Interaction of Metal Oxides with Carbon Monoxide and Nitric Oxide for Gas Sensing Applications

This dissertation involves the study of the interaction of carbon monoxide (CO) and nitric oxide (NO) on derivatives of low temperature conducting metal oxides, ruthenium and vanadium oxides. The interactions of these gases with the metal oxides lead to changes in conductivity which show promise for possible applications as a new class of resistive based ambient gas sensors that alleviate the current limitations of CO and NO sensors that operate at elevated temperatures. These sensors are based on hydrated ruthenium oxide (RuOx(OH)y) and vanadium pentoxide (V2O 5).;RuOx(OH)y was prepared a wet precipitation reaction involving ruthenium chloride with a base. This material was amorphous, made up of 20--50nm particles and contains Ru(III) and Ru(IV), as determined by XPS. Thick films were made of air and supercritical dried particles of RuOx(OH)y. The conductivity of these films decreased in the presence of CO in air and this change was reversible. Infrared spectroscopy showed the formation of carbonates and water in the presence of CO, which disappeared upon replacement of CO with air. Upon thermal treatment of RuO x(OH)y above 200°C, a decrease in the conductivity change in the presence of CO at room temperature is observed. These changes were accompanied by a conversion of the amorphous RuOx(OH)y to a crystalline RuO2 and consequently a conversion of Ru(III) to Ru(IV). This dissertation proposes the oxidation of CO on RuOx(OH) y leads to reduction of the ruthenium and subsequently a decrease in conductivity of the thick films. With the conversion to crystalline RuO 2, the material becomes metallic and conductivity changes are diminished. Changes in RuOx(OH)y conductivity with CO provides an opportune platform for an ambient CO sensor. The interferences from ambient concentrations of hydrocarbons, ammonia, CO2, NO and NO2, were shown to have no effect on the conductivity.;This dissertation also discusses the study of the interaction of NO with vanadium oxides. The V2O5 was used as received and the vanadium dioxide (VO2) was synthesized by reduction of V2 O5. The materials were both crystalline particles of 5 microm in V2O5 and 1x5 microm rods in VO2. The composition was determined to be predominantly V(V) in V2O 5 and V(V) and V(IV) in VO2, as determined by XPS. Thick films of V2O5 were made and the conductivity of these films decreased in the presence of 15 ppm NO and increased in the presence of 500 ppm NO in air reversibly. While the conductivity of VO2 decreased in the presence of 15 and 500 ppm NO. Upon heating V2O 5 to > 350°C, an increase in the conductivity is observed irrespective of concentration. This dissertation proposes the chemisorption of NO on the surface of V2O5 which leads to a change in the bulk donor density on the surface manifested as an inversion layer. This n to p transition explains the change in conductivity observed on V2O 5 with NO which does not occur on VO2 due to its metallic conductivity. The interferences from ambient concentrations of propane, ammonia, CO and acetone were shown to have no effect on the conductivity.