Experimental and theoretical study of TiO2 based nanostructured semiconducting humidity sensor

Semiconducting ceramic based humidity sensors are of great interest for many industrial automation applications. Regarding to these demands, TiO2 and copper (Cu) doped TiO2 films, as potential candidates for humidity sensor, are fabricated in the present study by sol-gel method. The effects of copper on crystalline structure, chemical properties, film structure, and optical properties are investigated in details to evaluate the humidity sensing capabilities. Based on the XRD results, the samples have nano-crystalline structure with nearly 20 nm crystallite size. SEM images show rough morphologies and higher surface areas which are obtained due to the nature of the applied sol-gel route and it can be arranged promising interfaces for sensor applications. XPS results point out that the doping agent Cu is successfully incorporated into TiO2 structure. Optical parameters (absorption and band gap) are determined by using UV-VIS spectrophotometer. The results reveal that the optical band gap of TiO2 films decreases with the Cu doping. As the main objective of the study, humidity sensing properties are analyzed using the QCM method. Based on the QCM results, the humidity sensing response is improved by the Cu doping. Additionally, the Langmuir kinetic model is conducted to evaluate adsorption kinetic factors for humidity changes between the relative humidity (RH) values 30% and 70%. Finally, it can be stated that the produced materials exhibited higher repeatability and sensitivity to humidity changes at room temperature. According to the density functional theory (DFT) calculations, Cu-doping leads to a decrease in the band gap value of TiO2 and enhance its sensitivity towards the water molecule.