Ultrasensitive NO<inf>2</inf>Sensor Based on Ohmic Metal-Semiconductor Interfaces of Electrolytically Exfoliated Graphene/Flame-Spray-Made SnO<inf>2</inf>Nanoparticles Composite Operating at Low Temperatures

Abstract

© 2015 American Chemical Society. In this work, flame-spray-made undoped SnO2nanoparticles were loaded with 0.1-5 wt % electrolytically exfoliated graphene and systematically studied for NO2sensing 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 SnO2phase. The 10-20 μm thick sensing films fabricated by spin coating on Au/Al2O3substrates were tested toward NO2at operating temperatures ranging from 25 to 350 °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 NO2with a short response time of 13 s and good recovery stabilization at a low optimal operating temperature of 150 °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 NO2at room temperature (25 °C). Furthermore, the sensors displayed very high NO2selectivity against H2S, NH3, C2H5OH, H2, and H2O. Detailed mechanisms for the drastic NO2response enhancement by graphene were proposed on the basis of the formation of graphene-undoped SnO2ohmic metal-semiconductor junctions and accessible interfaces of graphene-SnO2nanoparticles. Therefore, the electrolytically exfoliated graphene-loaded FSP-made SnO2sensor is a highly promising candidate for fast, sensitive, and selective detection of NO2at low operating temperatures.