Understanding the Anomalous Gas-Concentration-Driven Response Polarity Transition Behavior of a Room-Temperature-Operated TiO2-Based Alcohol Sensor

Abstract

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.