Ammonia Sensing Resistors Based on Fe₂ O₃-Modified ZnO Thick Films

Thick films of ZnO were prepared by a screen printing technique. Unmodified ZnO thick film was almost insensitive toNH₃. ZnO thick films were surface modified withFe³⁺by dipping them into an aqueous solution of ammonium ferric sulfate for different intervals of time and fired at 500∘ Cfor 24 h. The ammonium ferric sulfate would transform into ferric oxideFe₂ O₃upon firing. The grains ofFe₂ O₃dispersed around the grains of ZnO base material. Upon exposure toNH₃gas, the barrier heightFe₂ O₃- ZnOintergranular region decreases markedly due to the chemical transformation ofFe₂ O₃into ammonium ferric hydroxide, leading to a drastic increase in conductance. The ZnO film dipped for 5 min (with 0.74 mass % ofFe₂ O₃) was observed to be most sensitive toNH₃gas at 350∘ 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) ofNH₃gas at 350∘ 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 ofNH₃gas were studied and discussed.