectral broadening was attempted within the framework of the so-called coherent-potential approximation. Finally, electrochemical studies in 1,2-dichloroethane show a large electronic overlap between the nanocrystals and the metal substrate. Results obtained from electrochemical impedance spectroscopy strongly suggest that the electrostatic assembly of nanocrystal behaves like a 2D array of randomly distributed spherical nanoelectrodes.

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

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.

EPFL2004

Highly Loaded Mildly Edge‐Oxidized Graphene Nanosheet Dispersions for Large‐Scale Inkjet Printing of Electrochemical Sensors

Victor Costa Bassetto, Hubert Girault, Milica Jovic, Andreas Stephan Lesch, Bhawna Nagar, Horst Pick, Yingdi Zhu

Inkjet printing of electrochemical sensors using a highly loaded mildly edge‐oxidized graphene nanosheet (EOGN) ink is presented. An ink with 30 mg/mL EOGNs is formulated in a mixture of N‐methyl pyrrolidone and propylene glycol with only 30 min of sonication. The absence of additives, such as polymeric stabilizers or surfactants, circumvents reduced electrochemical activity of coated particles and avoids harsh post‐printing conditions for additive removal. A single light pulse from a xenon flash lamp dries the printed EGON film within a fraction of a second and creates a compact electrode surface. An accurate coverage with only 30.4 μg of EOGNs per printed layer and cm2 is achieved. The EOGN films adhere well to flexible polyimide substrates in aqueous solution. Electrochemical measurements were performed using cyclic voltammetry and differential pulse voltammetry. An all inkjet‐printed three‐electrode living bacterial cell detector is prepared with EOGN working and counter electrodes and silver‐based quasi‐reference electrode. The presence of E. coli in liquid samples is recorded with four electroactive metabolic activity indicators.

2020

Organisation and electrochemical reactivity at molecular interfaces

EPFL2004

Photoelectrochemical properties of sensitized thin films

Mohamad Hojeij

EPFL2009