Lanthanum doped SnO2 and ZnO thin films sensitive to ethanol and humidity

Thin SnO2 and ZnO films doped with 0–7 at.% La were deposited by spray pyrolysis. The electrical resistance change of films doped with different lanthanum concentrations was investigated in the presence of 1.5 vol% ethanol vapour in the air at temperatures varying between 150 and 400oC. The ethanol sensitivity of 3% La-doped SnO2 films was found to be about 30. Zinc oxide films doped with 3% La showed a much higher sensitivity (S = 350). All the films exhibited very short response and recovery times (less than 60 s.) at 300°C. Increasing of La content in the films up to 7% deteriorate the ethanol sensitivity. The enhancement of ethanol sensitivity for ZnO and SnO2 films doped with 3 at.% lanthanum can be explained by its double action on the films surface – increasing of surface basicity and decreasing of the size of crystallites. The undoped and doped SnO2 films show small change in resistance in the range of relative humidities 20–95% RH. The electrical resistance of La doped ZnO films is more strongly influenced by the relative humidity than the resistance of the doped SnO2 films, but these dependencies are not linear.