LAUSR

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.