Thickness dependent H₂S sensing properties of nanocrystalline ZnO thin films derived by advanced spray pyrolysis

Undoped zinc oxide (ZnO) thin films with various thicknesses were deposited onto glass substrates by advanced spray pyrolysis technique by varying the volume of spray solution. All the films were deposited at 473 K substrate temperature. These samples were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM) techniques, respectively. The X-ray diffraction analysis reveals that film growth is preferentially along (0 0 2) plane and thickness independent. All films exhibit hexagonal wurtzite structure, however, noticeable change is observed in crystallite size. An enhancement in the grain size with increase in film thickness was observed from the SEM studies. A two probe resistivity measurements exhibit decrease in resistivity with increase in film thickness. The response of ZnO samples toward H₂S gas has been investigated at different operating temperatures and gas concentrations. Highest sensitivity was obtained for the film with 135 nm film thickness at 20 ppm H₂S concentration when operating temperature is 573 K. The observed variation in sensitivity is related to the change in grain size with variation in film thickness.