Christophe Kinowski

Beyond the silica surface by direct silicon-29 dynamic nuclear polarization

Buried truth: High-field magic angle spinning dynamic nuclear polarization (MAS DNP) enhances the sensitivity of solid-state NMR spectroscopy, but only for protonated surfaces. Direct 29 Si DNP using the biradical TOTAPOL (see picture) circumvents this limitation by producing a 30-fold enhancement of subsurface 29 Si NMR signals in mesoporous silica, a material with applications in photonics, nanotechnology and catalysis.

H2-induced copper and silver nanoparticle precipitation inside sol-gel silica optical fiber preforms

Ionic copper- or silver-doped dense silica rods have been prepared by sintering sol-gel porous silica xerogels doped with ionic precursors. The precipitation of Cu or Ag nanoparticles was achieved by heat treatment under hydrogen followed by annealing under air atmosphere. The surface plasmon resonance bands of copper and silver nanoparticles have been clearly observed in the absorption spectra. The spectral positions of these bands were found to depend slightly on the particle size, which could be tuned by varying the annealing conditions. Hence, transmission electron microscopy showed the formation of spherical copper nanoparticles with diameters in the range of 3.3 to 5.6 nm. On the other hand, in the case of silver, both spherical nanoparticles with diameters in the range of 3 to 6 nm and nano-rods were obtained.

Environment segregation of Er3+ emission in bulk sol–gel-derived SiO2–SnO2 glass ceramics

Er-doped (100-x) SiO2–x SnO2 glass–ceramic monoliths were prepared using a sol–gel method. Raman spectroscopic measurements showed the structural evolution of the silica matrix caused by the formation and the growth of SnO2 nanocrystals. Analysis of the photoluminescence properties shows that the quantity of Er3+ ions embedded in the vicinity of SnO2 nanocrystals could be controlled by the SnO2 concentration. We give spectroscopic evidence of energy transfer to erbium ions provided by SnO2 nanocrystals in the silica matrix. The 4I13/2 level decay curves present a double-exponential profile with two lifetimes associated to rare-earth ions in two different environments.

Direct laser-assisted synthesis of localized gold nanoparticles from both Au (III) and Au (I) precursors within a silica monolith

This work presents a solvent-free and laser-assisted growth of gold nanoparticles (Au-NPs) within silica monoliths using both Au(III) and Au(I) precursors. The novelty of the synthesis method is that Au-NPs of about 20 nm in diameter were obtained well dispersed in the matrix with no need of either reducing or capping agents. Moreover, the laser-assisted synthetic procedure here described made it possible to obtain reproducible 2D and 3D patterns of Au-NPs. For this purpose, suitable Au(I) and Au(III) precursors, soluble in dichloromethane, were easily prepared following a well-known procedure. The mesoporous silica matrix was first loaded with the precursors via a simple impregnation and then irradiated using either a continuous laser (λ= 266 or 532 nm) or a pulsed laser (λ=800 nm; pulse: 120 fs; repetition rate: 1KHz). In all cases, a photothermal gold reduction was observed. The Au-NPs have been characterized using UV-vis absorption spectroscopy, x-ray diffraction and Transmission Electron Microscopy. Finally it is shown that the excess gold precursors can be removed after the Au-NP synthesis by a simple washing of the monolith with a few immersions in the pure solvent. The stability of the Au-NPs was further tested by a series of heat-treatments up to 500°C, showing that the silica monolith acts as an effective support to prevent the agglomeration of the nanoparticles.

Erbium-activated silica-tin oxide glass ceramics for photonic integrated circuits: fabrication, characterisation, and assessment

Er-doped (100-x) SiO2 - x SnO2 glass-ceramic monoliths were prepared using a sol-gel processing. The thermally induced growth of SnO2 nanocrystals was followed by Raman spectroscopic measurements. Using x-ray crystallography, the average crystal size was determined to be about 5nm for a heat-treatment at 1000°C. Analysis of the photoluminescence data shows that the amount of Er3+ ions incorporated in the SnO2 nanocrystals can be controlled by the tin dioxide concentration. In addition, spectroscopic evidence is provided of a transfer of energy from SnO2 nanocrystals to erbium ions within the silica matrix, thus confirming the crystalline environment of the rare-earth ions.

Zirconia coating for enhanced thermal stability of gold nanoparticles

This paper describes a rapid, simple and one-step method for the preparation of 2–4 nm diameter zirconia-coated gold nanoparticles at room temperature. These nanoparticles were synthesized by two simultaneous processes: the chemical reduction of tetrachloroauric acid with sodium borohydride and the formation of zirconia sol–gel matrices. All the gold nanoparticle sols were characterized by UV–visible absorption and transmission electron microscopy to determine the nanoparticle size and shape. The synthesis method is a combination of a polymeric structure of the amorphous zirconia and the use of a strong reducing agent, and it yields to very small quasi-spherical gold nanoparticles at room temperature. The thermal stability up to 1200 °C of the coated nanoparticles was studied by x-ray diffraction. The metastable tetragonal phase of the zirconia coating was obtained at 400 °C, and a progressive transformation from tetragonal to monoclinic phases of the zirconia coating was observed up to 1100 °C. After the heat treatment at 400 °C, the crystallite size of the gold nanoparticles was about 29 nm, and it remained unchanged from 400 °C to 1200 °C. These results are promising for the development of such materials as doping elements for optical fiber applications.

Combination of porous silica monolith and gold thin films for electrode material of supercapacitor

An all-solid electrical double layer supercapacitor was prepared, starting from a porous silica matrix coated with a gold thin-film. The metallization of the silica xerogel was performed by an original wet chemical process, based on the controlled growth of gold nanoparticles on two opposite faces of the silica monolith as a seed layer, followed by an electroless deposition of a continuous gold thin film. The thickness of the metallic thin film was assessed to be 700 nm. The silica plays two major roles: (1) it is used as a porous matrix for the gold electrode, creating a large specific surface area, and (2) it acts as a separator (non-metallized part of the silica). The silica monolith was soaked in a polyvinyl alcohol and phosphoric acid mixture which is used as polymer electrolyte. Capacitance effect was demonstrated by cyclic voltammetry experiments. The specific capacitance was found to be equal to 0.95 mF cm− 2 (9.5 F g−1). No major degradation occurs within more than 3000 cycles.