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This paper focused on the preparation of pure and Crdoped tungsten trioxide (WO 3 ) thin films using the spray pyrolysis method. Different techniques were adopted to analyze these films' structural and morphological properties. The X-ray detection analysis showed that the average crystallite size of the WO 3 -nanostructured thin films increased as the Cr doping concentration increased. The atomic force microscopy results showed that the root-mean-square roughness of the films increased with Cr doping concentration up to 3 wt % and then decreased. The increased roughness is favorable for gas-sensing applications. Surface morphology and elemental analysis of the films were studied by field emission scanning electron microscopy with energydispersive X-ray spectroscopy measurements. The 3 wt % Cr-WO 3 has a large nanoflake-like structure with high surface roughness and porous morphology. Gas-sensing characteristics of undoped and Cr-doped WO 3 thin films were investigated with various gases at room temperature. The results showed that 3 wt % Cr-doped WO 3 film performed the maximum response toward 50 ppm of xylene with excellent selectivity at room temperature. We believe that increased lattice defects, surface morphology, and roughness due to Cr doping in the WO 3 crystal matrix might be responsible for increased xylene sensitivity.

Pure and urea-modified zinc oxide thin films are prepared using the spray pyrolysis technique on microscopic glass substrates. We have added different urea concentrations as a modifier to the zinc acetate precursor for obtaining urea-modified ZnO thin films and investigated the effect of the urea concentration on the structural, morphological, optical, and gas-sensing properties. The gas-sensing characterization of pure and urea-modified ZnO thin films is tested in the static liquid distribution technique with 25 ppm of ammonia gas at an operating temperature of 27 °C. The prepared film with a concentration of 2 wt % of urea has shown the best sensing properties toward ammonia vapors due to more active sites for the reaction between chemiabsorbed oxygen and the target vapors.

Undoped and vanadium-doped tungsten trioxide (V–WO3) nanocrystalline thin films were prepared on glass substrates with the spray pyrolysis method. The synthesized films were characterized using X-ray diffraction (XRD), Raman spectroscopy, atomic force microscopy (AFM), field-emission scanning electron microscopy (FESEM), UV-visible spectroscopy, and X-ray photoelectron spectroscopy for micro-structural, topographical, optical, and chemical properties, respectively. XRD analysis confirmed that WO3-based thin films were hexagonal phases. The FESEM images showed the interconnected nanoflake-like morphology of the undoped and V-WO3 thin films. AFM investigation confirmed that the average surface roughness of the thin films ranged from 3.36 to 14.2 nm. The optical energy bandgap of the films was estimated from the optical spectrum of absorption. The static liquid distribution method was utilized to examine the gas-sensing characteristics of deposited films. The results show that the 4wt% vanadium-doped WO3 sensor responds to ethanol vapour at room temperature. The response and recovery times towards 50 ppm of ethanol gas were determined as 32 and 21 seconds, respectively. The 4wt.% V-WO3 film has shown an increased response from 1.3 to 2.8, which is 2.15 times more than the undoped WO3.

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