Enhancing Sensing Performance of SnO2-Based Gas Sensors Using Bismuth Ions As Catalytic Additives
Authors: Yang Haoyue , Suematsu Koichi , Watanabe Ken , Shimanoe Kengo 
Published: December 2024

Metal oxide semiconductor (MOS) based resistive-type gas sensors play a crucial role in monitoring and detecting various gases for a wide range of applications, from environmental monitoring to industrial safety. Among the various MOS materials, SnO2 has emerged as a prominent gas sensing material due to its high sensitivity, excellent stability, and low cost. In recent years, significant efforts have been directed towards further enhancing the performance of SnO2-based gas sensors. According to the gas sensing mechanism of resistive-type gas sensor, improving the receptor function can fundamentally modify the sensing performance to combustible gases. Surface functionalization using catalytic additives has been introduced as an effective approach to strengthen the receptor function of sensing material. In this study, Bismuth (Bi) ions are employed as one of foreign receptors to improve the performance of SnO2-based gas sensors due to their high catalytic activity. SnO2, Bi2O3-mixed SnO2, Bi-loaded SnO2 and Bi-doped SnO2 particles were successfully synthesized. XRD, SEM-EDS and N2 adsorption/desorption measurements were conducted to characterize the as-prepared samples. The results revealed that Bi2O3-mixed SnO2 and Bi-loaded SnO2 materials show similar crystal structure and specific surface area with pure-SnO2 NPs. Meanwhile, Bi2O3 particles were physically mixed with SnO2 nanoparticles in the Bi2O3-mixed SnO2 sample. For Bi-loaded SnO2 sample, no Bi2O3 particles are observed, and Bi ions are well dispersed onto the SnO2 surface. Additionally, Bi-doped SnO2 particles show significantly smaller crystallite size and a larger specific surface area than pure-SnO2 NPs, indicating that Bi ions were doped into SnO2 lattice during the Bi-doping process. Subsequently, thin-film gas sensors were fabricated using a screen-printing technique based on as-prepared materials, and gas sensing performance measurements were conducted from 200oC to 350oC. The results show that Bi-loaded and Bi-doped SnO2 sensors exhibit significantly higher sensitivity to C2H5OH than pure and Bi2O3-mixed SnO2 sensors. Generally, there are mainly two reaction pathways during C2H5OH oxidation, including dehydration to produce C2H4 and dehydrogenation to generate CH3CHO. According to the previous research, the oxidation of C2H5OH molecules on pure-SnO2 surface follows a large amount of dehydrogenation and partial dehydration. In this experiment, both Bi-loaded and Bi-doped SnO2 sensors exhibit comparable sensitivity to CH3CHO with C2H5OH accompanied by an extremely low response to C2H4, indicating that the oxidation of C2H5OH mainly depends on CH3CHO combustion. The results reveal that the catalytic activity of Bi ions improves the dehydrogenation process during C2H5OH oxidation on the SnO2 surface. Additionally, comparable sensitivity to all targeted gases is observed at 350oC for Bi-doped SnO2 sensor. The poor selectivity can probably be attributed to the modified electronic structure and chemical properties caused by the incorporation of bismuth ions into the SnO2 lattice, leading to overactive reactivity at higher temperature. Furthermore, Bi-loaded SnO2 sensor shows significantly lower sensitivity to CO, C7H8 and H2 than to C2H5OH at any operating temperature, illustrating excellent selectivity to C2H5OH. In conclusion, the dispersion of Bi ions onto the SnO2 surface improves sensitivity and selectivity to C2H5OH by accelerating dehydrogenation. This study provides valuable insights into the design and development of advanced gas sensing material with outstanding performance using catalytic additives.