Sensing behavior of CdS-TiO₂ thick films for the detection of hydrocarbons

In this article, the sensing behaviors of undoped titanium dioxide (TiO₂) and CdS-doped TiO₂ (CdS-TiO₂) thick films are discussed. Sensing pastes of 2 wt% CdS-TiO₂ and undoped TiO₂ were prepared in the laboratory and used to fabricate thick film gas sensors on an alumina substrate. The crystal structures of TiO₂ and CdS-TiO₂ samples were characterized by XRD and atomic force microscopy (AFM). The results indicated that the grain size and RMS roughness parameter were reduced by adding CdS contents. The sensing behaviors of the fabricated devices were studied at varying concentrations (0-5000 ppm) of different hydrocarbon gases, such as LPG, methanol, ethanol, toluene, and benzene, in ambient air at 300 K. The effect of humidity levels on the sensing properties of the sensors was also investigated. The sensor response value of CdS-TiO₂ for benzene was found to be 2.25 times higher than that of TiO₂-based sensing devices. Thus, CdS doping significantly enhanced the response and recovery times of the sensor. The TiO₂ film exhibited response and recovery times of 65 s and 180 s, respectively. In contrast, when doped with CdS, the response times were reduced to 15 s and 103 s, respectively, when exposed to benzene at a concentration of 5000 ppm at 300 K. The sensing mechanism has been discussed and the experimental results were validated using a model based on the Frenkel-Poole theory of electronic emission and catalytic oxidation. The obtained results demonstrate that TiO₂ structures doped with low concentrations of CdS exhibit superior sensitivity and selectivity to benzene gas under low humidity levels at room temperature (300 K).