Toluene detection characteristics by a gas sensor based on Al and Ti co-doped ZnO nanostructures

The ZnO nanostructures co-doped with 3 at.% aluminum and different concentrations of titanium (1%, 3%, and 5%) were successfully synthesized by the sol-gel method under supercritical conditions for toluene gas detection applications. The structural, morphological, and optical properties of the synthesized samples were studied. The X-ray diffraction patterns confirmed that all the crystallites belong to the hexagonal wurtzite phase of ZnO, while transmission and scanning electron microscopy observations revealed nanometer-sized particles and a spherical shape of the crystallites. The energy-dispersive X-ray analysis verified the elemental compositions of the samples, which were primarily formed of zinc, oxygen, aluminum, and titanium. The UV–visible–NIR characterization demonstrated maximum absorption in the ultraviolet and a reflectance band in the visible–NIR. Thus, it indicated a constant optical bandgap value around 3.32 eV as the titanium content increased. Finally, it was verified that the sample co-doped with 3 at.% Al and 3 at.% Ti exhibits the best toluene detection performance, displaying a higher and linear response with a maximum value of Igaz/Iair = 6.15 at 400°C for 500 ppm of toluene, a low detection limit below 100 ppm, and a reduced response time compared to the other synthesized samples.