Functionalized CNTs-Based Gas Sensors for BTX-Type Gases: How Functional Peripheral Groups Can Affect the Time Response through Surface Reactivity

Herein we present gas sensor responses of functionalized carbon nanotubes (CNTs) toward benzene and xylene vapors. The functional moieties are phthalocyanine and porphyrin derivatives which possess a central part delimited by the macroring and different peripheral groups (aryl or alkyls) surrounding the macroring. The sensor devices based on QCM (quartz crystal microbalance) and resistive transducers are performed simultaneously to elucidate the sensing mechanism. The sensing performance from the two transducers recorded at room temperature revealed different behavior but complementary to understand the mechanism. Our findings show that if the aromatic VOCs bear methyl groups (xylene), the desorption profile is slow and the response time too long, while in the absence of a methyl group (benzene) the desorption profile is very rapid with a shorter response time. This illustrates that the whole gas response is driven by an interplay between the π–π interaction and the π–alkyl or alkyl–alkyl interactions, i.e., van der Waals (VdW) interactions. The analysis of the adsorption–desorption profile shows that the interaction mechanism is based on a synergy between many interactions that proceed collectively or separately to strengthen or weaken the gas–material interaction. Thus, the presence of alkyl groups on both sides (VOCs and functional moieties) inherently contributes to define the gas material interaction.