Study of crystallographic, optical and sensing properties of Na₂WO₄ films deposited by thermal evaporation with several thickness

Na₂WO₄ films have been grown at 400 °C using thermal evaporation technique. Their structural properties were characterized by XRD, while their chemical composition was verified by both EDX and X-ray photoelectron spectroscopy (XPS). The evolution of crystallinity was studied as a function of film thickness that ranged from 500 nm to 3000 nm. The grain size increased with increasing film thickness. The surface morphology of the prepared films was studied using scanning electron microscope (SEM) and atomic force microscopy (AFM). It has been observed that the average transmittance of samples in the visible and near infrared range has varied from 90 % to 78 % with the film thickness. The optical band gap of the Na₂WO₄ films varied from 3.8 eV to 4.1 eV. The crystalline size increased with increasing thickness and showed better sensing response to gases. Thus, this study confirmed the possibility of using Na₂WO₄ thick films as a sensor element for detection of ethanol (C₂H₅OH), acetone (C₃H₆O) methanol (CH₃OH) and ammonia hydroxide (NH₄OH) vapor at room temperature, where thicker films exhibited sensing properties with a maximum sensitivity at 25 °C in air, especially for NH₄OH.