Understanding the Anomalous Gas-Concentration-Driven Response Polarity Transition Behavior of a...

Emerging p-type gas response of n-type oxide-based resistive gas sensors challenges classical theory, which posits that gas response polarity aligns with the metal oxide's conductivity type. This work investigates the anomalous gas-concentration-driven gas response polarity transition to alcohols with various chain lengths of a conventional TiO₂-based gas sensor at room temperature. A particular focus is on the relationship between the critical concentration and the gas molecular structures. The sensor shows anomalous p-type responses to four alcohols (methanol, ethanol, propanol, and butanol) and transitions to n-type as the alcohol concentration increases to approximately 9-20 K ppm, depending on the alcohol species. Interestingly, this critical concentration decreases generally as the alcohol chain length increases. This concentration-driven response polarity transition is attributed to a shift in the reaction sites governed by the gas concentration, specifically from the adsorbed water layer to the O₂ ^- on the TiO₂ surface. The p-type response is attributed to the disruption of proton migration in the adsorbed water layer on the TiO₂ surface by alcohol molecules. This process becomes saturated as the alcohol concentration increases and the reaction with the O₂ ^- on the TiO₂ surface progressively regains dominance, restoring the n-type response. Longer-chain alcohols show stronger hydrophobicity, reaching water saturation at lower concentrations, thereby showing a lower critical concentration. This work links the anomalous concentration-driven gas response polarity transition to alcohol chain length, providing critical theoretical insights into anomalous gas-sensing behaviors.