Self-assembled SnO2 micro- and nanosphere-based gas sensor thick films from an alkoxide-derived high purity aqueous colloid precursor.

Tin oxide is considered to be one of the most promising semiconductor oxide materials for use as a gas sensor. However, a simple route for the controllable build-up of nanostructured, sufficiently pure and hierarchical SnO2 structures for gas sensor applications is still a challenge. In the current work, an aqueous SnO2 nanoparticulate precursor sol, which is free of organic contaminants and sorbed ions and is fully stable over time, was prepared in a highly reproducible manner from an alkoxide Sn(OR)4 just by mixing it with a large excess of pure neutral water. The precursor is formed as a separate liquid phase. The structure and purity of the precursor is revealed using XRD, SAXS, EXAFS, HRTEM imaging, FTIR, and XRF analysis. An unconventional approach for the estimation of the particle size based on the quantification of the Sn-Sn contacts in the structure was developed using EXAFS spectroscopy and verified using HRTEM. To construct sensors with a hierarchical 3D structure, we employed an unusual emulsification technique not involving any additives or surfactants, using simply the extraction of the liquid phase, water, with the help of dry butanol under ambient conditions. The originally generated crystalline but yet highly reactive nanoparticles form relatively uniform spheres through self-assembly and solidify instantly. The spheres floating in butanol were left to deposit on the surface of quartz plates bearing sputtered gold electrodes, producing ready-for-use gas sensors in the form of ca. 50 µm thick sphere-based-films. The films were dried for 24 h and calcined at 300 °C in air before use. The gas sensitivity of the structures was tested in the temperature range of 150-400 °C. The materials showed a very quickly emerging and reversible (20-30 times) increase in electrical conductivity as a response to exposure to air containing 100 ppm of H2 or CO and short (10 s) recovery times when the gas flow was stopped.

Crack formation during post-treatment of nano- and microfibres prepared by sol-gel technique.

We report on the method of TiO2 nano- and microfibres preparation and their cracking during processing and post-treatment. Nano- and microfibres were fabricated by drawing from viscous alkoxide based oligomeric concentrate precursors with the following exposure into an atmosphere of 30-50% humidity. The fibres microstructure was analyzed with TEM, solid state NMR, X-ray diffraction tools, and AFM. Experiments on crack formation in TiO2 microfibres proved that fibres with diameter larger than 10 micron are fractured for chosen post-treatment regimes. In theoretical considerations sol-gel produced and thermally treated microfibres are modeled as core/shell structures. It is suggested that the formation of fibres starts via solidification of liquid jet through the appearance of a rigid solid shell, which reveals tensile mechanical stresses because of material shrinkage. The effect of post-treatment is taken into account by additional densification of the fibre surface layer. The stress intensity factor K(I) is calculated for the model core/shell structures and the dependence of K(I) on the fibre diameter is demonstrated. The results of modeling qualitatively confirm experimental data of microfibre cracking above a certain threshold diameter.