Optimizing ZnO–SnO2 thin-film ratios for high sensitivity to NO2 and NH3
摘要
In this study, thin films for NO2 and NH3 gas sensors were synthesized using a sol–gel spin-coating technique from the composition (ZnO)x(SnO2)1−x at various volumetric ratios (x = 0.1, 0.2, 0.3, and 0.4). The effects of these synthesis ratios on the structural, morphological, optical, electrical, and gas-sensing properties were investigated. XRD patterns and Raman analysis revealed that the samples exhibited a nanometric polycrystalline structure characterized by tetragonal rutile and hexagonal ilmenite-type phases. Energy-dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) validated the existence of zinc (Zn2+), tin (Sn4+), and oxygen (O2−) states in the deposited films. XPS measurements reveal a notable peak at a binding energy of 530 eV, associated with oxygen vacancies. EDS elemental mapping indicates that the surfaces of the coated thin films are rich in oxygen, suggesting that the adsorption process may improve their sensitivity to NO2 and NH3 gases. Field emission scanning electron microscopy and Atomic Force Microscopy measurements revealed a nanostructure morphology characterized by nanoscale spherical grains, ranging from 10 to 80 nm, exhibiting agglomeration. Additionally, an increase in roughness from 4.525 to 24.41 nm was observed at elevated x-mixing ratios, which facilitated the adsorption of atmospheric oxygen and the target gas. All films exhibited a decrease in absorbance values with increasing wavelength, while the measured band-gap energies were 3.75, 3.92, 3.85, and 3.81 eV. The PL studies indicated the existence of oxygen vacancy defects in the synthesized films. Hall Effect tests indicate that electrical mobility increases from 7.61 × 10–2 to 7.90 × 102 (cm2/Vs) as the mixing ratio increases from x = 0.1 to x = 0.2. The findings indicated that the mixing ratios had a significant impact on the growth characteristics, crystal structure, and carrier concentration of the nanostructured spin-coated thin films, thus improving their potential use as gas sensors. Measurements indicated a notable enhancement in sensitivity. Under ambient conditions (25 °C), the spin-coated films demonstrated a sensitivity of 83.15%. Increasing the temperature to 100 °C improved the sensitivity to 84.45%. The optimal sensing performance was achieved at 75 °C, yielding sensitivities of 92.28% for 60 ppm of NO2 and 52.15% for 7.5 ppm of NH3 at mixing ratios of x = 0.2 and 0.3, respectively. The optimal sensitivity for NO2 gas was recorded at an operating temperature of 75 °C, whereas for NH3 gas, it was observed at room temperature. The prepared films exhibited notable sensitivity to NO2 and NH3 gases at low temperatures. In conclusion, the porous (ZnO)x(SnO2)1−x spin-coated thin films serve as effective metal oxide semiconductors for detecting both NO2 and NH3 at room temperature. The findings indicate that ZnOx (SnO2)1−x nanostructured spin-coated thin films are viable candidates for advanced gas-sensing applications, exhibiting superior detection capabilities for volatile compounds and making significant contributions to the advancement of next-generation sensing technologies.