Papot Jaroenapibal

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.

Recent advances in Ag-doped WO₃ nanofibers have provided an opportunity to fabricate high quality and low-cost surface-enhanced Raman scattering (SERS) substrates. This work studied the performance and shelf-life of Ag-doped WO₃ nanofiber-based substrates. Methylene blue (MB) molecules were used as analytical references. The prepared substrates were stored in ambient air at room temperature for the duration of 12 months. The Raman enhancement factor (EF) of freshly prepared sample was calculated to be 6.2 × 10⁵. In the first 6 months, a small decrease in EF was observed. The EF was found to significantly decrease after samples were stored for 12 months. After 12 months, the relative standard deviations (RSD) of Raman signals changed from 10.35% to 50.29%, indicating non-uniform Raman enhancements at different locations of the sample. These suggested agglomerations of Ag nanoparticles, which deteriorated the Raman enhancement effect, after samples were stored for a prolonged period.

X-ray photoelectron spectroscopy (XPS) and Spectroscopic Ellipsometry (SE) were used to analyse the effect of oxygen plasma treatment on properties of aluminum oxide thin films. The aluminum oxide films were fabricated using a reactive sputtering system. The as-deposited films were treated with oxygen plasma powered by an RF generator. During the plasma treatment, the pressures were set at 1 x 10^-1 to 1x 10^-2 mbar, while the RF supplied powers at 100 W and 200 W. It was observed that lower plasma powers and higher pressures resulted in smoother films. The O/Al ratio of the films were found to decrease with increasing plasma powers and pressures. The thickness and refractive index of the films were significantly affected by the oxygen plasma treatment process, which could be related to the change in films’ packing density and the etching at the surface.