Katia Fajerwerg

Organometallic approach for the synthesis of nanostructures

Nanostructures are considered as chemical systems of high potential owing to their unusual properties at the interface of those of molecular species and bulk metals. Consequently, they are promising candidates for application in different domains such as catalysis, magnetism, medicine, opto-electronics or sensors. The control of the characteristics of nanostructures is a fundamental prerequisite if one envisages exploring their physical or chemical properties since they vary dramatically with size, shape and surface state. Thus, the development of efficient methods leading to reproducible nanostructures is presently one of the main objectives in the nanochemistry community. Although organometallic chemistry has been early involved, it arises only marginally in the field. Nevertheless, the concepts and techniques of organometallic chemistry appear to be well-adapted for the growth of well-controlled nanostructures. This will be discussed through recent advances in the synthesis of metal and metal oxide nanoparticles in terms of size dispersion, chemical composition, surface state, shape or organization, pointing out the role of ligands. Moreover their characterization at a molecular level and the development of their chemical/physical properties towards applications will be described. This review reflects more than 20 years of efforts of our team to achieve these goals.

Catalysts for aromatics hydrogenation in presence of sulfur: reactivities of nanoparticles of ruthenium metal and sulfide dispersed in acidic Y zeolites

Zeolite-supported ruthenium sulfide and ruthenium metal catalysts were prepared with various Si/Al ratios, with and without extra-framework Al species. They were characterized by means of NMR, HRTEM and FTIR. For the sulfided catalysts, the activity for the tetralin hydrogenation, carried out in presence of H2S was very high and roughly 10 times the activity (expressed per gram of catalyst) of an industrial hydrotreating catalyst, i.e. NiMo/Al2O3. The differences in activities within the series of zeolites were discussed in terms of dispersion of the active phase and acidity of the zeolitic support. The catalytic properties of the metal catalysts were much lower than those of the sulfided catalysts in similar testing conditions.

Acidic zeolites and Al-SBA-15 as supports for sulfide phases: application to hydrotreating reactions

Hydrotreating catalysts were prepared by supporting molybdenum, nickel molybdenum or nickel tungsten sulfides on HBEA zeolites or Al-SBA-15 mesoporous materials. The effect of the acidity (strength and number of acidic sites) on the properties of the sulfide phases and on their catalytic activity was discussed. An effect of the zeolite acidity on the electronic properties of the MoS 2 phase was observed, leading to a considerable enhancement in the hydrogenation and hydrodesulfurization activity. On the other side, for nickel molybdenum sulfide catalysts supported on an Al-SBA-15 support, no effect of acidities on hydrodesulfurization activity was observed. This result was assigned to the mild Bronsted acidity of Al-SBA-15. However, compared to silica support or commercial silica-alumina, an improvement of the hydrocracking activity ( n -decane hydrocracking) of the nickel tungsten sulfide phase (NiWS) and isomerization selectivity (4,6-dimethyldibenzothiophene hydrodesulfurization) of the nickel molybdenum phase (NiMoS) was observed for the Al-SBA-15 supports.

Organometallic Synthesis of CuO Nanoparticles: Application in Low-Temperature CO Detection

A metal-organic approach has been employed for the preparation of anisotropic CuO nanoparticles. These nanostructures have been characterized by transmission and high resolution transmission electron microscopy, field-emission scanning electron microscopy, X-ray powder diffraction, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. The CuO nanoparticles have been deposited as gas-sensitive layers on miniaturized silicon devices. At an operating temperature of 210 °C, the sensors present an optimum response toward carbon monoxide correlated with a fast response (Rn) and short recovery time. A high sensitivity to CO (Rn≈150 %, 100 ppm CO, RH 50 %) is achieved. These CuO nanoparticles serve as a very promising sensing layer for the fabrication of selective CO gas sensors working at a low temperature.

Ultra-sensitive SnO 2 gas sensors based on hierarchical octahedra

The controlled hydrolysis of Sn11 precursor ([Sn(NMe)2]2) yields micrometric Sn3O2(OH)2 octahedra, formed by a self-assembly process. The water content and alkylamine surfactants are major parameters for the growth process. These structures have been employed to prepare highly sensitive gas sensors, which present 7% variation of their resistance at only 0.25 ppm CO.

Influence of thermal treatment on the dispersion of MoS2 into MCM-41 and SBA-15 supports: TEM study and hydrotreating activity

MoS 2 catalysts supported on MCM-41 and SBA-15 were prepared by impregnation with ammonium heptamolybdate (≈8.0 wt % Mo) with the aim of obtaining a high Mo oxide dispersion, then a high MoS 2 dispersion after sulfidation for application in reaction of hydrodesulfurization. The influence of the thermal treatment (under air at 573 and at 723 and under He at 823K) on the nature and dispersion of the MoO x phase, and then of the sulfided phase was first studied with a commercial silica support, by X-ray diffraction, transmission electron microscopy and hydrodesufurization of dibenzothiophene. The best conditions of the thermal treatment, i.e. , under He at 823 K were then successfully applied for the preparation of MoS 2 /MCM-41 and MoS 2 /SBA-15 catalysts, which leads to a higher hydrodesulfurization activity of dibenzothiophene than with MoS 2 supported on commercial silica.

Air-Stable Anisotropic Monocrystalline Nickel Nanowires Characterized Using Electron Holography

Nickel is capable of discharging electric and magnetic shocks in aerospace materials thanks to its conductivity and magnetism. Nickel nanowires are especially desirable for such an application as electronic percolation can be achieved without significantly increasing the weight of the composite material. In this work, single-crystal nickel nanowires possessing a homogeneous magnetic field are produced via a metal-organic precursor decomposition synthesis in solution. The nickel wires are 20 nm in width and 1-2 μm in length. The high anisotropy is attained through a combination of preferential crystal growth in the ⟨100⟩ direction and surfactant templating using hexadecylamine and stearic acid. The organic template ligands protect the nickel from oxidation, even after months of exposure to ambient conditions. These materials were studied using electron holography to characterize their magnetic properties. These thin nanowires display homogeneous ferromagnetism with a magnetic saturation (517 ± 80 emu cm-3), which is nearly equivalent to that of bulk nickel (557 emu cm-3). Nickel nanowires were incorporated into carbon composite test pieces and were shown to dramatically improve the electric discharge properties of the composite material.

Controlled Growth and Grafting of High-Density Au Nanoparticles on Zinc Oxide Thin Films by Photo-Deposition

The integration of high-purity nano-objects on substrates remains a great challenge for addressing scaling-up issues in nanotechnology. For instance, grafting gold nanoparticles (NPs) on zinc oxide films, a major step process for catalysis or photovoltaic applications, still remains difficult to master. We report a modified photodeposition (P-D) approach that achieves tight control of the NPs size (7.5 ± 3 nm), shape (spherical), purity, and high areal density (3500 ± 10 NPs/μm2) on ZnO films. This deposition method is also compatible with large ZnO surface areas. Combining electronic microscopy and X-ray photoelectron spectroscopy measurements, we demonstrate that growth occurs primarily in confined spaces (between the grains of the ZnO film), resulting in gold NPs embedded within the ZnO surface grains thus establishing a unique NPs/surface arrangement. This modified P-D process offers a powerful method to control nanoparticle morphology and areal density and to achieve strong Au interaction with the metal oxide substrate. This work also highlights the key role of ZnO surface morphology to control the NPs density and their size distribution. Furthermore, we experimentally demonstrate an increase of the ZnO photocatalytic activity due to high densities of Au NPs, opening applications for the decontamination of water or the photoreduction of water for hydrogen production.

Micromolar nitrate electrochemical sensors for seawater analysis with silver nanoparticles modified gold electrode

Well dispersed AgNPs have been obtained on gold substrate by direct decomposition of silver amidinate precursor in liquid phase. This functionalization is very efficient for micromolar detection of nitrate ions in artificial seawater.

Organometallic Approach for the Preparation of Zinc Oxide Nanostructured Gas Sensitive Layers

A reproducible organometallic approach was used in order to prepare zinc oxide gas sensitive layers. Various ZnO nanostructures with well-defined morphology were prepared by controlled hydrolysis of suitable organometallic precursor. These nanomaterials were deposited on miniaturized gas sensors substrates by an ink-jet method. The as prepared devices were tested towards different reducing gases, namely: CO, C3H8, and NH3. We showed that the morphology of these nanostructures significantly influences the sensor response level and selectivity to the reducing gases.