Wet process-based fabrication of WO3 thin film for NO2 detection

It was tried to fabricate a WO3 thin film device through a wet process starting from an aqueous sol of WO3·2H2O. When mixed with polyethylene glycol (PEG, molecular weight 6000), the sol was made compatible to spin coating on an alumina substrate and the coating could be converted into a WO3 thin film by calcination at 300 °C for 2 h. Starting with a typical coating dispersion containing WO3·2H2O by 5 mass% on WO3 basis and 2 mass% PEG, the WO3 film obtained was 450 nm in mean thickness. The film was a slim pack of square plates, each of which was a stack of thin lamellar crystals of WO3. The device was sensitive enough to detect 50 ppb NO2 in air at 200 or 250 °C, although the response and recovery transients were rather sluggish. Unexpectedly, the transients were found to be sharpened drastically in humid atmosphere, while sensor response (sensitivity) to NO2 was hardly degraded with humidity.