Slow Transition Processes to Chemisorption on Oxide Surfaces Discriminate Vapor Aliphatic Carboxylic Acid Homologues

Molecular selectivity among volatile organic compounds (VOCs) remains a critical challenge for metal oxide (MOx) gas sensors, particularly in distinguishing molecules with identical functional groups. Carboxylic acids, in particular, pose a significant challenge due to the strong influence of their terminal groups. In this study, we demonstrate that the chain length of aliphatic carboxylic acids can be effectively distinguished at room temperature by analyzing their adsorption-desorption behaviors on ZnO nanowire arrays. By comparing GC-MS, FT-IR, and QCM observations, we revealed that a large portion of carboxylic acids with longer alkyl chains is initially physisorbed and gradually transitions to chemisorption. To elucidate this finding, we propose an adsorption model considering a transition from physisorption to chemisorption, which slows as the alkyl chain extends due to increased van der Waals (vdW) interactions, validated by molecular dynamics simulations. Additionally, their adsorption behaviors differentiated even when exposed to mixture vapors, highlighting the potential for identifying compositions in complex mixtures. Furthermore, our investigation of other MOx surfaces reveals that Lewis acidity is crucial for carbon-number selectivity. This study underscores the previously underestimated impact of vdW interactions between alkyl chains and the ZnO surface in adsorption behavior, offering new insights for designing next-generation gas sensors.