Formaldehyde Graphene Gas Sensors Modified by Thermally Evaporated Tin Oxides and Tin Compound Films

Tin oxides and tin compound (SnO x –Sn) films with various thicknesses were thermally evaporated onto the chemical vapor deposition (CVD) grown graphene films, and their sensitivity to the formaldehyde (HCHO) was investigated. It was found that the graphene substrate did not show any chemical inertness for the thermally evaporated SnO x –Sn films, which could be easily grown on the bare graphene films without any functionalization. The Sn was only partially oxidized, and the specific structure of Sn core and SnO x (including SnO 2 and SnO) outer shell as well as the mixture of the SnO x and Sn nanoparticles was proposed, in which SnO x acted as highly sensitive reaction centers while the metallic Sn in association with high conductive graphene substrates formed electron transporting networks. Abnormal p-type response of the SnO x –Sn/graphene sensors upon the exposure to reducing HCHO was observed probably due to the generation of holes from the reactions of oxygen vacancies and oxygen. The SnO x –Sn/graphene sensors sensitivity showed strong dependence on the compound films surface morphology, which was closely related to the films thickness and thermal evaporation rate. With the optimal thickness of 1 nm and thermal evaporation rate of 0.1 Å/s, SnO x –Sn/graphene sensors response of 24% upon the exposure to 10 ppm of HCHO and the low detection limit of 200 ppb could be obtained. The mechanism responsible for such sensitivity performance was discussed.