Nanostructured SnO₂-ZnO composite gas sensors for selective detection of carbon monoxide

A series of SnO₂-ZnO composite nanostructured (thin) films with different amounts of SnO₂ (from 0 to 50 wt %) was prepared and deposited on a miniaturized porous alumina transducer using the sol-gel and dip coating method. The transducer, developed by our research group, contains Au interdigital electrodes on one side and a Pt heater on the other side. The sensing films were characterized using SEM and AFM techniques. Highly toxic and flammable gases (CO, CO₂, CH₄, and C₃H₈) were tested under lab conditions (carrier gas was dry air) using a special gas sensing cell developed by our research group. The gas concentrations varied between 5 and 2000 ppm and the optimum working temperatures were in the range of 210-300 °C. It was found that the sensing performance was influenced by the amount of oxide components present in the composite material. Improved sensing performance was achieved for the ZnO (98 wt %)-SnO₂ (2 wt %) composite as compared to the sensors containing only the pristine oxides. The sensor response, cross-response and recovery characteristics of the analyzed materials are reported. The high sensitivity (R_S = 1.21) to low amounts of CO (5 ppm) was reported for the sensor containing a composite sensitive film with ZnO (98 wt %)-SnO₂ (2 wt %). This sensor response to CO was five times higher as compared to its response to CO₂, CH₄, and C₃H₈, thus the sensor is considered to be selective for CO under these test conditions.