Enhancement of CO sensitivity of indium oxide-based semiconductor gas sensor through ultra-thin cobalt adsorption

An In2O3-based thin film sensor was fabricated on alumina substrate for detecting CO gas and ultra-thin transition metal Co was adsorbed by sputtering and annealed as a surface activator as thick as 0.7–2.4 nm to enhance its sensitivity (S). As the thickness (t) of Co catalyst layer increased, the sensitivity of Co–In2O3 sensor for show the highest sensitivity of S=7.5 at t=2.1 nm at 350°C. For the comparison, C3H8 was used as a test gas of hydrocarbon contained gas and the maximum sensitivity for 1000 ppm C3H8 was S=13.5 at t=1.4 nm at 400°C. In consequence, it was found the possibility that CO and C3H8 can be detected without cross-talking by using hybrid type Co–In2O3 sensor. From X-ray photoelectron spectroscopy, adsorbed Co layer was oxidized CoO after 500°C annealing in air and to be covered with Co3O4. Such a formation of CoO (p-type)–In2O3 (n-type) junction was suggested as a main sensing mechanism to explain the enhanced sensitivity of Co–In2O3 sensor.