In this work, we used a commercial
manufacturing process
to obtain
a WO3 semiconductor gas sensor to realize NO2 detection in parts-per-billion concentration at room temperature
(25–27 °C). The radio-frequency (RF) sputtering process
was used to deposit an ultrathin (down to 5 nm) WO3 sensing
layer. With suitable control of the deposition ambient and the postannealing
condition, the WO3 ultrathin-film resistor with a 50-μm
line width can detect 100 ppb of NO2 without any heating
setup. Notably, no nanometer process is required; hence, the production
can be realized by current flat-panel display technology. By modulating
the deposition condition, we investigated the influences of surface
roughness, crystalline condition, and surface hydroxyl group levels
on the sensing response. To achieve parts-per-billion (ppb) level
detection, the ultrathin thickness is essential, and the high-level
crystal together with the low-level surface hydroxyl group also enhances
the sensitivity and the recovery. The humidity effect is also discussed
to show an almost unchanged response in relative humidity (RH) from
30 to 70%. In summary, the WO3 sensor shows good sensing
performance, including a high sensitivity, a wide detecting concentration
range (17 ppm to 100 ppb), and fast response/recovery (30 s/∼21
s) at low NO2 concentration (0.68 ppm). Finally, the good
enough selectivity, stability, and nondecayed sensing of the WO3-based gas sensor after 14 days were demonstrated.