Structural Evolution and Thickness Effect on CO2 Gas Detection in Tungsten Oxide Thin Films

In the current work, the microstructural evolution and CO2 sensing performance of tungsten trioxide (WO3) thin films synthesized by reactive DC magnetron sputtering are investigated. Three specific thicknesses of 42, 66, and 131 nm were obtained and annealed at 500 °C, resulting in a stable monoclinic P21/n phase with a strong (200) preferred orientation. Gas sensing tests toward 10,000 ppm of CO2 revealed that the 42 nm film achieves the highest sensitivity (92%) at an optimal operating temperature of 300 °C. Rietveld refinement and texture analysis (texture index, J) demonstrate that the superior performance of the thinnest film is driven by a synergy between its high surface porosity, a grain size comparable to the Debye length, and a high density of active sites on the (200) plane. While all films exhibit n-type semiconductor behavior, increasing thickness leads to microstructural densification and reduced texture, which hinders gas diffusion and operational stability. These findings establish thickness control as a critical parameter for engineering high-performance WO3-based CO2 sensors.