Impedance Spectroscopic Analysis of Methane Sensing Characteristics in Electrospun Tungsten Oxide Nanofibers with Particle Size Heterogeneity.

The sensing performance of nanostructured metal-oxide-based gas sensors is highly dependent on their morphology, which directly influences sensitivity and response dynamics. This study investigates the impedance characteristics of tungsten oxide (WO₃) nanofibers with varying particle sizes, synthesized through electrospinning. By adjusting precursor concentrations and calcination temperatures, nanofibers with an average diameter of 135 ± 29 nm were produced. These nanofibers exhibited particle sizes ranging from 29 to 100 nm, as confirmed by transmission electron microscopy (TEM), and crystallite sizes from 17 to 62 nm, as determined by X-ray diffraction (XRD) analysis. Impedance spectroscopy was employed to analyze electronic conduction and response dynamics, which are critical to the methane (CH₄) sensing mechanism. The optimal operating temperature of the sensors was found to be inversely related to particle size. The sensor with the smallest particles (29 nm) exhibited the lowest optimal operating temperature of 200 °C, while sensors with larger particles required higher temperatures, ranging up to 275 °C. Nanofibers with 29 nm particle sizes also demonstrated the highest sensitivity (S = R _air/R _gas) of 2.85 when exposed to 0.1% (1000 ppm) CH₄ at 200 °C. Additionally, smaller particle sizes were associated with reduced grain boundary relaxation times, leading to faster sensor responses. The enhanced performance and fast response dynamics of smaller particles are attributed to morphology-induced catalytic effects, which reduce activation energy and improve charge carrier mobility at grain boundaries. This study provides valuable insights into the factors governing the electrical response of nanostructured metal-oxide chemiresistive gas sensors.