Ammonia Sensing Resistors Based on ${\rm Fe}_{2}{\rm O}_{3}$-Modified ZnO Thick Films

Thick films of ZnO were prepared by a screen printing technique. Unmodified ZnO thick film was almost insensitive to ${\rm NH}_{3}$. ZnO thick films were surface modified with ${\rm Fe}^{3+}$ by dipping them into an aqueous solution of ammonium ferric sulfate for different intervals of time and fired at 500$^{\circ}{\rm C}$ for 24 h. The ammonium ferric sulfate would transform into ferric oxide ${\rm Fe}_{2}{\rm O}_{3}$ upon firing. The grains of ${\rm Fe}_{2}{\rm O}_{3}$ dispersed around the grains of ZnO base material. Upon exposure to ${\rm NH}_{3}$ gas, the barrier height ${\rm Fe}_{2}{\rm O}_{3}-{\rm ZnO}$ intergranular region decreases markedly due to the chemical transformation of ${\rm Fe}_{2}{\rm O}_{3}$ into ammonium ferric hydroxide, leading to a drastic increase in conductance. The ZnO film dipped for 5 min (with 0.74 mass % of ${\rm Fe}_{2}{\rm O}_{3}$) was observed to be most sensitive to ${\rm NH}_{3}$ gas at 350$^{\circ}{\rm C}$. 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 ${\rm NH}_{3}$ gas at 350$^{\circ}{\rm C}$, 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 ${\rm NH}_{3}$ gas were studied and discussed.