Highly selective room temperature ammonia sensors based on ZnO nanostructures decorated with graphene quantum dots (GQDs)

This paper presents a highly selective ammonia sensor based on ZnO nanostructures combined with graphene quantum dots (GQDs). Novel graphene quantum dots (GQDs) have an average lateral size distribution of 2.6 nm. Various amounts of GQDs were combined with the ZnO nanostructure surfaces. Prior to the ZnO:GQDs investigation, field emission scanning electron microscopy (FE-SEM), energy-dispersive spectroscopy (EDS), Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) were used to intensively characterize the surface morphologies, existence of functional groups, and chemical compositions. ZnO:GQD heterojunctions are crucial candidate materials due to their highly selective response to ammonia (NH3) vapor. The ammonia sensing characteristics of bare ZnO and ZnO:GQDs sensors at room temperature were systematically investigated via exposure to acetone and ethanol vapor. The ammonia sensing results show that ZnO:GQDs sensors with a volume of 15 μL have optimum sensor responses at an ammonia concentration of 1000 ppm with a value of 6047. The ammonia sensing properties of ZnO:GQDs sensors are due to the GQDs' carboxyl and hydroxyl groups, which produce more oxygen-containing groups leading to a high H+ molecule density. This further contributes to their highly responsive and selective performance for sensing ammonia at room temperature.