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 500degC 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 Fe₂O ₃ 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 NH ₃ gas at 350degC. 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 NH ₃ gas at 350degC, 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