Enhanced H2S gas sensing performance of Ca-doped Bismuth Ferrite thick films

In the present work, gas sensing properties of Calcium-doped Bismuth Ferrite (BFO) nano-particulate films have been investigated as a function of doping concentration, operating temperature and nature of the exposure gases. Pure Bismuth ferrite films were found to be sensitive towards H2S gas at 350 °C. With increase in calcium concentration, besides an increase in sensor response, from ∼4.5 in BFO to ∼226 in Bi0·7Ca0·3FeO3 on exposure to 50 ppm of H2S, a non-ideal p-type behavior is observed in response curves. Such a large enhancement has not been observed earlier in a p-type semiconductor. In BFO and Bi0·9Ca0·1FeO3, resistance of films is observed to increase in accordance to p-type behavior, followed by a small decrease before saturation in gas ambience. In samples with higher calcium content, on exposure to gas, resistance of the sample is observed to increase initially followed by a decrease in resistance while still in gas ambience (saturation in gas ambience is not at all observed). In order to investigate the reason for (i) the large enhancement in sensor response usually not observed in a p-type sensor, and (ii) origin of non-ideal p-type behavior, X-ray photoelectron spectroscopy analysis of Bi0·7Ca0·3FeO3 films was carried out, after gas exposure. Based on analysis of XPS results, an additional sensing mechanism involving chemical reaction between the cations of sensing material and H2S, was found; which could satisfactorily explain the non-ideal behavior as well as enhancement in sensor reponse.