3-D-electron microscopy configuration of TMOS wet silica gels prepared by the quick-freeze,deep-etching-rotary-replication technique

Silica gel provides a useful medium for crystal growth; solution growth is confined to pores left free by the polymer during its development. All growth steps depend on the gel structure, which is not completely known for crystal growth conditions. Therefore, a three-dimensional (3-D) visualization has been performed for two TMOS aqueous gels, which are rather fragile: the quick-freeze, deep-etching, rotary-replication method has been applied for sample preparation. An original surface labeling technique has been used for surface recognition. The results concern the distribution of macropores that are responsible for crystal nucleation; micropores whose total volume is larger have not been visualized due to the limits of the method. These results are discussed in comparison with previous data performed by light scattering.     

Buffer layer effect in nanostructured tin electrodeposition on insulating and conducting substrates

The electroplating techniques for metal aggregates and films deposition commonly use an electric current to reduce metal ions in solution, but are restricted to conducting substrate. This new electrochemical technique permits coating of insulating or conducting substrates with metals having controlled aggregate size and growth speed. The basis of our approach is the progressive outward growth of the metal from an electrode in contact with the substrate, with the cell geometry chosen in such a way that the electron current providing the reduction passes through the growing deposit. The nanostructured deposit is composed of branched nanoaggregates from a quasi-continuous film to a more dendritic morphology dependant on current conditions. This approach has been used to elaborate tin electrodeposited thin films composed of a homogeneous distribution of nanoparticles on conducting or insulating substrates. In our works, when a non-continuous buffer gold coating is used, spontaneous mixing of tin atoms into AuSn nanoparticles takes place even at room temperature forming a nanostructured fractal film, if the substrate is conducting or insulating. Without a gold buffer layer, the deposit is composed of large pure tin micro-crystals with a large size distribution, less adapted to tin oxide nanoparticle formation. Indeed, from these tin metal deposits, the final goal is to elaborate functional nanostructured tin oxide films by oxidation for gas sensor applications.