Temperature-Dependent n-to-p-Type Transition of 2D Mn Oxide Nanosheets toward NO₂ for Flexible Gas Sensor Application

Rapid and on-site detection of nitrogen dioxide (NO₂) is important for environmental monitoring as NO₂ is a highly toxic chemical emitted from automobiles and power plants. In this study, we proposed atomically thin two-dimensional (2D) Mn oxide nanosheets (NSs) assembled on a flexible heating substrate for application in flexible and wearable NO₂ sensors. A liquid-phase exfoliation technique was used to obtain individual Mn oxide layers that formed a homogeneous suspension. A flexible heater was fabricated by partially embedding Ag nanotubes (NTs) on the surface of a colorless polyimide (CPI) film for use as a sensor substrate. Temperature-dependent NO₂ sensing properties were investigated via control of the operating temperature using a Ag NT-embedded CPI heating film. As a result, the n-type sensing behavior of the Mn oxide NSs exhibited a response [(R_gas - R_air)/R_air × 100 (%)] of 1.20 ± 0.21% for 20 ppm NO₂ at room temperature (25 °C). Meanwhile, n-p transition occurred with p-type sensing property as the operating temperature increased to 150 °C with an improved response [(R_air - R_gas)/R_air × 100 (%)] of 4.10 ± 0.42% for 20 ppm NO₂. The characteristic n-p transition of Mn oxide NSs at different operating temperatures was evidenced by the surface-adsorbed oxygen ions (i.e., O₂^- and O^-) and nitrate species (NO₃^- and NO₃^2-).