Nanoporous carbons as gas sensors: Exploring the surface sensitivity

Ammonia sensing properties of a poly(sodium 4-styrene sulfonate) derived carbon and its air oxidized counterpart were studied. The carbons were used to fabricate chips-like devices that were further used for electrical resistance measurements when exposed to various concentrations of dry ammonia. The sulfur-containing carbon after oxidation had a nine times greater response to ammonia than before oxidation. This response was evaluated from the point of view of reactive adsorption and physisorption. The initial reactions of ammonia with the sulfonic groups on the non-oxidized carbon led to an irreversible increase in resistance, while on the oxidized carbon, deprived of sulfonic groups, physisorption caused an opposite effect. After equilibration and reactive adsorption, a reversible decrease in resistance upon exposure to ammonia was found on both samples. The extensive characterization of the carbons’ surfaces, and their performance as ammonia adsorbents, suggest that the sensing mechanism is governed by the physical adsorption of ammonia inside the micropores. Higher concentration of ammonia in air results in more pores being filled and thus, in the stronger response of the sensor.