Conversion of n-Type CuTCNQ into p-Type Nitrogen-Doped CuO and the Implication for Room-Temperature Gas Sensing

Sensors to detect toxic and harmful gases are usually based on metal oxides that are operated at elevated temperature. However, enabling gas detection at room temperature (RT) is a significant ongoing challenge. Here, we address this issue by demonstrating that microrods of semiconducting CuTCNQ (TCNQ = 7,7,8,8-tetracyanoquinodimethane) with nanostructured features can be employed as conductometric gas sensors operating at 50 degrees C for detection of oxidizing and reducing gases such as NO2 and NH3. The sensor is evaluated at RT and up to 200 degrees C. It was found that CuTCNQ is transformed into a N-doped CuO material with p-type conductivity when annealed at the maximum temperature. This is the first time that such a transformation, from a semiconducting charge transfer material into a N-doped metal oxide, is detected. It is shown here that both the surface chemistry and the type of majority charge carrier within the sensing layer are critically important for the type of response toward oxidizing and reducing gases. A detailed physical description of the NO2 and NH3 sensing mechanism at CuTCNQ and N-doped CuO is provided to explain the differences in the response. For the N-doped CuO sensor, detection limits of 1 ppm for NO2 and 10 ppm for NH3 are achieved.