Interaction of CO with hydrous ruthenium oxide and development of a chemoresistive ambient CO sensor

Hydrated ruthenium oxide (RuOx(OH)y), the material of interest in this study was prepared by reaction of an aqueous solution of ruthenium chloride with base. This material was amorphous, made up of 20-50 nm particles and contains Ru(III) and Ru(IV), as determined by X-ray photoelectron spectroscopy. The conductivity of thick films of RuOx(OH)y decreased in the presence of CO in a background of air and this change was reversible. Infrared spectroscopy showed the formation of carbonates in the presence of CO, which disappeared upon replacement of CO with O2. Upon heating RuOx(OH)y, there was a gradual conversion to crystalline RuO2 beyond 200 °C. With these heated materials, the resistance change in the presence of CO at room temperature also gradually diminished. We propose that oxidation of CO on RuOx(OH)y leads to reduction of the ruthenium and a decrease in conductivity. With the conversion to crystalline RuO2 upon heating, the material becomes metallic and conductivity changes are diminished. The change in conductivity of RuOx(OH)y with CO provides a convenient platform for an ambient CO sensor. Such a device also does not show interference from hydrocarbons (2000 ppm), ammonia (150 ppm), CO2 (2000 ppm), NO (15 ppm) and NO2 (15 ppm).