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 400 degrees C. 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 degrees 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. (C) 2000 Elsevier Science S.A, All rights reserved.