HIGHLY SENSITIVE AND SELECTIVE NOX AND NO2 SENSOR BASED ON CD-DOPED SNO2 THIN-FILMS

SnO2 thin films, doped with Cd between 1 and 6 at.%, are prepared by an original method allowing us to obtain a high surface area; the adsorption of low NO(x) and NO2 concentrations onto the thin-film surface has been extensively studied. The thin films are grown by means of thermal evaporation, the tin being deposited at a temperature higher than its melting point (tin rheotaxial growth) while the cadmium is deposited at room temperature; the metallic film is slowly oxidized up to a maximum temperature of 520-degrees-C. The XRD analysis shows that the thin films are polycrystalline with preferential (101) and (111) orientations for SnO2 and CdO respectively. Measurements of the thin-film electrical conductance in the presence of various gases show first that these films have a maximum sensitivity, defined as relative percentage variation of conductance, towards NO(x) and NO2 equal to 10 000 and 4000% respectively at a temperature of 300-degrees-C and 10 ppm gas concentration. At the same temperature the sensitivities towards some gases like CO, CH4, C4H10 and H2, present in an air flow at a 100 ppm concentration, always remains lower than 300%.