Fast detection of low concentration carbon monoxide using calcium-loaded tin oxide sensors

Solid state sensors with noble metal-loaded tin oxide as sensing elements are widely used for the detection of flammable and toxic gases and volatile organic compounds (VOCs). Apart from rendering high cost, incorporation of noble metals often increases the preconditioning time and introduces a drift in baseline properties of the sensors. Herein, we report on the development of stable and economically viable sensor modules for fast and efficient detection of carbon monoxide (CO) in air. The sensing layer comprises sonochemically synthesized calcium-loaded tin oxide (Ca-SnO2) nanocrystals, in which calcium primarily gets segregated at the SnO2 grain boundaries as calcium oxide (CaO) and thereby restricts the growth of SnO2 particles. Due to larger ionic radius of Ca2+ as compared to that of Sn4+, a minute quantity of calcium can be doped as well into the SnO2 lattice. The variation of calcium concentration in SnO2 has a prominent effect on the sensor performance, where 5 wt% calcium loading shows the highest sensor response. The sensors exhibit a lower detection limit of 1 ppm CO in air. The response time (10-12 s) and recovery time (30-45 s) of our sensors, for different concentrations of CO, are equivalent or less compared to those of commercially available metal-oxide sensors. Additionally, a highly stable baseline with minimal drift even after being operational for over 1.5 years is observed.