Room temperature synthesis and ammonia sensing of monodispersed hierarchical 1 and 3-dimensional Co3O4 nanostructures: switching from p to n-type sensing

This study report on a sonochemical synthesis of 1- and 3-dimensional hierarchical nanostructured cobalt oxide systems (Co₃O₄) and their application in ammonia sensing at room temperature (i.e., 30 °C). The Co₃O₄ nanostructures were synthesized via a room temperature-assisted precipitation and subsequent thermal treatment of the oxalate precursor. The resulted nanostructures were characterized by SEM, XRD, TEM, FTIR spectroscopy, BET, and TGA/DTA. The synthesis mechanism was proposed on the basis of morphology analyzed at various stages of the particle growth. It was observed that the final hierarchical microspheres structure resulted from the self-aggregation of the initially formed nanorods. The microspheres and nanorods were used as efficient room temperature gas sensors for ammonia detection in the concentration range of 0.01–500 ppm. The nanorod-based sensor showed an unusual n-type sensing behavior to ammonia in a temperature range of 30–300 °C. This transition of p to n-type was correlated to the formation of successive layers of physisorbed water molecules at the surface of the synthesized Co₃O₄. However, in case of the microspheres, the n-type behavior and superior sensitivity were observed at 30 °C followed by a negligible response up to 200 °C, while the intrinsic p-type behavior was recorded at an elevated temperature (200–300 °C). The observed unusual sensing performance may be associated with the crystallographic nature and lattice strain in the material structures. Additionally, the large specific surface area and the change in crystalline structure with temperature made the as prepared novel hierarchical Co₃O₄ structures a distinctive material for sensing ammonia at 30 °C.