Dependence of the Nitrogen Dioxide (NO₂) Sensitivity of SnO(x) -Sn/Graphene Gas Sensors on Vacuum Annealing and Ultraviolet (UV) Ozone Exposure.

Tin oxides and tin (SnO(x) -Sn) compound films were thermally evaporated onto chemical vapor deposition (CVD)-grown graphene films to obtain improved nitrogen dioxide (NO₂) gas sensitivity. The effects of the vacuum annealing and ultraviolet (UV) ozone (O₃) exposure of the bare graphene films prior to the thermal evaporation on the SnO(x) -Sn films’ sensitivities, bonding states, and surface morphologies were investigated. With increasing annealing time, the coverage of the SnO(x) -Sn nanoparticles on the graphene increased and the p to n sensitivity transition occurred when n-type SnO(x) -Sn nanoparticles became dominant instead of the p-type graphene films for sensors without O₃ exposure. Meanwhile, the opposite p-type sensitivity response was dominant with increasing annealing time for the O₃-treated sensors. The chemisorbed Sn on the graphene generated by O₃ exposure was oxidized by highly reactive NO₂, resulting in a p-type doping effect, which would lead to n- to p-type sensitivity transition when the hole concentration exceeded the initial electron concentration of the n-type SnO(x) -Sn compound films. Vacuum annealing and O₃ exposure also exhibited a tremendous influence on the SnO(x) -Sn films’ surface morphologies, which could be responsible for the subsequent sensitivity dependence.