▎ 摘　　要

In this work, flame-spray-made undoped SnO2 nanoparticles were loaded with 0.1-5 wt % electrolytically exfoliated graphene and systematically studied for NO2 sensing at low working temperatures. Characterizations by X-ray diffraction, transmission/scanning electron microscopy, and Raman and X-ray photoelectron spectroscopy indicated that high-quality multilayer graphene sheets with low oxygen content were widely distributed within spheriodal nanoparticles having polycrystalline tetragonal SnO2 phase. The 10-20 mu m thick sensing films fabricated by spin coating on Au/Al2O3 substrates were tested toward NO2 at operating temperatures ranging from 25 to 350 degrees C in dry air. Gas-sensing results showed that the optimal graphene loading level of 0.5 wt % provided an ultrahigh response of 26 342 toward 5 ppm of NO2 with a short response time of 13 s and good recovery stabilization at a low optimal operating temperature of 150 degrees C. In addition, the optimal sensor also displayed high sensor response and relatively short response time of 171 and 7 min toward 5 ppm of NO2 at room temperature (25 degrees C). Furthermore, the sensors displayed very high NO2 selectivity against H2S, NH3, C2H5OH, H-2, and H2O. Detailed mechanisms for the drastic NO2 response enhancement by graphene were proposed on the basis of the formation of graphene-undoped SnO2 ohmic metal semiconductor junctions and accessible interfaces of graphene SnO2 nanoparticles. Therefore, the electrolytically exfoliated graphene-loaded FSP-made SnO2 sensor is a highly promising candidate for fast, sensitive, and selective detection of NO2 at low operating temperatures.