Effect of film thickness on gas sensing properties of sprayed WO3 thin films

Abstract

Tungsten trioxide (WO₃) thin films were deposited on glass substrates for obtaining films of different thicknesses (0.5–2.5 µm) using spray pyrolysis technique. The structural and chemical properties, surface composition and morphology of these films were investigated using XRD, TEM, SEM, EDAX and Raman spectroscopy. XRD and SAED revealed the polycrystalline nature of the films exhibiting monoclinic structure. The crystallite size increased with the thickness. This was confirmed from TEM analysis and Raman spectroscopy as well. The SEM images showed that the surface morphology of the films changes along with film thickness. EDAX estimated the average atomic percentage ratio of W and O in the WO₃ films. Value of DC resistance confirmed the high amount of oxygen on the surface of WO₃ thin films. WO₃ thin film of thickness 0.8 µm emerged as the best NO₂ sensor at a low operating temperature of about 125 °C along with excellent reproducibility and very fast response–recovery property. This study analyzes the relationship between thickness, surface morphology, electrical properties and the gas sensing capability of these thin film based WO₃ gas sensors.