NO2 gas sensitivity of sol-gel-derived alpha-Fe2O3 thin films

alpha-Fe2O3 thin films have been coated on silicon and sapphire substrates with Ag, Au and Pt electrodes by sol-gel processing. Differential thermal analysis and X-ray diffraction have highlighted that the film crystallization occurs at temperatures higher than 400 degrees C. After heat treatments at 400, 500 and 600 degrees C for 30 min, the 200 nm-thick films have alpha-Fe2O3 cystallites about 50 nm in mean size, estimated from the half-width of the (104) diffraction peak. The electrical response of the films to various gases including NO2 and CO has been tested in the temperature range 100-300 degrees C. The sensors with Pt and Au electrodes show the largest resistance changes for NO2 at 210 degrees C and for CO at 300 degrees C, respectively. For instance, the sensitivity, defined as the ratio between the resistances in gas and in air, is 36 for a Pt-electrode sensor in 100 ppm NO2, and 1.6 for an Au-electrode sensor in 600 ppm CO. No CH4 and NO sensitivities have been found in the investigated samples. The electrode effect on the gas sensitivity has been reasoned in terms of the Pt and An catalyst reaction to NO2 and CO on alpha-Fe2O3 films.