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.

Etude par microscopie électronique de particules nanostructurées d'oxalate de cobalt précipitées en milieu aqueux

The understanding of material self-assembling and self-organization mechanisms, at mesoscale, is crucial for nanotechnologies development. Such structures, hierarchically organized in superlattices or colloïdal crystals, are observed in inorganic or organo-metallic precipitates. The classical characterization, using electron microscopy and X-ray diffraction, of a synthesized powder is inadequate to describe the hierarchically organized polycrystalline structure of the final particles. Here, we have highlighted the successive steps that lead to the spontaneous formation of a cobalt oxalate dihydrate (COD) colloïdal crystals that precipitate in aqueous media. The characterization of the final particles, made by X-ray and electronic diffraction, has allowed us to determine the controversial crystalline phase of the precipitated COD. The β and γ COD phases has been discriminated by comparing experimental results with simulations of X-ray diffractogram and electron diffraction patterns for these both structures. The precipitated COD corresponds to the γ phase indicating that it comes from solid dehydration of cobalt oxalate tetrahydrate. The final powder characterization, performed by X-ray diffraction (XRD), atomic force microscopy (AFM), low voltage high resolution scanning electron microscopy (LVHRSEM) and transmission electron microscopy (TEM), gives some diverging results about monocrystalline or polycrystalline nature of the precipitates. XRD and AFM analysis show a polycrystalline structure of particles whereas LVHRSEM, TEM and electron diffraction observations indicate a monocrystalline structure. This apparent contradiction has led us to elaborate a new characterization method allowing to follow the growth and the structural evolution of the precipitate as a function of time. The cryo- preparation techniques are well adapted as they allow freezing and direct observation of solid state suspension by electron microscopy. The freeze/drying method has been modified and used for both SEM and TEM, and has allowed us to study the morphological and structural evolution of the COD precipitate from nanoscale to mesoscale. In addition, TEM analysis of samples prepared by classical techniques has been performed to complement cryo-electron microscopy observations. The combination of these two methods show that COD precipitation is a complex multi-step process leading to the formation of a core/shell heterostructure. The anisotropic core is porous and partially crystalline. It is formed by agglomeration of isolated primary particles and agregated ones (15-20 nm sized secondary particles). The crystalline shell corresponds to the final particles faces and is made up by the layer by layer self-assembling and alignment of 5-7 nm sized crystalline primary particles. These nanoparticles are aligned and perfectly ordered in strings of nanograins in the layer structure. The self-assembling occurs in the last step of growth where we have a lower ionic strength and supersaturation inducing slower kinetics of growth and aggregation. Moreover, the crystalline order in the primary and secondary particles increase with the time of reaction consistent with a continuous process of primary particle nucleation and growth. Our results show that self-assembling occurs layer by layer with terraces, kinks and steps that are present on the faces. This reminds the layer by layer crystalline growth models but in this case, the buiding blocks are colloïdal instead of atomic or molecular. Based on these different results, we propose an original model describing the COD precipitate growth. In addition, we have studied the poly(methacrylic acid-sodium salt) effect on the COD precipitation. This additive (PMMA) appeared to be a very good growth inhibitor. The PMMA effect on the final particles morphology is dramatic. Its concentration variation in solution slows down the nanoparticles aggregation rates along a [101] preferential direction. The crystal habit can then be controlled and we have synthesized platelets, cubes and rods with tailored length. The use of cryogenic electron microscopy has been shown to be a powerful tool for the understanding of time dependent aspects of particle growth. The use of such techniques for the study of other systems, where the final precipitate appearance may hide particle substructures, will help to elucidate growth mechanisms and allow finer control of nanostructured materials for many applications. The complex self-assembling and self-organization processes could then be highlighted and studied on a nanometer to micrometer scale.

EPFL2004

Advanced characterization methods for self-assembled organization on nanoparticles

Zhi Luo

The complex organizations of amino acid residues on protein surfaces give rise to a wide space of functions for proteins. Great efforts are devoted in biology to understand such structure-property relationships. Many advanced characterization techniques have been developed for this purpose. Nanomaterials are regarded as a close resemblance to proteins, as they share similar sizes and, when coated with different types of ligand molecules, can present patchy surfaces (e.g. stripe-like, patchy, or Janus) that bear some similarities to the surfaces of proteins. So far, most characterization methods developed for nanoparticles come from classic material sciences and consequently deliver a precise description of the inorganic and well defined core of the nanoparticles. Yet big challenges remain to accurately describe the morphologies of the ligand shell of nanoparticles that is closer to a biological entity rather than small sized materials. This thesis establishes two new techniques for the characterization of the morphology of the ligand shell of nanoparticles. The first stems directly from structural biology, i.e. small angle neutron scattering combined with ab initio reconstructions. 3D models could be obtained by fitting the scattering data, allowing for the quantitative determination of different morphologies. The second one is based on the statistical analysis of the MALDI-TOF MS spectra. Adopting Monte Carlo calculations, the structures of mixed ligands could be visualized by fitting the fragmentation patterns that represents a sampling of the nanoparticle surfaces. Thanks to the advancement in characterization provided by these newly development methods, two phenomena are studied in this thesis. First, the evolution of the ligand shell morphology during a ligand replacement reaction called place-exchange is uncovered for the first time. Also the evolution of the ligand shell morphology upon gentle heating is uncovered. These results provide a new way to control the morphology of the nanoparticle ligand shells. Second, an in-depth study of the water structure around patchy surfaces is presented. Two nanoparticles with the same core size and composition but varying in the ligand shell structures are synthesized. The morphologies of the heterogeneous nanoparticle surfaces on top of chemical nature are thus directly correlated to the structure of interfacial water. This work offers molecular explanation on the structural factor in the interfacial energy of multiple component systems, which has not been discussed in physical chemistry textbooks before. This latter work offers new insights on how patchy nanoparticles could be used as model systems to understand the interfacial properties of proteins, which are often very difficult to be studied directly due to the complex and dynamic structures of the proteins.

EPFL2018

Surface Characterization of Colloidal Silica Nanoparticles by Second Harmonic Scattering: Quantifying the Surface Potential and Interfacial Water Order

Marie Bischoff, Cornelis Ulrich Lütgebaucks, Arianna Marchioro, Sylvie Roke

The microscopic description of the interface of colloidal particles in solution is essential to understand and predict the stability of these systems, as well as their chemical and electrochemical reactivity. However, this description often relies on the use of simplified electrostatic mean field models for the structure of the interface, which give only theoretical estimates of surface potential values and do not provide properties related to the local microscopic structure, such as the orientation of interfacial water molecules. Here we apply polarimetric angle-resolved second harmonic scattering (AR-SHS) to 300 nm diameter SiO2 colloidal suspensions to experimentally determine both surface potential and interfacial water orientation as a function of pH and NaCl concentration. The surface potential values and interfacial water orientation change significantly over the studied pH and salt concentration range, whereas zeta-potential (ζ) values remain constant. By comparing the surface and ζ-potentials, we find a layer of hydrated condensed ions present in the high pH case, and for NaCl concentrations ≥1 mM. For milder pH ranges (pH < 11), as well as for salt concentrations

Etude par microscopie électronique de particules nanostructurées d'oxalate de cobalt précipitées en milieu aqueux

EPFL2004

Advanced characterization methods for self-assembled organization on nanoparticles

Zhi Luo

EPFL2018