Wet process-based fabrication of WO₃ thin film for NO ₂ 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 2h. Starting with a typical coating dispersion containing WO ₃·2H₂O by 5mass% on WO₃ basis and 2mass% PEG, the WO₃ film obtained was 450nm 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 50ppb 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.