Effect of bulk and surface modification of SnO2 thin films with PdO catalyst on CO gas sensing characteristics prepared by vacuum evaporation process

Abstract

We have investigated various approaches of PdO addition on to SnO2 thin films for efficient detection of CO gas molecules. Bulk-doped and surface-decorated PdO/SnO2 thin films were prepared by the thermal oxidation of vacuum evaporated metal films in ambient air. For bulk-doping, Pd and Sn metals were co-evaporated followed by thermal oxidation. PdO surface-decoration was done by thermal oxidization of either Pd/Sn metal bilayer or Pd/SnO2 films. The SnO2 thin films are characterized with rutile crystalline structure and porous surface morphology. X-ray photoelectron spectroscopy confirms the dominant presence of PdO on SnO2 surface for the doped films. Pristine SnO2 film exhibited a maximum sensitivity of about 13% for 915 ppm of calibrated CO molecules in nitrogen gas with a response/recovery time of 69/49 s. The PdO bulk-doping in SnO2 film marginally increased the CO sensitivity, whereas, two to four-fold increase is achieved for PdO surface-decorated SnO2 thin films. It is also found that the Pd film thickness plays a critical role in determining the CO sensitivity and is optimized to be 4 nm. A maximum CO sensitivity of 52% with a lowest response/recovery time of 34/46 s is obtained for PdO/SnO2 structure formed through direct oxidation of Pd metal on SnO2 films. A possible gas sensing mechanism based on PdO/SnO2 hetero-junction is discussed.