CO2 sensing properties of WO3 powder: experimental and theoretical studies

Abstract

Tungsten oxide (WO3) powders were obtained in this work by both wet chemical synthesis and homogeneous precipitation with ultrasound-assisted radiation methods. Experimental and theoretical investigations were performed to study the effect of the synthesis method and molarity concentration on the structural, optical, electric, and gas sensing properties of WO3. X-ray powder diffraction and Raman spectroscopy confirmed the presence of the monoclinic γ-phase. Rietveld refinement and size/strain calculations were done to perform a complete powder diffraction data analysis. The bandgap was calculated based on UV–Visible Diffuse Reflectance Spectroscopy data, resulting in 2.55 and 2.58 eV for the prepared samples by wet chemical and homogeneous precipitation methods, respectively. These experimental measurements were explained by first-principles total energy calculations, and the structural and electric properties of WO3 (002) surface were determined. Five atomic models were built with the purpose of determining the most stable structure of this surface with different oxygen terminations. Sensing tests were carried out for all the WO3 samples when interacting with carbon dioxide (CO2) molecules to analyze their performance as gas detecting devices. Parameters such as the sensing response, surface resistance behavior and response/recovery times were investigated in detail. Experimental tests confirmed that the maximum sensing response is obtained at 500 ppm of CO2, when operated at 300 °C. Based on the characterizations and gas sensing results, a CO2 gas sensing mechanism of WO3 was proposed and discussed in this work. Finally, the competitive properties of WO3 as a semiconductor-based gas sensor for CO2 detection were confirmed.