Cr2O3-activated ZnO thick film resistors for ammonia gas sensing operable at room temperature

Pure ZnO thick films, prepared by screen-printing technique, were almost insensitive to NH3. Pure ZnO thick films were surface activated with Cr3+ by dipping them into an aqueous solution (0.01 M) of chromium trioxide CrO3 for different intervals of time and fired at 500 °C for 24 h. The CrO3 is known to be unstable above 197 °C and transforms into Cr2O3 upon firing above 197 °C. The grains of Cr2O3 would disperse around the grains of ZnO base material. The Cr3+-activated ZnO films dipped for 5 min were observed to be sensitive and highly selective to 300 ppm of NH3 gas at room temperature. The effects of surface microstructure and Cr3+ concentrations on the sensitivity, selectivity, response and recovery of the sensor in the presence of NH3 and other gases were studied and discussed. The better performance could be attributed to an optimum number of surface misfits in terms of Cr2O3 on the ZnO films.