Ammonia sensing resistors based on Fe2O3-modified ZnO thick films

Thick films of ZnO were prepared by a screen printing technique. Unmodified ZnO thick film was almost insensitive to NH3. ZnO thick films were surface modified with Fe3+ by dipping them into an aqueous solution of ammonium ferric sulfate for different intervals of time and fired at 500 degrees C for 24 h. The ammonium ferric sulfate would transform into ferric oxide Fe2O3 upon firing. The grains of Fe2O3 dispersed around the grains of ZnO base material. Upon exposure to NH3 gas, the barrier height Fe2O3 - ZnO intergranular region decreases markedly due to the chemical transformation of Fe2O3 into ammonium ferric hydroxide, leading to a drastic increase in conductance. The ZnO film dipped for 5 min (with 0.74 mass % of Fe2O3) was observed to be most sensitive to NH3 gas at 350 degrees C. The Fe-surface misfits, calcination temperature, and operating temperature can affect the microstructure and gas sensing performance of the sensor. The efforts are made in present paper to develop the sensor using surface-modified zinc oxide. An exceptional sensitivity was found to low concentrations (50 ppm) of NH3 gas at 350 degrees C, and no cross sensitivity was observed, even to high concentrations of other hazardous and polluting gases. The effects of microstructure and surfactant concentration on the sensitivity, selectivity, response, and recovery of the sensor in the presence of NH3 gas were studied and discussed.