On the influence of temperature gradient of annealing process on the nano-structure and sensing properties of WO3 thin films to NO2 gas and relative humidity

This research presents a detailed study of the influence of temperature gradient of the annealing process on nanostructure, porosity and the sensitivity of sputtered WO3 thin films to NO2 gas and relative humidity that can be used for the development of metal oxide gas sensors. WO3 thin films were deposited by DC reactive sputtering method and then post-annealed in the air at 500 °C with different temperature gradients (gradual, step by step and rapid annealing). Morphological and structural investigations were carried out on all samples by atomic force microscopy and X-ray diffraction method. Porosity and effective surface area were measured by physical adsorption isotherm. Electrical response of all prepared films was tested to 10 ppm NO2 gas at the operating temperature range of 50–250 °C and humidity at room temperature. Results showed that gradually annealed sample had the best sensitivity to NO2 gas and the relative humidity due to the most effective surface area.