A novel approach to low-temperature gas sensing using sol-gel spin-coated (NiO:ZnO:SnO2) thin films for NO2, H2S, and NH3 detection

Abstract

This research used a sol-gel spin-coating technique to synthesize thin films for gas sensors capable of detecting nitrogen dioxide, hydrogen sulfide, and ammonia. The films were composed of nickel oxide (NiO), zinc oxide (ZnO), and tin oxide (SnO₂) in volumetric ratios of (5:5:90), (10:10:80), (15:15:70), and (20:20:60). X-ray Diffraction (XRD) and Raman investigations revealed a nanometric polycrystalline structure consisting of tetragonal rutile, hexagonal ilmenite, and cubic bunsenite phases. Field Emission Scanning Electron Microscopy (FESEM) and Atomic Force Microscopy (AFM) demonstrated average nanoscale spherical grains measuring between 20 nm and 40 nm, alongside an increase in roughness from 4.830 nm to 20.29 nm at higher mixing ratios, hence augmenting gas adsorption. The films exhibited a decrease in absorbance with increasing wavelength, with band gap energies measured at 3.45 eV, 3.38 eV, 3.19 eV, and 3.39 eV. Photoluminescence (PL) results demonstrate that the intensity of photoluminescence decreases as the amounts of Ni and Zn increase. Hall effect tests demonstrate that electrical mobility rises from 4.45 × 10⁻⁴ to 1.98 × 10² (cm²/V·s), corresponding with the minimum energy-gap value at elevated Ni and Zn mixing ratios. Energy Dispersive Spectroscopy (EDS) and X-ray Photoelectron Spectroscopy (XPS) confirmed the presence of Ni²⁺, Zn²⁺, Sn⁴⁺, and O²⁻ in the films. XPS measurements indicate a significant peak with a binding energy of 529.44 eV, linked to oxygen vacancies. EDS elemental mapping reveals that the surfaces of the coated thin films are abundant in oxygen, implying that the adsorption process may enhance their sensitivity to H₂S, NO₂, and NH₃ gases. These porous (NiO:ZnO: SnO₂) thin films exhibit sufficient selectivity in detecting H₂S, NO₂, and NH₃ at room temperature, exceeding the detection capabilities for xylene, toluene, and isopropyl alcohol gases. The results demonstrated that the mixing ratios substantially influenced the growth features of the nanostructured thin films, their crystalline structure, and the carrier concentration, enhancing their efficacy as gas sensors. Measurements exhibited elevated sensitivity and selectivity at ultralow temperatures. The porous synthesized nanocomposite spin-coated thin films display distinct uniform spherical structures and the establishment of p-n-p heterojunctions at the interface of Ni-Zn-Sn metal oxides, illustrating their effectiveness as metal oxide semiconductors for detecting H₂S, NO₂, and NH₃ at room temperature.

Graphical Abstract