Investigation on the O3 sensitivity properties of WO3 thin films prepared by sol-gel, thermal evaporation and r.f. sputtering techniques

WO3 thin films have been deposited on alumina substrates provided with platinum interdigital electrodes by sol-gel (SG), r.f. sputtering (RFS), and vacuum thermal evaporation (VTE) techniques and annealed at temperatures between 500 degrees C and 600 degrees C for 1 to 30 h in static air. The morphology, crystalline phase and chemical composition of the films have been characterised using SEM, glancing XRD and XPS techniques. The electrical response has been measured exposing the films to O-3 (10-180 ppb), NO2 (0.2-1 ppm), NOx (27 ppm NO and 1 ppm NO,) at different operating temperatures ranging between 200 and 400 degrees C and humid air at 50% R.R. SG prepared films have shown bigger responses (S = I-Air/I-gas) with respect to VTE and RFS for all the investigated gases and operating temperatures. RFS prepared has resulted to be less sensitive, but faster in the response and more stable in terms of signal reproducibility. The response to O-3 has been found to be at maximum at 400 degrees C. At this temperature the response to 80 ppb of ozone has been: S = 35 (SG), S = 18 (VTE) and S = 5 (RFS). The NO2 and NOx response reached the maximum at 200 degrees C and becomes negligible at 400 degrees C. Improvements on the O-3 gas sensitivity and selectivity can be achieved by fixing the operating temperature of the films at 400 degrees C. (C) 2000 Elsevier Science S.A. All rights reserved.