Formaldehyde sensing with a parts-per-billion limit of detection by dielectric properties and crystal symmetry optimization in BiFeO3-based p-type solid solution

Regarding sensing properties, a well-accepted theory is that the dielectric constant plays a crucial role in the depletion layer width and sensing response. Nevertheless, this theory has not yet been experimentally well demonstrated and applied in the design of high-performance sensors due to the incompatibility in conductivity, because traditional dielectrics are insulating while sensors are semiconducting. Herein, a very promising formaldehyde sensor based on new types of dielectrics (multiferroics) was designed via the strategy of improving the dielectric properties. The as-prepared Bi0.95Sm0.05FeO3 sensors show a decent sensing response in detecting ppb level formaldehyde, which is the best-known indoor carcinogenic air pollutant. It also shows low humidity dependence without sacrificing the gas response, making it an excellent sensor in indoor air quality monitoring. The sensing mechanism is attributed to the noncentrosymmetric R3c phase, which exhibits a higher dielectric constant and conductive domain walls, favoring a thicker hole accumulation layer and high carrier mobility, respectively. In stark contrast, the centrosymmetric Pbnm samples exhibit poor sensing responses, which hardly respond to formaldehyde because of the lower dielectric constant and cancellation of the domain structure, even though they possess a smaller particle size.