Cr 2O 3-activated ZnO thick film resistors for ammonia gas sensing operable at room temperature

Abstract

Pure ZnO thick films, prepared by screen-printing technique, were almost insensitive to NH 3. Pure ZnO thick films were surface activated with Cr 3+ by dipping them into an aqueous solution (0.01 M) of chromium trioxide CrO 3 for different intervals of time and fired at 500 °C for 24 h. The CrO 3 is known to be unstable above 197 °C and transforms into Cr 2O 3 upon firing above 197 °C. The grains of Cr 2O 3 would disperse around the grains of ZnO base material. The Cr 3+-activated ZnO films dipped for 5 min were observed to be sensitive and highly selective to 300 ppm of NH 3 gas at room temperature. The effects of surface microstructure and Cr 3+ concentrations on the sensitivity, selectivity, response and recovery of the sensor in the presence of NH 3 and other gases were studied and discussed. The better performance could be attributed to an optimum number of surface misfits in terms of Cr 2O 3 on the ZnO films.