Exploring an n-type conducting polymer (BBL) as a potential gas sensing material for NH3 and H2S detection

Abstract

Abstract Conducting polymers (CPs) have garnered significant interest in being used as an active material in gas sensors mainly because of their structural flexibility, ease of synthesis, and enhanced performance at room temperature. The p-type CPs and their composites are mostly studied in gas sensing, which, unfortunately, exhibit limitations in terms of selectivity, stability, and sensitivity toward reducing gases. This study focuses on one of the widely studied n-type polymers, BBL(benzimidazobenzophenanthroline), as an active material for the detection of two reducing gases, namely, hydrogen sulfide (H $$_{2}$$ S) and ammonia (NH $$_{3}$$ ), theoretically. Through molecular dynamics (MD) simulation and density functional theory (DFT) approach, we understand the adsorption behavior and selectivity of H $$_{2}$$ S and NH $$_{3}$$ in the BBL film. The DFT calculated adsorption energy of the preferential site at the top of a $$\pi -\pi$$ stack for H $$_{2}$$ S and NH $$_{3}$$ are – 0.22 eV and – 0.33 eV, respectively, and at the sides of a $$\pi -\pi$$ stack for H $$_{2}$$ S and NH $$_{3}$$ are – 0.42 eV and – 0.47 eV, respectively. MD simulations show that adsorption takes place in the free voids within the thin films, and the overall structure of the polymer film remained almost unaltered upon gas adsorption without any apparent swelling or significant morphological changes in the film. Our results show that BBL displays remarkable adsorption along with a higher magnitude of charge transfer for ammonia over hydrogen sulfide gas and other common gases present in the air. Moreover, both H $$_{2}$$ S and NH $$_{3}$$ gas adsorption happen without compromising the size of the $$\pi -\pi$$ stacked crystallites within the polymer film, which indicates, upon detection of reducing gases, the generated free electrons via the redox reactions between the gas molecules and polymer, will be able to be smoothly transported through the $$\pi -\pi$$ stack network present in the film. The detailed theoretical insights obtained from this study indicate the suitability of the n-type conducting polymer, BBL, for detecting reducing gases, NH $$_{3}$$ and H $$_{2}$$ S.