Anomalous n-p Sensing Conversion in Cobalt Ferrite: The Role of Surface Adsorption and Carrier Scattering.

Cobalt ferrite stands out as a versatile gas-sensing material, offering tunable composition and dual redox couples. We report a remarkable n-p sensing conversion in cobalt ferrite-based sensors-an anomalous resistance modulation that contradicts conventional intrinsic semiconductor behavior. While occasionally observed in metal oxides, such phenomena remain largely unexplored in the context of cobalt ferrite. Here, by regulating the proportion of metal elements in cobalt ferrite, we systematically decouple the unconventional sensing switching and the intriguing dynamic selectivity induced by diverse volatile organic compounds (VOCs) across a temperature range of 140-230 °C. Leveraging in situ XPS, Raman spectroscopy, and photoluminescence (PL), we propose a mechanistic framework: the competitive interplay between surface adsorbed oxygen and carrier scattering mechanisms governs the electronic transport. This study not only elucidates the underlying physics of the sensing conversion phenomenon but also establishes a fundamental "structure-chemistry-property" framework for the rational design of advanced cobalt ferrite gas sensors.