Sol-gel-derived metal oxide thin films design of new materials for chemical sensing

Semiconductor gas sensors are used for the detection of reducing gases such as CO, H{dollar}sb2{dollar}, hydrocarbons, and alcohols. When these sensors are constructed from pure polycrystalline metal oxide semiconductors, they do not have adequate selectivity for analytical applications in complex gas streams. This research is devoted to the design of metal oxide thin film materials that will improve the sensitivity, selectivity, and stability of these sensors.; The materials chosen for this research are thin films synthesized from metal alkoxides using sol-gel techniques. This molecular level approach allows the creation of oxide materials, such as amorphous and mixed metal oxides, which are not accessible by more conventional fabrication methods. The initial phase of this research focuses on understanding the relationship between solution chemistry and the characteristics of films prepared from these solutions. The second phase focuses on modification of films in ways which are relevant to gas sensing.; Films are made from tin, titanium, or zirconium alkoxides and valeric acid. Solution reactions are studied with Fourier transform infrared spectroscopy (FTIR) and a combination of FTIR and X-ray photoelectron spectroscopy (XPS) is used to elucidate film structure and composition. Oxide films are prepared by either H{dollar}sb2{dollar}O{dollar}sb2{dollar} treatment or by calcination of as cast (or "valerate") films at temperatures up to 1000{dollar}spcirc{dollar}C. These are characterized with XPS, X-ray diffraction (XRD), UV-Vis spectrocopy, and ellipsometry. Lattice oxygen reactivity, which may be correlated to sensor selectivity, is evaluated with XPS after in situ H{dollar}sb2{dollar} reduction. XRD and angle resolved XPS are used to evaluate the homogeneity of mixed TiO{dollar}sb2{dollar}-SnO{dollar}sb2{dollar} films as a function of composition and calcination temperature.; Valerate and oxide films are transparent, stable, and exhibit uniform adhesion to quartz substrates. Valerate films are comprised of an amorphous metal-oxygen network coordinated with valerate ligands. The compositions of tin oxide and titanium oxide films are similar to those of polycrystalline SnO{dollar}sb2{dollar} and TiO{dollar}sb2{dollar} but are largely amorphous if calcined at temperatures {dollar}<{dollar}600{dollar}spcirc{dollar}C. Lattice oxygen reactivity of tin oxide films is inversely related to the temperature used to oxidize the films. Results of redox cycling experiments suggest that films undergo very little irreversible restructuring upon H{dollar}sb2{dollar} reduction. As cast TiO{dollar}sb2{dollar}- SnO{dollar}sb2{dollar} films and those calcined at temperatures {dollar}<{dollar}400{dollar}spcirc{dollar}C are amorphous and homogeneously mixed. Calcination at higher temperatures leads to the formation of mixed rutile phases and to a surface segregation of titanium oxide.