Cu–BTC MOF–graphene-based hybrid materials as low concentration ammonia sensors†
Ammonia sensing capability of previously developed hybrid materials consisting of Cu-based MOF and either graphite oxide or aminated graphite oxide was investigated for the first time. The chips were exposed to continuous cycles of three different ammonia concentrations, followed by purging with dry air. The change in a normalized resistance was measured. All chips showed an irreversible increase in the resistance when initially exposed to ammonia indicating the chemical reaction of the target with their components. This resulted in the collapse of the MOF component. After signal stabilization/equilibration the chips were further tested for ammonia sensing and a reversible increase in the resistance was observed for all samples. Even though the crystalline porous structure of the sensing materials was no longer present, the ability of the resulting amorphous phase to weakly adsorb ammonia enabled the recording of electrical signal changes. The specific structure of the hybrid materials combined with the proximity of the graphene phase, resulted in carrier mobility. A hybrid material with the smallest content of graphene phase exhibited the largest signal change upon exposure to ammonia. This was linked to the more developed MOF units in the case of this material, and therefore to the larger involvement of the amorphous phase released by the collapse of MOF in the sensing mechanism. A linear relationship between the response of the sensors and the ammonia concentration was found. Combining the adsorption capacity of the hybrid materials with an electrical signal allows their application as components of safety devices.