Gas sensing properties of tin oxide thin films fabricated from hydrothermally treated nanoparticles: Dependence of CO and H2 response on film thickness

Thin film SnO2 sensors with various thicknesses were spin-coated from a hydrothermally treated SnO2 sol suspension. The FE-SEM observation revealed that the films prepared from the 1.8 wt.% SnO2 containing sol suspension had good uniformity packed with nano-crystalline SnO2 even after calcination at 600°C when its thickness was up to about 300 nm, though many cracks were observed for thicker films. The gas sensing characteristics to H2 and CO were evaluated as a function of film thickness (80–300 nm) in dry air. The electrical resistance of thin films decreased monotonically with increasing film thickness. It was found that the sensor response to H2 similarly decreased with an increase in film thickness, while the CO response was found to be almost independent of the film thickness. The relationship between gas sensing properties and film thickness was discussed on the basis of diffusivity and reactivity of the gases inside the oxide films.