The most extensively explored gas sensors are metal oxide semiconductor (MOS) materials owing to their high selectivity, stability, cost-effectiveness, and simple synthesis techniques.

Toluene gas is the most toxic and affects the respiratory system of humans, and thereby, its detection at lower levels is an important task. Herein, we report a room temperature-operatable indium oxide-based chemiresistive gas sensor, which detects 50 ppm toluene vapors. Nanocrystalline indium oxide (In₂O₃) films were sprayed on a pre-cleaned glass substrate using a cost-effective spray pyrolysis method at different substrate temperatures in the range of 350–500 °C. The X-ray diffraction studies confirmed that the sprayed thin films, which were deposited at different substrate temperatures, exhibit a cubic structure. The preferred orientation was aligned along the (222) orientation. Average crystallite size calculation based on the Scherrer formula indicates that the crystallite size increases with the enhancement of substrate temperature. FESEM analysis showed that the indium oxide thin films possess uniform grain distribution, which persists over the entire substrate. As the substrate temperature is increased, a partial agglomeration in the film morphology was observed. The deposited film’s nanostructured nature was confirmed by transmission electron microscopy, and the polycrystalline nature was confirmed from the selected area electron diffraction pattern. Root mean square roughness of the samples was determined from the atomic force microscopy studies. From the Raman spectra, characteristic vibrational modes appeared at 558.61, 802.85, and 1097.18 cm^–1 in all the samples, which confirms the cubic structure of indium oxide thin films. Photoluminescence emission spectra have been recorded with an excitation wavelength of 280 nm. The optical band gap was measured using the Tauc plot. The band gap was found to decrease with an increase in the substrate temperature. The gas-sensing performance of indium oxide films sprayed at various substrate temperatures has demonstrated a better response toward 50 ppm toluene gas at room temperature with good stability, and the response and recovery times were determined using a transient response curve.

ZnO/V₂O₅ nanocomposite thin films were synthesised by the spray pyrolysis technique with optimised deposition parameters by varying the concentration of vanadium pentoxide.

Tin oxide (SnO₂) and 2 mole% Ce doped SnO₂ nanoparticles synthesised by the hydrothermal method to detect ammonia vapours at room temperature. X-ray diffraction investigations have been confirmed that the synthesised nanoparticles are polycrystalline in nature with tetragonal rutile phase. The particle size is determined using Scherrer’s formula and it is found to increase with the ‘Ce’ dopant. Scanning electron microscopy observations reveal that these samples have spherical morphology composed of fine crystallites. The EDX spectra reveal the dominant presence of Sn and O in the case of pure SnO₂ nanoparticles. Whereas in the case of doped sample, the EDX reveals the existence of Ce in lesser percentage in comparison with the Sn and O. Atomic force microscopy studies reveals that the route mean square roughness of the nanoparticles is increased from 4.4nm to 15.3 nm due to the Ce dopant in the SnO₂ matrix. Optical bandgap is calculated with the Tauc plot and it is increased with the doping of cerium atoms and it is due to the Burstein -Moss effect. Gas sensing characterization has been performed using static liquid distribution technique against various volatile organic compounds (VOC’s) such as ammonia, ethanol, methanol and toluene in the range of 1 to 25ppm at room temperature.

This paper focused on the preparation of pure and Cr-doped tungsten trioxide (WO₃) 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 WO₃-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 energy-dispersive X-ray spectroscopy measurements. The 3 wt % Cr-WO₃ has a large nanoflake-like structure with high surface roughness and porous morphology. Gas-sensing characteristics of undoped and Cr-doped WO₃ thin films were investigated with various gases at room temperature. The results showed that 3 wt % Cr-doped WO₃ 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 WO₃ crystal matrix might be responsible for increased xylene sensitivity.