CO2 sensing of La0.875Ca0.125FeO3 in wet vapor: a comparison of experimental results and first-principles calculations

Abstract

Experimental results show that with an increase of relative humidity, the resistance of La_0.875Ca_0.125FeO₃ decreases at room temperature but increases at higher temperatures (140–360 °C). The humid effect at room temperature is due to the movement of H⁺ or H₃O⁺ inside of the condensed water layer on the surface of La_0.875Ca_0.125FeO₃. Regarding the humid effect at high temperatures, the density functional theory (DFT) calculations show that H₂O can be adsorbed onto the La_0.875Ca_0.125FeO₃ surface in the molecular and dissociative adsorption configurations, where the La_0.875Ca_0.125FeO₃ surface gains some electrons from H₂O or its dissociative products, consistent with our observation. Experimental results also show that CO₂ sensing response at high temperatures decreases with an increase of room-temperature relative humidity. DFT calculations indicate that CO₂ adsorbed onto the La_0.875Ca_0.125FeO₃(010) surface, where high concentration oxygen adsorption occurs without water adsorption nearby, releases some electrons into the semiconductor surface, playing the role of a donor. The interaction between CO₂ and the local La_0.875Ca_0.125FeO₃(010) surface with pre-adsorption of H₂O nearby results in some electron transfer from the La_0.875Ca_0.125FeO₃ surface to CO₂, which is responsible for the weakening of CO₂ response at high temperatures for La_0.875Ca_0.125FeO₃ with an increase of room-temperature relative humidity.